Description of the sedimentation scheme used operationally in all Météo-France NWP models

In this paper, a cost-efficient sedimentation algorithm is described. The numerical resolution of the sedimentation equation is replaced by three probabilities of transfer associated with the following three types of precipitation: (i) precipitation present in the layer at the beginning of the time step; (ii) precipitation coming from the layer above and crossing the layer under consideration and (iii) precipitation produced locally during the time step. This scheme can be considered as a special case of the more general statistical approach where a simple step function is chosen as the basis for these probabilities. The main advantage is a near-perfect reproduction of advective (Eulerian or Lagrangian) classical sedimentation schemes at low computation cost. The practical use of the scheme in the operational models of Météo-France is described. We believe that this approach can be used to compute sedimentation in any microphysical scheme. Moreover, the simplicity of the algorithm makes it possible to consider improvements which would have been complex to implement in the original schemes.