The COnvective Precipitation Experiment (COPE)
Contributed by Alan Blyth, Lindsay Bennett and Chris Collier
Main goal of COPE
The main goal of COPE is to improve forecasting of the location, timing and intensity of cumulus clouds that lead to heavy showers and flash flooding causing severe property damage and loss of life. The field component of this three-year project took place in Devon and Cornwall during July and August 2013. The location of the measurements was chosen since this part of the United Kingdom is prone to flash floods often brought about by the formation and persistence of convergence lines triggering showers and thunderstorms such as that which occurred at Boscastle, Cornwall in August, 2004.
What we do not understand about the formation of rain
Although a lot is known about how rain forms and develops within clouds, much remains to be understood. For example, we still do not know exactly how ice particles are produced in clouds. Also the detailed mechanisms that turn these particles into heavy rain and specifically the rates of this development remain unclear. Cloud drops may remain liquid at temperatures below 0oC, referred to as supercooled drops, until they encounter an ice nucleus, i.e. small aerosol particles such as dust, to freeze around. The properties of these particles need to be understood because they start the process of rain formation via ice particles. More importantly, however, once a few ice particles have formed, under the right conditions, there can be a rapid production of ice particles which leads to heavy rain. Importantly, raindrops produced by collision of cloud drops and subsequent coalescence (the so-called warm rain process) can short-circuit this process of ice particle development. Furthermore, surprisingly, the heavy rain in Cornwall and Devon often occurs as a warm rain in clouds that contain no ice particles at all. We need to know how fast all of these particles grow.
Improving river flow forecasts
The application of an ensemble approach to generating hydrological model simulations has led to the recognition of the uncertainties in model structures, parameter calibration and the input data. It is clear that the resulting total uncertainty is significant, and there remains a lack of understanding of how this information may be processed to improve the overall variance of hydrological forecasts. Part of the COPE programme aims to explore the assumptions behind the ensemble technique using measurements in the River Inny catchment (figure below).
Measurements made in COPE
Ground-based measurements included those from the NCAS mobile Doppler dual-polarisation X-band radar, a wind profiler, a Doppler lidar, a mico-rain radar, aerosol sensors and conventional meteorological instruments located at Davidstow on Bodmin Moor, Cornwall.
In addition radar data were also available from the Met Office / Environment Agency Cobbacombe Doppler dual polarisation C-band radar, and the Chilbolton Doppler dual band S-band radar. Three research aircraft made measurements: the FAAM BAe 146 operated by the Met Office and NCAS, The University of Wyoming King Air and the Met Office Cessna.
Scientists from the National Centre for Atmospheric Science based at the University of Leeds operated the mobile radar at Davidstow and are leading the project (Professor Alan Blyth is the PI), and NCAS scientists from the University of Reading organised the operation of the Chilbolton radar. Scientists from the University of Manchester, the Met Office and Purdue and Wyoming are leading other major parts of the programme.
Now that the field programme is over, work is focussed upon the data processing and analysis. The calibration and inter-comparison of the radar data is underway, and raingauge and flow data from the Environment Agency are being assembled. So far cases of very large rain drops have been identified by the aircraft, and work is underway to reconcile these observations with those from the radars.
Comment by Chris Collier submitted on Sep 18, 2013: Flash flooding due to convection can cause incredible damage to property and endanger life. At present we cannot attribute specific extreme weather events to climate change, although evidence is mounting that such events are becoming more frequent. Although we know a lot about how rain forms and develops in clouds our knowledge is incomplete. Major field campaigns such as the COnvective Precipitation Experiment (COPE) carried out in the UK this summer are contributing to our knowledge. However it would be useful through HEPEX if we can briing together worldwide knowledge of flash floods in order to build upon our existing understanding. Perhaps a user group or similar is needed?