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.
Detection of Entrainment Influences on Surface-Layer Measurements and Extension of Monin–Obukhov Similarity Theory
Boer, A. van de; Moene, A.F. ; Graf, A. ; Schüttemeyer, D. ; Simmer, C. - \ 2014
Boundary-Layer Meteorology 152 (2014)1. - ISSN 0006-8314 - p. 19 - 44.
convective boundary-layer - temperature-humidity correlation - sonic anemometer - analytical-model - mixed-layer - turbulence - fluxes - transport - water - heat
We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution
Quantifying the transport of subcloud layer reactants by shallow cimulus clouds over the Amazon
Ouwersloot, H.G. ; Vilà-Guerau De Arellano, J. ; Stratum, B.J.H. van; Krol, M.C. ; Lelieveld, J. - \ 2013
Journal of Geophysical Research: Atmospheres 118 (2013)23. - ISSN 2169-897X - p. 13041 - 13059.
large-eddy simulation - topped mixed layers - convective boundary-layer - diurnal cycle - surface - land - fluxes - model - chemistry - campaign
We investigate the vertical transport of atmospheric chemical reactants from the subcloud layer to the cumulus cloud layer driven by shallow convection over the Amazon during the dry season. The dynamical and chemical assumptions needed for mesoscale and global chemistry transport model parametrizations are systematically analyzed using a Large Eddy Simulation model. We quantify the mass flux transport contribution to the temporal evolution of reactants. Isoprene, a key atmospheric compound over the tropical rain forest, decreases by 8.5% h-1 on average and 15% h-1 at maximum due to mass¿flux¿induced removal. We apply mass flux parametrizations for the transport of chemical reactants and obtain satisfactory agreement with numerically resolved transport, except for some reactants like O3, NO, and NO2. The latter is caused by the local partitioning of reactants, influenced by UV radiation extinction by clouds and small¿scale variability of ambient atmospheric compounds. By considering the longer¿lived NOx (NO + NO2), the transport is well represented by the parametrization. Finally, by considering heterogeneous surface exchange conditions, it is demonstrated that the parametrizations are sensitive to boundary conditions due to changes in the boundary layer dynamics.
Towards a Validation of Scintillometer Measurements: The LITFASS-2009 Experiment
Beyrich, F. ; Bange, J. ; Hartogensis, O.K. ; Raasch, S. ; Braam, M. ; Dinther, D. van; Gräf, D. ; Kesteren, A.J.H. van; Kroonenberg, A.C. van den; Maronga, B. ; Martin, S. ; Moene, A.F. - \ 2012
Boundary-Layer Meteorology 144 (2012)1. - ISSN 0006-8314 - p. 83 - 112.
large-aperture scintillometer - heterogeneous land-surface - convective boundary-layer - sensible heat-flux - temperature-humidity correlation - structure-function parameters - eddy-covariance measurements - flevoland field experiment - index-structure parameter
Scintillometry has been increasingly used over the last decade for the experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres. Nevertheless, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation, in particular over heterogeneous terrain. Moreover, a validation of the path-averaged structure parameters derived from scintillometer data (and forming the basis for the flux calculations) by independent measurements is still missing. To achieve this, the LITFASS-2009 field campaign has been performed around the Meteorological Observatory Lindenberg – Richard-Aßmann-Observatory of the German Meteorological Service (DWD) in July 2009. The experiment combined tower-based in-situ turbulence measurements, field-scale laser scintillometers, long-range optical (large-aperture) and microwave scintillometers, and airborne turbulence measurements using an automatically operating unmanned aircraft. The paper describes the project design and strategy, and discusses first results. Daytime near-surface values of the temperature structure parameter, CT2C2T, over different types of farmland differ by more than one order of magnitude in their dependence on the type and status of the vegetation. Considerable spatial variability in CT2C2T was also found along the flight legs at heights between 50 and 100 m. However, it appeared difficult to separate the effects of heterogeneity from the temporal variability of the turbulence fields. Aircraft measurements and scintillometer data agreed in magnitude with respect to the temporal variation of the path-averaged CT2C2T values during the diurnal cycle. The decrease of CT2C2T with height found from the scintillometer measurements close to the surface and at 43 m under daytime convective conditions corresponds to free-convection scaling, whereas the aircraft measurements at 54 and 83 m suggest a different behaviour.
