Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Hydrometeorological multi-model ensemble simulations of the 4 November 2011 flash flood event in Genoa, Italy, in the framework of the DRIHM Project
    Hally, A. ; Caumont, O. ; Garrote, L. ; Richard, E. ; Weerts, A.H. ; Delogu, F. ; Fiori, E. ; Rebora, N. ; Parodi, A. ; Mihalovic, A. ; Ivkovic, M. ; Dekic, L. ; Verseveld, W.J. ; Nuissier, O. ; Ducrocq, V.P. ; Agostino, D. d'; Galizia, A. ; Danovaro, E. ; Clematis, A. - \ 2015
    Natural Hazards and Earth System Sciences 15 (2015). - ISSN 1561-8633 - p. 537 - 555.
    distributed hydrological model - convection-permitting ensemble - rainfall-runoff model - precipitating events - forecasting system - prediction system - extreme rainfall - mesoscale - scheme - parameterization
    The e-Science environment developed in the framework of the EU-funded DRIHM project was used to demonstrate its ability to provide relevant, meaningful hydrometeorological forecasts. This was illustrated for the tragic case of 4 November 2011, when Genoa, Italy, was flooded as the result of heavy, convective precipitation that inundated the Bisagno catchment. The Meteorological Model Bridge (MMB), an innovative software component developed within the DRIHM project for the interoperability of meteorological and hydrological models, is a key component of the DRIHM e-Science environment. The MMB allowed three different rainfall-discharge models (DRiFt, RIBS and HBV) to be driven by four mesoscale limited-area atmospheric models (WRF-NMM, WRF-ARW, Meso-NH and AROME) and a downscaling algorithm (RainFARM) in a seamless fashion. In addition to this multi-model configuration, some of the models were run in probabilistic mode, thus giving a comprehensive account of modelling errors and a very large amount of likely hydrometeorological scenarios (> 1500). The multi-model approach proved to be necessary because, whilst various aspects of the event were successfully simulated by different models, none of the models reproduced all of these aspects correctly. It was shown that the resulting set of simulations helped identify key atmospheric processes responsible for the large rainfall accumulations over the Bisagno basin. The DRIHM e-Science environment facilitated an evaluation of the sensitivity to atmospheric and hydrological modelling errors. This showed that both had a significant impact on predicted discharges, the former being larger than the latter. Finally, the usefulness of the set of hydrometeorological simulations was assessed from a flash flood early-warning perspective.
    The Challenge of Forecasting the Onset and Development of Radiation Fog Using Mesoscale Atmospheric Models
    Steeneveld, G.J. ; Ronda, R.J. ; Holtslag, A.A.M. - \ 2015
    Boundary-Layer Meteorology 154 (2015)2. - ISSN 0006-8314 - p. 265 - 289.
    boundary-layer - climate models - prediction - resolution - weather - system - parameterization - detrainment - sensitivity - simulation
    The numerical weather prediction of radiation fog is challenging, as many models typically show large biases for the timing of the onset and dispersal of the fog, as well as for its depth and liquid water content. To understand the role of physical processes, i.e. turbulence, radiation, land-surface coupling, and microphysics, we evaluate the HARMONIE and Weather Research and Forecasting (WRF) mesoscale models for two contrasting warm fog episodes at the relatively flat terrain around the Cabauw tower facility in the Netherlands. One case involves a radiation fog that arose in calm anticyclonic conditions, and the second is a radiation fog that developed just after a cold front passage. The WRF model represents the radiation fog well, while the HARMONIE model forecasts a stratus lowering fog layer in the first case and hardly any fog in the second case. Permutations of parametrization schemes for boundary-layer mixing, radiation and microphysics, each for two levels of complexity, have been evaluated within the WRF model. It appears that the boundary-layer formulation is critical for forecasting the fog onset, while for fog dispersal the choice of the microphysical scheme is a key element, where a double-moment scheme outperforms any of the single-moment schemes. Finally, the WRF model results appear to be relatively insensitive to horizontal grid spacing, but nesting deteriorates the modelled fog formation. Increasing the domain size leads to a more scattered character of the simulated fog. Model results with one-way or two-way nesting show approximately comparable results.
    Effects of Irrigation in India on the Atmospheric Water Budget
    Tuinenburg, O.A. ; Hutjes, R.W.A. ; Stacke, T. ; Wiltshire, A. ; Lucas-Picher, P. - \ 2014
    Journal of Hydrometeorology 15 (2014)3. - ISSN 1525-755X - p. 1028 - 1050.
    soil-moisture - part i - precipitation - climate - monsoon - scheme - models - cycle - parameterization - representation
    The effect of large-scale irrigation in India on the moisture budget of the atmosphere was investigated using three regional climate models and one global climate model, all of which performed an irrigated run and a natural run without irrigation. Using a common irrigation map, year-round irrigation was represented by adding water to the soil moisture to keep it at 90% of the maximum soil moisture storage capacity, regardless of water availability. For two focus regions, the seasonal cycle of irrigation matched that of the reference dataset, but irrigation application varied between the models by up to 0.8 mm day(-1). Because of the irrigation, evaporation increased in all models, but precipitation decreased because of a strong decrease in atmospheric moisture convergence. A moisture tracking scheme was used to track individual evaporated moisture parcels through the atmosphere to determine where these lead to precipitation. Up to 35% of the evaporation moisture from the Ganges basin is recycling within the river basin. However, because of a decreased moisture convergence into the river basin, the total amount of precipitation in the Ganges basin decreases. Although a significant fraction of the evaporation moisture recycles within the river basin, the changes in large-scale wind patterns due to irrigation shift the precipitation from the eastern parts of India and Nepal to the northern and western parts of India and Pakistan. In these areas where precipitation increases, the relative precipitation increase is larger than the relative decrease in the areas where precipitation decreases. It is concluded 1) that the direct effects of irrigation on precipitation are small and are not uniform across the models; 2) that a fraction of up to 35% of any marginal evaporation increase (for example, due to irrigation) will recycle within the river basin; and 3) that when irrigation is applied on a large scale, the dominant effect will be a change in large-scale atmospheric flow that decreases precipitation in eastern India and increases it in western and northern India.
    Effect of rooting depth, plant density and planting date on maize (Zea Mays L.) yield and water use efficiency in semi-arid Zimbabwe: Modelling with AquaCrop
    Nyakudya, I.W. ; Stroosnijder, L. - \ 2014
    Agricultural Water Management 146 (2014). - ISSN 0378-3774 - p. 280 - 296.
    fao crop model - management options - tillage - soils - parameterization - conservation - simulation - paradigm
    Under low and poorly distributed rainfall higher food production can be achieved by increasing crop water use efficiency (WUE) through optimum soil fertility management and selection of deep-rooting cultivars, appropriate plant density and planting dates. We explored AquaCrop's applicability in selecting adaptive practices for improving maize yield and WUE under rainfed smallholder farming in semi-arid Zimbabwe. AquaCrop was first tested using field measurements without calibration. The model was subsequently applied to estimate the effect of effective rooting depth (ERD), plant density and planting date on maize yield. Simulations were done with daily rainfall data for 25 seasons. During model testing AquaCrop simulated canopy cover development well and simulated biomass accumulation showed good agreement with measured values. The model overestimated soil water, and observed final biomass and grain yield were 96 and 92% of simulated values, respectively. Model application showed that increasing ERD from 0.40 m at 32,500 plants ha-1 to 0.60 m at 44,400 plants ha-1 increased grain yield from 6.0 to 7.8 t ha-1, biomass water use efficiency by 20.5%, grain water use efficiency by 23.6% and transpiration water use efficiency by 26.8%. At 0.60 and 0.80 m ERD and 44,400 plants ha-1, biomass and grain yield, and WUE, were similar. Drainage below the rootzone was =40% of non-productive water losses in normal and wet seasons whilst soil evaporation contributed 47% in dry seasons at 0.80 m ERD. To improve yield and WUE, we recommend: incorporation of deep-rooting legumes, deeper-rooting cultivars (=0.60 m effective rooting depth) and practices that improve ERD, a plant density of 44,400 plants ha-1; and practices that reduce soil evaporation e.g. mulching and addition of organic fertilisers to improve soils’ available water capacity and enhance response to mineral fertilisers. Further research should include field testing of results from this study with farmers.
