| A simple
two-dimensional rainfall model, based on advection and conservation 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 and surface weather
observations, together with forecasts from a mesoscale numerical weather
prediction model, to obtain frequently updated forecasts of rainfall fields.
Such data assimilation helps compensate for the simplified model dynamics and,
taken together, provides a practical real-time forecasting scheme for catchment
scale applications. Various ways are explored for using information from a
numerical weather prediction model (16.8 km grid) within the higher resolution
model (5 km grid). A number of model variants is considered, ranging from simple
persistence and advection methods used as a baseline, to different forms of the
dynamic rainfall model. Model performance is assessed using data from the Wardon
Hill radar in Dorset for two convective events, on 10 June 1993 and 16 July
1995, when thunderstorms occurred 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 from the dynamic model, using "inferred updraughts", provides
the best performance overall. Updraught velocity is inferred at the forecast
origin from the last two radar fields, using the mass-balance equation and
associated data and is held constant over the forecast period. This inference
model proves superior to the buoyancy parameterisation of updraught employed in
the original formulation. A selection of the different rainfall forecasts is
used as input to a catchment flow forecasting model, the IH PDM (Probability
Distributed Moisture) model, to assess their effect on flow forecast accuracy
for the 135 km2 Brue catchment in Somerset.
Keywords: rainfall forecasting, flood forecasting, weather radar, satellite,
storm model |