Relationships between Radar Reflectivity Factor and Liquid-Equivalent Snowfall Rate Derived by Direct Comparison of X-Band Radar and Disdrometer Observations in Niigata Prefecture, Japan

Abstract

The relationships between the radar reflectivity factor for horizontal polarization (Z_h) at the X-band and the liquid-equivalent snowfall rate (R) are presented for six hydrometeor classes of solid precipitation. These relationships were derived by comparing the values of Z_h, R, and the hydrometeor class obtained by performing simultaneous observations in Niigata Prefecture, Japan. The relationships were assumed to be of the form Z_h = B R^1.67, where B denotes an experimentally determined coefficient. The values of R and the hydrometeor class were determined using a high-resolution precipitation gauge and an optical disdrometer, respectively, set up inside a windshield net. The average Z_h value was determined for an analysis area of approximately 100 km², located upwind of the ground-observation site. The prevailing hydrometeor class, its representative size, and the fall speed, along with the average Z_h, R, and B values, were determined for 48 cases considered over three winters. The average B value for the “heavily rimed snow aggregate” was smaller than that for the “rimed snow aggregate”. The largest B value was derived for a case of aggregates of unrimed dendritic particles (unrimed-D class). The case involving aggregates of unrimed low-temperature-type crystals (unrimed-C class) showed the smallest B value. For graupel cases, the average B value was roughly twice that of the rimed and heavily rimed snow aggregate classes and much smaller than that of the unrimed-D class. Snow aggregates demonstrated a stronger or weaker backscattering than graupel depending on the riming degree and types of constituent ice crystals in the X-band.