Jean-Yves Grandpeix, CNRM, France – University of Copenhagen

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Atmospheric complexity > Complex Fluids Seminar > 2017 > Jean-Yves Grandpeix, C...

Jean-Yves Grandpeix, CNRM, France

Representing cold pools and their interactions with moist convection in a Global Circulation Model.

The talk will be about the cold pool parametrization developped at LMD (IPSL, Paris) and CNRM (Météo-France, Toulouse) during the years 2000 - 2010. More generally, it will present the representation of deep convection and of the various processes with which it is coupled: moist boundary layer thermals and evaporative cold pools. All the developments where made by comparing parametrization results with high resolution simulations.

Deep convection generates precipitation which evaporates while falling, generating downdraughts. When these downdraughts reach the surface they create cold pools that spread horizontally as density currents. These evaporative cold pools (or wakes) impact the atmosphere thermodynamics in two ways: (i) cold air is trapped within cold pools, enabling moist convection to develop outside cold pools; (ii) warm air is lifted at the cold pool edge, which may induce new convective towers, provided the lifting is strong enough. Thus cold pools appear as key components of the atmosphere moist thermodynamics.

I'll first present the cold pool parametrization (JY Grandpeix, JP Lafore, JAS, 2010). The state variables are the surface fraction covered by cold pools and the vertical profiles of the temperature and humidity differences between the wake and off-wake regions. The model is statiscal and depends on a key parameter: the wake number density (the number of wakes per unit area).

Then I'll describe the coupling between the cold pool parametrization and the deep convection scheme. First will come an analysis of the concepts of convective triggering and closure and their (very simple) formulation in the present scheme: the triggering is governed by the Available Lifting Energy (ALE in J/kg) and the closure is driven by the Available Lifting Power (ALP in W/m2). Then, in order to take into account the size of the 
global model grid cell, I'll introduce the ALP conditionned on the presence of a gust front within the grid cell.

Next the other lifting process feeding deep convection will come into play, namely the moist boundary layer thermals, together with its contributions to ALE and ALP. Putting it all together, I'll present the whole set up, the thermals and wakes feeding ALE and ALP to the convective scheme, the wake scheme modifying the environments of both the thermals and deep convection and the convective scheme providing the differential heating feeding the cold pools.

Finally I'll introduce some statiscal aspects of the field of boundary layer thermals which will lead to the statiscal triggering of deep convection and to the ALP conditionned on actual triggering within the grid cell (N Rochetin et al., JAS, 2014). 

To conclude I'll sketch a statistical view of the interactions between the fields of cumulus, cumulonimbus and cold pools with a prospect of parametrizing a cold pool population dynamics. Will this lead to a representation of convective aggregation? The question is open.