    Evaluation of the Weather Research and Forecasting mesoscale model for GABLS3: Impact of boundary-layer schemes, boundary conditions and spin-up
    Kleczek, M.A. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2014
    Boundary-Layer Meteorology 152 (2014)2. - ISSN 0006-8314 - p. 213 - 243.
    nonlocal closure-model - low-level jets - wrf model - part i - vertical diffusion - diurnal cycles - sea-ice - turbulence - sensitivity - parameterization
    We evaluated the performance of the three-dimensional Weather Research and Forecasting (WRF) mesoscale model, specifically the performance of the planetary boundary-layer (PBL) parametrizations. For this purpose, Cabauw tower observations were used, with the study extending beyond the third GEWEX Atmospheric Boundary-Layer Study (GABLS3) one-dimensional model intercomparison. The WRF model (version 3.4.1) contains 12 different PBL parametrizations, most of which have been only partially evaluated. The GABLS3 case offers a clear opportunity to evaluate model performance, focusing on time series of near-surface weather variables, radiation and surface flux budgets, vertical structure and the nighttime inertial oscillation. The model results revealed substantial differences between the PBL schemes. Generally, non-local schemes tend to produce higher temperatures and higher wind speeds than local schemes, in particular, for nighttime. The WRF model underestimates the 2-m temperature during daytime (about TeX K) and substantially underestimates it at night (about TeX K), in contrast to the previous studies where modelled 2-m temperature was overestimated. Considering the 10-m wind speed, during the night turbulent kinetic energy based schemes tend to produce lower wind speeds than other schemes. In all simulations the sensible and latent heat fluxes were well reproduced. For the net radiation and the soil heat flux we found good agreement with daytime observations but underestimations at night. Concerning the vertical profiles, the selected non-local PBL schemes underestimate the PBL depth and the low-level jet altitude at night by about 50 m, although with the correct wind speed. The latter contradicts most previous studies and can be attributed to the revised stability function in the Yonsei University PBL scheme. The local, turbulent kinetic energy based PBL schemes estimated the low-level jet altitude and strength more accurately. Compared to the observations, all model simulations show a similar structure for the potential temperature, with a consistent cold bias (TeX2 K) in the upper PBL. In addition to the sensitivity to the PBL schemes, we studied the sensitivity to technical features such as horizontal resolution and domain size. We found a substantial difference in the model performance for a range of 12, 18 and 24 h spin-up times, longer spin-up time decreased the modelled wind speed bias, but it strengthened the negative temperature bias. The sensitivity of the model to the vertical resolution of the input and boundary conditions on the model performance is confirmed, and its influence appeared most significant for the non-local PBL parametrizations
    Seasonal dependence of the urban heat island on the street canyon aspect ratio
    Theeuwes, N.E. ; Steeneveld, G.J. ; Ronda, R.J. ; Heusinkveld, B.G. ; Hove, L.W.A. van; Holtslag, A.A.M. - \ 2014
    Quarterly Journal of the Royal Meteorological Society 140 (2014)684. - ISSN 0035-9009 - p. 2197 - 2210.
    warmte - steden - stedelijke planning - warmtebalans - seizoenvariatie - meteorologie - nederland - heat - towns - urban planning - heat balance - seasonal variation - meteorology - netherlands - boundary-layer - energy-balance - climate zones - model - temperature - parameterization - simulation - schemes - cabauw - field
    In this paper we study the relation between the urban heat island (UHI) in the urban canyon and street geometry, in particular the aspect ratio. Model results and observations show that two counteracting processes govern the relation between the nocturnal UHI and the building aspect ratio: i.e. trapping of longwave radiation and shadowing effects. In general, trapping of longwave radiation supports the UHI, whereas shadowing effects reduce the UHI. The net effect depends on the UHI definition and the amount of available shortwave radiation penetrating the canyon. In summer, autumn and spring the shadowing effects can already reduce the UHI starting at an aspect ratio between 0.5 and 1. The analysis is carried out using several methods. Firstly, the single-column model version of the Weather Research and Forecasting model (WRF) is used extensively. Two separate runs, one rural and one urban, are used to estimate the UHI. Secondly, the urban canyon temperature at the two meter level is introduced, which allows for direct comparison between modelled and observed air temperatures within the urban canyon. Finally, the model is evaluated for all four seasons. The results of this research provide important insights for urban planning on how to use the aspect ratio to mitigate the UHI in the urban canyon
    Off-line algorithm for calculation of vertical tracer transport in the troposphere due to deep convection
    Belikov, D.A. ; Maksyutov, S. ; Krol, M.C. ; Fraser, A. ; Rigby, M. ; Bian, H. ; Agusti-Panareda, A. ; Bergmann, D. ; Bousquet, P. ; Cameron-Smith, P. ; Chipperfield, M.P. ; Fortems-Cheiney, A. ; Gloor, E. ; Haynes, K. ; Hess, P. ; Houweling, S. ; Kawa, S.R. ; Law, R.M. ; Loh, Z. ; Meng, L. ; Palmer, P.I. ; Patra, P.K. ; Prinn, R.G. ; Saito, R. ; Wilson, C. - \ 2013
    Atmospheric Chemistry and Physics 13 (2013)3. - ISSN 1680-7316 - p. 1093 - 1114.
    general-circulation model - observed radon profiles - cumulus convection - atmospheric transport - climate simulations - meteorological data - cloud ensemble - precipitation - rn-222 - parameterization
    A modified cumulus convection parametrisation scheme is presented. This scheme computes the mass of air transported upward in a cumulus cell using conservation of moisture and a detailed distribution of convective precipitation provided by a reanalysis dataset. The representation of vertical transport within the scheme includes entrainment and detrainment processes in convective updrafts and downdrafts. Output from the proposed parametrisation scheme is employed in the National Institute for Environmental Studies (NIES) global chemical transport model driven by JRA-25/JCDAS reanalysis. The simulated convective precipitation rate and mass fluxes are compared with observations and reanalysis data. A simulation of the short-lived tracer Rn-222 is used to further evaluate the performance of the cumulus convection scheme. Simulated distributions of Rn-222 are evaluated against observations at the surface and in the free troposphere, and compared with output from models that participated in the TransCom-CH4 Transport Model Intercomparison. From this comparison, we demonstrate that the proposed convective scheme in general is consistent with observed and modeled results.
    Stable atmospheric boundary layers and diurnal Cycles-Challenges for Weather and Climate Models
    Holtslag, A.A.M. ; Svensson, G. ; Baas, P. ; Basu, S. ; Beare, B. ; Beljaars, A.C.M. ; Bosveld, F.C. ; Cuxart, J. ; Lindvall, J. ; Steeneveld, G.J. ; Tjernstrom, M. ; Wiel, B.J.H. van de - \ 2013
    Bulletin of the American Meteorological Society 94 (2013). - ISSN 0003-0007 - p. 1691 - 1706.
    low-level jets - land-surface - contrasting nights - soil-moisture - ecmwf model - sea-ice - turbulence - cases-99 - parameterization - fluxes
    The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2 m temperature and 10 m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind-speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, we review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art, as well as findings and recommendations from three inter-comparison studies held within the “Global Energy and Water Exchanges (GEWEX)” Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models and Large Eddy Simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas and at Cabauw in the Netherlands. From these studies, we find that even for the non-cloudy boundary layer important parameterization challenges remain.
    Convective boundary layer wind dynamics and inertial oscillations: the influence of surface stress
    Schröter, J.S. ; Moene, A.F. ; Holtslag, A.A.M. - \ 2013
    Quarterly Journal of the Royal Meteorological Society 139 (2013)676. - ISSN 0035-9009 - p. 1694 - 1711.
    low-level jets - large-eddy simulations - nocturnal jet - models - momentum - parameterization - parametrization - entrainment - fluxes - shear
    Investigating the influence of surface friction on the inertial oscillation (IO) of an extratropical, non-growing, convective boundary layer (CBL), we paid particular attention to the stability-dependent interactive coupling of shear-induced turbulence and turbulent friction, which leads to a nonlinear relationship between friction and velocity. We showed that in contrast to common perception, IO damping is controlled not only by friction but also by the dependence of friction on velocity. Furthermore, we found that surface friction not only causes damping but also modifies the restoring force. Using these basic principles, we studied the oscillatory properties (equilibrium, periodicity and damping) of the CBL by means of a model based on Monin–Obukhov surface-layer similarity (MOS) and the mixed-layer approximation. We found that the model complies with a quadratic surface stress–velocity relationship (QS) in the neutral limit, and a linear surface stress–velocity relationship (LS) in the proximity of the free-convective limit. Dynamically, the LS leads to a harmonic oscillation with a constant periodicity and exponential damping of the IO. However, the QS displays rather complex anharmonic behaviour; in comparison with the LS it produces a 50% stronger overall damping and a 100% larger contribution to the restoring force. Considering CBLs of arbitrary stability, we found that the MOS stress–velocity relation can be very well approximated by a much simpler linear combination of the LS and the QS which, respectively, represent the convective and the shear-induced contributions to friction. This enabled us to link the set of the external parameters (surface roughness, surface buoyancy flux and boundary layer depth) to a set of three effective parameters: the equilibrium velocity, the convective friction constant and the neutral friction constant. Together with the Coriolis coefficient, these parameters completely determine the IO.
