Information content of the 95‐GHz cloud radar signals: Theoretical assessment of effects of nonsphericity and error evaluation of the discrete dipole approximation
Information content of the 95‐GHz cloud radar signals: Theoretical assessment of effects of nonsphericity and error evaluation of the discrete dipole approximation
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95GHz云雷达信号的信息内容：非球形影响的理论评估和离散偶极近似的误差评估
DOI：
10.1029/2001jd001386
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发表时间：
2002
影响因子：
--
通讯作者：
H. Okamoto
中科院分区：
文献类型：
--
作者：
H. Okamoto
[1]离散偶极子近似（DDA）的适用性进行检查，以估计随机取向的冰晶在水平面（2-D）和定向在三维空间（3-D）的各种尺寸和纵横比在95 GHz（3.16 mm）的频率的后向散射签名。以前的研究表明，准确估计的后向散射特性似乎是有问题的颗粒与尖锐的边缘和固定在一个方向上相对于入射雷达波。与此相反，在二维和三维方向的后向散射计算的误差大大减少，原来是小于10%的雷达反射率因子Ze和线性退偏比（LDR）。例外的是具有2-D取向的六边形板的LDR，其中不可能实现收敛解。然后，大小平均Ze和LDR为各种纵横比的有效半径reff的函数估计，第一次，与精度的信心。不同形状或取向的Ze差异在2 dB（40%）以内，因此尺寸是reff 100 μm的控制参数，Ze和LDR显示出对形状和尺寸的强烈依赖性。由于Ze和LDR的局部最小值和最大值出现在不同的reff处，这取决于形状，非球形对于reff > 100 μm起主要作用，其中不同形状的Ze之间的最大差异为8 dB，LDR的最大差异为14 dB。当reff > 1000 μm时，不同形状的LDR之间的最大差异为5 dB，而方向的影响变得相当，尽管这种较大的粒子在卷云中很少见。
[1] The applicability of the discrete dipole approximation (DDA) is examined to estimate backscattering signatures of ice crystals randomly oriented in horizontal plane (2-D) and oriented in three-dimensional space (3-D) for various sizes and aspect ratios at the frequency of 95 GHz (3.16 mm). Previous studies have shown that accurate estimation of the backscattering properties seem to be problematic for particles with sharp edges and fixed in one orientation with respect to the incident radar wave. Contrary, the errors in the backscattering calculations for 2-D and 3-D orientations are drastically reduced and turn out to be less than 10% for both radar reflectivity factor Ze and linear depolarization ratio (LDR). Exception is the LDR for hexagonal plates with 2-D orientation, where it is not possible to achieve a converging solution. Then the size-averaged Ze and LDR for various aspect ratios are estimated as a function of effective radius reff, for the first time, with a confidence of accuracy. The differences in Ze for different shapes or orientations are within 2 dB (40%), and thus the size is a control parameter for reff 100 μm, Ze and LDR show strong dependence on shape as well as size. Due to the fact that the local minimum and maximum of Ze and LDR occur at different reff depending on the shape, nonsphericity plays a major role for reff > 100 μm, where the maximum differences between Ze for the different shapes are 8 dB and those for LDR are 14 dB. For reff > 1000 μm, the maximum differences between the LDRs for the different shapes are 5 dB, while the effect of orientations becomes comparable, although such larger particles are rare in cirrus clouds.