Observations Suggestive of Ice Production through Secondary Processes in Convective Clouds in Southwest England

Abstract:

Although the UK is well known for persistent drizzle, summertime in the UK is often accompanied by heavy convective precipitation. Summertime convective storms over the southwestern peninsula of the UK were the subject of the COnvective Precipitation Experiment (COPE), conducted in July and August of 2013. With an aim to improving quantitative precipitation forecasts, one of the primary objectives of COPE was to document the microphysical evolution of convective storms and unravel the contributions of the many microphysical pathways that can lead to heavy convective rainfall.

Convective storms over southwest England are somewhat unusual, with low but cool cloud bases that can still provide ample time for development of precipitation through warm processes. The production of precipitation between cloud base through (and somewhat above) the 0 °C level plays an important role in the further microphysical evolution of the cloud as ascent continues. In some cases, we observed a rapid transition from cloud liquid to ice, presumably amplified through secondary ice production. In others, we found very little production of ice at similar temperature levels.

Here we investigate details of the microphysics and dynamics from select cases on four days from COPE using in situ observations from an instrumented aircraft and high-resolution measurements from an airborne radar. In particular, we investigate how frozen raindrops in the Hallet-Mossop zone (-3 to -8 °C) influences the production of small ice crystals. We further investigate the expected time these frozen drops, or graupel embryos, persist in the H-M zone to understand the role updraft speed plays in secondary ice production leading to rapid glaciation of the cloud.