Land use/land cover changes and climate: modeling analysis and observational evidence
Pielke sr., R.A. ; Pitman, A. ; Niyogi, D. ; Mahmood, R. ; McAlpine, C. ; Hossain, F. ; Kabat, P. - \ 2011
Wiley Interdisciplinary Reviews: Climate Change 2 (2011). - ISSN 1757-7780 - p. 828 - 850.
thermally forced circulations - convective boundary-layer - large-eddy simulations - northern great-plains - us corn-belt - united-states - atmosphere interaction - landscape variability - stomatal conductance - research priorities
Agreat deal of attention is devoted to changes in atmospheric composition and the associated regional responses. Less attention is given to the direct influence by human activity on regional climate caused by modification of the atmosphere’s lower boundary—the Earth’s surface. Land use/land cover change (LULCC), as discussed in this article, concerns human-caused changes that affect the biophysics, biogeochemistry, and biogeography of the terrestrial surface and its affect on the atmosphere.1–3 Vast areas of the Earth’s terrestrial surface have undergone LULCC.4,5 LULCC effects on climate include direct alterations in surface solar and longwave radiation and in atmospheric turbulence which result in changes
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
Land-atmosphere coupling explains the link between pan evaporation and actual evapotranspiration trends in a changing climate
Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. ; Teuling, A.J. - \ 2010
Geophysical Research Letters 37 (2010). - ISSN 0094-8276 - 5 p.
convective boundary-layer - united-states - hydrologic-cycle - soil-moisture - terrestrial - regions - dry
Decreasing trends in pan evaporation are widely observed across the world as a response of the climate system to changes in temperature, precipitation, incoming radiation and wind speed. Nevertheless, we only partially understand how trends in actual evapotranspiration are linked to those trends. Here, we use a model to show that regulation of the near-surface temperature and humidity by land-atmosphere feedbacks results in a strong connection between pan evaporation, actual evapotranspiration and vapor pressure deficit (VPD) depending on the climate forcings. When climate change occurs, the feedbacks direct the system towards a different combination of the three variables. If we know the trends in pan evaporation, VPD and wind speed, we can therefore infer the change in the forcings and estimate the trend in actual evapotranspiration.
Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications
Heus, T. ; Heerwaarden, C.C. van; Jonker, H.J.J. ; Siebesma, A.P. ; Axelsen, S. ; Dries, C.L.A.M. van den; Geoffroy, O. ; Moene, A.F. ; Pino, D. ; Roode, S.R. de; Vilà-Guerau de Arellano, J. - \ 2010
Geoscientific Model Development 3 (2010)2. - ISSN 1991-959X - p. 415 - 444.
convective boundary-layer - shallow cumulus convection - characteristic length scales - topped mixed layers - liquid water path - aircraft observations - marine stratocumulus - radiative-transfer - simple parameterization - turbulent dispersion
The current version of the Dutch Atmospheric Large-Eddy Simulation (DALES) is presented. DALES is a large-eddy simulation code designed for studies of the physics of the atmospheric boundary layer, including convective and stable boundary layers as well as cloudy boundary layers. In addition, DALES can be used for studies of more specific cases, such as flow over sloping or heterogeneous terrain, and dispersion of inert and chemically active species. This paper contains an extensive description of the physical and numerical formulation of the code, and gives an overview of its applications and accomplishments in recent years
Sensible and latent heat flux from radiometric surface temperatures at the regional scale: methodology and validation
Miglietta, F. ; Gioli, B. ; Brunet, Y. ; Hutjes, R.W.A. ; Matese, A. ; Sarrat, C. ; Zaldei, A. - \ 2009
Biogeosciences 6 (2009). - ISSN 1726-4170 - p. 1975 - 1986.
klimaatverandering - kooldioxide - temperatuur - modellen - climatic change - carbon dioxide - temperature - models - convective boundary-layer - evapotranspiration rates - eddy covariance - carbon-dioxide - water-vapor - aircraft - model - airborne - field - emissivity
The CarboEurope Regional Experiment Strategy (CERES) was designed to develop and test a range of methodologies to assess regional surface energy and mass exchange of a large study area in the south-western part of France. This paper describes a methodology to estimate sensible and latent heat fluxes on the basis of net radiation, surface radiometric temperature measurements and information obtained from available products derived from the Meteosat Second Generation (MSG) geostationary meteorological satellite, weather stations and ground-based eddy covariance towers. It is based on a simplified bulk formulation of sensible heat flux that considers the degree of coupling between the vegetation and the atmosphere and estimates latent heat as the residual term of net radiation. Estimates of regional energy fluxes obtained in this way are validated at the regional scale by means of a comparison with direct flux measurements made by airborne eddy-covariance. The results show an overall good matching between airborne fluxes and estimates of sensible and latent heat flux obtained from radiometric surface temperatures that holds for different weather conditions and different land use types. The overall applicability of the proposed methodology to regional studies is discussed
Flux estimates of isoprene, methanol and acetone from airborne PTR-MS measurements over the tropical rainforest during the GABRIEL 2005 campaign
Eerdekens, G. ; Ganzeveld, L.N. ; Vilà-Guerau de Arellano, J. ; Klüpfel, T. ; Sinha, V. - \ 2009
Atmospheric Chemistry and Physics 9 (2009). - ISSN 1680-7316 - p. 4207 - 4227.
