Estimating medium-range catchment flood potential

A methodology is presented for estimating the probability of a flood occurring on a particular catchment from heavy convective rain, at a lead-time of up to seven days. The procedure is based upon the physical characteristics of the land surface, which are remotely measured at a resolution of the order of 1 km. The sensible heat flux derived from these measurements is used to initialise a simple convective scheme and the vertical velocities of ascending parcels of air at the top of the atmospheric boundary layer estimated. The precipitable water content of the atmosphere is parameterised by a probability density function. The combination of this with the vertical velocity of the air leads to the derivation of the probability of occurrence of a flood-producing convective rainfall event. The system has been tested for a limited period over the River Irwell catchment in the Northwest of England and the results show some correlation between the predicted probabilities and actual convective rainfall observed. However, as the result of the procedure is probabilistic, a much longer trial is required to assess its reliability. This paper describes the physical basis for the proposed scheme and the limitations and weaknesses of the procedure are outlined. The case study is presented and followed by a discussion of the steps required to develop the model further and prepare it for possible operational situations.