INFLUENCE OF TURBULENCE PARAMETERIZATION ON THE MIXING LAYER HEIGHT PREDICTION WITH A MESOSCALE MODEL

摘要

In the case study presented here, a maximal MLH hit rate of 88 % in the CBL over the northern part of the model domain is achieved. However, despite of its tolerant definition, the maximal MLH hit rate over the mountainous terrain in the South does not exceed 39 %. Possible explanations for the low hit rate may be shortcomings of the applied parameterizations schemes over complex, inhomogeneous terrain, grid resolution and model forcing. As the prognostic TKE closure does not provide basically better results than the countergradient schemes, we conclude that the characteristic velocity scale is properly considered in the countergradient schemes. Therefore, the low hit rate may be also adressed to the parameterization of the integral length scale. This is suggested by sensitivity studies with the TKE closure, showing a strong influence of the integral turbulence length scale on the MLH prediction. Nevertheless, the verification results presented here are similar to findings from Sorensen and Rasmussen (1997), who exemplary quote a correlation coefficient of r ≈ 0.4 for their 24-h MLH forecast using the DMI-HIRLAM NWP model (Danish Meteorological Institute, High Resolution Limited Area Model) verified against ETEX radiosound-ing data (European Tracer Experiment 1994). Our findings are also in the range of evaluation results from Schaller and Wenzel (1999) for five regional atmospheric chemistry transport models showing correlation coefficients between 0.4 and 0.6 and overall hit rates between 49 and 69 % for a 24-h MLH forecast period of TRACT, 16.09.92.