Mass divergence in tropical weather systems Paper II: Large‐scale controls on convection

Abstract

AbstractComposited summertime deep cumulus convective weather systems are investigated over three tropical oceans. The occurrence and intensity of the convection are governed by various large‐scale forcing mechanisms, such as ITCZ convergence, easterly waves, and differences in diabatic heating between the disturbance and its clear or mostly cloud‐free surrounding region. Vertical profiles of divergence and vertical velocity for the weather systems are analysed from the point of view of trying to establish the relative importance of each large scale control on convection.The dominant forcing in the Western Pacific and the GATE region is ITCZ convergence, with important secondary roles being played by differential radiative‐convective heating, easterly waves and convective feedback. In the Western Atlantic trade wind region there is no contribution from ITCZ convergence; the vertical motion of a typical weather system is consequently much weaker than in the other regions.In all three regions the contribution to vertical velocity from frictional convergence is shown to be quite small.A large diurnal modulation of convection is observed. In the GATE region a diurnal variation is documented for the low‐level convergence into the ITCZ. A physical mechanism is proposed to explain this phenomenon.It is shown that the actual observed vertical motion of the summer‐time tropical oceanic weather systems comes from a combination of forcing mechanisms and does not result from any individual large‐scale control acting by itself. It is important that any idealized picture of tropical weather systems be broad and flexible enough to account for the large regional and multi‐scale forcing mechanism differences which exist. A simple ‘easterly wave’ or ‘ITCZ’ model of organized tropical convection is inadequate.