The continuous melting process in a cloud-scale model using a bin microphysics scheme

Simulations of an idealized stratiform cloud system are performed with the 3D cloud-scale model DESCAM-3D (DEtailed SCAvenging Model) using a bin-resolved microphysics scheme for ice, water and aerosol particles. This idealized case corresponds to numerical simulations of moist airflow over a narrow isolated mountain and a gentle slope in order to obtain a stratiform situation where radar bright-band phenomena are often visible. For this dynamical situation the impact of a detailed time-dependent melting scheme has been studied. This scheme allows the calculation of a water/ice ratio within each mass bin for the melting ice crystals. The results obtained with this detailed melting process are compared with corresponding results from simulations involving instantaneous melting at the 0 degrees C isotherm level. The new detailed melting scheme allows a penetration of the ice phase into the positive temperature areas. Most of the mass of falling ice particles melts over a distance of a few hundred metres. The non-instantaneous melting scheme also produces a deeper layer of latent cooling that slightly modifies the dynamical field. With the detailed melting scheme, a simple backscattering scheme is implemented in order to consider the wetted ice particles and simulate the bright-band phenomenon. It supposes that the backscattering proportions of the wetted ice particles are similar to those of raindrops. The association of the non-instantaneous melting process with this backscattering scheme permits us to reproduce the radar bright-band phenomenon. This study will enable a better understanding of the complex microphysics inside the melting layer and then will help to reduce the radar uncertainties in the precipitation estimation at the ground.