An Observational Study of the First-Order Vorticity Dynamics in a Tropical Easterly Wave

GATE Phase III A/B- and B-scale data were analyzed and used to diagnostically test the first-order vorticity equation proposed by Cho et al. The results are given in the form of a composite easterly wave and the analysis shows that the first-order vorticity equation is capable of describing the vorticity dynamics of the composite easterly wave. Cumulus heating produces significant large-scale vertical motion, and it is demonstrated that the effect of cumulus clouds on the large-scale vorticity field is through vortex-tube stretching by cloud-induced vertical motion. The composite analysis of the first-order vorticity equation also shows that the vorticity dynamics of tropical easterly waves are adequately described by a linearized first-order vorticity equation and dominated by the advection of the perturbed vorticity by the relative zonal mean wind, the advection of the zonal mean vorticity by the perturbed wind, and the vortex-tube stretching induced by cumulus activity. It is explained how, during Phase III of GATE, the westward convergence and eastward divergence of the wave trough observed just above the surface and the appearance of midlevel divergence before and convergence after the passage of the wave trough axis are associated with the vorticity dynamics of tropical easterly waves.