Towards an understanding of the interaction between convection and the larger-scale in the tropics

Some aspects of the interaction of moist convection with the larger-scale motions in the tropics are reviewed, primarily based on studies with numerical models (as well as observational studies) it is emphasised that cloud physical and dynamical processes in cumulus convection (such as rainwater production drag force and cooling due to evaporation of rainwater) may play an important role not only in the mesoscale organisation of cumulus convection but also in persistence of mesoscale convective activity and the development of a larger-scale circulation. The well-known CISK mechanism, to which frictional convergence is essential, appears to be applicable only when the vorticity Is very large, as in the case of the eyewall of tropical cyclones. In addition, it is suggested that the so-called Wave CISK theories for Rossby waves and gravity waves need to be improved or the validity be clarified by further studies. It appears that interaction of the outflow of cool downdraft air with a low-level larger-scale flow is important with regard to the persistence of convection and the development of larger-scale circulation. The effects of surface friction and the vertical shear of an environmental flow on the above mechanism are also discussed. It is also suggested that there exists a dynamically significant mode of organised cumulus convection, in this paper this is referred to as Mesoscale Convection (MC). Various types of mesoscale phenomena and larger-scale disturbances may be interpreted as a spatial and/or temporal ensemble of MC. The properties of MC (such as lifetime) strongly depend on the vertical shear of a larger-scale flow and on the vorticity. The latter mechanism is manifested not only as the effect of inertial stability but also as that of frictional inflow into convection. Finally, some results from a numerical model with a new scheme of implicit representation of cumulus convection (but with explicit treatment of mesoscale convection) are presented to show mesoscale and large-scale behaviour in the inter-tropical convergence zone under a simplified situation.