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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    The Wageningen Lowland Runoff Simulator (WALRUS): application to the Hupsel Brook catchement and Cabauw polder
    Brauer, C.C. ; Torfs, P.J.J.F. ; Teuling, A.J. ; Uijlenhoet, R. - \ 2014
    Hydrology and Earth System Sciences 18 (2014). - ISSN 1027-5606 - p. 4007 - 4028.
    surface parameterization schemes - distributed hydrological model - flow route contributions - land-surface - groundwater interactions - spatial variability - rainfall - scale - netherlands - validation
    The Wageningen Lowland Runoff Simulator (WALRUS) is a new parametric (conceptual) rainfall–runoff model which accounts explicitly for processes that are important in lowland areas, such as groundwater-unsaturated zone coupling, wetness-dependent flowroutes, groundwater–surface water feedbacks, and seepage and surface water supply (see companion paper by Brauer et al., 2014). Lowland catchments can be divided into slightly sloping, freely draining catchments and flat polders with controlled water levels. Here, we apply WALRUS to two contrasting Dutch catchments: the Hupsel Brook catchment and the Cabauw polder. In both catchments, WALRUS performs well: Nash–Sutcliffe efficiencies obtained after calibration on 1 year of discharge observations are 0.87 for the Hupsel Brook catchment and 0.83 for the Cabauw polder, with values of 0.74 and 0.76 for validation. The model also performs well during floods and droughts and can forecast the effect of control operations. Through the dynamic division between quick and slow flowroutes controlled by a wetness index, temporal and spatial variability in groundwater depths can be accounted for, which results in adequate simulation of discharge peaks as well as low flows. The performance of WALRUS is most sensitive to the parameter controlling the wetness index and the groundwater reservoir constant, and to a lesser extent to the quickflow reservoir constant. The effects of these three parameters can be identified in the discharge time series, which indicates that the model is not overparameterised (parsimonious). Forcing uncertainty was found to have a larger effect on modelled discharge than parameter uncertainty and uncertainty in initial conditions.
    The Effect of Surface Heterogeneity on the Structure Parameters of Temperature and Specific Humidity: A Large-Eddy Simulation Case Study for the LITFASS-2003 Experiment
    Maronga, B. ; Hartogensis, O.K. ; Raasch, S. ; Beyrich, F. - \ 2014
    Boundary-Layer Meteorology 153 (2014)3. - ISSN 0006-8314 - p. 441 - 470.
    convective boundary-layer - monin-obukhov similarity - large-aperture scintillometer - flevoland field experiment - index-structure parameter - heat-flux - sensible heat - turbulence structure - land-surface - small-scale
    We conduct a high-resolution large-eddy simulation (LES) case study in order to investigate the effects of surface heterogeneity on the (local) structure parameters of potential temperature C2T and specific humidity C2q in the convective boundary layer (CBL). The kilometre-scale heterogeneous land-use distribution as observed during the LITFASS-2003 experiment was prescribed at the surface of the LES model in order to simulate a realistic CBL development from the early morning until early afternoon. The surface patches are irregularly distributed and represent different land-use types that exhibit different roughness conditions as well as near-surface fluxes of sensible and latent heat. In the analysis, particular attention is given to the Monin–Obukhov similarity theory (MOST) relationships and local free convection (LFC) scaling for structure parameters in the surface layer, relating C2T and C2q to the surface fluxes of sensible and latent heat, respectively. Moreover we study possible effects of surface heterogeneity on scintillometer measurements that are usually performed in the surface layer. The LES data show that the local structure parameters reflect the surface heterogeneity pattern up to heights of 100–200 m. The assumption of a blending height, i.e. the height above the surface where the surface heterogeneity pattern is no longer visible in the structure parameters, is studied by means of a two-dimensional correlation analysis. We show that no such blending height is found at typical heights of scintillometer measurements for the studied case. Moreover, C2q does not follow MOST, which is ascribed to the entrainment of dry air at the top of the boundary layer. The application of MOST and LFC scaling to elevated C2T data still gives reliable estimates of the surface sensible heat flux. We show, however, that this flux, derived from scintillometer data, is only representative of the footprint area of the scintillometer, whose size depends strongly on the synoptic conditions.
