Short period forecasting of catchment-scale precipitation. Part II: a water-balance storm model for short-term rainfall and flood forecasting

A simple two-dimensional rainfall model, based on advection and conservatio n of mass in a vertical cloud column, is investigated for use in short-term rainfall and flood forecasting at the catchment scale under UK conditions. The model is capable of assimilating weather radar, satellite infra-red an d surface weather observations, together with forecasts from a mesoscale nu merical weather prediction model, to obtain frequently updated forecasts of rainfall fields. Such data assimilation helps compensate for the simplifie d model dynamics and, taken together, provides a practical real-time foreca sting scheme for catchment scale applications. Various ways are explored fo r using information from a numerical weather prediction model (16.8 km grid ) within the higher resolution model (5 km grid). A number of model variant s is considered, ranging from simple persistence and advection methods used as a baseline, to different forms of the dynamic rainfall model. Model per formance is assessed using data from the Warden Hill radar in Dorset for tw o convective events, on 10 June 1993 and 16 July 1995, when thunderstorms o ccurred over southern Britain. The results show that (i) a simple advection -type forecast may be improved upon by using multiscan radar data in place of data from the lowest scan, and (ii) advected, steady-state predictions f rom the dynamic model, using "inferred updraughts", provides the best perfo rmance overall. Updraught velocity is inferred at the forecast origin from the last two radar fields, using the mass-balance equation and associated d ata and is held constant over the forecast period. This inference model pro ves superior to the buoyancy parameterisation of updraught employed in the original formulation. A selection of the different rainfall forecasts is us ed as input to a catchment flow forecasting model, the IH PDM (Probability Distributed Moisture) model, to assess their effect on flow forecast accura cy for the 135 km(2) Brue catchment in Somerset.