CUMULUS EFFECTS ON THE LARGE-SCALE VORTICITY AND MOMENTUM FIELDS IN THE TROPICAL ATMOSPHERE

The relationship between the residual of large-scale vorticity budget and cumulus convection in the tropical eastern Atlantic is studied using the data taken during the Phase III of GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE). The appearance of large vorticity budget residual in the upper and middle troposphere is clearly associated with intense deep cumulus convection. The association becomes less clear in the lower troposphere particularly when there is strong vertical wind shear. An effort is made to recover the cumulus-induced time rate of change of the rotational part of wind from the vorticity budget residual. It is found that cumulus convection strongly decelerates large-scale flow especially in the upper troposphere.;A positive peak in the mean vertical profile and a horizontal dipole pattern of the vorticity budget residual are the major features found near the 200 mb level. These features are produced by cumulus detrainment at this level. In the middle troposphere near 400 mb, negative vorticity budget residual is produced by the vertical advection of large-scale vorticity due to cumulus-induced subsidence. This phenomenon is unique to the GATE region where there is a strong vertical gradient of vorticity in the middle troposphere. In the lower troposphere, a secondary positive peak of vorticity budget residual appears on the average. This is also caused by the vertical advection effects. Large negative values of vorticity budget residual exist near the sea surface regardless of the presence or absence of cumulus convection, suggesting the effects of turbulence in the boundary layer.;Parameterized expressions for the cumulus-induced time rate of change of momentum and for the vorticity budget residual, Z, are derived in a mutually consistent manner. The parameterizations are expressed in terms of cumulus mass flux, detrainment, momentum as well as large-scale momentum and vorticity. A simple entrainment model is used to calculate the cumulus momentum. The values calculated by the parameterized expression for Z with the diagnosed cumulus properties are compared with the observed Z. General agreement between the observed and parameterized Z is obtained in the upper and middle troposphere.