Showing papers by "Albert A. M. Holtslag published in 2006"

An intercomparison of large-eddy simulations of the stable boundary layer

Abstract: Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL.

Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer

Abstract: 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.

Modeling the evolution of the atmospheric boundary layer coupled to the land surface for three contrasting nights in CASES-99

Abstract: The modeling and prediction of the stable boundary layer over land is a persistent, problematic feature in weather, climate, and air quality topics. Here, the performance of a state-of-the-art single-column boundary layer model is evaluated with observations from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) field experiment. Very high model resolution in the atmosphere and the soil is utilized to represent three different stable boundary layer archetypes, namely, a fully turbulent night, an intermittently turbulent night, and a radiative night with hardly any turbulence (all at clear skies). Each archetype represents a different class of atmospheric stability. In the current model, the atmosphere is fully coupled to a vegetation layer and the underlying soil. In addition, stability functions (local scaling) are utilized based on in situ observations. Overall it is found that the vertical structure, the surface fluxes (apart from the intermittent character) and the surface tem...

Exploring Self-Correlation in Flux-Gradient Relationships for Stably Stratified Conditions

Abstract: In this paper, the degree of scatter in flux–gradient relationships for stably stratified conditions is analyzed. It is generally found that scatter in the dimensionless lapse rate ϕh is larger than in the dimensionless shear ϕm when plotted versus the stability parameter z/Λ (where Λ is the local Obukhov length). Here, this phenomenon is explained to be a result of self-correlation due to the occurrence of the momentum and the heat flux on both axes, measurement uncertainties, and other possibly relevant physical processes left aside. It is shown that the ratio between relative errors in the turbulent fluxes influences the orientation of self-correlation in the flux–gradient relationships. In stable conditions, the scatter in ϕm is largely suppressed by self-correlation while for ϕh this is not the case (vice versa for unstable stratification). An alternative way of plotting is discussed for determining the slope of the linear ϕm function.

An Automated Microlysimeter to Study Dew Formation and Evaporation in Arid and Semiarid Regions

Abstract: The development of a simple and low-cost portable weighing microlysimeter that makes use of a load cell for automated recording and for studying daily dew formation, rate of accumulation, and subsequent evaporation in arid or semiarid regions during rainless seasons is presented. The sampling cup is 3.5 cm deep, with the load cell itself situated at 20-cm depth to minimize temperature effects. The device was tested in a sand dune experimental station situated near Nizzana, northwest Negev Desert, Israel, during which extensive micrometeorological measurements were collected. One microlysimeter was placed in a playa and a second was installed on the stabilized midslope of an adjacent linear sand dune. To assess the performance of the load cell microlysimeters (LCM), one pair of manual microlysimeters was installed next to each LCM. A third pair was installed at a point between the LCMs and a fourth pair above the midslope LCM. Sixteen overnight measurements were carried out within a 6-week period....

Surface Fluxes and Characteristics of Drying Semi-Arid Terrain in West Africa

Abstract: This study examines the seasonal cycle of the components of the surface energy balance in the Volta basin in West Africa as part of the GLOWA-Volta project. The regional climate is characterized by a strong north-south gradient of mean annual rainfall and the occurrence of pronounced dry and wet seasons within one annual cycle, causing a strong seasonal variation in the natural vegetation cover. The observations are conducted with a combined system, consisting of a Large Aperture Scintillometer (LAS) for areally aver- aged sensible heat flux, radiometers and sensors for soil heat flux. For comparisons the eddy-covariance (EC) method providing the fluxes of momentum, sensible and latent heat is utilized as well. The measurements of a seasonal cycle in 2002/2003 were gathered including the rapid wet-to-dry transition after the wet season at two locations in Ghana, one in the humid tropical southern region and one in the northern region. A direct comparison and the energy balance closure of the two methods are investigated for daytime and nighttime separately. An attempt is made to understand and explain the differences between the two methods and the closure of energy budget found for these. It is found that the two systems correspond well during daytime. During nighttime the LAS seems to perform more realis- tically than the EC system. Considering the fact that a LAS system is much easier to use in the climate conditions of the Volta basin, it is concluded that the LAS approach is very suitable in this type of climate conditions. Surface conductances are estimated by rearranging the Penman-Monteith equation and compared to a Jarvis-type model optimised for savan- nah conditions. It is found that temperature dependence should be included in the conduc- tance formulation in contrast to earlier findings. Based on the findings the gathered dataset can be used for further model studies of the climate and environment of West Africa.

Modelling the Arctic Stable Boundary Layer and its Coupling to the Surface

Abstract: 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.

