Development of a precipitation nowcasting algorithm based upon optical flow techniques

A new advection-based nowcasting scheme for precipitation has been developed for the Gandolf system. The method employed removes the need to split the radar analysis into contiguous rain areas (CRAʹs) and uses a smoothness constraint. Thus, it is similar to the Variational Echo Tracking approach. Optical flow ideas are used to diagnose the advection velocity of blocks within rain analyses, which involve the direct solution of the Lagrangian persistence equation. In block-based approaches, the radar analyses are partitioned a-priori and a CRA may be split over a number of blocks. This scheme is compared with the old Gandolf advection scheme, which is based on CRAʹs, and the new scheme performs better-both in cases associated with severe flooding and over a continuous verification period of 3 months. The benefit is conjectured to be due mainly to the difficulty in unambiguously identifying CRAʹs, which is particularly troublesome on small domains, such as the UK. Thus, it is concluded that block-based methods are likely to be superior to object-based methods in the majority of cases.