Detecting the Onset of Drizzle with Joint Lidar and Radar Measurements

Simultaneous lidar and radar observations of clouds and precipitation are becoming more common, and to take advantage of such opportunities, radar–lidar-estimated diameter (RLED) was introduced in earlier work to describe relevant droplet size. Here, we use in situ cloud and drizzle probe measurements of drop size distribution (DSD) to show that RLED, the ratio of the sixth to the second moment of DSD (D₆₂), is sensitive to collision–coalescence, making it a potential indicator of drizzle onset. Remarkably, despite differences in sample volumes, in situ estimates of RLED agree closely with those based solely on lidar–radar measurements. Additionally, RLED is sensitive to DSD dispersion as we demonstrate on remote sensing observations. This is significant as DSD dispersion plays an essential role in numerical weather prediction models and terrestrial radiative transfer.