Intercomparison of single-column models for GABLS3: preliminary results

The GEWEX Atmospheric Boundary-Layer Study (GABLS) focus on the representation of stable boundary layers in atmospheric models (Holtslag, 2006). One of the main goals of GABLS is to provide a mondial platform for the atmospheric boundary layer research community through the organisation of model intercomparisons. Here we focus on single column models (SCM's), which can be both research models and SCM's derived from operational weather and climate models. Two SCM intercomparison case studies have been performed so far. One highly idealised case over snow with prescribed surface temperature (Cuxart et al., 2006) and a second case based on observations taken during the CASES 99 stable boundary layer experiment also with prescribed surface temperature (Svensson and Holtslag, 2007). In these studies it was found that especially the complexity of real world boundary conditions and the lack of interaction with the surface makes it difficult to confront the models with observed evaluation parameters. A reasonable ideal case was found in the long observational dataset of the meteorological site Cabauw in the Netherlands (Baas et al., 2008). To make comparison with observations possible care was taken to prescribe realistic advective tendency terms to the SCM's (Bosveld et al., 2008). These were estimated from both local observations and hind casts of several 3D NWP models. The specific characteristics of the Cabauw site with its flat topography (van Ulden and Wieringa, 1995; Beljaars and Bosveld, 1997) makes it well suited to study decoupling around sunset, inertial oscillation and low level jet and the morning time transition to convective conditions (Angevine et al. 2002). Preliminary results will be presented of an intercomparison between SCM's and an evaluation of the models with observations from the Cabauw site. Special attention in this study is on the moment of decoupling around sunset, the inertial oscillation and the morning time transition

Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment

We present the main results from the second model intercomparison within the GEWEX (Global Energy andWater cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today’s numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled nearsurface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identifie

Evaluation of the diurnal cycle in the atmospheric boundary layer over land as represented by a variety of single-column models: the second GABLS experiment

Postprint (published version

Single-column modeling versus mesoscale modeling for an episode with three diurnal cycles in CASES99

From GABLS2 and previous intercomparison studies it is obvious that single column models are a useful tool to study specific parts of the physics. For selected cases where the dynamic forcing plays a minor role, results from a single column model can be useful for the development of the physics. 1D experiments gain insight in the performance of the physics but how much we can learn from them in the perspective of correct dynamic forcings and the selection of the case-study? Following the GABLS2 single-column intercomparisons, we evaluate a 3-D version of HIRLAM on its performance in the atmospheric boundary layer and compare the 3D results with the single-column model results.The single-column model was run according to the specifications from the GABLS2 experiment. HIRLAM uses the maximum horizontal resolution for a hydrostatic model (10 km) and a vertical resolution which is close to the operational setting. A sensitivity experiment with a higher vertical resolution has been conducted with the single-column model. First a single 3D-run has been made where soil moisture was obtained from climatology. A second 3D-run was also made in which the soil moisture has been adapted to surface observations by a surface analysis scheme over a larger number of cycles. These two experiments show the importance of a correct division between sensible and latent heating for almost all parameters within the boundary layer. Results from 3-D HIRLAM divert significantly from the single column version. Wind and temperature profiles from the 3-D runs are in better accordance with the observations. For instance the representation of the Low-Level Jet is surprisingly good in the 3D experiment. We try to clarify the differences between the 3D and 1D experiment. The selection of a period with only small dynamical tendencies is crucial for the comparison of 1D modeloutput with observations. The geostrophic wind is an important factor for the development of the windprofile in the nocturnal boundary layer. It is clear from the 3D study that the dynamic forcing, and thereby the change in the geostrophic wind, is not small in this experiment. Therefore it makes sense that results from the 1D model differ from the 3D model. Consequently it is doubtful to compare 1D results directly with observations. The use of other cases with smaller dynamic forcing will enable a more direct comparison of the observations with the 1D model output and may help us to learn more from 1D model intercomparisons

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

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

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part A: Case Selection and Set-Up

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 dat

Evaluation of limited-area models for the representation of the diurnal cycle and contrasting nights in CASES-99

This study evaluates the ability of three limited-area models [the fifth-generation Pennsylvania State University¿National Center for Atmospheric Research Mesoscale Model (MM5), the Coupled Ocean¿Atmosphere Mesoscale Prediction System (COAMPS), and the High-Resolution Limited-Area Model (HIRLAM)] to predict the diurnal cycle of the atmospheric boundary layer (ABL) during the Cooperative Atmosphere¿Surface Exchange Study (CASES-99) experimental campaign. Special attention is paid to the stable ABL. Limited-area model results for different ABL parameterizations and different radiation transfer parameterizations are compared with the in situ observations. Model forecasts were found to be sensitive to the choice of the ABL parameterization both during the day and at night. At night, forecasts are particularly sensitive to the radiation scheme. All three models underestimate the amplitude of the diurnal temperature cycle (DTR) and the near-surface wind speed. Furthermore, they overestimate the stable boundary layer height for windy conditions and underestimate the stratification of nighttime surface inversions. Favorable parameterizations for the stable boundary layer enable rapid surface cooling, and they have limited turbulent mixing. It was also found that a relatively large model domain is required to model the Great Plains low-level jet. A new scheme is implemented for the stable boundary layer in the Medium-Range Forecast Model (MRF). This scheme introduces a vegetation layer, a new formulation for the soil heat flux, and turbulent mixing based on the local scaling hypothesis. The new scheme improves the representation of surface temperature (especially for weak winds) and the stable boundary layer structure

Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment

We present the main results from the second model intercomparison within the GEWEX (Global Energy andWater cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today’s numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled nearsurface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identifie

The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B: Results and Process Understanding

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 model