Forecasting of hourly accumulations of precipitation by optimal extrapolation of radar maps

Abstract The forecast of radar rainfall rate maps is optimized when the maps to be forecast are averaged over an area A = L 2 such that L = kT λ where L is in kilometres, T is the forecast period in minutes, and k and λ are empirical coefficients that are usually restricted to 1.0 ⩽ k ⩽ 1.3 and 0.7 ⩽ λ ⩽ 0.8. Application of this relationship to the generation of forecast 1 h rainfall accumulation maps has consistently reduced r.m.s. errors by at least 10% when compared with those derived with the originally unsmoothed data.