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Characteristics of Raindrop Size Distribution for a Squall Line at Chuzhou of Anhui During Summer
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摘要: 选取2014年7月31日安徽滁州一次飑线过程，使用地基雨滴谱仪资料分析此次过程的雨滴谱特征。根据雷达回波和地面降水强度将这次降水过程划分为对流降水、过渡性降水和层云降水，并以10 mm·h-1为临界值将对流降水进一步划分为对流前沿降水、对流中心降水、对流后沿降水。结果表明：对流中心降水、过渡性降水、层云降水的质量加权直径均比较稳定，平均值分别为1.8 mm, 1.0 mm, 1.7 mm。对流降水的标准化截距相比层云降水更大。对流中心降水各粒径段雨滴数浓度均较高；层云降水小雨滴浓度较低，且有少量大雨滴；过渡性降水由小雨滴组成。当雨水含量相同时，层云降水的质量加权直径相比对流降水更大。当雨强相同时，层云降水的反射率因子相比对流中心降水更大。更为精细的降水类型划分可有效改善Z-I关系。Abstract: Characteristics of raindrop size distribution are analyzed using a ground-based disdrometer for a mid-latitude squall line at Chuzhou of Anhui on 31 Jul 2014. The observational precipitation are classified into convective rain, transition rain and stratiform rain based on the radar reflectivity and rain rate at surface. The convective rain is divided into leading edge, convective center and trailing edge according to a threshold rain rate 10 mm·h-1. The raindrop spectrum characteristics in different precipitation regions are studied. Results show that the mass-weighted diameter for convective center, transition region, stratiform region are stable with mean values of 1.8 mm, 1.0 mm and 1.7 mm, respectively. The generalized intercepting parameter Nw of convective precipitation is larger compared with stratiform precipitation, indicating a larger number concentration of drops. The μ value is the largest for transition precipitation but the smallest for convective precipitation. The raindrop spectrums are different for varied rain type. For convective precipitation, it shows a highest concentration of raindrop within each size range, especially for small size of raindrops, which results in a smaller raindrop size than tropic region. For stratiform precipitation consists of a lower number concentration of small drops and less large raindrops, therefore, the spectrum curve is flat. For transition precipitation, the number concentration of small drops is close to stratiform precipitation without large drops, results in a steep spectrum. The mass-weighted diameter for leading edge is large probably caused by gravity separation at the early stage of precipitation. The rainwater content of stratiform precipitation is smaller compared with convective precipitation. The mass-weighted diameter of stratiform precipitation is larger compared with convective precipitation, and it increases more rapidly compared with convective precipitation as the rain water content increasing. The reflectivity is larger for stratiform precipitation compared with convective center precipitation at the same rain rate. The Z-I relationship of stratiform precipitation is Z=409I1.48 when partitioning the rain into convective rain, transition rain and stratiform rain, but Z=395I1.51 when the partitioning is blurred. The Z-I relationship is improved and the accuracy of radar rainfall estimates is enhanced by dividing the rain type more exactly. In summary, although the raindrop size distribution for a squall line at ground is discussed, the knowledge on microphysical process of mid-latitude squall line is still insufficient. Dual polarization radar can be applied to further investigate the microphysical process in different types of internal cloud precipitation in the future.
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图 1 合肥雷达2014年7月31日0.5°仰角的PPI图像
(三角形为雨滴谱仪所在位置)
Fig. 1 PPI images of Hefei radar with a elevation of 0.5° on 31 Jul 2014
(the black triangle is the location of disdrometer)
图 2 2014年7月30日23:30—31日02:50雨强随时间变化
Fig. 2 Changes of rain rate from 2330 BT 30 Jul to 0250 BT 31 Jul in 2014
图 3 2014年7月30日23:30—31日02:50谱分布及各微物理参数随时间的变化 (a) 谱分布 (彩色代表数浓度，黑色实线代表Dm)，(b) 雨强，(c) lgNw，(d) lgN0，(e)μ
Fig. 3 Time change of raindrop distribution and microphysical parameters from 2330 BT 30 Jul to 0250 BT 31 Jul in 2014 (a) spectral distribution (the shaded denotes number concentration, the black solid line denotes Dm), (b)R, (c) lgNw, (d) lgN0, (e)μ
图 4 2014年7月31日不同降水类型的雨滴谱分布 (实线代表Gamma分布拟合曲线)
Fig. 4 Raindrop spectra of different precipitation type on 31 Jul 2014(solid lines represent fitting curve of Gamma)
图 5 2014年7月30日23:58—31日02:27 μ-Λ关系，
(黑色实线对应Dm=(4+μ)/Λ中Dm为1.0，1.5，2.0 mm)
Fig. 5 μ-Λ relationship from 2358 BT Jul 30 to 0227 BT Jul 31 in 2014
(black solid lines correspond to the relationship Dm=(4+μ)/Λ given Dm of 1.0, 1.5, 2.0 mm)
图 7 2014年7月30日23:58—31日02:27降水Dm-W关系 (实线是对流中心和对流后沿降水的拟合曲线，虚线是层云降水的拟合曲线)
Fig. 7 Relationship of Dm-W from 2358 BT 30 Jul to 0227 31 Jul in 2014(the solid line denotes the fitting curve of convective center and trailing edge, the dashed line denotes the fitting curve of stratiform)
图 8 2014年7月30日23:58—31日02:27降水Z-I关系 (a) 层云降水，(b) 对流降水
Fig. 8 Relationship between Z and I from 2358 BT Jul 30 to 0227 BT 31 Jul in 2014 (a) stratiform precipitation, (b) convective precipitation
表 1 2014年7月30日23:58—31日02:27各参数平均值和标准差
Table 1 Mean values and standard deviations of parameters from 2358 BT 30 Jul to 0227 BT 31 Jul in 2014
参数 对流前沿降水 (4个样本) 对流中心降水 (42个样本) 对流后沿降水 (13个样本) 过渡性降水 (33个样本) 层云降水 (54个样本) 平均值 标准差 平均值 标准差 平均值 标准差 平均值 标准差 平均值 标准差 I/(mm·h-1) 6.8 54.0 5.0 0.4 2.3 Nt/m-3 351 103 2524 1294 411 75 92 44 104 22 W/(g·m3) 0.29 0.12 2.40 1.37 0.29 0.11 0.03 0.01 0.11 0.03 Dm/mm 2.0 0.07 1.8 0.15 1.4 0.16 1.0 0.13 1.7 0.14 lgNw/(mm-1·m-3) 3.1 0.19 4.2 0.21 3.8 0.06 3.3 0.27 3.0 0.13 lgN0/(mm-1-μ·m-3) 3.5 0.50 4.7 0.31 5.9 0.88 7.5 1.72 4.4 0.72 μ 1.7 1.78 2.6 0.86 6.4 1.66 9.2 3.43 5.2 1.84 Λ/mm-1 2.9 0.98 3.6 0.77 7.7 2.07 13.2 3.87 5.6 1.58 注：Nt为雨滴总数浓度。 
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