A Method Using Radar Reflectivity Data for the Objective Classificationof Precipitation during the Baiu Season

Abstract

A method for classifying precipitation is proposed to clarify the complicated properties of rainfallduring the Baiu season. With three-dimensional radar reflectivity data in the Kanto area of Japanthrough the Baiu period of 1995, the Bright Band Fraction (BBF) classification method, which focuseson the vertical structure of a radar echo, was improved and applied to the Baiu rainfall. As a result ofthis improvement, main rainfall types in the Baiu period were divided into three categories: convective,stratiform, and mixed.
To grasp the comprehensive properties of precipitation, four diagnostic parameters were introduced.By a fuzzy-clustering analysis, which uses the parameters as characteristics of rainfall samples, theclassification of the main rainfall types based on the improved BBF was verified to be reasonable, anddetailed patterns within the three types were further distinguished. Stratiform rain can barely be dividedinto patterns, which shows the consistency of its properties. In contrast, convective rain can be dividedinto a few patterns, one of which always produces the heaviest rain intensity at each of the selectedAutomated Meteorological Data Acquisition System stations in the Kanto area. Mixed rain usually has afew patterns that show the variation of its properties.
Applying the improved method of BBF to a rainfall case observed off the western coast of the NagasakiPrefecture in the Baiu season of 1997 with dual Doppler radar, the kinematic properties of therainfall of the three types were shown to correspond with their given definitions. The mixed type is recognizedas one of the important types of rainfall during the Baiu period, because it contains shallowconvective shape with relatively large rainfall amount.