    Pricing rainbow, green, blue and grey water: tree cover and geopolitics of climatic teleconnections
    Noordwijk, M. van; Namirembe, S. ; Catacutan, D. ; Williamson, D. ; Gebrekirstos, A. - \ 2014
    Current Opinion in Environmental Sustainability 6 (2014). - ISSN 1877-3435 - p. 41 - 47.
    tropical deforestation - vapor transport - land-surface - policy - precipitation - management - ecosystem - dynamics - forests - africa
    Atmospheric moisture (“rainbow water”) is the source of all green, blue and grey water flows. Current water-related legislation and policies have moved beyond blue (water allocation) and grey (waste water treatment) water concerns to incorporate the green water concept of additional water use by fast-growing trees; it may require further change to incorporate rainbow water relations as evident in recent literature on short-cycle rainfall derived from evapotranspiration over land. Specific teleconnections relate rainfall dynamics at any specific site to land use and sea conditions elsewhere. Government-mandated water use charges for payments for ecosystem services (PES) exist in some African countries but their use in enhancing actual water related ecosystem services covering the full hydrological cycle is still evolving as rainbow water science is new.
    Introduction to the Third GEWEX Atmospheric Boundary Layer Study (GABLS3)
    Holtslag, A.A.M. - \ 2014
    Boundary-Layer Meteorology 152 (2014)2. - ISSN 0006-8314 - p. 127 - 132.
    low-level jets - diurnal cycles - land-surface - sea-ice - model - radiation - weather
    The atmospheric boundary layer (ABL) plays a dominant role in the exchange of energy, water vapour, trace gases and momentum between the earth’s surface and the overlying atmosphere. Consequently, the ABL is an important part of any numerical model in use for atmospheric and climate research, for operational weather forecasting, and for air - quality and wind - energy studies. For all these applications an overall representation is needed for boundary-layer turbulence and near-surface processes, as well as for vertical diffusion above the boundary layer. This representation is typically referred as the parametrization of vertical diffusion and turbulent mixing. It appears that models at various research groups and operational centres use rather different methods to represent turbulence and vertical diffusion and the reasons behind this diversity are not that easy to unravel. Most likely, this originates for historical reasons due to the outcome of various tuning exercises and to the num
    The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding
    Bosveld, F.C. ; Baas, P. ; Steeneveld, G.J. ; Holtslag, A.A.M. ; Angevine, W.M. ; Bazile, E. ; Bruijn, E.I.F. de; Deacu, D. ; Edwards, J.M. ; Ek, M. ; Larson, V.E. ; Pleim, J.E. ; Raschendorfer, M. ; Svensson, G. - \ 2014
    Boundary-Layer Meteorology 152 (2014)2. - ISSN 0006-8314 - p. 157 - 187.
    stably stratified conditions - observed evening transition - nonlocal closure-model - low-level jets - land-surface - morning transition - vertical diffusion - radiative-transfer - energy-balance - ecmwf model
    We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models
    Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation
    Miralles, D. ; Teuling, A.J. ; Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. - \ 2014
    Nature Geoscience 7 (2014). - ISSN 1752-0894 - p. 345 - 349.
    boundary-layer - land-surface - summer - europe - evaporation - extremes - moisture - impact - wave
    The recent European mega-heatwaves of 2003 and 2010 broke temperature records across Europe1, 2, 3, 4, 5. Although events of this magnitude were unprecedented from a historical perspective, they are expected to become common by the end of the century6, 7. However, our understanding of extreme heatwave events is limited and their representation in climate models remains imperfect8. Here we investigate the physical processes underlying recent mega-heatwaves using satellite and balloon measurements of land and atmospheric conditions from the summers of 2003 in France and 2010 in Russia, in combination with a soil–water–atmosphere model. We find that, in both events, persistent atmospheric pressure patterns induced land–atmosphere feedbacks that led to extreme temperatures. During daytime, heat was supplied by large-scale horizontal advection, warming of an increasingly desiccated land surface and enhanced entrainment of warm air into the atmospheric boundary layer. Overnight, the heat generated during the day was preserved in an anomalous kilometres-deep atmospheric layer located several hundred metres above the surface, available to re-enter the atmospheric boundary layer during the next diurnal cycle. This resulted in a progressive accumulation of heat over several days, which enhanced soil desiccation and led to further escalation in air temperatures. Our findings suggest that the extreme temperatures in mega-heatwaves can be explained by the combined multi-day memory of the land surface and the atmospheric boundary layer
    The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part A: Case Selection and Set-Up
    Bosveld, F.C. ; Baas, P. ; Meijgaard, E. van; Bruijn, E.I.F. de; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2014
    Boundary-Layer Meteorology 152 (2014)2. - ISSN 0006-8314 - p. 133 - 156.
