Lake Breeze of Small Lakes in North-East Germany in COSMO-CLM simulations with non-diffusive dynamics and numerics

Abstract

We present results of analysis of simulated Lake Breeze (LB) and Land-Lake Wind (LLW) at M¨uritz Lake (100km2) and Lausitz Ostsee (22km2) in North East Germany in a series of 1 km Regional Climate Simulations (RCS) with COSMO-CLM 5.0-HOS over North-East Germany in 2006 and 2010 and discuss the requirements of modelling. We demonstrate the ability of the model to simulate LB and LLW in a physically consistent way. Best resuls are obtained for non-diffusive dynamics and numerics, no convection parameterisation and oppropriate turbulence parameterisation simulates LB and LLW at highest efficiency with respect to forcing. Higher surface heat flux or weaker horizontal and/or vertical turbulence parameterisation decrease the efficiency of generation of the phenomena. Numerical diffusion disturbes the dynamics and reduces the intensity and frequency of LB and LLW by approximately 50%. Each day of the year is associated with Small Scale Variability (SV) or Ekman Spiral (ES) regime (70-85% of days), dominated by processes independent on the land-lake contrast, or Convecitve Instability regime (15-30% of the days), characterised by relatively high ∆rΘ ≥ 10◦ and ∆rT2m > 2K values. Lake Breeze (LB) and Land-Lake Wind (LLW) regimes are found at lake locations only at 50% of days in Convective Instability regime The LLW regime (3-5% of days) at M¨uritz Lake exhibits the theoretically expected typical 10m wind direction change ∆rΘ ≃ 90◦ and 2m temperature difference between land and lake of 6.9 K. We found a limitation of ∆rT2m ∼ −0.2vg for geostrophic wind vg ≥ 6m/s and an exponential increase of ∆rΘ ∼ exp(∆rT2m) with 2m temperature difference in all regimes.