The 9th June case.

Damian Wilson

This is a convective case, with widespread showers formed from weak convection. The radar returns clearly have a melting layer, which suggests graupel formation and strong updraughts do not exist in this case.

Observational Data

Radar data, from the Chilbolton radar, for this event can be found via the University of Reading Radar Group website. Aircraft data should follow soon.

Model simulation

• The model appears to struggle somewhat more in this case. This may be more due to a problem in comparing model and observations in a convective situation, where a model cross section may miss a convective cell altogether. Also, we have been trying to compare the large-scale schemes, whereas a lot of the condensate will be diagnosed from the convection scheme. However, the model does get the convective region in the correct place. If we make the assumption that the convective condensate can be considered as ice above the melting layer and has the same microphysical characteristics as the large-scale water contents (probably quite reasonable in this case) then we can calculate a combined reflectivity . This gives reflectivities which are a few dB too large, at over 35 dBZ in ice cloud. This suggests there is too much condensate diagnosed by the convection scheme in this case. This can be reduced a little if some of the condensate is assumed to be liquid instead of ice.
• The 3C scheme appears to give better results than the 3B scheme over the Borders region when compared to the Nimrod analysis, but we are not sure why.
• Running with the precipitation part of the convection scheme switched off produces siginificant degredation of the forecast in terms of the positioning of the precipitation. The large-scale water contents are, as expected, much increased as a result, and compare somewhat to the values inferred from the Chilbolton radar.
• If we searching for some convective regions in the model then we can reproduce significant evaporative regions underneath ice cloud, which were observed by the radar. We don't get much supercooled water in the 3B scheme, but get more in the 3C scheme and in reality, which shows supercooled water to the top of the convective plumes.
• Overall, the model needs to get the convection correct in order for the large-scale to be correct. There is more of a problem in comparing the radar and the model in this case due to the convection. Also note that the subgrid scale part of the convective microphysics scheme is vitally important in producing a good forecast (the large-scale microphysics would be quite reasonable in the convective regions in this case). The 3C scheme is probably slightly better than 3B.