    low-level jets - land-surface - weather research - radiation fog - cabauw - climate - parametrizations - netherlands - performance - prediction
    We describe a novel methodology on the selection and composition of a single-case observational dataset from the comprehensive measurement program at the Cabauw observatory field site located in the Netherlands. The case can be regarded as the basis of the third case study conducted within the framework of the GEWEX (Global Energy and Water Exchange) Atmospheric Boundary-Layer Study (GABLS) and is meant to be used for the evaluation of single-column models. The ideal case is supposed to cover a period of at least 24 h with clear skies, moderate near-surface winds and a stable stratification during nighttime. From the multi-year data archive with Cabauw observations data for 1–2 July 2006 were found to best match the requirements, and were consequently selected for analysis. The dates contains a 24-h period with a nearly constant geostrophic wind of ˜ 7 m s -1 , and a considerable wind shear in the vertical. It is also characterized by a pronounced low-level jet related to an inertial oscillation that developed around sunset when the atmosphere had decoupled from the surface. Detailed initial conditions, surface conditions and dynamical forcings are derived on the basis of local observations and the outcome of a conceptual and a three-dimensional atmospheric model. It appears that a very precise prescription of the forcings is a prerequisite to enable a meaningful evaluation of models against observational data
    Emission ratio and isotopic signatures of molecular hydrogen emissions from tropical biomass burning
    Haumann, F.A. ; Batenburg, A.M. ; Pieterse, G. ; Gerbig, C. ; Krol, M.C. ; Rockmann, T. - \ 2013
    Atmospheric Chemistry and Physics 13 (2013)18. - ISSN 1680-7316 - p. 9401 - 9413.
    atmospheric hydrogen - assimilation system - land-surface - amazon basin - trace gases - tall tower - model tm5 - h-2 - chemistry - plants
    In this study, we identify a biomass-burning signal in molecular hydrogen (H-2) over the Amazonian tropical rainforest. To quantify this signal, we measure the mixing ratios of H-2 and several other species as well as the H-2 isotopic composition in air samples that were collected in the BARCA (Balanco Atmosferico Regional de Carbono na Amazonia) aircraft campaign during the dry season. We derive a relative H-2 emission ratio with respect to carbon monoxide (CO) of 0.31 +/- 0.04 ppb ppb(-1) and an isotopic source signature of -280 +/- 41 parts per thousand in the air masses influenced by tropical biomass burning. In order to retrieve a clear source signal that is not influenced by the soil uptake of H-2, we exclude samples from the atmospheric boundary layer. This procedure is supported by data from a global chemistry transport model. The Delta H-2/Delta CO emission ratio is significantly lower than some earlier estimates for the tropical rainforest. In addition, our results confirm the lower values of the previously conflicting estimates of the H-2 isotopic source signature from biomass burning. These values for the emission ratio and isotopic source signatures of H-2 from tropical biomass burning can be used in future bottom-up and top-down approaches aiming to constrain the strength of the biomass-burning source for H-2. Hitherto, these two quantities relied only on combustion experiments or on statistical relations, since no direct signal had been obtained from in-situ observations.
    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.
