2. Proposed methods and required collaboration with partners

As planned in Task 2.1 of the FASTEX-CSS Work Programme, a set of short-range (typically 12 hours) simulations with mesoscale models (e.g. HIRLAM, UKMO, MesoNH), which are able to describe many more microphysical processes than those retained in Fig.1, should be performed by at least two of the project partners. A horizontal resolution of 10 kilometers for those simulations would permit keeping the simulation domain large enough (namely 1500$\times$1500 km²), and the time step not too small, on one hand, and to resolve most of the mesoscale structure of the cloud systems, on the other hand. It might also be relevant to perform some additional simulations in which the ice microphysics is by-passed, in order to assess the importance of representing the ice phase as such, in the model.

The main characteristics of the runs are reported in the following table:

MESOSCALE MODEL RUNS
SIMULATION DURATION	12 hours
HORIZONTAL RESOLUTION	10 km
VERTICAL RESOLUTION	40-50 levels
NUMBER OF LONGITUDES	151
NUMBER OF LATITUDES	151

The outputs from all these simulations will allow to validate the simplifying assumptions made before building the proposed cloud scheme, then to tune the free parameters of the scheme, and finally to validate its results.

Météo-France intends to provide initial and boundary conditions to the participating partners in the form of ARPEGE 4D-VAR reanalyses of the selected FASTEX cases. These reanalysis data should be available from January 1999, and will include all observations from the FASTEX experiment. Slight format and resolution adjustments of the reanalyses can be considered, in order to fulfil the requirements of the different models.

In addition to the aforementioned purpose of validation, Météo-France will intercompare the different models as regards their ability to simulate the observed cloud systems.

Prior to any run, all participants to the intercomparison will need to agree on a common list of model output fields to be delivered to Météo-France. A draft list is proposed in section 4.

In addition to the intercomparison, Météo-France encourages CETP and JCMM to validate each mesoscale simulation against aircraft and radar measurements, collected during the FASTEX field experiment.