Rotating convective disturbances in the trades



Observations of rotating convective disturbances in the trade-wind region just east of Hawaii are presented. Rotation results from stretching of vertical ambient vorticity in the convective regions. Surface friction is shown to play a significant dissipative role in these disturbances. Ekman pumping associated with the rotation is probably only of secondary importance. Convection is found to be associated with weak local maxima in the boundary-layer equivalent potential temperature. This suggests that convection occurs where such anomalies make the buoyancy of surface parcels larger than average. Convective available potential energies are found to be small, which means that such equivalent-potential-temperature anomalies are important in determining parcel buoyancy. It is hypothesized that mesoscale circulations induced by the ascending motion of convection reinforce the anomalies, resulting in the intensification of the circulations. The equivalent potential temperature of the boundary layer results from a balance between surface fluxes and entrainment from above. Ascending motion apparently reduces entrainmcnt, thus allowing the equivalent potential temperature to rise, whereas subsidence has the opposite effect.