A density current parameterization coupled with Emanuel's convection Scheme

We present the recent advances achieved in France concerning the representation of density currents in global circulation models (GCMs). We will first summarize the basic concepts, assumptions and resulting equations governing this new wake parameterization coupled with the Emanuel's convective scheme (Emanuel 1991) as detailed in Grandpeix and Lafore (2010).

The model represents a population of identical circular cold pools (the wakes) with vertical frontiers. The wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts. The budget equations for mass, energy, and water yield evolution equations for the prognostic variables (the vertical profiles of the temperature and humidity differences between the wakes and their exterior). They also provide additional terms for the equations of the mean variables. The driving terms of the wake equations are the differential heating and drying due to convective drafts. The action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precipitating downdrafts to the wake region and the effect of the saturated drafts to their exterior. Conversely, the action of wakes on convection is implemented by introducing two new variables representing the convergence at the leading edge of the wakes. The available lifting energy (ALE) determines the trigger of deep convection: convection occurs when ALE exceeds the convective inhibition. The available lifting power (ALP) determines the intensity of convection; it is equal to the power input into the system by the collapse (spread ?) of the wakes. The ALE/ALP closure, together with the splitting of the convective heating and drying, implement the full coupling between wake and convection. The result is thus a coupled wake–convection scheme, which conveys a more realistic representation of moist convective processes, and prepares the coupling of convection with boundary layer and orographic processes, as well as the simulation of the propagation of convective systems.

In a second step we will show a few tests of this wake parameterization coupled with Emanuel's convection scheme, in a single column framework for continental and maritime convective systems (Grandpeix et al. 2010). The case definitions and reference simulations are provided by cloud resolving models (CRM). For both cases, the wake scheme yields cold pools with temperature and humidity differences relative to the environment in reasonable agreement with observations (with wake depth of the order of 2 km over land and 1 km over ocean). The coupling with the convection scheme yields convective heating, drying and precipitation similar to those simulated by the CRM. Thus the representation of the action of the wakes on convection through ALE and ALP appears satisfactory.

This wake scheme has also been implemented in the LMDZ4 GCM, and some preliminary results on its impact will be presented. Emphasis is put on the diurnal cycle and especially on late evening convection.

Finally we will point out the remaining key issues to be addressed, in order to fully benefit of this new wake scheme: shear impact, wake density, representation of the propagation of density currents from grid cell to grid cell, enhancement of surface fluxes by wakes. The ALE and ALP appear as powerful concepts for the trigger and closure of the convection scheme. Current efforts are underway to account for other processes such as the boundary layer or the orography to contribute to the ALE and ALP.