Cumulus Ensemble Effects on the Large-Scale Vorticity and Momentum Fields of GATE. Part I: Observational Evidence

Abstract

The relationship between the residual of the large-scale vorticity budget, Z, and organized cumulus convectionis studied using the data taken during Phase III of GARP Atlantic Tropical Experiment (GATE). The appearanceof large vorticity budget residuals is clearly associated with intense cumulus convection. The association isclearest in the upper and middle troposphere. A positive peak appears in the Phase III-mean vedcal profile of 2 near the 200 mb level. Large day-to-dayvariations, however, exist in the time series of the area-mean 2 near this level. The horizontal distributions of2 at the same level for major convective events show distinct dipole patterns over areas of deep convection.These features are attributed to the detrainment of weaker horizontal momentum from deep clouds. Near 400mb, negative values of 2 are dominant. Peaks of n-egative 2 are associated with the activity of deep cumulusconvection and positive vertical vorticity gradient, δ?/δp, suggesting the effects of vertical advection of the e-scale vorticity due to cumulus-induced subsidence in the environment. In the lower troposphere where δ?/δpk generally negative, the horizontal distributions of 2 tend to be localized and positive values of 2 dominatein the convective area. These are attributable to the vertical advection process and detrainment from shallowclouds. Large negative values of 2 near the sea surface are caused by boundary layer turbulence. The cumulus-induced time rate of change of the rotational part of the wind is estimated from the vorticity budget residual.Organized cumulus convection is found to strongly decelerate the large-scale flow particularly in the uppertroposphere and reduce the vertical wind shear in the lower troposphere.