by Maria Barrera Verdejo, Moritz Haarig, William Greg Blumberg, Dimitra Mamali, and Linda Forster

The conversion of cloud droplets (approx. 10 µm) to drizzle (approx. 50 µm) is associated with the starting point of precipitation. In a non-precipitating cloud the largest cloud droplets are close to the cloud top, due to the droplet growth by condensation. Further growing of the droplets by the process of collision-coalescence results in a downward movement of these droplets, which enhances even further the growth by colliding with other droplets. Drizzle is formed and the cloud will start precipitating.

Figure 1: Cloud top and base height over Jülich on 23 April 2013 with the height of maximum radar reflectivity (Z_max). The cloud base height was measured with a ceilometer, the cloud top height and height of maximum radar reflectivity with a cloud radar (MIRA-36).

The radar reflectivity Z is extremely sensitive to droplet size (Z proportional D⁶). Exploiting this sensitivity it is easy to detect the larger droplets in a cloud. If the largest droplets (and so the highest radar reflectivity) are close to the cloud top, precipitation is very unlikely as seen in fig. 1 between 8:30 and 9:00 UTC. The downward moving of the maximum radar reflectivity’s height (and so the position of the largest droplets in the cloud) towards the cloud base can be associated with the formation of precipitation. This is the case between 9:00 and 10:30 UTC in our example of the 23 April 2013. Later on that day when the maximum radar reflectivity is close to the cloud base the continuation of precipitation is very likely.