K. Heinke Schlünzen

Modelling the arctic convective boundary-layer with different turbulence parameterizations

Different parameterizations of subgrid-scale fluxes are utilized in a nonhydrostatic and anelastic mesoscale model to study their influence on simulated Arctic cold air outbreaks. A local closure, a profile closure and two nonlocal closure schemes are applied, including an improved scheme, which is based on other nonlocal closures. It accounts for continuous subgrid-scale fluxes at the top of the surface layer and a continuous Prandtl number with respect to stratification. In the limit of neutral stratification the improved scheme gives eddy diffusivities similar to other parameterizations, whereas for strong unstable stratifications they become much larger and thus turbulent transports are more efficient. It is shown by comparison of model results with observations that the application of simple nonlocal closure schemes results in a more realistic simulation of a convective boundary layer than that of a local or a profile closure scheme. Improvements are due to the nonlocal formulation of the eddy diffusivities and to the inclusion of heat transport, which is independent of local gradients (countergradient transport).

Relevance of sub‐grid‐scale land‐use effects for mesoscale models

The dependence of the performance of mesoscale-α and mesoscale-β models on model resolutionand parameterisation of sub-grid-scale land-use effects is investigated for a meteorological situationthat is strongly influenced by local effects. Two mesoscale models of different horizontal resolutionand model physics are applied to the same meteorological situation, and the results are comparedwith observations. Sensitivity studies are performed to estimate the effect of resolution (4 km or 5 kmversus 18 km), parameterisation of sub-grid-scale land-use effects and initial state on the results ofboth models. Results show that the accuracy improves with resolution when simple parameterisationsof sub-grid-scale land-use effects are applied. The inclusion of sub-grid-scale land-use effects withapplication of a flux aggregation method provides the best solution, which is nearly independent of theresolutions used. The use of parameter averaging to account for sub-grid-scale land-use effects wasworse than just using one (main) land-use per grid cell. Initial conditions were also found to have asignificant impact on the results.

Evaluation and Bias Correction of Regional Climate Model Results Using Model Evaluation Measures

AbstractFor the assessment of regional climate change the reliability of the regional climate models needs to be known. The main goal of this paper is to evaluate the quality of climate model data that are used for impact research. Temperature, precipitation, total cloud cover, relative humidity, and wind speed simulated by the regional climate models Climate Local Model (CLM) and Regional Model (REMO) are evaluated for the metropolitan region of Hamburg in northern Germany for the period 1961–2000. The same evaluation is performed for the global climate model ECHAM5 that is used to force the regional climate models. The evaluation is based on comparison of the simulated and observed climatological annual cycles and probability density functions of daily averages. Several model evaluation measures are calculated to assure an objective model evaluation. As a very selective model evaluation measure, the hit rate of the percentiles is introduced for the evaluation of daily averages. The influence of interann...

A prognostic physico-chemical model of secondary and marine inorganic multicomponent aerosols II. Model tests

The accuracy and efficiency of the sectional multicomponent aerosol model SEMA, described in the first part of this paper, are tested. Comparisons of results of an equilibrium version of SEMA with results of the equilibrium models SEQUILIB and AIM show good agreement for relative humidities above 60% and thus validate the thermodynamic portion of the model. Tests of the dynamic portion of SEMA show the reliability of the model down to a minimum number of four sections. The results of a model application give evidence that kinetic limitations may be important in the formation of secondary aerosol species by condensation of sulphuric acid, nitric acid, and ammonia on sea salt aerosol. The concentrations of the chemical components of marine aerosol may be substantially different from their thermodynamic equilibrium concentrations in the polluted coastal atmosphere.

Modification of dry deposition in a developing sea-breeze circulation—A numerical case study

Abstract In this paper a numerical mesoscale model is used to study the temporal and spatial changes in the dry deposition of pollutants (particularly SO2) at a coastline with mudflats in the North Sea region during a developing sea-breeze situation. The amount of deposited material depends on the deposition velocity and the concentration. Both are controlled by the atmospheric stability and the wind field which are strongly affected by terrain-forced local circulation systems. The dry deposition velocity is assumed in the present model to depend on surface, tracer, and atmospheric properties. The model results indicate that the deposition velocity over water remains nearly unchanged during the entire day. Over land and mudflats, the deposition velocity is strongly correlated with vertical mixing, with maxima in the diffusion coefficient resulting in maxima in the deposition velocity. Additionally, it shows a diurnal variation, e.g. the deposition velocity for SO2 overland is about tripled at the day compared to the values at night. A maximum in the SO2 deposition overland can be expected in the morning when the deposition velocity is quite high and the atmospheric boundary layer is still low, resulting in comparatively high concentrations near the ground. In view of the dependence of deposition velocity on local circulation systems, accurate deposition modelling requires meteorological data with a high resolution in space and time.