    The role of snow-surface coupling, radiation, and turbulent mixing in modeling a stable boundary layer over Arctic sea ice
    Sterk, H.A.M. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2013
    Journal of Geophysical Research: Atmospheres 118 (2013)3. - ISSN 2169-897X - p. 1199 - 1217.
    regional climate models - land-surface - polar amplification - longwave radiation - atmospheric models - energy budget - closure-model - part i - sheba - feedback
    To enhance the understanding of the impact of small-scale processes in the polar climate, this study focuses on the relative role of snow-surface coupling, radiation and turbulent mixing in an Arctic stable boundary layer. We extend the GABLS1 (GEWEX Atmospheric Boundary-Layer Study 1) model intercomparison for turbulent mixing with the other relevant physical processes in the stable boundary layer over sea ice. We use the Single Column Model (SCM) version of the Weather Research and Forecasting (WRF) mesoscale meteorological model and run different combinations of boundary layer and radiation schemes, using a state-of-the art land surface scheme. With this intercomparison of schemes, we confirm a wide variety in the state of the atmosphere and the surface variables for the selected parameterization schemes. To understand this variety, a sensitivity analysis for one particular combination of parameterization schemes is performed, using a novel analysis method of process diagrams. The variation between the sensitivity runs indicates a relative orientation of model sensitivities to variations in each of the overning processes and these can explain the variety of model results obtained in the intercomparison of different parameterization schemes. Moreover, we apply the same method for several geostrophic wind speeds to represent a large range of synoptic conditions. Results indicate a shift in process significance for different wind regimes. For low wind regimes, the model sensitivity is larger for surface coupling and radiation, while for high wind speeds, the largest sensitivity is found for the turbulent mixing process. An interesting non-linear feature was found for turbulent mixing for frequently occurring wind speeds and low wind speed cases, where the 2m temperature increases for decreased amounts of mixing.
    Systematic errors in ground heat flux estimation and their correction
    Gentine, P. ; Entekhabi, D. ; Heusinkveld, B.G. - \ 2012
    Water Resources Research 48 (2012)9. - ISSN 0043-1397 - 15 p.
    large-aperture scintillometer - apparent thermal-diffusivity - energy-balance closure - soil-temperature - net-radiation - land-surface - exchanges - forest
    Incoming radiation forcing at the land surface is partitioned among the components of the surface energy balance in varying proportions depending on the time scale of the forcing. Based on a land-atmosphere analytic continuum model, a numerical land-surface model and field observations we show that high-frequency fluctuations in incoming radiation (with period less than 6 hours, for example due to intermittent clouds) are preferentially partitioned towards ground heat flux. These higher frequencies are concentrated in the 0-1 centimeter surface soil layer. Subsequently, measurements even at a few centimeters deep in the soil profile miss part of the surface soil heat flux signal. The attenuation of the high-frequency soil heat flux spectrum throughout the soil profile leads to systematic errors in both measurements and modeling, which require a very fine sampling near the soil surface (0-1 centimeter). Calorimetric measurement techniques introduce a systematic error in the form of artificial band-pass filter if the temperature probes are not placed at appropriate depths. In addition the temporal calculation of the change in the heat storage term of the calorimetric method can further distort the reconstruction of the surface soil heat flux signal. A correction methodology is introduced which provides practical application as well as insights into the estimation of surface soil heat flux and the closure of surface energy balance based on field measurements.
    A generic method for hydrological drought identification across different climate regions
    Huijgevoort, M.H.J. van; Hazenberg, P. ; Lanen, H.A.J. van; Uijlenhoet, R. - \ 2012
    Hydrology and Earth System Sciences 16 (2012). - ISSN 1027-5606 - p. 2437 - 2451.
    environment simulator jules - conterminous united-states - soil-moisture - model description - multimodel ensemble - land-surface - 20th-century - precipitation - temperature - definition
    The identification of hydrological drought at global scale has received considerable attention during the last decade. However, climate-induced variation in runoff across the world makes such analyses rather complicated. This especially holds for the drier regions of the world (both cold and warm), where, for a considerable period of time, zero runoff can be observed. In the current paper, we present a method that enables to identify drought at global scale across climate regimes in a consistent manner. The method combines the characteristics of the classical variable threshold level method that is best applicable in regions with non-zero runoff most of the time, and the consecutive dry days (period) method that is better suited for areas where zero runoff occurs. The newly presented method allows a drought in periods with runoff to continue in the following period without runoff. The method is demonstrated by identifying droughts from discharge observations of four rivers situated within different climate regimes, as well as from simulated runoff data at global scale obtained from an ensemble of five different land surface models. The identified drought events obtained by the new approach are compared to those resulting from application of the variable threshold level method or the consecutive dry period method separately. Results show that, in general, for drier regions, the threshold level method overestimates drought duration, because zero runoff periods are included in a drought, according to the definition used within this method. The consecutive dry period method underestimates drought occurrence, since it cannot identify droughts for periods with runoff. The developed method especially shows its relevance in transitional areas, because, in wetter regions, results are identical to the classical threshold level method. By combining both methods, the new method is able to identify single drought events that occur during positive and zero runoff periods, leading to a more realistic global drought characterization, especially within drier environments.