volatile organic-compounds - methyl vinyl ketone - reaction mass-spectrometry - convective boundary-layer - general-circulation model - chemistry-climate model - atmospheric chemistry - shallow cumulus - initiated oxidation - monoterpene fluxes
Tropical forests are a strong source of biogenic volatile organic compounds (BVOCs) to the atmosphere which can potentially impact the atmospheric oxidation capacity. Here we present airborne and ground-based BVOC measurements representative for the long dry season covering a large area of the northern Amazonian rainforest (6-3° N, 50-59° W). The measurements were conducted during the October 2005 GABRIEL (Guyanas Atmosphere-Biosphere exchange and Radicals Intensive Experiment with the Learjet) campaign. The vertical (35 m to 10 km) and diurnal (09:00-16:00) profiles of isoprene, its oxidation products methacrolein and methyl vinyl ketone and methanol and acetone, measured by PTR-MS (Proton Transfer Reaction Mass Spectrometry), have been used to empirically estimate their emission fluxes from the forest canopy on a regional scale. The mixed layer isoprene emission flux, inferred from the airborne measurements above 300 m, is 5.7 mg isoprene m-2 h-1 after compensating for chemistry and ~6.9 mg isoprene m-2 h-1 taking detrainment into account. This surface flux is in general agreement with previous tropical forest studies. Inferred methanol and acetone emission fluxes are 0.5 mg methanol m¿2 h¿1 and 0.35 mg acetone m-2 h-1, respectively. The BVOC measurements were compared with fluxes and mixing ratios simulated with a single-column chemistry and climate model (SCM). The inferred isoprene flux is substantially smaller than that simulated with an implementation of a commonly applied BVOC emission algorithm in the SCM.
Mean and Flux Horizontal Variability of Virtual Potential Temperature, Moisture, and Carbon Dioxide: Aircraft Observations and LES Study
Górska, M. ; Vilà-Guerau de Arellano, J. ; LeMone, M.A. ; Heerwaarden, C.C. van - \ 2008
Monthly Weather Review 136 (2008). - ISSN 0027-0644 - p. 4435 - 4451.
convective boundary-layer - large-eddy simulations - sensible heat - land-surface - entrainment - scale - ihop-2002 - soil - heterogeneity - cases-97
The effects of the horizontal variability of surface properties on the turbulent fluxes of virtual potential temperature, moisture, and carbon dioxide are investigated by combining aircraft observations with large-eddy simulations (LESs). Daytime fair-weather aircraft measurements from the 2002 International H2O Project¿s 45-km Eastern Track over mixed grassland and winter wheat in southeast Kansas reveal that the western part of the atmospheric boundary layer was warmer and drier than the eastern part, with higher values of carbon dioxide to the east. The temperature and specific humidity patterns are consistent with the pattern of surface fluxes produced by the High-Resolution Land Data Assimilation System. However, the observed turbulent fluxes of virtual potential temperature, moisture, and carbon dioxide, computed as a function of longitude along the flight track, do not show a clear east¿west trend. Rather, the fluxes at 70 m above ground level related better to the surface variability quantified in terms of the normalized differential vegetation index (NDVI), with strong correlation between carbon dioxide fluxes and NDVI.
Relative Humidity as an Indicator for Cloud Formation over Heterogeneous land surfaces
Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. - \ 2008
Journal of the Atmospheric Sciences 65 (2008)10. - ISSN 0022-4928 - p. 3263 - 3277.
