The more extreme nature of the Boscastle case is linked to three factors: (1) higher rain rates, associated with a warmer and moister tropospheric column and deeper convective clouds; (2) a more stationary system, due to slower evolution of the large-scale flow; and (3) distribution of the heaviest precipitation over fewer river catchments. For the 2010 case, simulations performed using a 1.5-km grid-length configuration of the Met Office Unified Model reveal that convection was repeatedly initiated through lifting of low-level air parcels along a quasi-stationary coastal convergence line. Sensitivity tests are used to show that this convergence line was a sea breeze front which temporarily stalled along the coastline due to the retarding influence of an offshore-directed background wind component. Several deficiencies are apparent in the 1.5-km model's representation of the storm system, including delayed initiation; however, significant improvements are observed when the grid-length is reduced to 500 m. These result in part from an improved representation of the convergence line, which enhances the associated low-level ascent allowing air parcels to more readily reach their level of free convection. The implications of this finding for forecasting convective precipitation are considered.
Supplementary URL: http://www.met.reading.ac.uk/~hy010960/phd/presentations/AMS_Mesoscale_Processes_20130809.ppt