    Atmosphere-ocean ozone fluxes during the TexAQS 2006, STRATUS 2006, GOMECC 2007, GasEx 2008, and AMMA 2008 cruises
    Helmig, D. ; Lang, E.K. ; Bariteau, L. ; Boylan, P. ; Fairall, C.W. ; Ganzeveld, L.N. ; Hare, J.E. ; Hueber, J. ; Pallandt, M. - \ 2012
    Journal of Geophysical Research: Atmospheres 117 (2012). - ISSN 2169-897X - 15 p.
    marine boundary-layer - dry deposition - surface - atlantic - iodine - water - coastal - parameterization - enhancement - chemistry
    A ship-based eddy covariance ozone flux system was deployed to investigate the magnitude and variability of ozone surface fluxes over the open ocean. The flux experiments were conducted on five cruises on board the NOAA research vessel Ronald Brown during 2006-2008. The cruises covered the Gulf of Mexico, the southern as well as northern Atlantic, the Southern Ocean, and the persistent stratus cloud region off Chile in the eastern Pacific Ocean. These experiments resulted in the first ship-borne open-ocean ozone flux measurement records. The median of 10 min oceanic ozone deposition velocity (v(d)) results from a combined similar to 1700 h of observations ranged from 0.009 to 0.034 cm s(-1). For the Gulf of Mexico cruise (Texas Air Quality Study (TexAQS)) the median v(d) (interquartile range) was 0.034 (0.009-0.065) cm s(-1) (total number of 10 min measurement intervals, N-f = 1953). For the STRATUS cruise off the Chilean coast, the median v(d) was 0.009 (0.004-0.037) cm s(-1) (N-f = 1336). For the cruise from the Gulf of Mexico and up the eastern U. S. coast (Gulf of Mexico and East Coast Carbon cruise (GOMECC)) a combined value of 0.018 (0.006-0.045) cm s(-1) (N-f = 1784) was obtained (from 0.019 (-0.014-0.043) cm s(-1), N-f = 663 in the Gulf of Mexico, and 0.018 (-0.004-0.045) cm s(-1), N-f = 1121 in the North Atlantic region). The Southern Ocean Gas Exchange Experiment (GasEx) and African Monsoon Multidisciplinary Analysis (AMMA), the Southern Ocean and northeastern Atlantic cruises, respectively, resulted in median ozone v(d) of 0.009 (-0.005-0.026) cm s(-1) (N-f = 2745) and 0.020 (-0.003-0.044) cms(-1) (N-f = 1147). These directly measured ozone deposition values are at the lower end of previously reported data in the literature (0.01-0.12 cm s(-1)) for ocean water. Data illustrate a positive correlation (increase) of the oceanic ozone uptake rate with wind speed, albeit the behavior of the relationship appears to differ during these cruises. The encountered wide range of meteorological and ocean biogeochemical conditions is used to investigate fundamental drivers of oceanic O-3 deposition and for the evaluation of a recently developed global oceanic O-3 deposition modeling system.
    Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment
    Svensson, G. ; Holtslag, A.A.M. ; Kumar, V. ; Mauritsen, T. ; Steeneveld, G.J. ; Angevine, W.M. ; Bazile, E. ; Beljaars, A. ; Bruijn, E.I.F. de; Cheng, A. - \ 2011
    Boundary-Layer Meteorology 140 (2011)2. - ISSN 0006-8314 - p. 177 - 206.
    turbulence closure scheme - large-eddy simulations - part i - contrasting nights - morning transition - parameterization - mesoscale - cases-99 - surface - system
    We present the main results from the second model intercomparison within the GEWEX (Global Energy andWater cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today’s numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled nearsurface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identified
    Understanding the daily cycle of evapotranspiration: a method to quantify the influence of forcings and feedbacks
    Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. ; Gounou, A. ; Guichard, F. ; Couvreux, F. - \ 2010
    Journal of Hydrometeorology 11 (2010)6. - ISSN 1525-755X - p. 1405 - 1422.
    convective boundary-layer - land-atmosphere interaction - diurnal time scales - soil-moisture - surface - model - evaporation - parameterization - entrainment - inversion
    A method to analyze the daily cycle of evapotranspiration over land is presented. It quantifies the influence of external forcings, such as radiation and advection, and of internal feedbacks induced by boundary layer, surface layer, and land surface processes on evapotranspiration. It consists of a budget equation for evapotranspiration that is derived by combining a time derivative of the Penman–Monteith equation with a mixed-layer model for the convective boundary layer. Measurements and model results for days at two contrasting locations are analyzed using the method: midlatitudes (Cabauw, Netherlands) and semiarid (Niamey, Niger). The analysis shows that the time evolution of evapotranspiration is a complex interplay of forcings and feedbacks. Although evapotranspiration is initiated by radiation, it is significantly regulated by the atmospheric boundary layer and the land surface throughout the day. In both cases boundary layer feedbacks enhance the evapotranspiration up to 20 W m-2 h-1. However, in the case of Niamey this is offset by the land surface feedbacks since the soil drying reaches -30 W m-2 h-1. Remarkably, surface layer feedbacks are of negligible importance in a fully coupled system. Analysis of the boundary layer feedbacks hints at the existence of two regimes in this feedback depending on atmospheric temperature, with a gradual transition region in between the two. In the low-temperature regime specific humidity variations induced by evapotranspiration and dry-air entrainment have a strong impact on the evapotranspiration. In the high-temperature regime the impact of humidity variations is less pronounced and the effects of boundary layer feedbacks are mostly determined by temperature variations
    Surface Wind Regionalization over Complex Terrain: Evaluation and Analysis of a High-Resolution WRF Simulation
    Jiménez, P.A. ; González-Rouco, J.F. ; García-Bustamante, E. ; Navarro, J. ; Montávez, J.P. ; Vilà-Guerau de Arellano, J. ; Dudhia, J. ; Muñoz-Roldan, A. - \ 2010
    Journal of Applied Meteorology and Climatology 49 (2010)2. - ISSN 1558-8424 - p. 268 - 287.
    speed distributions - objective analysis - diagnostic model - isolated hill - flow - forecast - climate - fields - energy - parameterization
    This study analyzes the daily-mean surface wind variability over an area characterized by complex topography through comparing observations and a 2-km-spatial-resolution simulation performed with the Weather Research and Forecasting (WRF) model for the period 1992–2005. The evaluation focuses on the performance of the simulation to reproduce the wind variability within subregions identified from observations over the 1999–2002 period in a previous study. By comparing with wind observations, the model results show the ability of the WRF dynamical downscaling over a region of complex terrain. The higher spatiotemporal resolution of the WRF simulation is used to evaluate the extent to which the length of the observational period and the limited spatial coverage of observations condition one’s understanding of the wind variability over the area. The subregions identified with the simulation during the 1992–2005 period are similar to those identified with observations (1999–2002). In addition, the reduced number of stations reasonably represents the spatial wind variability over the area. However, the analysis of the full spatial dimension simulated by the model suggests that observational coverage could be improved in some subregions. The approach adopted here can have a direct application to the design of observational networks.
    Estimation of orographically induced wave drag in the stable boundary layer during the CASES-99 experimental campaign
    Steeneveld, G.J. ; Nappo, C.J. ; Holtslag, A.A.M. - \ 2009
    Acta Geophysica 57 (2009)4. - ISSN 1895-6572 - p. 857 - 881.
    self-correlation - turbulence - model - simulations - surface - wind - parameterization - intermittency - level - flow
    This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and ¿wave drag events¿ is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flux
    Mesoscale modelling of the CO2 interactions between the surface and the atmosphere applied to the April 2007 CERES field experiment
    Sarrat, C. ; Noilhan, J. ; Lacarrère, P. ; Ceschia, E. ; Ciais, P. ; Dolman, A.J. ; Elbers, J.A. ; Gerbig, C. ; Gioli, B. ; Lauvaux, T. ; Miglietta, F. ; Neininger, B. ; Ramonet, M. ; Vellinga, O.S. ; Bonnefonds, J.M. - \ 2009
    Biogeosciences 6 (2009)4. - ISSN 1726-4170 - p. 633 - 646.
    hapex-mobilhy - regional-scale - fluxes - parameterization - simulation - resolution - database - strategy
    This paper describes a numerical interpretation of the April 2007, CarboEurope Regional Experiment Strategy (CERES) campaign, devoted to the study of the CO2 cycle at the regional scale. Four consecutive clear sky days with intensive observations of CO2 concentration, fluxes at the surface and in the boundary layer have been simulated with the Meso-NH mesoscale model, coupled to ISBA-A-gs land surface model. The main result of this paper is to show how aircraft observations of CO2 concentration have been used to identify surface model errors and to calibrate the CO2 driving component of the surface model. In fact, the comparisons between modelled and observed CO2 concentrations within the Atmospheric Boundary Layer (ABL) allow to calibrate and correct not only the parameterization of respired CO2 fluxes by the ecosystem but also the Leaf Area Index (LAI) of the dominating land cover. After this calibration, the paper describes systematic comparisons of the model outputs with numerous data collected during the CERES campaign, in April 2007. For instance, the originality of this paper is the spatial integration of the comparisons. In fact, the aircraft observations of CO2 concentration and fluxes and energy fluxes are used for the model validation from the local to the regional scale. As a conclusion, the CO2 budgeting approach from the mesoscale model shows that the winter croplands are assimilating more CO2 than the pine forest, at this stage of the year and this case study.