    The Minimum Wind Speed for Sustainable Turbulence in the Nocturnal Boundary Layer
    Wiel, B.J.H. van de; Moene, A.F. ; Jonker, H.J.J. ; Baas, P. ; Basu, S. ; Donda, J.M.M. ; Sun, J. ; Holtslag, A.A.M. - \ 2012
    Journal of the Atmospheric Sciences 69 (2012)1. - ISSN 0022-4928 - p. 3116 - 3127.
    large-eddy simulations - temperature-fluctuation method - surface-energy balance - low-level jet - stable conditions - intermittent turbulence - land-surface - heat-flux - regimes - cases-99
    The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s-1, depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s-1 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated.
    The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer
    Wiel, B.J.H. van de; Moene, A.F. ; Jonker, H.J.J. - \ 2012
    Journal of the Atmospheric Sciences 69 (2012)1. - ISSN 0022-4928 - p. 3097 - 3115.
    large-eddy simulations - temperature-fluctuation method - low-level jet - intermittent turbulence - atmospheric models - surface-layer - land-surface - part i - regimes - cases-99
    The mechanism behind the collapse of turbulence in the evening as a precursor to the onset of the very stable boundary layer is investigated. To this end a cooled, pressure-driven flow is investigated by means of a local similarity model. Simulations reveal a temporary collapse of turbulence whenever the surface heat extraction, expressed in its nondimensional form h/L, exceeds a critical value. As any temporary reduction of turbulent friction is followed by flow acceleration, the long-term state is unconditionally turbulent. In contrast, the temporary cessation of turbulence, which may actually last for several hours in the nocturnal boundary layer, can be understood from the fact that the time scale for boundary layer diffusion is much smaller than the time scale for flow acceleration. This limits the available momentum that can be used for downward heat transport. In case the surface heat extraction exceeds the so-called maximum sustainable heat flux (MSHF), the near-surface inversion rapidly increases. Finally, turbulent activity is largely suppressed by the intense density stratification that supports the emergence of a different, calmer boundary layer regime.
    Some observational evidence for dry soils supporting enhanced relative humidity at the convective boundary layer top
    Westra, D. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2012
    Journal of Hydrometeorology 13 (2012). - ISSN 1525-755X - p. 1347 - 1358.
    diurnal time scales - land-surface - cumulus onset - vertical diffusion - model - precipitation - entrainment - prediction - moisture - column
    The tendency of the relative humidity at the top of a clear convective boundary layer (RHtop) is studied as an indicator of cloud formation is studied over a semi-arid region within the conceptual framework introduced by Ek and Holtslag (2004). Typically the tendency of RHtop increases if the evaporative fraction at the land surface increases, which supports boundary layer moistening but only when boundary-layer growth is limited by atmospheric factors. This regime was supported by Cabauw observations in the original study. Here we provide new observational evidence that the tendency of RHtop can also increase as the surface becomes more dry as is consistent with another regime of the conceptual framework. The observations used are from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. In addition, we evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for a typical day in the AMMA campaign. It appears that the model confirms that dryer soils can support cloud formation.
    Radiation balance at the surface in the city of São Paulo, Brazil: diurnal and seasonal variations
    Ferreira, M.J. ; Oliveira, A.P. de; Soares, J. ; Codato, G. ; Wilde Barbaro, E. ; Escobedo, J.F. - \ 2012
    Theoretical and Applied Climatology 107 (2012)1-2. - ISSN 0177-798X - p. 229 - 246.