Scaling Variances of Scalars in a Convective Boundary Layer Under Different Entrainment Regimes

Abstract: For the presentation and analysis of atmospheric boundary-layer (ABL) data, scales are used to non-dimensionalise the observed quantities and independent variables. Usually, the ABL height, surface sensible heat flux and surface scalar flux are used. This works well, so long as the absolute values of the entrainment ratio for both the scalar and temperature are similar. The entrainment ratio for temperature naturally ranges from −0.4 to −0.1. However, the entrainment ratio for passive scalars can vary widely in magnitude and sign. Then the entrainment flux becomes relevant as well. The only customary scalar scale that takes into account both the surface flux and the entrainment flux is the bulk scalar scale, but this scale is not well-behaved for large negative entrainment ratios and for an entrainment ratio equal to −1. We derive a new scalar scale, using previously published large-eddy simulation results for the convective ABL. The scale is derived under the constraint that scaled scalar variance profiles are similar at those heights where the variance producing mechanisms are identical (i.e., either near the entrainment layer or near the surface). The new scale takes into account that scalar variance in the ABL is not only related to the surface flux of that scalar, but to the scalar entrainment flux as well. Furthermore, it takes into account that the production of variance by the entrainment flux is an order of magnitude larger than the production of variance by the surface flux (per unit flux). Other desirable features of the new scale are that it is always positive (which is relevant when scaling standard deviations) and that the scaled variances are always of order 1–10.

Single column modeling of the diurnal cycle based on CASES99 data - GABLS second intercomparison project

Abstract: The second GABLS (GEWEX Atmospheric Boundary Layer Study) intercomparison case was selected from the CASES99 data set. The purpose of the study is to intercompare boundary layer closures performance for a diurnal cycle in a highly controlled single column setting. To keep the simulation simple, all models were forced with the observed surface temperature and a constant geostrophic wind base on the observations. The simulations covered two full diurnal cycles and the first night plus a full diurnal cycle it is possible to compare with the observations, the rest of the simulation may be used for model intercomparison. In this study, 23 different models so far are participating and there is a good spread in their turbulence closures, with first order models as well as models with prognostic TKE. The participants are from research groups but many operational centers have also contributed to this study. In additions, some of the models are run with both high and operational (low) vertical resolution. The analysis shows that even for this very controlled simulation there is a large variability in the model results especially when it comes to the turbulent fluxes and the boundary layer height and the deviations from the observations are large. There is higher agreement between the models when it comes to the 2m-temperature (naturally because of the surface temperature forcing) and the 10m-windspeed and direction. However, the models in general do not show the observed increase in wind speed during the daytime. Thus, the modeled wind speed is to high during the night (due to less stability than observed) and to low during the daytime. To further add to the information of the case, a three-dimensional simulation of the selected period was performed with one of the contributing models, COAMPS. The analysis of this simulation gives some indication of how 3-dimensional the flow situation was and what can or can't be expected by the single column models. In addition, an effort is made by another group to make a LES simulation of this diurnal cycle. If simulations are completed in time, the results from this simulation will also be compared with the single-column model results.

Area-Averaged Surface Fluxes in a Semiarid Region with Partly Irrigated Land: Lessons Learned from EFEDA

Abstract: 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 stron...

Atmospheric Boundary‐Layer Climates and Interactions with the Land Surface

Abstract: The purpose of this article is to provide an overview of the relevant processes and interactions in the Atmospheric Boundary Layer (ABL) over land. Modeling principles for the ABL are summarized and discussed, including the formulation and role of the surface fluxes. It is shown that both the ABL and the coupled soil-vegetation-atmosphere system can be well represented in the well-defined cases examined. The interactions between the ABL and the land surface are illustrated with the role of soil moisture on boundary-layer cloud initiation. As such, a new analytical development is discussed and illustrated for the tendency of relative humidity at boundary-layer top. Typically, increased soil moisture leads to a higher potential for boundary-layer clouds, but in some cases, the largest potential for boundary-layer clouds is predicted over dry soils. For readers not familiar with atmospheric turbulence and meteorological definitions, some background is given as well. Keywords: atmospheric boundary layer (ABL); atmospheric turbulence; boundary-layer modeling; boundary-layer cloud initiation; land surface climate; surface fluxes; soil-vegetation-atmosphere; soil moisture

Contribution of dew to the water budget and ecology of a grassland area in the Netherlands

Abstract: The annual amount of dew input to the water budget in the midlatitudes has been neglected, possibly because direct dew measurements are very difficult and time-consuming. As The Netherlands has a very high frequency of dew events, a grassland area was selected to determine whether dew input could be significant. The study site is situated within the Wageningen University meteorological station. Dew measurement experiments were carried out in 2004. Data were used to calibrate a surface energy dew model, which was then applied to a 12-year data set. A mean annual dew amount of 37 mm was obtained with a standard deviation of 8 mm, while the mean annual precipitation was 830 mm with a standard deviation of 200 mm. Dew contributed about 4.5 % of the mean annual precipitation. The average number of dew nights per year was 250 with a standard deviation of 25 nights. Thus nearly 70% of all nights experienced a dew episode, which impacts significantly on the leaf wetness and possible vegetation diseases.

