Can flash floods in England be produced by warm rain?

Key findings will be presented from the COnvective Precipitation Experiment (COPE) which took place during July and August, 2013 in the south-west peninsula of England, a region prone to convection forced along convergence lines. There were some surprising results, including radar reflectivities of about 55 dBZ in warm clouds with tops below the 0-deg level, very large raindrops, high values of liquid water content, high concentrations of cloud drops, and multiple cells packed closely together along the convergence line. It is clear that warm rain is important in the convective clouds: the warm rain was at times surprisingly heavy for English convective clouds; and the mobile dual-polarisation radar frequently observed vertical columns of supercooled raindrops embedded in deeper clouds suggesting a link to ice and graupel particle formation.

Severe flash floods have occurred in the sout-west of England on several occasions, most recently at Boscastle in 2004 where a convergence line remained stationary for about 4 hours as new cells of relatively deep convection continuously formed along the line. The ultimate goal of COPE is to improve forecasts of such flash flooding due to heavy convective precipitation. This goal is being addressed by studying the cloud dynamics and detailed aerosol and microphysical processes as well as the dynamics and thermodynamics of the convergence lines.

Three aircraft were involved in the project. Two of the aircraft, the Facility for Airborne Atmospheric Measurement (FAAM) BAe 146 and the University of Wyoming King Air (WKA), made coordinated penetrations through developing convective clouds, making measurements of aerosols, cloud microphysics and dynamics. In addition, the BAe 146 carried a Small Ice Detector (SID), a 2D-S, and a CDP with BCPOL, all of which aid in detecting small ice particles. The WKA operated its Wyoming Cloud Radar and Cloud Lidar for high-resolution measurements of cloud structure and composition.

Ground-based facilities included the National Centre for Atmospheric Science (NCAS) mobile X-band, Doppler, dual-polarisation radar deployed at a fixed location in the centre of the COPE domain. It has a beam width of less than 1 deg. It made complete volume scans to a range of 150 km approximately every 4 minutes making it possible to observe the development of precipitation with relatively good time resolution. The Chilbolton Advanced Meteorological Radar (CAMRa) and the Met Office radars near the COPE domain provided additional coverage. The characteristics of the aerosol particles in the sub-cloud layer -- potential CCN and IN particles -- were measured near the ground with a comprehensive suite of aerosol instruments and also on the BAe 146 aircraft, both operated by the University of Manchester. In addition a range of other surface-based instruments, including a Doppler lidar were deployed near the NCAS radar and high-resolution models were run in forecast mode.