Diabatic divergence profiles in western Pacific mesoscale convective systems

Heating in the atmosphere can be expressed as diabatic divergence δ_d, which is nearly equal to the actual horizontal divergence δ in tropical convection. High-quality δ profile measurements from airborne Doppler radar "purls' in ten mesoscale convective systesm (MCS) observed during TOGA-COARE are examined, and the mean profile is compared with rawinsonde array measurements. Young convective features have strong near-surface convergence, while older cells with better-developed downdrafts and stratiform precipitation areas have their peak convergence aloft. In the mean, then, surface flow is only weakly convergent or even divergent, so that the main convergence into MCSs is deep and peaked aloft, with a sharp "melting convergence' at 0°C. Simple linear models of Hadley and Walker circulations forced by observed MCS δ_d profiles illustrate the importance of the elevated convergence peak to large-scale circulations, particularly to low-level wind fields. -from Authors