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

Abstract

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.

Characteristics of convective precipitation over tropical Africa in storm‐resolving global simulations

“…This is ensured by defining the convective adjustment time scale (which regulates the convective mass flux) as the product of and an empirical scaling function f that depends on the grid spacing (similar to Kwon and Hong, 2017), developed through a collaboration with the German national meteorological service (Deutscher Wetterdienst, DWD): Thus, at grid spacings smaller than 8 km, the parametrization of deep convection includes a smooth reduction of the parametrized convective mass flux, and therefore a transition to resolved convection with increasing resolution. This empirical formulation is consistent with the full mass flux formulation at high resolution, that is, without the small‐area approximation (Malardel and Bechtold, 2019). This is important because the small‐area approximation is not fulfilled at storm‐resolving scales, as it assumes that the convective area fraction is much smaller than the environmental area fraction.…”

Long‐term single‐column model intercomparison of diurnal cycle of precipitation over midlatitude and tropical land

“…SCMs that produce most of nocturnal precipitation from convection (SKIM, TaiESM1, CMC, ICON and EAMv1.trigger) all allow convection to be triggered above the boundary layer. Although a few recent studies have considered convection moving across grids (Malardel and Bechtold, 2019; McTaggart‐Cowan et al ., 2019b), most SCMs and their parent GCMs still struggle to propagate convection from one grid to another. The capability to detect mid‐level instability above the boundary layer is important for them to simulate nocturnal precipitation.…”

How grid‐spacing and convection representation affected the wind speed forecasts of four polar lows

“…The parametrization of convection is based on the bulk mass‐flux approach (Tiedtke, 1993; Bechtold et al ., 2008), with a a modified CAPE closure leading to an improved diurnal cycle of convection (Bechtold et al ., 2014). The momentum transport by convective downdraughts are included in the parametrization scheme (recently discussed in Malardel and Bechtold, 2019) and the resulting gustiness is contributing to the model wind gusts ECMWF (2016). The scheme includes a scaling factor dependent on the horizontal resolution (Malardel and Bechtold, 2019), which scales the convective mass flux for a 5 km grid to about 65% of the non‐scaled flux.…”

“…The momentum transport by convective downdraughts are included in the parametrization scheme (recently discussed in Malardel and Bechtold, 2019) and the resulting gustiness is contributing to the model wind gusts ECMWF (2016). The scheme includes a scaling factor dependent on the horizontal resolution (Malardel and Bechtold, 2019), which scales the convective mass flux for a 5 km grid to about 65% of the non-scaled flux. The convection scheme separately considers shallow, mid-level and deep convection, with an experimental option to switch off the deep convection part.…”