The coupling of deep convection with the resolved flow via the divergence of mass flux in the IFS

Malardel, Sylvie ; Bechtold, Peter

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<p align=justify>The resolution of the European Centre for Medium-range Weather Forecast (ECMWF) integrated forecast system (IFS) is expected to reach 5 km in the coming decade. Assumptions in the parametrization of deep convection, such as that all of the compensating environmental flow occurs in the grid column, i.e. the convective and environmental mass fluxes cancel each other in term of mass transport, have to be challenged. In this paper, we further develop the original concept of separating the convective updraught from the subsiding branch of the overturning convective circulation and apply it to the global hydrostatic equations of the IFS. In practice, this constitutes a revised convection-dynamics coupling where the mass flux subsidence of the dynamical variables is not computed locally by the convection scheme, but instead is recomputed from the revised continuity equation and is effective through the semi-Lagrangian advection of the dynamical core. Therefore horizontal divergence/convergence is also generated in the dynamics at the top/bottom of the convective columns, thus adding to the representation of deep convection a three-dimensional character which is not present in traditional schemes. The proposed physics-dynamics coupling is intended to be applicable to any mass flux convection scheme and within any regional or global model. We first demonstrate the accuracy of the revised physics-dynamics coupling in terms of global temperature and moisture budgets. The potential impact of the coupling on the convective organization is demonstrated for an idealized squall line case at high horizontal resolution using the small planet testbed. Model reforecasts at 9 km and 5 km resolution confirm the viability of the method in terms of forecast skill and model climate. However, the model impacts are limited as the main factor that still determines the convective stabilization and organization is the current conceptual model of subgrid mass flux which, actually, remains unchanged.</p>
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