Exploring variability of model results in the GEWEX Atmospheric Boundary Layer Study (GABLS)

Abstract: Within the GEWEX Atmospheric Boundary Layer Study (GABLS) intercomparisons are organized for the performance of boundary-layer schemes. Sofar this has been done by using a prescribed surface temperature in onedimensional (column) models (see Cuxart et al, 2006; Svensson and Holtslag, 2006). It appears that the results for both the first and second GABLS study show a significant variability in the surface fluxes and atmospheric profiles, despite the relatively simple boundary condition and the forcing conditions. This is directly related to the different parameterizations of the various models (Holtslag, 2006). In stead of prescribing the surface temperature, one may also consider to prescribe the surface heat flux. However, in particular for night time conditions over land, the surface heat flux depends strongly on the surface layer turbulence. In addition, the surface temperature and the surface heat flux are strongly related (Van de Wiel et al, 2003; Steeneveld et al, 2006a). Thus in reality neither the surface temperature nor the surface heat flux are true external boundary conditions. Therefore in this paper we address surface feed-back by solving the surface energy balance. As such we investigate to what extent the degree of variability among the models is influenced by this. The set up of the second GABLS intercomparison case is used to study three diurnal cycles of the boundary layer over land under clear skies.

A Formula for the Depth of the Stable Boundary layer: Evaluation and Dimensional Analysis

Abstract: The height (h) of the stable boundary layer (SBL) is of major importance to understand the relevant processes that govern the SBL development. The SBL depth is the layer in which turbulence transport takes place, and thus governs the vertical structure of the lower atmosphere. Furthermore, release of pollutants below h during periods of weak mixing results in very high pollutant concentrations. For meteorological preprocessors for air quality models, h is the most critical quantity to estimate. This study evaluates the performance of two so-called multi-limit equations for h, and presents an alternative formulation (based on dimensional analysis) that depends on surface friction velocity, surface buoyancy flux, Coriolis parameter and the stratification above the boundary layer. The formulations are tested against four observational data sets (with varying roughness, latitude and land use) and Large Eddy Simulation from the first GEWEX Atmospheric Boundary Layer Study (GABLS). The multi-limit formulations show a significant negative bias for shallow SBLs. We find that the required parameters in the multi-limit Equations cannot be calibrated in a robust manner. The main advantages of the new formulation lies in a robust calibration, a significant reduced bias for small SBL depth and reduction of scatter compared over the whole range of observations to the two multi-limit Equations.

Mesoscale Model Intercomparison and Observational Evaluation for Three Contrasting Diurnal Cycles in CASES-99: Focus on the Stable Boundary Layer

Abstract: The boundary-layer scheme in Numerical Weather Prediction (NWP) models is an essential connection between the atmosphere and the land-surface. Despite its relevance, the representation of the boundary-layer and the diurnal cycle in NWP models is rather poor, especially for stable conditions. Following the GABLS single-column and LES-model intercomparisons, we evaluate three state-of-the-art meso-scale models: MM5 (using 4 boundary-layer schemes, and three radiation schemes), COAMPS and HIRLAM on their performance in the atmospheric boundary layer, for three contrasting diurnal cycles during CASES-99. All models use maximum possible horizontal resolution, and increased vertical resolution near the surface compared to operational settings. Soil moisture availability was modified to realistic conditions according to the observations. The first diurnal cycle contains a night with intermittent turbulence, while the second night is continuous turbulent. Finally, the last cycle contains a night with extremely small amount of turbulence and highly driven by the radiative forcing. We find that during daytime the TKE-closure schemes show too cold and shallow boundary layers. During nighttime TKE models show a more realistic vertical mixing compared to operational first-order schemes. All models show a high sensitivity of the boundary layer development to the soil moisture availability. Contrary to earlier findings, the representation of the Low-Level jet is surprisingly good. A general model deficiency is the underestimated longwave incoming radiation. Furthermore,it is found that the minimum temperature modeled with MM5 depend strongly on the chosen radiation scheme (up to 5 K during weak winds), with favorable results when using RRTM or CCM3. This sensitivity is an extra contribution to the already known strong sensitivity for turbulent mixing in stable conditions.