Characteristics of the raindrop size distribution in a squall line measured by Thies optical disdrometers

Temporal evolutions and spatial variations of the raindrop size distributions (DSDs) and integral variables in a parallel stratiform squall line are analyzed based on observations of four Thies optical disdrometers and CINRADA/SA Doppler weather radar data. DSDs and time series of integral variables from each individual disdrometer observations are investigated. Statistical characteristics of the Z-R relationship, and relationships between the γ distribution parameters and the rain rate are studied. Typical characteristics of the raindrop size distribution and integral variables over the convective area of the squall line have been revealed in this study. In the area where R is increasing, the DSDs are characterized by lower and higher number concentrations of small and large size raindrops, respectively; the raindrop number is small, the reflectivity factor is high, and the slope λ and the shape parameter μ of γ function are gentle. In contrast, in the area where R is decreasing, the DSDs are characterized by higher and lower number concentrations of small and large size raindrops, respectively; the raindrop number is high, the reflectivity factor is low, and the slope λ and the shape parameter μ are steep. Power functions are good fits for N0-R, λ-R and μ-R relationships. However, N0 increases with the increase in rainfall rate, while λ and μ decrease with the increase in rainfall rate. The λ-μ relationship can be empirically described by a polynomial of second degree. The mean Z-R relationship Z=324R1.60 is derived from the fitting of the convective DSD-obtained rainfall rate and the reflectivity. Compared to the standard Z-R relationship Z=300R1.4, the fitted mean Z-R relationship in the parallel stratiform squall line has a similar value of coefficient and a higher value of exponent. The spatial variability of raindrop size distribution in different parts of the squall line has a significant influence on the Z-R relationship, but has little influence on the λ-μ relationship. Compare to that in the convective region in a trailing stratiform squall line, DSDs and integral variables in different parts of the parallel stratiform squall line have some similar characteristics while obvious differences can also be found. More studies are necessary to determine whether the unique characteristics of the parallel stratiform squall line are related to the dynamical structure of the squall line.