Séminaire
It is important to move towards understanding trade-wind clouds in regimes that are less strongly defined by the subsiding branch of the circulation, but by local convection and local circulations. But those regimes are challenging to simulate, because the vertical development of convection should no longer be strongly constrained by the setup. In this talk I discuss the importance of different processes, convection and precipitation itself, as well as the large-scale dynamic and thermodynamic atmosphere, in controlling the boundary layer profiles of temperature, humidity, and cloudiness. The large-eddy simulations discussed are performed on a 50x50 km^2 domain which supports the emergence of organized and deeper shallow convection. The development of deeper clouds has a strong influence on the boundary layer structure, because (a) the deeper clouds detrain moisture above the inversion rather than in the cloud layer, and (b) are associated with changes in the updraft-downdraft structure resulting in broader areas of slowly subsiding-motion, both of which lead to a drying of the cloud layer. In simulations with stronger wind this drying is strong enough to cause the collapse of the boundary layer, followed by a recovery period with very shallow convection. While precipitation increases with the depth of convection, precipitation and the associated organization of convection therefore limit the depth of the boundary layer and hence also the precipitation rate realized. Unless strongly forced externally, surface precipitation never increases much beyond 1 mm/d. Observations from the Barbados Cloud Observatory are used to illustrate the simulation results.
marie-pierre.lefebvre@lmd.jussieu.fr