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Notes from the working group on convection held in Toulouse, on 17-23-24 May 2002Some proposals for a renewal of the parameterization of condensation processes Basis· Anticipating the march to smaller scales (typically 5 km) : large-scale precipitations (LSP) and updraughts are to be handled simultaneously, with cloud microphysics variables as input · Writing a new scheme for shallow convection · Avoiding a too complex iterative process (expensive and eventually not converging) · Keeping as much as possible benefit from previous work, e.g. with respect to the Bougeault scheme Constraints· Trying to ensure some constraints on "moist enthalpy", hv : · Downdraughts are to be handled separately, with the result of "LSP + updraughts + shallow convection (possibly saved from the previous time-step) + non-organised evaporation" as input (closure); retro-action from downdraughts will be taken into account only at the next time-step · Rain and snow are to be handled separately from cloud liquid water and ice, i.e. introduced as prognostic variables later And ...Work on operational problems must go on ! and is the first priority ! Confrontation with the experience of other teams is very valuable (e.g. in the EUROCS framework, with the ARPEGE-Climat team, with HIRLAM, ...). Jean-François Geleyn proposed the following approach to create, if the general framework is agreed upon, a "baseline version", compatible with these constraints and still close to the present operational version, which should be able to welcome new developments : First steps· Merging LSP (large scale precipitations) and updraughts parameterizations, introducing a diagnostic cloudiness variable (consistent with micro-physics) and non-organized evaporation under precipitations (as long as required) · Isolating the parameterization of downdraughts, introducing improvements to have it at a level of sophistication comparable to that of as the updraughts, at least for the compensation phenomena Afterwards· Design of a new scheme for shallow convection, based for example on a mass-flux approach. It should be parallel to the downdraughts computations (with closure problems solved either by saving fluxes to the next time-step or having a short iterative process). Temporary tricks may be used for the first tests of the new architecture, while waiting for a satisfying scheme. · Design of a simple microphysics, handling prognostically only liquid water and ice, e.g. by adaptation of the scheme of P. Lopez · Introducing downdraughts in the "Functional Boxes" approach at the same level as micro-physics Longer term actions· General interface with dynamics, with the use of other new prognostic variables, with the new turbulence scheme, ... · Further work on the previous scheme of L. Gérard (some more validation, towards a full 3d cloud fraction field, ...) · Handling precipitating water · Full forecasting scheme for draughts (new scheme developed by L. Gérard)
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