CWVC Flight V (28 February 2001) radar synopsis

Robin Hogan 18 July 2002

Introduction

The cloud observed on this day occurred almost exclusively below 2.5 km and according to the sonde ascent, was associated with a well-mixed cloudy boundary layer. The temperature at the surface was around 0 degrees, and at cloud top was around -12 degrees. Patches of weak precipitation were indicated by the Frontiers system. Later in the experiment the cloud measured by the Chilbolton radar began to break up.

Microphysics

High values of ZDR were observed throughout the day, tending to be highest at cloud top and decreasing through the cloud, presumably as the pristine crystals started to aggregate. Peak values were around 7 dB, indicating that plates rather than columns were responsible for the signal (plates can produce a theoretical maximum Zdr of 10 dB, while columns can manage only 4 dB). It is possible that the cloud was rather similar to stratocumulus in nature but with considerable ice being formed from cloud top.

Dynamics

The scanning radar showed little in the way of shear, but the ground-based 94-GHz radar had just started measuring Doppler parameters (please excuse the glitched rays), and the vertical velocity (second panel) showed fluctuations that seemed to penetrate the whole of the cloud in the vertical, and showed more small-scale structure close to the surface, supporting the idea that the cloud was similar to stratocumulus in a well-mixed boundary layer, with overturning curculations maintaining the presence of the liquid water. Note that superimposed on the velocity fluctuations is the downward velocity associated with sedimenting ice. The fluctuations are also visible in the scanning reflectivity data.