convective boundary-layer - large-eddy-simulation - entrainment - turbulence - shear - vegetation - evolution - models - driven - scale
The influence of land surface heterogeneity on potential cloud formation is investigated using relative humidity as an indicator. This is done by performing numerical experiments using a large-eddy simulation model (LES). The land surface in the model was divided into two patches that had the same sum of latent and sensible heat flux but different Bowen ratios to simulate heterogeneous land surfaces. For heterogeneity in the meso-¿ scale (2¿20 km), sensitivity analyses were carried out on the heterogeneity amplitude (Bowen ratio difference between contrasting areas) and the inversion strength of potential temperature and specific humidity. The competition between absolute temperature decrease by ABL growth and dry air entrainment in heterogeneous conditions is analyzed using the LES results. First, it is shown that entrainment is located and enhanced over patches with higher Bowen ratios (warm patches) than their surroundings (cold patches). The heterogeneity-induced strong thermals can further penetrate the inversion at the ABL top, thereby reaching lower absolute temperatures than in homogeneous conditions. Second, because of the heterogeneity-induced circulations the moisture is located over the warm patch, and higher time-averaged RH values at the ABL top (RHzi) than over the cold patches are found here, even for dry atmospheres. These RHzi exceed values found over homogeneous land surfaces and are an indication that surface heterogeneity may facilitate cloud formation. In vertical profiles of RH, few differences are found between the homogeneous and heterogeneous cases, but the essential heterogeneity-induced modifications are within the domain variability.
Diurnal and vertical variability of the sensible heat and carbon dioxide budgets in the atmospheric surface layer
Casso-Torralba, P. ; Vilà-Guerau de Arellano, J. ; Bosveld, F. ; Soler, M.R. ; Vermeulen, A. ; Werner, C. ; Moors, E.J. - \ 2008
Journal of Geophysical Research: Atmospheres 113 (2008). - ISSN 2169-897X - 15
warmtestroming - kooldioxide - atmosfeer - bovenlagen - meteorologische waarnemingen - heat flow - carbon dioxide - atmosphere - surface layers - meteorological observations - convective boundary-layer - eddy-covariance - solfatara volcano - water-vapor - exchange - forest - advection - moisture - cabauw - fluxes
The diurnal and vertical variability of heat and carbon dioxide (CO2) in the atmospheric surface layer are studied by analyzing measurements from a 213 m tower in Cabauw (Netherlands). Observations of thermodynamic variables and CO2 mixing ratio as well as vertical profiles of the turbulent fluxes are used to retrieve the contribution of the budget terms in the scalar conservation equation. On the basis of the daytime evolution of turbulent fluxes, we calculate the budget terms by assuming that turbulent fluxes follow a linear profile with height. This assumption is carefully tested and the deviation from linearity is quantified. The budget calculation allows us to assess the importance of advection of heat and CO2 during day hours for three selected days. It is found that, under nonadvective conditions, the diurnal variability of temperature and CO2 is well reproduced from the flux divergence measurements. Consequently, the vertical transport due to the turbulent flux plays a major role in the daytime evolution of both scalars and the advection is a relatively small contribution. During the analyzed days with a strong contribution of advection of either heat or carbon dioxide, the flux divergence is still an important contribution to the budget. For heat, the quantification of the advection contribution is in close agreement with results from a numerical model. For carbon dioxide, we qualitatively corroborate the results with a Lagrangian transport model. Our estimation of advection is compared with traditional estimations based on the Net Ecosystem-atmosphere Exchange (NEE).
Role of nocturnal turbulence and advection in the formation of shallow cumulus over land
Vilà-Guerau de Arellano, J. - \ 2007
Quarterly Journal of the Royal Meteorological Society 133 (2007)628. - ISSN 0035-9009 - p. 1615 - 1627.
convective boundary-layer - low-level jet - diurnal cycle - model - simulation - inversion - moisture
Shallow cumuli over land are normally studied from a diurnal perspective. However, the thermodynamic vertical profiles of the morning transition may play an important role in setting up favourable conditions for the formation of shallow cumuli. In turn, these profiles are highly dependent on the evolution of the nocturnal boundary characteristics and of their layer aloft. By analysing thermodynamic profiles measured by radiosondes launched every three hours at four different stations, we are able to determine how horizontal advection and turbulent mixing modify the atmospheric stability and the differences in potential temperature and specific humidity at the interface between the atmospheric boundary layer and the layer above it. Two consecutive nights are studied. They show very similar boundary-layer development; but variations in the layer aloft by a low-level-jet advection event during the second night, and intense turbulent mechanical mixing, lead to the development of two diurnal boundary layers with very different characteristics: the first one clear, the second cloudy. To complete the observational study, we perform a sensitivity analysis using a mixed-layer model to examine the role of the morning initial conditions in the formation of shallow cumuli over land. The complexity and subtlety of the observed situation - namely, the interaction of a strongly-mixed nocturnal boundary layer and horizontal advection - make this case suitable for testing the capacity of mesoscale models to reproduce cloudy boundary layers that are largely dependent on conditions during the previous night.
