A method using radar reflectivity data for the objective classification of precipitation during the Baiu season

Abstract

A method for classifying precipitation is proposed to clarify the complicated properties of rainfall during the Baiu season. With three-dimensional radar reflectivity data in the Kanto area of Japan through the Baiu period of 1995, the Bright Band Fraction (BBF) classification method, which focuses on the vertical structure of a radar echo, was improved and applied to the Baiu rainfall. As a result of this 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, the classification of the main rainfall types based on the improved BBF was verified to be reasonable, and detailed patterns within the three types were further distinguished. Stratiform rain can barely be divided into patterns, which shows the consistency of its properties. In contrast, convective rain can be divided into a few patterns, one of which always produces the heaviest rain intensity at each of the selected Automated Meteorological Data Acquisition System stations in the Kanto area. Mixed rain usually has a few 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 Nagasaki Prefecture in the Baiu season of 1997 with dual Doppler radar, the kinematic properties of the rainfall of the three types were shown to correspond with their given definitions. The mixed type is recognized as one of the important types of rainfall during the Baiu period, because it contains shallow convective shape with relatively large rainfall amount.