Simulation of the dynamics and composition of secondary and marine inorganic aerosols in the coastal atmosphere

Results of an extended version of the three-dimensional Eulerian Mesoscale Transport, Chemistry, and Stream Model (METRAS) for simulating size-segregated inorganic aerosols are presented. The extended version of METRAS includes aerosol transport by advection, diffusion, gravitational settling, and dry deposition. Aerosols are produced by homogeneous nucleation and by sea spray. They are modified by condensation and evaporation of inorganic aerosol precursor gases. Altogether, 73 inorganic and organic gas species are treated in the model by taking into account transport, deposition, emissions, and gas-phase reactions. The model is applied to simulate the mixing of marine and continental air in the coastal atmosphere over the German Bight. The simulation results give evidence for the importance of the uptake of nitric acid and ammonia by sea-salt aerosol for the dynamics of nitrogen compounds in the coastal atmosphere. As a result of the mixing of polluted continental air masses with marine air masses, the pH of the sea-salt aerosol reaches values as low as pH = 2.1.

On the validation of high-resolution atmospheric mesoscale models

Abstract The central ideas of a validation concept are outlined. The concept is applicable to high-resolution atmospheric mesoscale models, which calculate wind, temperature and humidity fields as well as pollutant concentrations and which might be used as regulatory models. The validation concept takes into account model characteristics, the model realization and results from selected case studies. It includes five criteria: completeness, comprehensibility, code quality, result quality and result control . The concept may be used by model developers and users to determine the validity of their models and to evaluate its applicability for the simulation of atmospheric phenomena, developing in an area of horizontal extension between 10 and 300 km in the lower troposphere in mid-latitudes.

Evaluation of a Mesoscale Model with Different Surface Parameterizations and Vertical Resolutions for the Bay of Valencia

Abstract Two different setups of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used and the results of 71 forecasts are evaluated with a focus on the accuracy of meteorological surface data including wind, temperature, dewpoint temperature, and pressure. The forecasts, which cover the Valencia coastal region with a 3-km grid, are nested into two coarser grids of 9- and 27-km mesh size. The integration time is 18 h. The ability to correctly simulate the sea breeze plays a major role for this area. Different model setups are used: in the M1 simulations 34 atmospheric vertical sigma levels combined with a five-layer land surface model (5L LSM) are applied for the first half of the year 2006. In total, 52 atmospheric vertical sigma levels and the Noah LSM are used for the second half of 2006 (setup M2). The accuracy improves for all meteorological parameters except pressure with the M2 setup. However, negative biases of temperature, dewpoint temperature, and wind speed wors...

Weather Pattern Classification to Represent the Urban Heat Island in Present and Future Climate

AbstractA classification of weather patterns (WP) is derived that is tailored to best represent situations relevant for the urban heat island (UHI). Three different types of k-means-based cluster methods are conducted. The explained cluster variance is used as a measure for the quality. Several variables of the 700-hPa fields from the 40-yr ECMWF Re-Analysis (ERA-40) were tested for the classification. The variables as well as the domain for the clustering are chosen in a way to explain the variability of the UHI as best as possible. It turned out that the combination of geopotential height, relative humidity, vorticity, and the 1000–700-hPa thickness is best suited. To determine the optimal cluster number k several statistical measures are applied. Except for autumn (k = 12) an optimal cluster number of k = 7 is found. The WP frequency changes are analyzed using climate projections of two regional climate models (RCM). Both RCMs, the Regional Model (REMO) and Climate Limited-Area Model (CLM), are driven ...

Field measurements within a quarter of a city including a street canyon to produce a validation data set

Air pollutants and meteorological parameters were measured continuously by in situ instruments, path-averaging techniques (up to three DOAS systems), and SODAR inside a street canyon and in the surrounding area of 1 km x 1 km (Gottinger Strasse in Hanover, Germany) from 2001 until 2003 which are available in the data bank ValiData for validation of microscale models. During three IOPs tracer experiments with a SF6 line source, sampling techniques of up to 15 sites and path-averaging FTIR spectrometry were performed. Concentration measurement results at roof level were anti-correlated with SODAR mixing layer heights, while those inside the street canyon are not. Re-circulation flow patterns inside the street canyon were studied together with corresponding wind tunnel experiments.