    urban heat-island - land-surface - longwave radiation - solar-radiation - energy-balance - satellite data - st-louis - climate research - air-pollution - rural-areas
    The main goal of this work is to describe the diurnal and seasonal variations of the radiation balance components at the surface in the city of São Paulo based on observations carried out during 2004. Monthly average hourly values indicate that the amplitudes of the diurnal cycles of net radiation (Q*), downwelling and upwelling shortwave radiation (SWDW, SWUP), and longwave radiations (LWDW, LWUP) in February were, respectively, 37%, 14%, 19%, 11%, and 5% larger than they were in August. The monthly average daily values indicate a variation of 60% for Q*, with a minimum in June and a maximum in December; 45% for SWDW, with a minimum in May and a maximum in September; 50% for SWUP, with a minimum in June and a maximum in September; 13% for LWDW, with a minimum in July and a maximum in January; and 9% for LWUP, with a minimum in July and a maximum in February. It was verified that the atmospheric broadband transmissivity varied from 0.36 to 0.57; the effective albedo of the surface varied from 0.08 to 0.10; and the atmospheric effective emissivity varied from 0.79 to 0.92. The surface effective emissivity remained approximately constant and equal to 0.96. The albedo and surface effective emissivity for São Paulo agreed with those reported for urban areas Europe and North America cities. This indicates that material and geometric effects on albedo and surface emissivity in São Paulo are similar to ones observed in typical middle latitudes cities. On the other hand, it was found that São Paulo city induces an urban heat island with daytime maximum intensity varying from 2.6°C in July (16:00 LT) to 5.5°C in September (15:00 LT). The analysis of the radiometric properties carried out here indicate that this daytime maximum is a primary response to the seasonal variation of daily values of net solar radiation at the surface.
    Confronting the WRF and RAMS mesoscale models with innovative observations in the Netherlands-Evaluating the boundary-layer heat budget
    Steeneveld, G.J. ; Tolk, L. ; Moene, A.F. ; Hartogensis, O.K. ; Peters, W. ; Holtslag, A.A.M. - \ 2011
    Journal of Geophysical Research: Atmospheres 116 (2011). - ISSN 2169-897X - 16 p.
    large-eddy simulations - co2 mixing ratios - land-surface - sensible heat - contrasting nights - vertical diffusion - weather forecasts - transport models - regional-scale - diurnal cycles
    The Weather Research and Forecasting Model (WRF) and the The Weather Research and Forecasting Model (WRF) and the Regional Atmospheric Mesoscale Model System (RAMS) are frequently used for (regional) weather, climate and air quality studies. This paper covers an evaluation of these models for a windy and calm episode against Cabauw tower observations (The Netherlands), with a special focus on the representation of the physical processes in the atmospheric boundary layer (ABL). In ad-dition, area averaged sensible heat flux observations by scintillometry are utilized which enables evaluation of grid scale model fluxes and flux observations at the same horizontal scale. Also, novel ABL height observations by ceilometry and of the near surface longwave radiation divergence are utilized. It appears that WRF in its basic set-up shows satisfactory model results for nearly all atmospheric near surface variables compared to field observations, while RAMS needed refining of its ABL scheme. An important inconsistency was found regarding the ABL daytime heat budget: Both model versions are only able to correctly forecast the ABL thermodynamic structure when the modeled surface sensible heat flux is much larger than both the eddy-covariance and scintillometer observations indicate. In order to clarify this discrepancy, model results for each term of the heat budget equation is evaluated against field observations. Sensitivity studies and evaluation of radiative tendencies and entrainment reveal that possible errors in these variables cannot explain the overestimation of the sensible heat flux within the current model infrastructure.
    Screen level temperature increase due to higher atmospheric carbon dioxide in calm and windy nights revisited
    Steeneveld, G.J. ; Holtslag, A.A.M. ; McNider, R.T. ; Pielke sr., R.A. - \ 2011
    Journal of Geophysical Research: Atmospheres 116 (2011). - ISSN 2169-897X - 7 p.
    nocturnal boundary-layer - land-surface - simulation - scale - urban - model - cases-99 - terrain
    Long-term surface observations over land have shown temperature increases during the last century, especially during nighttime. Observations analyzed by Parker [2004] show similar long-term trends for calm and windy conditions at night, and on basis of this it was suggested that the possible effect of urban heat effects on long-term temperature trends are small. On the other hand, a simplified analytic model study by Pielke and Matsui [2005, henceforth PM05] suggests that at night the resultant long-term temperature trends over land should depend on height and strongly on wind speed (mostly due to alterations in the rate of nocturnal cooling in the stable boundary layer (SBL)). In this paper we expand the PM05 study by using a validated atmospheric boundary-layer model with elaborated atmospheric physics compared to PM05, in order to explore the response of the SBL over land to a change in radiative forcing. We find that the screen level temperature response is surprisingly constant for a rather broad range of both geostrophic wind speed (5-15 ms-1) and 10 meter wind (2-4.0 ms-1). This is mostly due to land surface-vegetation-atmosphere feedbacks taken into account in the present study which were not considered by PM05.
