Quasi-stationary convective systems in the UK

• Main Author: Warren, Robert A.
• Published: University of Reading 2014
• Subjects: 551.554
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625438

Quasi-stationary convective systems (QSCSs) are lines or clusters of convective cells in which the repeated development of new cells upstream of their predecessors results in near-zero system velocity. These storms can produce extreme precipitation accumulations in a matter of hours, leading in some cases to deadly flash flooding. Little is known about the frequency, regional distribution, environmental characteristics, and formation mechanisms of QSCSs in the UK. Here, this knowledge gap is addressed through a climatology, a case study, and idealised numerical simulations. First, a five-year climatology of QSCSs in the UK is constructed using a combination of automated and manual analysis of radar imagery. A total of 88 events are identified, giving a mean frequency of 17.6 per year, with the vast majority taking place over land during summertime afternoons. Systems are observed to be more common in the vicinity of coastlines and prominent orography suggesting that these features often play a role in the repeated triggering of convection. A region which appears to be particularly favourable for these storms is the Southwest Peninsula of England. QSCSs are found to occur under a diverse range of synoptic patterns; however, on average their environments feature greater instability and weaker winds than those which typify summertime convective episodes in the UK. Next, a case study of a QSCS over the Southwest Peninsula is undertaken. The system, which occurred on 21 July 2010, showed remarkable similarity to the flash flood-producing Boscastle storm of 16 August 2004, but did not produce extreme rainfall or flooding itself. This difference is linked to three factors: lower rain rates, a shorter period of stationarity, and distribution of the rainfall over more river catchments. Numerical simulations of the event reveal that, as in the Boscastle case, convection was repeatedly initiated by lifting along a quasi-stationary sea-breeze front (SBF). This feature is under-resolved with a grid length of 1.5 km (the shortest currently used for operational forecasting in the UK) leading to substantial errors in the simulated representation of the QSCS. Major improvements are obtained when the grid length is reduced to 500 m, highlighting the need for continuing increases in model resolution. The final part of this work uses idealised simulations to investigate the mechanisms by which quasi-stationary SBFs may form over a peninsula. A simple west-to-east orientated strip of land is considered with prescribed vertical profiles of temperature and wind velocity, and a diurnally varying land surface heat flux. Three key mechanisms are identified, all of which likely contribute to the formation of QSCSs over the Southwest Peninsula. These are associated with (1) downstream advection of the north- and south-coast sea breezes in primarily along-peninsula flow , (2) collision of the north and south-coast sea breezes, and (3) cancellation between a cross-peninsula ambient wind and the sea breeze on the downstream coast. For purely along-peninsula flow , the evolution of the SBFs can be accurately predicted using a simple scaling which relates the front velocity to the surface heat flux integrated along air-parcel trajectories.