    Interactions between dry-air entrainment, surface evaporation and convective boundary-layer development
    Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. ; Moene, A.F. ; Holtslag, A.A.M. - \ 2009
    Quarterly Journal of the Royal Meteorological Society 135 (2009)642. - ISSN 0035-9009 - p. 1277 - 1291.
    equilibrium evaporation - land-surface - soil-moisture - feedbacks - dynamics - models - parameterization - sensitivity - simulations - heat
    The influence of dry-air entrainment on surface heat fluxes and the convective boundary-layer (CBL) properties is studied for vegetated land surfaces, using a mixed-layer CBL model coupled to the Penman¿Monteith equation under a wide range of conditions. In order to address the complex behaviour of the system, the feedback mechanisms involved were put into a mathematical framework. Simple expressions for the evaporative fraction and the Priestley¿Taylor parameter were derived, based on the concept of equilibrium evaporation. Dry-air entrainment enhances the surface evaporation under all conditions, but the sensitivity of the evaporation rate to the moisture content of the free troposphere falls as temperature rises. Due to the evaporation enhancement, shallower CBLs develop beneath dry atmospheres. In all cases, dry-air entrainment reduces the relative humidity at the land surface and at the top of the CBL. However, because of dry-air entrainment-induced land¿atmosphere feedback mechanisms, relative humidity at the top of the CBL responds nonlinearly to temperature rise; it decreases as temperature rises beneath a moist free troposphere, whereas it increases beneath a dry free troposphere. Finally, it was found that in certain conditions the evolution of the surface fluxes, relative humidity and CBL height can be as sensitive to the free tropospheric moisture conditions as to the land-surface properties. Therefore, studies of the land surface and of convective clouds have to take into account the influence of dry-air entrainment through land¿atmosphere feedback mechanisms
    On inferring isoprene emission surface flux from atmospheric boundary layer concentration measurements
    Vilà-Guerau de Arellano, J. ; Dries, K. van den; Pino, D. - \ 2009
    Atmospheric Chemistry and Physics 9 (2009). - ISSN 1680-7316 - p. 3629 - 3640.
    model - hydrocarbons - entrainment - inversion - forest - parameterization - dynamics - cumulus - amazon - site
    We examine the dependence of the inferred isoprene surface emission flux from atmospheric concentration on the diurnal variability of the convective boundary layer (CBL). A series of systematic numerical experiments carried out using the mixed-layer technique enabled us to study the sensitivity of isoprene fluxes to the entrainment process, the partition of surface fluxes, the horizontal advection of warm/cold air masses and subsidence. Our findings demonstrate the key role played by the evolution of boundary layer height in modulating the retrieved isoprene flux. More specifically, inaccurate values of the potential temperature lapse rate lead to changes in the dilution capacity of the CBL and as a result the isoprene flux may be overestimated or underestimated by as much as 20%. The inferred emission flux estimated in the early morning hours is highly dependent on the accurate estimation of the discontinuity of the thermodynamic values between the residual layer and the rapidly forming CBL. Uncertainties associated with the partition of the sensible and latent heat flux also yield large deviations in the calculation of the isoprene surface flux. Similar results are obtained if we neglect the influence of warm or cold advection in the development of the CBL.We show that all the above-mentioned processes are non-linear, for which reason the dynamic and chemical evolutions of the CBL must be solved simultaneously. Based on the discussion of our results, we suggest the measurements needed to correctly apply the mixed-layer technique in order to minimize the uncertainties associated with the diurnal variability of the convective boundary layer.
    Sensitivity analysis of leaf wetness duration within a potato canopy
    Jacobs, A.F.G. ; Heusinkveld, B.G. ; Kessel, G.J.T. ; Holtslag, A.A.M. - \ 2009
    Meteorological Applications 16 (2009)4. - ISSN 1350-4827 - p. 523 - 532.
    estimating dew duration - meteorological models - parameterization - field
    A description and analysis is given of a wetness duration experiment, carried out in a potato field in the centre of the Netherlands in September 2005. The observations are used to design and evaluate a within-canopy dew model which provides the leaf wetness distribution within the canopy caused by dew processes and by precipitation. This withincanopy dew model consists of three layers (bottom, centre, top) each with equal contribution to the leaf area index. The model results compared favourably with experimental evidence. The sensitivity of the dew and precipitation interception on the amount of free water and the duration of the leaf wetness was analysed by varying the leaf area index and some important weather variables. The findings suggest that the leaf area index affects the amount of free water, but is barely sensitive to leaf wetness duration. Wind speed has hardly any effect on the amount of free water collection as well as on leaf wetness duration. The net radiation, however, appears to be sensitive to the amount of collected free water as well as the leaf wetness duration
    Exploring the possible role of small scale terrain drag on stable boundary layers over land
    Steeneveld, G.J. ; Holtslag, A.A.M. ; Nappo, C.J. ; Wiel, B.J.H. van de; Mahrt, L. - \ 2008
    Journal of Applied Meteorology and Climatology 47 (2008). - ISSN 1558-8424 - p. 2518 - 2530.
    atmospheric gravity-waves - complex terrain - critical-level - surface-layer - general-circulation - contrasting nights - turbulent-flow - form drag - model - parameterization
    This paper addresses the possible role of unresolved terrain drag, relative to the turbulent drag on the development of the stable atmospheric boundary layer over land. Adding a first-order estimate for terrain drag to the turbulent drag appears to provide drag that is similar to the enhanced turbulent drag obtained with the so-called long-tail mixing functions. These functions are currently used in many operational models for weather and climate, although they lack a clear physical basis. Consequently, a simple and practical quasi-empirical parameterization of terrain drag divergence for use in large-scale models is proposed and is tested in a column mode. As an outcome, the cross-isobaric mass flow (a measure for cyclone filling) with the new scheme, using realistic turbulent drag, appears to be equal to what is found with the unphysical long-tail scheme. At the same time, the new scheme produces a much more realistic less-deep boundary layer than is obtained by using the long-tail mixing function.
    Evaluation of limited-area models for the representation of the diurnal cycle and contrasting nights in CASES-99
    Steeneveld, G.J. ; Mauritsen, T. ; Bruijn, E.I.F. de; Vilà-Guerau de Arellano, J. ; Svensson, G. ; Holtslag, A.A.M. - \ 2008
    Journal of Applied Meteorology and Climatology 47 (2008). - ISSN 1558-8424 - p. 869 - 887.
    atmospheric boundary-layer - low-level jet - surface-energy balance - mesoscale model - climate model - turbulent fluxes - national center - land-surface - sensitivity - parameterization
    This study evaluates the ability of three limited-area models [the fifth-generation Pennsylvania State University¿National Center for Atmospheric Research Mesoscale Model (MM5), the Coupled Ocean¿Atmosphere Mesoscale Prediction System (COAMPS), and the High-Resolution Limited-Area Model (HIRLAM)] to predict the diurnal cycle of the atmospheric boundary layer (ABL) during the Cooperative Atmosphere¿Surface Exchange Study (CASES-99) experimental campaign. Special attention is paid to the stable ABL. Limited-area model results for different ABL parameterizations and different radiation transfer parameterizations are compared with the in situ observations. Model forecasts were found to be sensitive to the choice of the ABL parameterization both during the day and at night. At night, forecasts are particularly sensitive to the radiation scheme. All three models underestimate the amplitude of the diurnal temperature cycle (DTR) and the near-surface wind speed. Furthermore, they overestimate the stable boundary layer height for windy conditions and underestimate the stratification of nighttime surface inversions. Favorable parameterizations for the stable boundary layer enable rapid surface cooling, and they have limited turbulent mixing. It was also found that a relatively large model domain is required to model the Great Plains low-level jet. A new scheme is implemented for the stable boundary layer in the Medium-Range Forecast Model (MRF). This scheme introduces a vegetation layer, a new formulation for the soil heat flux, and turbulent mixing based on the local scaling hypothesis. The new scheme improves the representation of surface temperature (especially for weak winds) and the stable boundary layer structure.
    Water balance versus land surface model in the simulation of Rhine river discharges
    Hurkmans, R.T.W.L. ; Moel, H. de; Aerts, J.C.J.H. ; Troch, P.A. - \ 2008
    Water Resources Research 44 (2008). - ISSN 0043-1397 - 14
    hydrologie van stroomgebieden - afvoer - rijn - klimaatverandering - modellen - catchment hydrology - discharge - river rhine - climatic change - models - climate-change - parametrization schemes - hydrological model - basin - impact - parameterization - variability - management - streamflow - resources
    Accurate streamflow simulations in large river basins are crucial to predict timing and magnitude of floods and droughts and to assess the hydrological impacts of climate change. Water balance models have been used frequently for these purposes. Compared to water balance models, however, land surface models carry the potential to more accurately estimate hydrological partitioning and thus streamflow, because they solve the coupled water and energy balance and are able to exploit a larger part of the information provided by regional climate model output than water balance models. Owing to increased model complexity, however, they are also more difficult to parameterize. The purpose of this study is to investigate and compare the accuracy of streamflow simulations of a water balance approach (Spatial Tools for River basins and Environment and Analysis of Management (STREAM)) and a land surface model (Variable Infiltration Capacity (VIC)) approach. Both models are applied to the Rhine river basin using regional climate model output as atmospheric forcing, and are evaluated using observed streamflow and lysimeter data. We find that VIC is more robust and less dependent on model calibration. Although STREAM performs better during the calibration period (Nash-Sutcliffe efficiency (E) of 0.47 versus E = 0.29 for VIC), VIC more accurately simulates discharge during the validation period, including peak flows (E = 0.31 versus E = 0.21 for STREAM). This is the case for most locations throughout the basin, except for the Alpine part where both models have difficulties due to the complex terrain and surface reservoirs. In addition, the annual evaporation cycle at the lysimeters is more realistically simulated by VIC.
