Radar Echo Characteristics and Processes of Oblique Intense Echo Formation between Upper and Lower Bright Bands

– A Case Study –

Abstract

　For quantitative precipitation estimation (QPE) using weather radar, the phenomenon known as the bright band (BB) appearing around the 0 °C level is recognized as an area of overestimation. Understanding the factors contributing to BB formation and its radar echo characteristics is crucially important to improve the accuracy of radar rainfall measurements by removing and correcting the BB region. We conducted observations of precipitation clouds during 2013–2019 using an X-band marine radar with high temporal and spatial resolution and a Micro Rain Radar in Sapporo, Hokkaido, Japan. During this period, we frequently observed phenomena such as multilayered BBs and oblique strong echo bands that slope diagonally from the upper to lower BBs.
　For this study, we conducted a case analysis of the oblique strong echo band which appeared on April 3, 2016. The oblique strong echo band is presumably to be composed of melting particles because both the downward Doppler vertical velocity and the differential reflectivity increased. Also, the cross-correlation coefficient decreased in the band. The oblique strong echo band was formed in the convergence zone of cold and moist air flowing from the southern region of the radar site towards the lower layer. Warm and dry air originates from the northern side and comprises melting particles transported by the lower-level jet within the cold air. Within the oblique strong echo band, the particles were vertically classified in the degree of melting while transported by a strong wind. Particles that had melted completely from upper BB were also observed simultaneously. The upper BB persisted for a long time during the approach of a warm front, where warmer air with positive temperatures was advected above the lower 0 °C level. Also, locale-scale meteorological conditions strongly influenced the formation of the lower BB and the oblique strong echo band.
　To improve the accuracy of QPE and to detect the vertical–horizontal multilayer structure of the rain–snow boundary, detailed case analyses of BB distributions must be accumulated using radar with high vertical resolution and using MP radars with hydrometeor classification capability.