The Vorticity Budget of a Composite African Tropical Wave Disturbance

Abstract

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 m...