    Global reactive nitrogen deposition from lightning NOx
    Shepon, A. ; Gildor, H. ; Labrador, L.J. ; Butler, T. ; Ganzeveld, L.N. ; Lawrence, M.G. - \ 2007
    Journal of Geophysical Research: Atmospheres 112 (2007). - ISSN 2169-897X - 14 p.
    general-circulation model - dry deposition - atmospheric chemistry - organic nitrogen - climate-change - trace gases - distributions - parameterization - cycle - emissions
    We present results of the deposition of nitrogen compounds formed from lightning (LNO x ) using the global chemical transport Model of Atmospheric Transport and Chemistry¿Max Planck Institute for Chemistry version. The model indicates an approximately equal deposition of LNO x in both terrestrial and oceanic ecosystems, primarily in the tropics and midlatitudes open ocean, despite much higher intensities of lightning flashes above landmasses. The highest values of deposition are due to wet convective deposition, with highest values concentrated in the tropical continents. Nonconvective wet deposition, associated with large-scale weather patterns, occurs over large areas of the ocean amid lower values per square meter, manifesting the long-range transport of NO y , including long-lived species such as HNO3 at high altitudes and PAN. Dry deposition is concentrated primarily above landmasses, yet oceanic deposition over wide areas is still observed. Combined together, the total LNO x deposition exhibits maximal influx values over land, whereas oceanic deposition over wider areas renders the integrated deposition over both ecosystems almost identical. Peaks of terrestrial deposition values (located in Africa, South America, and Asia) show seasonal variability by meridionally penetrating the northern or southern midlatitude following the corresponding summer hemisphere, in accordance with the migration of LNO x production sites. On land, wet and dry deposition rates are more or less equal with a small bias toward wet deposition, whereas above the ocean, wet deposition is markedly higher because of a small water uptake efficiency and relatively small surface roughness. Further work of modeling additional species and obtaining more information on different compounds is required
    Water-side turbulence enhancement of ozone deposition to the ocean
    Fairall, C.W. ; Helmig, D. ; Ganzeveld, L.N. ; Hare, J. - \ 2007
    Atmospheric Chemistry and Physics 2007 (2007)7. - ISSN 1680-7316 - p. 443 - 451.
    dry deposition - boundary-layer - gas-exchange - wind-speed - parameterization - surface - fluxes - trends
    A parameterization for the deposition velocity of an ocean-reactive atmospheric gas (such as ozone) is developed. The parameterization is based on integration of the turbulent-molecular transport equation (with a chemical source term) in the ocean. It extends previous work that only considered reactions within the oceanic molecular sublayer. The sensitivity of the ocean-side transport to reaction rate and wind forcing is examined. A more complicated case with a much more reactive thin surfactant layer is also considered. The full atmosphere-ocean deposition velocity is obtained by matching boundary conditions at the interface. For an assumed ocean reaction rate of 103 s¿1, the enhancement for ozone deposition by oceanic turbulence is found to be up to a factor of three for meteorological data obtained in a recent cruise off the East Coast of the U.S.
    Estimation of the regional evaporative fraction over the Tibetan Plateau area by using Landsat-7 ETM data and the field observations
    Ma, Y. ; Song, M. ; Ishikawa, H. ; Yang, K. ; Koike, T. ; Jia, L. ; Meneti, M. ; Su, Z. - \ 2007
    Journal of the Meteorological Society of Japan 85A (2007). - ISSN 0026-1165 - p. 295 - 309.
    soil heat-flux - surface-energy balance - heterogeneous landscape - net-radiation - model - parameterization - game/tibet - profiles - length - budget
    This study proposed a parameterization methodology based on Landsat-7 ETM data and field observations and tested it for deriving an evaporative fraction (EF) over a heterogeneous landscape. As a case study, the methodology was applied to the experimental area of CAMP/Tibet located on the central Tibetan Plateau. Four scenes of Landsat-7 ETM data were used in the study. Scenes of 9 June 2002 and 28 August 2002 were selected as case examples of summer and autumn, respectively; the scene of 2 December 2002 was selected as a winter case; and 24 March 2003 was selected as a spring case (or pre-monsoon period). To validate the proposed methodology, the Landsat-7 ETM derived EFs were compared to ground-measured values in four different months that spanned a wide range of surface conditions and surface features. This comparison revealed that the predictions were in good accordance with the ground measurements with absolute percent differences of less than 9.5%. It was concluded that the proposed methodology successfully facilitates the retrieval of EF using Landsat-7 ETM data and field observations over the study area.
    Wind profiles, momentum fluxes and roughness lengths at Cabauw revisited
    Verkaik, J.W. ; Holtslag, A.A.M. - \ 2007
    Boundary-Layer Meteorology 122 (2007)3. - ISSN 0006-8314 - p. 701 - 719.
    grenslaagmeteorologie - windsnelheid - fluctuaties - experimenten - boundary-layer meteorology - wind speed - fluctuations - experiments - atmospheric boundary-layer - surface-layer - complex terrain - land-surface - parameterization - similarity - turbulence - footprint - location - exchange
    We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (<40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.
    Technical Note: An implementation of the dry removal processesDRY DEPosition and SEDImentation in the Modular EarthSubmodel System (MESSy)
    Kerkweg, A. ; Buchholz, J. ; Ganzeveld, L.N. ; Pozzer, A. ; Tost, H. ; Jöckel, P. - \ 2006
    Atmospheric Chemistry and Physics 6 (2006)12. - ISSN 1680-7316 - p. 4617 - 4632.
    general-circulation model - parameterization - chemistry
    We present the submodels DRYDEP and SEDI for the Modular Earth Submodel System (MESSy). Dry deposition of gases and aerosols is calculated within DRYDEP, whereas SEDI deals with aerosol particle sedimentation. Dry deposition velocities depend on the near-surface turbulence and the physical and chemical properties of the surface cover (e.g. the roughness length, soil pH or leaf stomatal exchange). The dry deposition algorithm used in DRYDEP is based on the big leaf approach and is described in detail within this Technical Note. The sedimentation submodel SEDI contains two sedimentation schemes: a simple upwind zeroth order scheme and a first order approach
    Evaluation of Peroxide Exchanges over a Coniferous Forest in a Single-Column Chemistry-Climate Model
    Ganzeveld, L.N. ; Valverde-Canossa, J. ; Moortgat, G. ; Steinbrecher, R. - \ 2006
    Atmospheric Environment 40 (2006)Supp.1. - ISSN 1352-2310 - p. S68 - S80.
    reactive trace gases - general-circulation model - red spruce needles - dry deposition - hydrogen-peroxide - flux experiments - ozone - nox - parameterization - emissions
    A single-column chemistry-climate model has been applied to evaluate peroxide exchanges measured over a coniferous forest during the BEWA2000 field campaign, July¿August 2001. Simulations indicate that for suppressed nocturnal turbulent mixing, the H2O2 mixing ratios are sensitive to the representation of sources and sinks, e.g., non-stomatal uptake and chemical transformations, the latter tightly linked to atmosphere¿biosphere NOx exchanges through its control on HO2 production. Comparison of observed and simulated H2O2 fluxes suggests that the commonly applied method to estimate uptake resistances results in a significant underestimation of the dry deposition flux. By using a very small surface uptake resistance, as observed, the modeled surface fluxes are still too low due to an underestimation of the simulated turbulent transport. Further, a reasonable agreement between simulated and observed methylhydroperoxide and hydroxymethylhydroperoxide mixing ratios in and above the canopy air is observed. Our analysis indicates the important role of daytime as well as nocturnal turbulent exchanges, which control the efficiency of dry deposition and downward transport of peroxides that are chemically produced higher up in the boundary layer. In turn, this chemical production depends on the upward transport of emitted precursor gases and their oxidization products. This demonstrates that improved simulations of atmosphere¿biosphere peroxide exchanges rely heavily on improved model representations of boundary layer and canopy turbulent exchanges.
    The sensitivity of aerosol in Europe to two different emission inventories and temporal distribution of emissions
    Meij, A. de; Krol, M.C. ; Dentener, F. ; Vignati, E. ; Cuvelier, C. ; Thunis, P. - \ 2006
    Atmospheric Chemistry and Physics 6 (2006)12. - ISSN 1680-7316 - p. 4287 - 4309.
    anthropogenic sulfate aerosols - sea-salt aerosol - zoom model tm5 - optical-properties - transport - chemistry - simulation - ozone - parameterization - climate
    The sensitivity to two different emission inventories, injection altitude and temporal variations of anthropogenic emissions in aerosol modelling is studied, using the two way nested global transport chemistry model TM5 focussing on Europe in June and December 2000. The simulations of gas and aerosol concentrations and aerosol optical depth (AOD) with the EMEP and AEROCOM emission inventories are compared with EMEP gas and aerosol surface based measurements, AERONET sun photometers retrievals and MODIS satellite data. For the aerosol precursor gases SO2 and NOx in both months the model results calculated with the EMEP inventory agree better ( overestimated by a factor 1.3 for both SO2 and NOx) with the EMEP measurements than the simulation with the AEROCOM inventory ( overestimated by a factor 2.4 and 1.9, respectively). Besides the differences in total emissions between the two inventories, an important role is also played by the vertical distribution of SO2 and NOx emissions in understanding the differences between the EMEP and AEROCOM inventories. In December NOx and SO2 from both simulations agree within 50% with observations. In June SO4= evaluated with the EMEP emission inventory agrees slightly better with surface observations than the AEROCOM simulation, whereas in December the use of both inventories results in an underestimate of SO4 with a factor 2. Nitrate aerosol measured in summer is not reliable, however in December nitrate aerosol calculations with the EMEP and AEROCOM emissions agree with 30%, and 60%, respectively with the filter measurements. Differences are caused by the total emissions and the temporal distribution of the aerosol precursor gases NOx and NH3. Despite these differences, we show that the column integrated AOD is less sensitive to the underlying emission inventories. Calculated AOD values with both emission inventories underestimate the observed AERONET AOD values by 20 - 30%, whereas a case study using MODIS data shows a high spatial agreement. Our evaluation of the role of temporal distribution of anthropogenic emissions on aerosol calculations shows that the daily and weekly temporal distributions of the emissions are only important for NOx, NH3 and aerosol nitrate. However, for all aerosol species SO4=, NH4+, POM, BC, as well as for AOD, the seasonal temporal variations used in the emission inventory are important. Our study shows the value of including at least seasonal information on anthropogenic emissions, although from a comparison with a range of measurements it is often difficult to firmly identify the superiority of specific emission inventories, since other modelling uncertainties, e. g. related to transport, aerosol removal, water uptake, and model resolution, play a dominant role.
