Development of a short range QPF algorithm based upon optic flow techniques
Bowler, N. ; Pierce, C.E.
A new rain advection based nowcast scheme has been developed for Gandolf. This uses optic flow ideas [Horn & Schunck, 1981] to improve forecasting skill. The method employed removes the need to split the radar derived rain analysis into continuous rain areas, but uses a smoothness constraint to ensure that the diagnosed velocity field varies smoothly across the analysis. The scheme has been shown to improve nowcast of precipitation associated with cyclonic circulation around low pressure areas, and has also demonstrated superior performance during cases of embedded convection. Optic flow methods derive from a direct application of the Optical Flow Constraint (OFC) equation. The terms in this equation are evaluated over a square block, which is considered to move with a single velocity. This velocity is found using a least squares method (see [Bab-Hadiashar & Suter, 1998]). Block velocities are smoothed, and an instantaneous rain rate forecast is produced. The...
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