The raindrop size distribution characteristics and its applications to radar rainfall estimation in Taiwan area

The oblateness and the raindrop size distribution (DSD) have great variation in different types of rainfall condition. The shape and DSD can determine not only the different moment of rainfall integral parameters but also the measurements of polarimetric radar. Through the calculation from DSD, the coefficient No, m, Λ of Gamma distribution can be retrieved. The Z-R relations and the median volume diameter (D₀) were also derived through the coefficient No, m, Λ.

From year 2001 to 2002, we had analyzed two years DSD data of 2d-video disdrometer. The DSD characteristics of different rain types were obtained through statistics. The analysis of median volume diameter indicated that the D₀ increases with increasing rainfall rate. However, the D₀ remains the value about 1.7 mm when the rainfall rate greater than 60 mm/hr. It means that the heavy rain event is composed of lots of median and small raindrops rather than giant raindrops. The microphysics process of collision and breakup may be the main reason for this unique D₀ value.

We also have applied the disdrometer derived Z-R relations to estimate rainfall from the radar reflectivity. Before this calculation, the correlation between the disdrometer derived reflectivity and the radar observed reflectivity was checked. During the heavy rain period, radar reflectivity was about 3.0 dBZ less than the disdrometer. The smoothing in horizontal area and the difference of reflectivity between 1.75km and surface may cause the underestimation. During the weak rainfall period there was very small difference between disdrometer and radar. The area rainfall accumulation is quite satisfactory by using the disdrometer derived Z-R relation and corrected radar reflectivity.