Airborne dual-Doppler radar observations of horizontal velocity fields in growing cumulus

Continental summer cumuli were sampled via the 95 GHz Wyoming Cloud Radar on board the Wyoming KingAir research aircraft. The airborne installation allowed for fine-scale dual-Doppler syntheses across horizontal planes with data resolution on the order of 30 to 45 m, and velocity uncertainty on the order of 1-2 m/s. The investigated clouds were a few kilometers deep and wide, forming over land in a fairly dry environment with bases above 4000 m MSL.

The retrieved kinematic fields revealed gradients and length scales comparable to those previously observed in vertical planes. Regions of diverging flow were collocated with weak radar echoes. They appear to mark the tops of the rising bubbles. Calculations of mass fluxes through the lateral boundaries of the thermals indicate a rate of rise of about half the maximum updraft speed.

Intense vortical structures were observed to persist for periods on the order of 5-10 minutes, indicating the important role played by these features in the evolution of the clouds. Pairs of counter-rotating vortices often enclosed areas of stronger reflectivity and were located at the periphery of the updraft core regions. The size and intensity of the resolved eddies and the organization of the air flow in the horizontal plane were comparable to those in vertical planes.

The velocity and reflectivity fields demonstrate how the kinematic patterns contributed to a spatial organization of the hydrometeors, just as it was observed in the vertical plane. Moreover, entrainment is promoted by the vortical motion at the lateral edges of the clouds. Dry air intrusion paths are revealed by weaker radar echoes and smaller reflectivity gradients associated with the inflow quadrants of the vortices.

The in situ and remote sensing data confirm and extend the conceptual model of toroidal thermals. A possible source of vertical vorticity is the tilting of the thermal azimuthal vorticity by ambient shear, in some cases associated with thermal-to-thermal interaction.