    Representing Sheared Convective Boundary Layer by Zeroth- and First-Order-Jump Mixed-Layer Models: Large-Eddy Simulation Verification
    Pino, D. ; Vilà-Guerau de Arellano, J. ; Kim, S.W. - \ 2006
    Journal of Applied Meteorology and Climatology 45 (2006)9. - ISSN 1558-8424 - p. 1224 - 1243.
    entrainment zone - wind shear - cumulus convection - kinetic-energy - part ii - inversion - growth - dynamics - parameterization - budget
    Dry convective boundary layers characterized by a significant wind shear on the surface and at the inversion are studied by means of the mixed-layer theory. Two different representations of the entrainment zone, each of which has a different closure of the entrainment heat flux, are considered. The simpler of the two is based on a sharp discontinuity at the inversion (zeroth-order jump), whereas the second one prescribes a finite depth of the inversion zone (first-order jump). Large-eddy simulation data are used to provide the initial conditions for the mixed-layer models, and to verify their results. Two different atmospheric boundary layers with different stratification in the free atmosphere are analyzed. It is shown that, despite the simplicity of the zeroth-order-jump model, it provides similar results to the first-order-jump model and can reproduce the evolution of the mixed-layer variables obtained by the large-eddy simulations in sheared convective boundary layers. The mixed-layer model with both closures compares better with the large-eddy simulation results in the atmospheric boundary layer characterized by a moderate wind shear and a weak temperature inversion. These results can be used to represent the flux of momentum, heat, and other scalars at the entrainment zone in general circulation or chemistry transport models.
    Modelling the Arctic Stable boundary layer and its coupling to the surface
    Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2006
    Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 357 - 378.
    large-eddy simulation - intermittent turbulence - land-surface - atmospheric models - energy-balance - parameterization - fluxes - oscillations - temperature - validation
    The impact of coupling the atmosphere to the surface energy balance is examined for the stable boundary layer, as an extension of the first GABLS (GEWEX Atmospheric Boundary-Layer Study) one-dimensional model intercomparison. This coupling is of major importance for the stable boundary-layer structure and its development because coupling enables a realistic physical description of the interdependence of the surface temperature and the surface sensible heat flux. In the present case, the incorporation of a surface energy budget results in stronger cooling (surface decoupling), and a more stable and less deep boundary layer. The proper representation of this is a problematic feature in large-scale numerical weather prediction and climate models. To account for the upward heat flux from the ice surface beneath, we solve the diffusion equation for heat in the underlying ice as a first alternative. In that case, we find a clear impact of the vertical resolution in the underlying ice on boundary-layer development: coarse vertical resolution in the ice results in stronger surface cooling than for fine resolution. Therefore, because of this impact on stable boundary-layer development, the discretization in the underlying medium needs special attention in numerical modelling studies of the nighttime boundary layer. As a second alternative, a bulk conductance layer with stagnant air near the surface is added. The stable boundary-layer development appears to depend heavily on the bulk conductance of the stagnant air layer. This result re-emphasizes the fact that the interaction with the surface needs special attention in stable boundary-layer studies. Furthermore, we perform sensitivity studies to atmospheric resolution, the length-scale formulation and the impact of radiation divergence on stable boundary-layer structure for weak windy conditions
    Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer
    Cuxart, J. ; Holtslag, A.A.M. ; Beare, R.J. ; Bazile, E. ; Beljaars, A. ; Cheng, A. ; Conangla, L. ; Ek, M.B. ; Freedman, F. ; Hamdi, R. ; Kerstein, A. ; Kitagawa, H. ; Lenderink, G. ; Lewellen, D. ; Mailhot, J. ; Mauritsen, T. ; Perov, V. ; Schayes, G. ; Steeneveld, G.J. ; Svensson, G. ; Taylor, P. ; Weng, W. ; Wunsch, S. ; Xu, K.M. - \ 2006
    Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 273 - 303.
    turbulence closure-model - large-eddy simulation - part i - scheme - parameterization - formulation - diffusion - dynamics - surfaces - system
    The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored
    Simulating of leaf wetness duration within a potato canopy
    Jacobs, A.F.G. ; Heusinkveld, B.G. ; Kessel, G.J.T. - \ 2005
    NJAS Wageningen Journal of Life Sciences 53 (2005)2. - ISSN 1573-5214 - p. 151 - 166.
    gewasteelt - aardappelen - gewasbescherming - fungiciden - meteorologie - micrometeorologie - meteorologische factoren - neerslag - weer - dauw - relatieve vochtigheid - pesticiden - toepassing - toepassingsdatum - simulatie - modellen - simulatiemodellen - phytophthora - phytophthora infestans - crop management - potatoes - plant protection - fungicides - meteorology - micrometeorology - meteorological factors - precipitation - weather - dew - relative humidity - pesticides - application - application date - simulation - models - simulation models - phytophthora - phytophthora infestans - estimating dew duration - meteorological models - parameterization - field
    A leaf wetness duration experiment was carried out in a potato field in the centre of the Netherlands during the growing season of 2003. A within-canopy dew simulation model was applied to simulate leaf wetness distribution in the canopy caused by dew and rainfall. The dew model is an extension of an earlier-developed energy budget model, distinguishing three layers within the potato canopy. To run the dew model successfully, information on the above-canopy wind speed, air temperature, humidity and net radiation as well as the within-canopy temperature and humidity must be available. In most cases leaf wetting starts in the top layer followed by the centre and the bottom layer, in that order. Leaf drying shortly after sunrise takes place in the same order. Leaf wetness lasted longest in the bottom layer. Rainfall was accounted for by applying an interception model. The results of the dew model agreed well with leaf wetness recorded with a resistance grid
    Transport and chemical transformations influenced by shallow cumulus over land
    Vilà-Guerau de Arellano, J. ; Kim, S.W. ; Barth, M.C. ; Patton, E.G. - \ 2005
    Atmospheric Chemistry and Physics 5 (2005). - ISSN 1680-7316 - p. 3219 - 3231.
    large-eddy-simulation - convective boundary-layer - cloud - turbulence - ozone - model - flux - photodissociation - parameterization - atmosphere
    The distribution and evolution of reactive species in a boundary layer characterized by the presence of shallow cumulus over land is studied by means of two large-eddy simulation models: the NCAR and WUR codes. The study focuses on two physical processes that can influence the chemistry: the enhancement of the vertical transport by the buoyant convection associated with cloud formation and the perturbation of the photolysis rates below, in and above the clouds. It is shown that the dilution of the reactant mixing ratio caused by the deepening of the atmospheric boundary layer is an important process and that it can decrease reactant mixing ratios by 10 to 50 percent compared to very similar conditions but with no cloud formation. Additionally, clouds transport chemical species to higher elevations in the boundary layer compared to the case with no clouds which influences the reactant mixing ratios of the nocturnal residual layers following the collapse of the daytime boundary layer. Estimates of the rate of reactant transport based on the calculation of the integrated flux divergence range from to ¿0.2 ppb hr-1 to ¿1 ppb hr-1, indicating a net loss of sub-cloud layer air transported into the cloud layer. A comparison of this flux to a parameterized mass flux shows good agreement in mid-cloud, but at cloud base the parameterization underestimates the mass flux. Scattering of radiation by cloud drops perturbs photolysis rates. It is found that these perturbed photolysis rates substantially (10¿40%) affect mixing ratios locally (spatially and temporally), but have little effect on mixing ratios averaged over space and time. We find that the ultraviolet radiance perturbation becomes more important for chemical transformations that react with a similar order time scale as the turbulent transport in clouds. Finally, the detailed intercomparison of the LES results shows very good agreement between the two codes when considering the evolution of the reactant mean, flux and (co-)variance vertical profiles.
