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", h_v :

· 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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