A new method for identifying the main type of solid hydrometeors contributing to snowfall from measured size-fall speed relationship

Abstract

This paper presents a new method for identifying the type of solid hydometeor mainly contibuting to snowfall from the measured size and fall speed data. The main type is determined from the relationship between measured size and fall speed by considering the contributions of various hydrometeor types to precipitation, including graupel, graupel-like snow, aggregates at different riming stages, and small particles such as single snow crystals. The mass flux of each hydrometeor, defined as the product of its mass and fall speed, is needed to evaluate its contribution; however, it is practically difficult to measure. In this study, we estimate mass flux from the empirical relationships between size and mass and between size and fall speed. The mass flux distribution in the size-fall speed coordinates for all measured hydrometeors is found to accurately reflect the characteristics of types of hydrometeors and their contribution to observed precipitation. Considering these results, we introduce a new variable, the center of mass flux distribution (CMF), in the size-fall speed coordinates. The CMF, which is the average of size and fall speed weighted by the mass flux, can be obtained in the same way as the centerof gravity in mechanics. We believe that it indicates the size and fall speed of the principal hydrometeors among all particles in the observation period. This new method allows the quantitative identification of the main hydrometeor types from the locations of CMFs in the coordinates of size and fall speed. We verify this method by its application to different types of observed snowfall events. Although there is some ambiguity in estimating the mass flux, the method is expected to be useful foridentifying the main hydrometeor types in snowfall events and for quantitatively interpreting returned radar power. © 2013, Meteorological Society of Japan.