Abstract
Hong Kong Observatory has been operating an in-house developed rainfall nowcasting system called “Short-range Warning of Intense Rainstorms in Localized Systems (SWIRLS)” to support rainstorm warning and rainfall nowcasting services. A crucial step in rainfall nowcasting is the tracking of radar echoes to generate motion fields for extrapolation of rainfall areas in the following few hours. SWIRLS adopted a correlation-based method in its first operational version in 1999, which was subsequently replaced by optical flow algorithm in 2010 and further enhanced in 2013. The latest optical flow algorithm employs a transformation function to enhance a selected range of reflectivity for feature tracking. It also adopts variational optical flow computation that takes advantage of the Horn–Schunck approach and the Lucas–Kanade method. This paper details the three radar echo tracking algorithms, examines their performances in several significant rainstorm cases and summaries verification results of multi-year performances. The limitations of the current approach are discussed. Developments underway along with future research areas are also presented.
Highlights
In the early flood season from March to May, surface frontal system and monsoonal trough, in tandem with active mid-tropospheric westerly jets, induce squall lines that lead to heavy rain, severe convective gusts and hail to Hong Kong and the Pearl River Delta (PRD) region
Heavy rain is typically associated with southwest monsoon, tropical cyclones, as well as the wind convergence zone between a low pressure system near Beibu Wan and a subtropical ridge extending from the western Pacific
The red and black signals warn the public of heavy rain which is likely to bring bring about serious road flooding and traffic congestion
Summary
Located at the southern coast of China (Figure 1), Hong Kong is affected by heavy rain and severe weather every year. In the early flood season from March to May, surface frontal system and monsoonal trough, in tandem with active mid-tropospheric westerly jets, induce squall lines that lead to heavy rain, severe convective gusts and hail to Hong Kong and the Pearl River Delta (PRD) region. The urban areas and satellite towns where most people reside are very densely populated. With such a setting, even highly localized downpour could affect a significant portion of the population. There is no dearth of events in which heavy rain and the associated weather have ignited a chain reaction of flooding, landslides, infrastructure failures, fallen trees, traffic congestions, accidents and the like that gravely affected livelihood of the general public.
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