Fine Structures of Vertical Air Motion and Warm Rain Formation Mechanism in Small Cumulus Revealed by a High Resolution Airborne FMCW Radar

To investigate dynamics and microphysics in shallow precipitating cumulus clouds, an upward facing airborne W-band FMCW radar with range resolution as high as 10 meters was employed in field experiment conducted in Key West of Florida in May 2012. After removing aircraft motion, the Mie notch technique is applied to radar observed Doppler spectra to retrieve vertical profiles of vertical air velocity in clouds. Observed detailed fine structures in both Doppler spectra and retrieved velocity profiles reveal for the first time that vertical air velocity structures in the precipitating areas of cumuli with cloud depths of about 1 km are not spatially uniform and exhibit changes with height. Multiple layers of depths of 20 to 250 meters were observed in these clouds. In each of these layers upward vertical air velocity increases with height in the lower portion of the layer and decreases with height in the upper portion. The magnitude of velocity variation between two consecutive turning points within a layer can be as high as 2.8 ms-1. For the May 23 case in Key West, the mean vertical velocity in clouds is about 3 ms-1 upward and mean standard deviation is about 0.5 ms-1. Similar structures are also observed from Doppler spectra and velocity profiles obtained in clouds observed upwind from Barbados where the radar range resolution is 24 meters. The local convergence implied by these observed structures, may contribute to an enhancement of the collision-coalescence between hydrometers. This process could repeat in each of the multiple layers and may help account for the relatively heavy precipitation falling out of these shallow clouds. It has been realized for decades that turbulence may play a significant role in warm rain formation. This study, for the first time, shows observational evidence that the collision-coalescence associated with turbulence or oscillations in the vertical air velocity may be an important process in the formation of warm rain precipitation.