Improvements to the parametrization of snow in AROME in the context of ice crystal icing

Ice crystal icing (ICI) poses a threat nowadays for airplane pilots crossing the anvils of tropical mesoscale convective systems (MCSs). The use of fine-scale operational numerical weather predictions as provided by the French limited-area model AROME could help to better understand this phenomenon and to help its anticipation. To enable AROME to simulate ICI-prone conditions, modifications of its single-moment microphysical scheme Intercity-Express 3 (ICE3) are tested. Using a temperature-dependent snow particle distribution deeply impacts the organization and the ice phase of the simulated MCS. Notably, while the size of convective regions decreases, the size of anvil clouds increases and the low stratiform rain increases as well. As a result, by increasing the quantity of snow and decreasing the quantity of graupel, the simulation of ICI-prone conditions in the anvils of convective systems is enabled. Using this parametrization, further modifications fine-tune the representation of snow and further increase the size of the anvil cloud. The Marshall-Palmer snow distribution is replaced by a generalized gamma and the terminal fall velocities of snow hydrometeors are parametrized so that they are in closer agreement with observations.