1. Main task of Météo-France in FASTEX-CSS

One of the main task of Météo-France within the FASTEX-CSS Project (Task 2.4 of the Work Programme) will consist in developing a "simplified" large-scale condensation and cloud parameterization which shall treat cloud water and cloud ice in an explicit manner.

The simplicity of the designed parameterization will make its use possible in an operational forecast model for assimilating cloud water measurements from satellites, on one hand, and in climate simulations with an AGCM, on the other hand.

In the framework of short-range assimilation, simplicity is required to ease the definition of the adjoint of the cloud scheme. In the climatic context, the implementation of a simple parameterization will allow to keep the extra computational cost at a reasonable level.

A preliminary version of the simplified cloud scheme is shown in the following schematic:

  

Figure 1: Schematic diagram of the proposed "simplified" cloud scheme.

Autoconversion and accretion/aggregation processes can be treated in a simple way, whereas all quasi-instantaneous transfers between cloud condensate and water vapor should depend on an "efficient" supersaturation ratio, computed from large-scale variables. Furthermore, whether snow should be treated prognostically or diagnostically is still undetermined.

It is planned to implement the large-scale condensation scheme applying the new modular concept of "functional boxes", currently under testing at Météo-France (CNRM/GMAP).