Implementation and Testing of the Building Energy Parameterization Model (bep) in the Climate Local Model (clm)

In this study the Building Energy Parameterization Scheme (BEP) (Martilli et al. ) was implemented in the regional Climate Local Model (CLM) which is the climate version of the German Weather Service numerical weather forecast model COSMO (COnsortium for Small-scaleMOdeling) formerly known as “Local Model” (Steppeler et al. ). Currently in the standard CLM cities are parameterized bymeans of a bulk-transfer scheme as natural land surfaces but with an increased surface roughness length (1m), reduced vegetation cover (%) and leaf area index (1mm) in order to account for the increased vertical momentum flux and reduced evapotranspiration, respectively. The advantage of this approach is the relatively low demand on input parameters and the simplicityof its couplingwith the atmospheric model (Masson ). However, this simple parameterization is not able to fully represent the characteristics of urban areas that influence the atmosphere (Best ), such as a significant increase in heat storage and small values of the nocturnal sensible heat flux. Therefore characteristics of the urban planetary boundary layer (PBL) such as the urban heat island (UHI) aswell as a near-neutral nocturnal vertical temperature profile and its downwind advection cannot be simulated sufficiently well (Best ). Also, bulk-transfer schemes do not resolve the vertical effects of buildings on the urban canopy air and do not differentiate between several urban land use / land cover (LULC) classes (Liu et al. ). This leads to limitations for some model application usually related to air quality, air temperature variability within urban areas and human comfort (Masson ). In contrast, the multi-layer scheme BEP recognizes aspects of the three-dimensional nature of urban surfaces and the fact that buildings vertically distribute sources and sinks of heat,moisture, andmomentum through the urban canopy layer. BEP uses the geometry of a generic street canyon that is characterized by roadwidth, buildingwidth and building height distribution. Exchangeswith the atmosphere occur at multiple vertical levels within the urban canopy by directly modifying the prognostic differential equations of the regional atmospheric model to include additional terms for the urban drag force, heating, turbulent kinetic energy production and dissipation. At each