    Quantification of the dry aeolian deposition of dust on horizontal surfaces: an experimental comparison of theory and measurements
    Goossens, D. - \ 2005
    Sedimentology 52 (2005)4. - ISSN 0037-0746 - p. 859 - 873.
    wind-tunnel - particles - accumulation - velocity - gauge - flux - parameterization - sedimentation - resuspension - performance
    Eight techniques to quantify the deposition of aeolian dust on horizontal surfaces were tested in a wind tunnel. The tests included three theoretical techniques and five measurement techniques. The theoretical techniques investigated were: the gradient technique, the inferential technique without grain-shape correction, and the inferential technique corrected for grain shape. The measuring techniques included the following surrogate surfaces: a water surface, a glass surface, a metal surface, a vertical array of metal plates, and an inverted frisbee filled with glass marbles. The efficiency of the techniques was investigated for the sediment as a whole (all grain sizes together) as well as for a large number of grain sizes extending from 1 to 104 m. The surrogate surfaces showed more or less comparable catch efficiencies, although the water surface nearly always caught the highest quantities of dust and the marble-filled frisbee and the vertical array of metal plates the lowest quantities of dust. The dust fluxes calculated by theoretical methods were markedly different from those obtained by direct measurements. The fluxes calculated by the inferential technique approximated those of the direct measurements only for grain sizes between 30 and 40 m. For smaller and coarser grains, deviations from the measured fluxes were high. The gradient method, in its turn, provided extremely low calculated fluxes for grains in all size classes investigated. The latter technique was not considered very reliable for the dust used in the tests
    Impact of Amazonian deforestation on atmospheric chemistry
    Ganzeveld, L.N. ; Lelieveld, J. - \ 2004
    Geophysical Research Letters 31 (2004). - ISSN 0094-8276 - 5 p.
    general-circulation model - dry deposition - emissions - parameterization - ozone
    A single-column chemistry and climate model has been used to study the impact of deforestation in the Amazon Basin on atmospheric chemistry. Over deforested areas, daytime ozone deposition generally decreases strongly except when surface wetness decreases through reduced precipitation, whereas nocturnal soil deposition increases. The isoprene and soil nitric oxide emissions decrease although nitrogen oxide release to the atmosphere increases due to reduced canopy deposition. Deforestation also affects vertical transport causing substantial ozone and hydroxyl changes, also depending on soil moisture. The analysis shows that assessment of the impact of land cover and land use changes on atmospheric chemistry requires the development of explicitly coupled chemistry and meteorological models including surface trace gas exchanges, micro-meteorology and the hydrological cycle
    An updated length-scale formulation for turbulent mixing in clear and cloudy boundary layers
    Lenderink, G. ; Holtslag, A.A.M. - \ 2004
    Quarterly Journal of the Royal Meteorological Society 130 (2004)604. - ISSN 0035-9009 - p. 3405 - 3427.
    grenslaagmeteorologie - wolken - turbulentie - kinetische energie - boundary-layer meteorology - clouds - turbulence - kinetic energy - shallow cumulus convection - large-eddy simulation - atmospheric models - parameterization - scheme - stratocumulus - fluxes - transport - impacts - wind
    A new mixing-length scale is presented for turbulence-closure schemes, with special emphasis on neutral-to-convective conditions in clear and cloudy boundary layers. The length scale is intended for a prognostic turbulent-kinetic-energy closure. It is argued that present-day length-scale formulations may easily fail in one of two limiting situations. Schemes based on a local stability measure (e.g.the Richardson number) display unrealistic behaviour and instabilities in the convective limit. This strongly limits the representation of mixing in cloudy boundary layers. On the other hand, it is shown that non-local parcel methods may misrepresent mixing near the surface. The new length-scale formulation combines local and non-local stability in a new way; it uses vertical integrals over the stability (the Richardson number) in a simple 'parcel' framework. The length scale matches with surface-layer similarity for near-neutral conditions and displays a realistic convective limit. The use of the length-scale formulation can be extended easily to cloudy boundary layers. The scheme is numerically stable and computationally cheap. The behaviour of the length scale is evaluated in a single-column model (SCM) and in a high-resolution limited-area model (LAM). The SCM shows good behaviour in three cases with and without boundary-layer clouds. The prediction of the near-surface wind and temperature in the LAM compares favourably with tower measurements at Cabauw (the Netherlands).
    An evaluation of mass flux closures for diurnal cycles of shallow cumulus
    Neggers, R.A.J. ; Siebesma, A.P. ; Lenderink, G. ; Holtslag, A.A.M. - \ 2004
    Monthly Weather Review 132 (2004)11. - ISSN 0027-0644 - p. 2525 - 2538.
    large-eddy simulation - convective adjustment scheme - planetary boundary-layer - atmospheric radiation - cloud - parameterization - subcloud - model - heat - validation
    Three closure methods for the mass flux at cloud base in shallow cumulus convection are critically examined for the difficult case of a diurnal cycle over land. The closure methods are first evaluated against large-eddy simulations (LESs) by diagnosing all parameters appearing in the closure equations during simulations of two different observed diurnal cycles of shallow cumulus. This reveals the characteristic behavior of each closure mechanism purely as a result of its core structure. With these results in hand the impact of each closure on the development of the cloudy boundary layer is then studied by its implementation in an offline single-column model of a regional atmospheric climate model. The LES results show that the boundary layer quasi-equilibrium closure typically overestimates the cloud-base mass flux after cloud onset, due to the neglect of significant moisture and temperature tendencies in the subcloud layer. The convective available potential energy (CAPE) adjustment closure is compromised by its limitation to compensating subsidence as the only CAPE breakdown mechanism and the use of a constant adjustment time scale. The closure method using the subcloud convective vertical velocity scale gives the best results, as it catches the time development of the cloud-base mass flux as diagnosed in LES.
    Analyis of the role of the planetary boundary layer schemes during a severe convective storm
    Wisse, J.S.P. ; Vilà-Guerau de Arellano, J. - \ 2004
    Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences 22 (2004)6. - ISSN 0992-7689 - p. 1861 - 1874.
    numerical-simulation - model - sensitivity - parameterization - precipitation - evaporation - catalonia - flood - event - tests
    The role played by planetary boundary layer (PBL) in the development and evolution of a severe convective storm is studied by means of meso-scale modeling and surface and upper air observations. The severe convective precipitation event that occurred on 14 September 1999 in the northeast of the Iberian Peninsula was simulated by means of the mesoscale model MM5 (version 3) using three different PBL schemes. The numerical results show a large impact of the PBL schemes on the precipitation fields associated to the convective storm. The schemes are based on different physical assumptions: the nonlocal first order Medium-Range Forecast (MRF) and Blackadar (BLA) scheme and the local, one-and-a-half order ETA scheme. Surface and radar observations are used to validate the model results. The comparison focuses on three aspects: the evolution, the spatial distribution and the 24-h accumulated precipitation. The comparison with rain gauge observations shows that the MRF, BLA and ETA schemes predicted most of the precipitation during the morning, whereas the rain gauge stations recorded rainfall during the evening. The evaluation performed with the radar data shows that all three numerical simulations produced a realistic spatial accumulated precipitation distribution. According to the quantity distribution, all three numerical simulations were able to predict precipitation quantities comparable to the rain gauge measurements. The MRF scheme predicted the largest average accumulated precipitation and the largest average precipitation rate, whereas the ETA scheme predicted the smallest accumulated precipitation and average precipitation rate. However, the ETA scheme yielded the highest extreme precipitation rates. The performance of the three schemes is analyzed in terms of the vertical distribution of potential temperature, specific humidity and conserved variables, like equivalent potential temperature and total water content. The MRF scheme showed more evidence of enhanced mixing than did the other schemes. Due to this process, more moisture was more efficiently transported to the free atmosphere. Consequently, the MRF scheme predicts more widespread precipitation. Furthermore, the enhanced mixing led to a less sharp capping inversion. However, the stronger inversion resulting from suppressed mixing processes in the case of the ETA scheme yielded higher values of convective available potential energy (CAPE) than did the other two schemes. Consequently, the more extreme precipitation rates are simulated by MM5 when the ETA scheme is used.
    Entrainment process of carbon dioxide in the atmospheric boundary layer
    Vilà-Guerau de Arellano, J. ; Gioli, B. ; Miglietta, F. ; Jonker, H.J.J. ; Klein Baltink, H. ; Hutjes, R.W.A. ; Holtslag, A.A.M. - \ 2004
    Journal of Geophysical Research: Atmospheres 109 (2004). - ISSN 2169-897X - p. D18110 - D18110.
    large-eddy simulation - sensible heat - flux measurements - aircraft - turbulence - model - parameterization - moisture - budget - latent
    Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations we were able to estimate the vertical profiles of the fluxes, the correlation coefficients, and the skewness of the virtual potential temperature, the specific humidity, and the carbon dioxide. These profiles indicate that important entrainment events occurred during the observed period. The data were also used to estimate the budgets for heat, moisture, and carbon dioxide. By studying this observational data we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer. This process is particularly important in the morning hours which are characterized by a rapidly growing boundary layer. The observations show that the CO2 concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by CO2 uptake by the vegetation. We quantify this effect by calculating the ratio of the entrainment flux of CO2 to the surface flux of CO2(beta(c)=-((wc) over bar)(e)/((wc) over bar)(o)). A value of beta(c) equal to 2.9 is estimated at around 1300 UTC from the vertical profile of the carbon dioxide flux. We corroborate this observational evidence by reproducing the observed situation using a mixed layer model. The mixed layer model also yields the variation in time of beta(c). During the morning the ventilation process is more important than the CO2 uptake by the vegetation (beta(c)>1), whereas in the afternoon the assimilation by grass at the surface becomes the dominant process (beta(c) <1). This research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide.
