A First Order Vorticity Equation for Tropical Easterly Waves

A first order vorticity equation tropical disturbances is derived. The equation indicates that the evolution of the vorticity field in the tropical atmosphere is influenced mainly by the horizontal advection of vorticity, and a vorticity source produced by cumulus convection. The A/B-scale vorticity budgets during Phase III of the GARP Atlantic Tropical Experiment (GATE) are analyzed to provide a consistency check between the first order vorticity equation and observations. The results show that the equation agrees well with the observed evolution of the large-scale vorticity field, at least in the diagnostic sense. The simple vorticity equation is also used to study the vorticity dynamics of tropical easterly waves observed in GATE; certain features of the vorticity and divergence fields are related through the first order vorticity equation. In particular, the pronounced mid-level divergence often observed in GATE daring convectively active periods is found to be associated with the vorticity advection by the west African easterly jet at the 650 mb level.