Error growth at the convective scale

Severe convection in the United Kingdom is the primary cause of flash

flooding. Increasing computational power has made possible the use of grid spacings of

the order of 1 km over large domains, with potential great benefit for the forecasting of

such storms. However, the error doubling times are short (hours) compared

to the lead times necessary for useful warnings. The flood of August 16 2004

over the coastal town of Boscastle, UK is a particularly interesting case not

only because of the extremely high accumulations (200 mm over 5 hours) but also

because it was characterised by the almost continuous formation, over the coast,

of new cells that moved over the small river catchment where the town is

located. The predictability of the event is investigated by running the Met Office Unified

Model with a horizontal grid spacing of 1 km. Ensemble members are

generated by perturbing the potential temperature as the simulation progresses and by

altering the microphysics parameterisation and the surface roughness. The

predictability of the event is then assessed by comparison with the control run of the accumulations

over the duration of the event simulated by the different ensemble members.