    The microphysical structure of extreme precipitation as inferred from ground-based raindrop spectra
    Uijlenhoet, R. ; Smith, J.A. ; Steiner, M. - \ 2003
    Journal of the Atmospheric Sciences 60 (2003). - ISSN 0022-4928 - p. 1220 - 1238.
    drop-size distributions - radar reflectivity - catastrophic rainfall - convective clouds - conceptual-model - tropical rain - stratiform - equilibrium - disdrometer - parameterization
    The controls on the variability of raindrop size distributions in extreme rainfall and the associated radar reflectivity-rain rate relationships are studied using a scaling-law formalism for the description of raindrop size distributions and their properties. This scaling-law formalism enables a separation of the effects of changes in the scale of the raindrop size distribution from those in its shape. Parameters controlling the scale and shape of the scaled raindrop size distribution may be related to the microphysical processes generating extreme rainfall. A global scaling analysis of raindrop size distributions corresponding to rain rates exceeding 100 mm h(-1), collected during the 1950s with the Illinois State Water Survey raindrop camera in Miami, Florida, reveals that extreme rain rates tend to be associated with conditions in which the variability of the raindrop size distribution is strongly number controlled (i.e., characteristic drop sizes are roughly constant). This means that changes in properties of raindrop size distributions in extreme rainfall are largely produced by varying raindrop concentrations. As a result, rainfall integral variables (such as radar reflectivity and rain rate) are roughly proportional to each other, which is consistent with the concept of the so-called equilibrium raindrop size distribution and has profound implications for radar measurement of extreme rainfall. A time series analysis for two contrasting extreme rainfall events supports the hypothesis that the variability of raindrop size distributions for extreme rain rates is strongly number controlled. However, this analysis also reveals that the actual shapes of the (measured and scaled) spectra may differ significantly from storm to storm. This implies that the exponents of power-law radar reflectivity-rain rate relationships may be similar, and close to unity, for different extreme rainfall events, but their prefactors may differ substantially. Consequently, there is no unique radar reflectivity-rain rate relationship for extreme rain rates, but the variability is essentially reduced to one free parameter (i.e., the prefactor). It is suggested that this free parameter may be estimated on the basis of differential reflectivity measurements in extreme rainfall.
    Representing the atmospheric boundary layer in climate models of intermediate compexity
    Ronda, R.J. ; Haarsma, R.J. ; Holtslag, A.A.M. - \ 2003
    Climate Dynamics 21 (2003)3-4. - ISSN 0930-7575 - p. 327 - 335.
    general-circulation model - surface fluxes - part i - parameterization - simulations - turbulence - land - diffusion - schemes - energy
    In this study the role of atmospheric boundary layer schemes in climate models is investigated. Including a boundary layer scheme in an Earth system model of intermediate complexity (EMIC) produces only minor differences in the estimated global distribution of sensible and latent heat fluxes over land (upto about 15% of the net radiation at the surface). However, neglecting of boundary layer processes, such as the development of a well-mixed layer over land or the impact of stability on the exchange coefficient in the surface layer, leads to erroneous surface temperatures, especially in convective conditions with low wind speeds. As these conditions occur frequently, introducing a boundary layer scheme in an EMIC gives reductions in June-July-August averaged surface temperature of 1¿2 °C in wet areas, to 5¿7 °C in desert areas. Even a relatively simple boundary layer scheme provides reasonable estimates of the surface fluxes and surface temperatures. Detailed schemes that solve explicitly the turbulent fluxes within the boundary layer are only required when vertical profiles of potential temperature are needed.
    The contribution of shear to the evolution of a convective boundary layer
    Pino, D. ; Vilà-Guerau de Arellano, J. ; Duynkerke, P.G. - \ 2003
    Journal of the Atmospheric Sciences 60 (2003)16. - ISSN 0022-4928 - p. 1913 - 1926.
    large-eddy simulation - mixed-layer - density interface - wind shear - entrainment - inversion - models - parameterization - dynamics - top
    The role of shear in the development and maintenance of a convective boundary layer is studied by means of observations and large eddy simulations (LESs). Particular emphasis is given to the growth of the boundary layer and to the way in which this growth is affected by surface fluxes of heat and moisture and entrainment fluxes. This paper analyzes the processes that drive the latter mechanism, which accounts for approximately 30% of the growth of the mixing layer. Typically, it is found that under pure convective conditions, without shear, the entrainment buoyancy flux at the inversion is about ¿20% of the surface buoyancy flux. This value is widely used for entrainment rate closures in general circulation models. The data collected during the Atmospheric Radiation Measurement campaign allow one to introduce realistic vertical profiles and surface fluxes into the LES runs and to compare the simulation results with the observed evolution of the boundary layer height during a convective situation with high entrainment rates and high geostrophic winds. The analysis of the turbulent kinetic energy (TKE) budget shows that the inclusion of geostrophic winds, which produce shear at the surface and in the entrainment zone, modifies the vertical profile of the various terms in the TKE budget. As a consequence, the entrainment flux is enhanced, resulting in increased growth of the boundary layer. The numerical experiments and the observations enable one to validate the efficiency of earlier representations, based on the TKE equation, which describe the evolution of the ratio between entrainment and surface buoyancy fluxes. The proposed parameterization for the entrainment and surface buoyancy flux ratio (ß), which includes the main buoyancy and shear contributions, is in good agreement with the LES results. Some aspects of the parameterization of ß, for instance, the absence of entrainment flux and its behavior during the transition between convective to neutral conditions, are discussed
    SVAT modeling over the Alpilles-ReSeDA experiment: comparing SVAT models over wheat fields
    Olioso, A. ; Braud, I. ; Chanzy, A. ; Courault, D. ; Demarty, J. ; Kergoat, L. ; Lewan, E. ; Ottle, C. ; Prevot, L. ; Zhao, W.G.G. ; Calvet, J.C. ; Cayrol, P. ; Jongschaap, R.E.E. ; Moulin, S. ; Noilhan, J. ; Wigneron, J.P. - \ 2002
    Agronomie 22 (2002)6. - ISSN 0249-5627 - p. 651 - 668.
    soil-water - surface-temperature - land - evaporation - vegetation - parameterization - prediction - moisture - equation - zone
    Remote sensing is an interesting tool for monitoring crop production, energy exchanges and mass exchanges between the soil, the biosphere and the atmosphere. The aim of the Alpilles-ReSeDA program was the development of such techniques combining remote sensing data, and soil and vegetation process models. This article focuses on SVAT models (Soil-Vegetation-Atmosphere Transfer models) which may be used for monitoring energy and mass exchanges by using assimilation of remote sensing data procedures. As a first step, we decided to implement a model comparison experiment with the aim of analyzing the relationships between the models' complexity, validity and potential for assimilating remote sensing data. This experiment involved the definition of three comparison scenarios with different objectives: (i) test the models' capacity to accurately describe processes using input parameters as measured in the field; (ii) test the portability of the models by using a priori information on input parameters (such as pedotransfer functions), and (iii) test the robustness of the models by a calibration/validation procedure. These 3 scenarios took advantage of the experimental network that was implemented during the Alpilles experiment and which combined measurements on different fields that may be used for calibration of models and their validations on independent data sets. The results showed that the models' performances were close whatever their complexity. The simpler models were less sensitive to the specification of input parameters. Significant improvements in the models' results were achieved when calibrating the models in comparison with the first scenario
    Simulation of global sulfate distribution and the influence of effective cloud drop radii with a coupled photochemistry-sulfur cycle model
    Roelofs, G.J. ; Lelieveld, J. ; Ganzeveld, L.N. - \ 1998
    Tellus Series B: Chemical and Physical Meteorology 50 (1998)3. - ISSN 0280-6509 - p. 224 - 242.
    general-circulation model - climate model - parameterization - aerosols - methanesulfonate - chemistry - troposphere - atmosphere - gases - sensitivity
    A sulfur cycle model is coupled to a global chemistry-climate model. The simulated surface sulfate concentrations are generally within a factor of 2 of observed concentrations, and display a realistic seasonality for most background locations. However, the model tends to underestimate sulfate and overestimate surface SO2 at relatively polluted locations. A possible explanation for this is that additional oxidation reactions not considered in the model, may be important. Calculated tropospheric sulfate column abundances exceed those of previous studies, which is predominantly associated with a less efficient nucleation scavenging in wet convective updrafts. Through the simultaneous calculation of the sulfur cycle and tropospheric photochemistry, simulated H2O2 and SO2 concentrations are strongly linked, especially in polluted areas. The coupled model simulates a stronger oxidant limitation and, consequently, a smaller contribution to sulfate formation by H2O2 oxidation of SO2 when compared to sulfur cycle models that use monthly averaged oxidant distributions as input. In the polluted NH, the differences in calculated sulfate columns are largest in winter and relatively small in summer. Therefore, the coupling between the sulfur cycle and the oxidant chemistry is expected to have a minor impact on the calculation of the indirect and direct radiative forcing by sulfate. An empirical relation between sulfate concentration and cloud drop number concentration, derived from cloud measurements at Grean Dun Fell (UK), is applied to the simulated cloud and sulfate fields to derive distributions of effective could drop radii. Additionally, a relation between wind speed and cloud drop number concentration is applied over marine regions to account for the effect of seasalt aerosol on cloud formation when sulfate concentrations are relatively low. Calculated droplet radii are systematically underestimated by about 10-20% in the NH compared to satellite derived values, but they agree relatively well in the SH.
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