Use of DYMECS Observations to Validate the Representation of Convection over the UK in High Resolution Versions of the Unified Model

The DYMECS (DYnamics and Microphysical Evolution of Convective Storms) project has gathered data from hundreds of convective showers/storms over 40 cases using the Chilbolton research radar in central southern England. The aim was to compare statistics of various aspects of convective clouds and showers with the Unified Model (UM) running at various gridlengths and with different mixing and microphysics parameterisations.

Results will be presented showing statistical comparisons of convective cell sizes with observations using both surface rainrate data from the operational radar network and 3D radar scans from Chilbolton. It is found that the UK 1.5km operational model (the UKV) often produces reasonable statistics for larger clouds and showers but struggles with smaller showers, as would be expected from its gridlength. Higher resolution models (down to 100m gridlength) cope better with smaller showers but have problems producing larger cells, tending to break them up into smaller ones. It is clear that the results from the models are very dependent on the subgrid mixing employed - the models currently use a Smagorinsky type scheme. By running the high resolution models with a longer mixing length they can be made to look like lower resolution ones (which removes the problems with larger cells but re-creates the problems that the low resolution models have with small cells).

The DYMECS data is being used to attempt to estimate the vertical velocity in convective cells using Doppler velocities. Some comparisons of these estimated vertical velocities with those in the models at varying resolutions will also be presented.