The Vorticity Budget of a Composite African Tropical Wave Disturbance

Abstract

Composite fields of large-scale variables derived from synoptic-scale wave disturbances observed during Phase III of the GARP Atlantic Tropical Experiment over Africa and the eastern Atlantic are used to determine the vorticity budget of the composite African wave. The velocity, vorticity and divergence fields used are “combined region” composites of the wave disturbances derived by Norquist et al. (1977).The vorticity budget is made for all eight categories (phases) of the wave at the reference latitude (∼11°N) as well as the latitudes 4° to the north and south of the reference latitude. The large-scale fields are decomposed into a zonal mean and deviations from the zonal mean to separate contributions from the wave and the basic flow in which it is embedded. Cumulus mass fluxes (derived by R. Johnson) are determined from a thermodynamic budget and a spectral cloud model. The average vorticity in the clouds is determined from a simple one-dimensional single-cloud model using the given cumulus man fluxes. Since cumulus mass fluxes and vorticities are determined independently of the large-scale vorticity budget, the parameterized vorticity source due to cumulus is not forced to equal the apparent vorticity source derived from the large-scale balance.It is found that the large-scale vorticity balance for the wave is linear, with the nonlinear horizontal advective terms approximately cancelling due to the presence of a quasi-nondivergent, single-propagating wave component. The linear waves are approximately advected by the horizontal wind at all levels above 850 mb, even to the south of the easterly jet where the mean zonal wind is small. The curvature term contributes significantly to the balance. The parameterized form of the vertical advection of vorticity due to cumulus accounts well for most features of the apparent vorticity source obtained from the law-scale budget when the vertical cumulus mass flux is specified only for deep clouds. The large apparent source ahead of the trough in the middle troposphere at 11°N is reflected in the parameterized form. Cumulus-scale twisting effects are not needed to explain the major part of the large-scale apparent vorticity source, except possibly near the tropopause.