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基于二维视频粒子谱仪(2DVD)观测的北京降雪微物理特征研究

Winter Snowfall Microphysics Characterized by a 2-D Video Disdrometer (2DVD) in Beijing

  • 摘要: 研究降雪微物理特征,对理解冬季降水机制和优化天气模式中的云微物理参数化方案具有重要意义。目前国内针对降雪微物理特征的相关研究仍相对匮乏。为了深入了解北京地区降雪微物理特征,本文基于二维视频粒子谱仪(2DVD)观测数据对北京六次降雪过程的粒子谱分布(PSD)、下落末速度(V)、密度(ρ)及轴比(ar)等微物理参数及其与等效直径(D)的关系进行了详细分析。结果表明:(1)当中值粒径(D0)小于1.5 mm时,雪粒子密度低于已有文献报道值;V(D)关系的系数与密度呈正相关,且下落速度略大于科罗拉多州地区而略小于南京地区的观测结果。(2)采用指数分布和标准化gamma分布拟合了各次降雪过程的5分钟PSD,两种分布所得到的斜率参数(Λ)及数浓度参数(N0Nw)随时间变化趋势基本一致,但gamma 分布在反演D0、粒子总数浓度(Nt)和降雪率(SR)方面与2DVD实测结果更接近,误差更小、一致性更高。(3)在Nw-D0关系中引入ρ可有效提高二者之间的相关性。(4)本文首次系统给出了北京地区的雪花轴比特征,平均ar为0.76,当D > 2.5 mm时,ar趋近于1,与其他地区观测结果较为一致。(5)当雷达反射率(Ze)不变时,SR的增加伴随D0的减小以及Nwρ的增加;在SR不变时,Ze的增加与D0增大、ρNw减小密切相关。与南京地区Ze-SR关系相比,本文得出的关系式(Ze=851 SR1.33)在相同Ze下给出更低的SR估计值。上述特征说明,相较于降雨,基于雷达的定量降雪估测具有更大的不确定性。

     

    Abstract: The study of snow microphysics holds great significance in understanding the mechanism of winter precipitation and optimizing the cloud microphysical parametrization scheme in the weather model. However, at present, very few studies have analyzed the microphysical characteristics of snowfall in China. To thoroughly understand the microphysical characteristics of snowfall in Beijing, this paper presents a detailed analysis of particle size distributions (PSDs), fall velocity (V), snow density (ρ) and axis ratio (ar), and their relationships with equivalent diameter (D) for six snowfall events, using data from a 2D video disdrometer (2DVD). The results are as follows: (1) The density of snow particles with a median volume diameter (D0) less than 1.5 mm is lower than values reported in other literature. The coefficient in the V(D) relationship is related to density, increasing with it, and the terminal velocity is slightly greater than that observed in Colorado but less than that in Nanjing. (2) The 5-minute PSDs for each case align well with the exponential and normalized gamma distribution models. The temporal variations of the slope parameter (Λ) and the concentration parameter (N0 for exponential, Nw for gamma) were generally consistent between the two models, while the snowfall rate (SR), total number concentration (Nt) and D0 calculated from gamma PSD are better consistency with the direct 2DVD measurements. (3) Introducing density (ρ) into the Nw-D0 relationship improved the correlation. (4) This study presents the first measurements of snowflake aspect ratio for Beijing, showing a mean ar of 0.76 and a tendency to approach 1 for diameters more than 2.5 mm, in agreement with other studies. (5) When the radar reflectivity (Ze) is constant, an increase in SR is accompanied by a decrease in D0 and increases in Nw and ρ. Conversely, when SR is constant, an increase in Ze is closely related to an increase in D0 and decreases in ρ and Nw. Compared to the ZeSR relationship derived for Nanjing, the relationship obtained in this study (Ze = 851 SR1.33) gives lower SR estimates for the same Ze. These features demonstrate that quantitative snowfall estimation using radar entails greater uncertainty compared to quantitative rainfall estimation.

     

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