Role of Shear and the Inversion Strength During Sunset Turbulence Over Land: Characteristic Length Scales, pre print
Pino, D. ; Jonker, H.J.J. ; Vilà-Guerau de Arellano, J. ; Dosio, A. - \ 2006
Boundary-Layer Meteorology 121 (2006)3. - ISSN 0006-8314 - p. 537 - 556.
convective boundary-layer - large-eddy simulation - homogeneous isotropic turbulence - decay - transition - transport
The role of shear and inversion strength on the decay of convective turbulence during sunset over land is systematically studied by means of large-eddy simulations. Different decay rates have been found for the vertical and horizontal velocity fluctuations, resulting in an increase of the anisotropy for all the studied cases. Entrainment, which persists during the decay process, favours the appearance of vertical upward movements associated with a conversion from kinetic to potential energy. Particular attention is paid to the evolution of the characteristic length scale of the various turbulent variables during this process. The length scale evolution is found to depend on the wind shear characteristics, but not on the strength of the inversion. In general the length scales of the variables grow during decay because small-scale fluctuations dissipate faster than large-scale fluctuations. Only the length scale of the vertical velocity component remains nearly constant during decay. Spectral analysis of the variance budgets shows that pressure correlations are responsible for fixing this length scale, effectively compensating the strong but oscillating influence of buoyancy. In the shear cases, after an initial period of growth, the length scales start to decrease once the buoyancy-generated variance has sufficiently subsided. Also here the effect of pressure redistribution is crucial, as it transfers the spectral influence of shear to the other velocity components
Area-Averaged Surface Fluxes in a Semiarid Region with Partly Irrigated Land: Lessons Learned from EFEDA
Jochum, A.M. ; Debruin, H.A.R. ; Holtslag, A.A.M. ; Calera Belmonte, A. - \ 2006
Journal of Applied Meteorology and Climatology 45 (2006)6. - ISSN 1558-8424 - p. 856 - 874.
desertification-threatened area - convective boundary-layer - european field experiment - hapex-mobilhy - nonhomogeneous terrain - meteorological models - ground measurements - turbulent fluxes - water-vapor - evaporation
The European Field Experiment in a Desertification-Threatened Area (EFEDA) provides a comprehensive land surface dataset for a semiarid Mediterranean environment with natural vegetation and cultivated dry and irrigated land. This paper discusses the methods and practical aspects of deriving area-averaged fluxes for a range of areas from the whole EFEDA region to several numerical weather prediction model grid cells (on 10¿100-km scales). A time series of grid-scale surface fluxes for the entire observational period of 1 month was obtained from weighted surface averages, using a crop phenology¿based land use classification together with a homogenized set of surface observations representative of the four major vegetation classes. The flux-aggregated surface observations were compared with two other approaches to obtain grid-scale fluxes (airborne flux observations and radiosondes in conjunction with a simple mixed-layer model). The area-aggregated fluxes (in particular of latent heat) depend strongly on the location of the area boundaries whenever a significant fraction of irrigated land is present. This result confirms clearly the importance of adequately accounting for tiles of irrigated land in surface schemes and corresponding physiographic databases of large-scale models. A simple way to accommodate for minimum information on the canopy water status is proposed in terms of the distinction of at least two seasonal classes of irrigated crops¿one of spring and one of summer growing cycles. The main lesson from this aggregation exercise concerns the role of irrigation. First, this study quantifies the uncertainties in the space¿time pattern and its effects on aggregated surface fluxes for the first time on the grounds of observational data. Second, it demonstrates practical ways to accomplish the parameterization of irrigation in flux aggregation schemes, by identifying the key data along with their possible sources and by defining a practical implementation procedure
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.
Bridging the gap between atmospheric physics and chemistry in studies of small-scale turbulence
Vilà-Guerau de Arellano, J. - \ 2003
Bulletin of the American Meteorological Society 84 (2003)1. - ISSN 0003-0007 - p. 51 - 56.
convective boundary-layer - large-eddy simulation - surface-layer - chemical-reactions - fluxes - nitrogen - plume - ozone - deposition - diffusion
The current understanding of the influence of atmospheric turbulence on chemical reactions is briefly reviewed. The fundamentals of this influence and the consequences for the transport and mixing of the reactants are discussed. A classification of the turbulent reacting flows is proposed in terms of the values of dimensionless numbers. These numbers depend on the characteristic timescale of the dynamics and the chemistry. The main findings obtained by modeling studies of various atmospheric boundary layer flows and chemical mechanisms, in particular the ones done by means of the large eddy simulation technique, are summarized. Based on the conclusions of these studies, the need to carry out intensive and comprehensive atmospheric field campaigns and laboratory experiments to corroborate the numerical results is discussed. Specific open questions are posed to improve, by combining observational experiments and modeling, our knowledge of the role played by physical processes on the transformations of reactive species in the atmospheric boundary layer