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首页> 外文期刊>Journal of atmospheric and oceanic technology >Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain
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Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

机译：雨中的液滴形状，模型比较和极化雷达参数的计算

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摘要

Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (D_(eq)) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into D_(eq) intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for D_(eq) > 4 mm, the larger drops having a more flattened base, in good agreement with the equilibrium (nonoblate) shape model of Beard and Chuang. Deviations were noted from the Beard and Chuang model shapes for diameters larger than 6 mm. However, the 2DVD measurements of the most probable contour shapes are the first to validate the Beard and Chuang model shapes for large drops, and further to demonstrate the differences from the equivalent oblate shapes. The purpose of this paper is to document the differences in radar polarization parameters and the range of error incurred when using the equivalent oblate shapes versus the most probable contoured shapes measured with the 2DVD especially for drop size distributions (DSDs) with large median volume diameters ( >2 mm). The measured contours for D_(eq) > 1.5 mm were fitted to a modified conical equation, and scattering calculations were performed to derive the complex scattering amplitudes for forward and backscatter for H and V polarizations primarily at 5.34 GHz (C band) but also at 3 GHz (S band) and 9 GHz (X band). Calculations were also made to derive the relevant dual-polarization radar parameters for measured as well as model-based drop size distributions. When comparing calculations using the contoured shapes against the equivalent oblate spheroid shapes, good agreement was obtained for cases with median volume diameter (D_0) less than around 2 mm. Small systematic differences in the differential reflectivity (Z_(dr)) values of up to 0.3 dB were seen for the larger D_0 values when using the oblate shapes, which can be primarily attributed to the shape differences in the resonance region, which occurs in the 5.5-7-mm-diameter range at C band. Lesser systematic differences were present in the resonance region at X band (3-4 mm). At S band, the impact of shape differences in the polarimetric parameters were relatively minor for D_0 up to 2.5 mm. Unusual DSDs with very large D_0 values ( >3 mm) (e.g., as can occur along the leading edge of severe convective storms or aloft due localized "big drop" zones) can accentuate the Z_(dr) difference between the contoured shape and the oblate spheroid equivalent, especially at C band. For attenuation-correction schemes based on differential propagation phase, it appears that the equivalent oblate shape approximation is sufficient using a fit to the axis ratios from the 80-m fall experiment given in this paper. For high accuracy in developing algorithms for predicting D_0 from Z_(dr), it is recommended that the fit to the most probable contoured shapes as given in this paper be used especially at C band.
机译：介绍了使用二维视频测速仪（2DVD）从先前进行的人工降雨实验中得出的水滴形状。实验涉及到液滴落在80 m的距离上以达到最终速度以及稳态振荡的目的。先前的研究分析了在1.5至9毫米范围内的超过11.5万个液滴的测得的轴比（即最大垂直弦与最大水平弦的比）与等体积球滴直径（D_（eq））的关系。在本文中，报告了液滴的实际轮廓形状，同时考虑了仪器的有限量化极限。这些形状来自2DVD的快速线扫描相机。将液滴分类为0.25mm宽度的D_（eq）间隔，并将每个液滴类别的平滑轮廓相互叠加，以获得其最可能的形状以及由于液滴振荡而引起的变化。对于D_（eq）> 4 mm，最可能的形状显示出与扁球体的偏差，较大的液滴具有更平坦的底部，这与Beard和Chuang的平衡（非扁圆形）形状模型非常吻合。注意到直径大于6毫米的Beard和Chuang模型形状存在偏差。但是，最可能的轮廓形状的2DVD测量值是第一个验证大液滴的Beard和Chuang模型形状的方法，并进一步证明了与等效扁圆形状的区别。本文的目的是记录使用2DVD测量的等效扁圆形状与最可能的轮廓形状时雷达极化参数的差异以及误差范围，特别是对于中值体积直径较大的液滴尺寸分布（DSD）（ > 2毫米）。将D_（eq）> 1.5 mm的测量轮廓拟合到修改后的圆锥方程式，并进行散射计算以得出H和V极化的正向和反向散射的复杂散射幅度，主要在5.34 GHz（C波段），但在3 GHz（S频段）和9 GHz（X频段）。还进行了计算以得出用于测量以及基于模型的墨滴尺寸分布的相关双极化雷达参数。将轮廓形状与等效扁球体形状的计算结果进行比较时，对于中位体积直径（D_0）小于2 mm的情况，可以获得很好的一致性。当使用扁圆形状时，对于较大的D_0值，可看到高达0.3 dB的微分反射率（Z_（dr））值的系统小差异，这主要归因于共振区域中的形状差异，这种差异发生在C波段直径范围为5.5-7毫米。在X波段（3-4毫米）的共振区域存在较小的系统差异。在S波段，对于2.5mm以下的D_0，偏振参数中形状差异的影响相对较小。具有非常大的D_0值（> 3 mm）的异常DSD（例如，可能在强对流风暴的前缘或由于局部“大滴”区域而高空发生）会加剧轮廓形状与目标之间的Z_（dr）差异。扁球体等效，特别是在C波段。对于基于差分传播相位的衰减校正方案，使用本文给出的80米跌落实验的轴比进行拟合，似乎等效的扁圆形状近似就足够了。为了开发用于从Z_（dr）预测D_0的算法的高精度，建议特别是在C波段使用本文给出的最可能轮廓形状的拟合。

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