    Effects of soil moisture gradients on the path and the intensity of a West African squall line
    Wolters, D. ; Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. ; Cappelaere, B. ; Ramier, D. - \ 2010
    Quarterly Journal of the Royal Meteorological Society 136 (2010)653. - ISSN 0035-9009 - p. 2162 - 2175.
    mesoscale convective complexes - land-surface - monsoon - variability - sahel - model - precipitation - dynamics - system - field
    During the West African monsoon season, precipitation is strongly coupled to soil moisture availability. This interaction is generally understood as a positive feedback mechanism, and has been considered on very different spatial and temporal scales. Past research has mainly focused on this feedback in terms of the effects on general precipitation patterns, not on a single convective system. In this research, a single squall line is reproduced using the Weather Research and Forecasting Advanced Research (WRF-ARW) mesoscale weather model. Model results are analyzed and compared with surface and upper-air observations. A sensitivity analysis on the influence of soil moisture on the squall line is performed through five numerical experiments. In four experiments, soil moisture is increased or decreased with respect to a control experiment. This is done in two manners: by affecting soil moisture most strongly in the wetter places in the modelled domain and by affecting soil moisture most strongly in the drier places. Minor deviations occur in the path of the squall line after modifying soil moisture most strongly in the wetter places. Systematic deviations occur in its path after increasing soil moisture most strongly in the drier places. A mechanism is proposed that connects the applied soil moisture modifications to larger-scale flow patterns that determine the path of the squall line. In all five experiments, the precipitation intensity of the squall line strongly declines when the systemmoves towards western areaswith lower soil moisture values. It is concluded that a positive effect of local soil moisture on precipitation intensity in passing squall lines is likely on the considered length-scale of 100 km. Until now, this mechanism has only been shown for much smaller spatial scales.
    Impact of Surface Flux Formulations and Geostrophic Forcing on Large-Eddy Simulations of Diurnal Atmospheric Boundary Layer Flow
    Kumar, V. ; Svensson, G. ; Holtslag, A.A.M. ; Meneveau, C. ; Parlange, M.B. - \ 2010
    Journal of Applied Meteorology and Climatology 49 (2010)7. - ISSN 1558-8424 - p. 1496 - 1516.
    profile relationships - complex terrain - land-surface - model - wind - temperature - turbulence - cases-99
    The impact of surface flux boundary conditions and geostrophic forcing on multiday evolution of flow in the atmospheric boundary layer (ABL) was assessed using large-eddy simulations (LES). The LES investigations included several combinations of surface boundary conditions (temperature and heat flux) and geostrophic forcing (constant, time varying, time and height varying). The setup was based on ABL characteristics observed during a selected period of the Cooperative Atmosphere–Surface Exchange Study—1999 (CASES-99) campaign. The LES cases driven by a constant geostrophic wind achieved the best agreement with the CASES-99 observations specifically in terms of daytime surface fluxes and daytime and nighttime profiles. However, the nighttime fluxes were significantly overestimated. The LES cases with the surface temperature boundary condition and driven by a time- and height-varying geostrophic forcing showed improved agreement with the observed nighttime fluxes, but there was less agreement with other observations (e.g., daytime profiles). In terms of the surface boundary condition, the LES cases driven by either surface temperature or heat fluxes produced similar trends in terms of the daytime profiles and comparisons with data from soundings. However, in reproducing the fluxes and nighttime profiles, the agreement was better with imposed temperature because of its ability to interact dynamically with the air temperature field. Therefore, it is concluded that surface temperature boundary condition is better suited for simulations of temporally evolving ABL flow as in the diurnal evolution of the ABL
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