Coupling the atmospheric model Meso-NH-v5.5 with the Monte-Carlo solver of conductive-radiative-convective heat exchanges stardis-v0.11.1 to calculate the surface energy balance of complex geometries

Abstract. The meteorological impact on humans and infrastructure in urban areas is strongly influenced by the air temperature, humidity, wind velocity, and radiative fluxes. These can be strongly heterogeneous in urban environments with complex 3-D building and vegetation geometry. Micrometeorological building-resolving models have been developed to solve the prognostic equations of the atmospheric variables in urban areas at metric resolution. They take into account the radiative exchanges in the 3-D urban geometry and heat conduction in the building and ground surfaces with separate deterministic approaches. An alternative introduced in this study is to solve the 3-D heat conduction, radiation, and convection with the stochastic Monte-Carlo Method (MCM) in a unified algorithm. Such MCM solvers have become available thanks to recent breakthroughs in the Monte-Carlo community, which allow to establish a connection between conductive and radiative random walks. The advantage of the MCM is that the calculation is independent of the complexity of the urban geometry, the calculations can deal with a large range of scales, and are easy to parallelise. In this study, the coupling between the atmospheric model Meso-NH and the Monte-Carlo solver stardis of conductive-radiative-convective heat exchanges is presented. Humid processes cannot yet be considered with the new model. The coupled model is evaluated against observational data for dry heat wave conditions at the impervious Comprehensive Outdoor Scale MOdel (COSMO) urban-like site in the northern outskirts of Tokyo (Japan). Evaluation results show that the model represents well the temporal evolution of the sensible heat flux and the vertical profiles of air temperature in the urban roughness sublayer. Future developments could focus on including urban vegetation and moist processes such as to enable the study of adaptation measures in urban areas.