A Climatology of Heavy Rain-Producing Convective Systems in the UK

Flash flooding represents one of the greatest natural hazards to life and property due to its rapid onset which gives little time for warning dissemination and response. Compounding the issue is the long-recognised difficulty in accurately forecasting the small-scale convective systems which typically cause these floods. While many regional numerical weather prediction models now operate at sufficiently fine horizontal resolution to permit the explicit representation of deep, moist convection, significant difficulties remain in determining the exact timing and location of these storms. For such high-risk, low-predictability events, climatologies can be a valuable component of the forecaster's toolbox, providing an expectation of when and where hazardous weather might occur under different large-scale conditions. We present a climatology of long-duration convective rain events in the UK for the period 2008–12. These are identified in archived radar imagery using an object-based technique with three main criteria: (1) rainfall accumulations of at least 15 mm over an area of 100 km² or more; (2) large rainfall gradients within the causative rainfall objects; and (3) a rainfall duration of at least three hours. Following rejection of false identifications (associated with non-convective precipitation and spurious radar echoes), events are subjectively placed into one of four categories based on the observed nature of the convective rainfall. The categories are: (1) back-building quasi-stationary systems; (2) non-stationary training convective lines; (3) slowly evolving, near-stationary systems; and (4) widespread scattered convection. For each category, the events' geographical, seasonal, and diurnal distributions are examined along with their duration and intensity. ERA-Interim analyses are subsequently used to identify the large-scale conditions which favour their occurrence.