一次飑线过程的雨滴谱特征研究

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

中文关键词:  平行层状飑线  雨滴谱  Thies激光雨滴谱仪

英文关键词:Parallel stratiform squall line  Raindrop size distributions  Thies optical disdrometers

中文摘要:

      利用4台Thies激光雨滴谱仪组成的观测网和CINRADA/SA多普勒雷达观测资料,通过单点雨滴谱和积分参数时间序列分析、以及γ谱拟合参数和Z-R关系等的统计分析,研究成熟平行层状飑线不同部位雨滴谱和积分参数的演变特征。结果表明,不同部位雨滴谱和积分参数演变特征存在明显差别,但有一致的基本特征,即在雨强增大阶段为较小的小粒子数浓度,较大的大粒子数浓度和谱宽,而雨强减弱阶段为较大的小粒子数浓度,较小的大粒子数浓度和谱宽,所以,雨强增大阶段具有较低的雨滴浓度和较大的雷达反射率因子,以及较小的γ谱斜率参数λ和形状参数μ。但有时雨强减弱阶段存在较大的大粒子数浓度和谱宽,因此,具有较大的雷达反射率因子;统计表明,γ谱三参数N₀、μ、λ与雨强的关系可以用幂函数拟合, N₀随雨强增大而增大,μ、λ随着雨强的增大而减小。λ-μ关系可以用二次多项式拟合,对流云Z-R关系为Z=324R^1.60。不同部位雨滴谱演变特征的差异对Z-R关系等统计关系影响明显,但对λ-μ关系影响较小。平行层状飑线不同部位雨滴谱和积分参数演变特征与拖曳层状飑线对流带典型雨滴谱演变特征类似,但也存在一些明显差别,这些差异是否与平行层状飑线动力结构的不同有关,尚需进一步的研究。

英文摘要:

      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=324R^1.60 is derived from the fitting of the convective DSD-obtained rainfall rate and the reflectivity. Compared to the standard Z-R relationship Z=300R^1.4, 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.