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 (Zh) 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 Zh, R, and the hydrometeor class obtained by performing simulta-neous observations in Niigata Prefecture, Japan. The relationships were assumed to be of the form Zh = B R1.67, 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 wind-shield net. The average Zh 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 Zh, 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 backscatter-ing than graupel depending on the riming degree and types of constituent ice crystals in the X-band.