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COPE - the Convective Precipitation Experiment

A convective shower line over the SW Peninsula at 1400Z, 10 May 2011.

July 2013 - An international field campaign to study convective precipitation over the SW Peninsula during July and August 2013.

Scientific background

Convective clouds are those that form as a result of atmospheric thermal instability. Warm air rises initially in dry thermals, but then forms the characteristic heaped formations of cumulus clouds once condensation of water vapour occurs. If the clouds are able to develop strongly in the vertical, they may give rise to locally heavy rain, exceeding several tens of millimetres per hour. In some circumstances, such clouds can become organized into linear structures and this organization can act to further increase the local accumulation of rainfall. This may occur both where the line itself is slow-moving or where growing cells move along the line repeatedly causing precipitation over the same area. In severe cases, this can lead to incidents of flash-flooding, a recent example of which was the Boscastle Storm and floods of 16 August 2004.

The duration and intensity of precipitation from such a system is controlled by the interaction of a range of separate physical processes including the boundary-layer transports of heat and moisture, aerosol-cloud microphysical interactions and mesoscale dynamical processes that contribute to the spatial organization of the clouds. The aim of COPE - the COnvective Precipitation Experiment - is to bring to bear a wide range of observational facilities to study the interactions amongst these processes in convective clouds over SW England. Together with a supporting program of numerical model development, this will drive improvements in the quantitative precipitation forecasts derived from operational kilometre-scale versions of the Unified Model such as the UKV and from convection-permitting kilometre-scale ensembles.

Amongst the questions to be asked are:

  • What is the role of "warm-rain" (that is, rain produced by the collision and coalescence of liquid-phase cloud droplets) in precipitation production from these systems?
  • Where and when do the first ice particles form in the cloud, why is this commonly only when the cloud top temperature has fallen to -10C or colder?
  • Do the models correctly represent the subsequent growth of the ice-phase, including the growth of graupel (or "soft hail" - ice particles that grow by the accretion of supercooled liquid droplets)?
  • How does entrainment - the mixing of cloudy air with its unsaturated environment - affect the subsequent microphysical evolution?
  • Does the operational UKV model correctly reproduce the boundary-layer convergence lines or other mesoscale structures such as sea-breeze fronts that are associated with the triggering of convective cloud formation?

Observing facilities

The FAAM BAe146 research aircraft will operate from Exeter airport during July 2013 and from its home base of Cranfield during August. This deployment will be lead by scientists from the Cloud Physics of Observation Based Research. Additional microphysical and aerosol observations (including the analysis of filter samples for the activity of ice-nucleating aerosol, IN) will be provided on the aircraft by groups from the Universities of Manchester and Leeds.The FAAM BAe146-301 research aircraft in-flight carrying a large range of Met Office and University instrumentation.

The King Air aircraft of the University of Wyoming will be located at Exeter for the whole of July and August 2013 with financial support from the US National Science Foundation. It carries a sophisticated 94 GHz cloud radar system that can provide information on the vertical distribution of cloud an precipitation above and below the aircraft enabling in-situ measurements to be put into a wider spatial and temporal context. The BAe146 and the King Air will operate in coordination with each other so as to provide both observations of the same cloud from different vertical levels and spatially separated observations of clouds in different parts of an evolving system.The University of Wyoming King-Air research aircraft in flight, showing the radar antenna housing on the rear fuselage and cloud physics probes under the wing tips (Vanda Grubisic).

Met Office civil contingency response aircraft commissioned will also be used on some occasions to obtain additional measurements of the boundary layer structure below cloud base, typically operating at an earlier time than the other two aircraft.

On the ground, one of the key facilities will be the new X-band Doppler weather radar operated by FGAM will be located at Davidstow, Cornwall and operated by a team from the University of Leeds. This facility will be linked by satellite to the aircraft and will provide real-time guidance on the location and state of cloud and precipitation development so as to direct the aircraft into the most favourable regions. The Meteorological Research Unit will operate an instrumented van able to move to locations of interest as determined during mission planning meetings. This van will provide both mobile radiosonde launches and Doppler lidar observations of the boundary-layer turbulence structure as it evolves during the day. In addition, the campaign will exploit a wide range of observations from the Met Office operational observing network, including Doppler weather radars (principally at Cobbacombe Cross, Devon), radiosondes, wind profilers and surface observations.The FGAM transportable X-band Doppler weather radar on its site at Davidstow airfield, Cornwall.

Forecasting and Model development

The operational UKV model will provide forecast guidance out to T+36hrs with other operational UM forecasts providing guidance on longer time scales. Forecasting guidance for planning research flights is being provided by staff from the Convective-Scale Data Assimilation and Nowcasting team and Reading University using these systems.

Following the campaign, higher-resolution versions of the Unified Model with resolutions down to 100m and below may be run for specific case studies. In addition, scientists at Manchester and Leeds Universities in the UK and Wyoming and Purdue Universities in the USA will use a variety of specialized numerical models to study detailed microphysical processes to address the key science questions noted above.

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