Use of Doppler radar to assess ice cloud particle fall velocity‐size relations for remote sensing and climate studies
Use of Doppler radar to assess ice cloud particle fall velocity‐size relations for remote sensing and climate studies
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使用多普勒雷达评估冰云颗粒下落速度与尺寸关系，用于遥感和气候研究
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发表时间：
2000
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通讯作者：
A. Heymsfield
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作者：
S. Matrosov;A. Heymsfield
了解冰晶终端速度（无论是单个晶体还是尺寸分布）对于在气候模型中充分表示冰粒沉积非常重要。虽然已经测量了简单形状的单个晶体的终端速度 (υt)，但使用空气动力阻力表达式获得的 υt = ADB（其中 D 是最大颗粒尺寸）形式的理论关系通常更有用，因为它们可以应用于大范围的颗粒尺寸、高度和大气中的温度。对于高对流层冰云，系数 A 变化超过 1 个数量级；指数B一般在0.7-1.4范围内。气动阻力计算表明A和B是相关的。 A 和 B 还可用于表征尺寸分布的终端速度-颗粒特征尺寸关系。在本研究中，我们使用冰云雷达反射率、多普勒速度和红外亮度温度的并置垂直指向测量来估计云颗粒特征尺寸、云冰水含量和系数 A 的垂直平均值的垂直剖面，重点关注卷云。我们分析单个颗粒的终端速度尺寸关系的变化以及颗粒整体的相应变化：例如，反射率加权终端速度与中值体积尺寸之间的关系以及质量加权终端速度与中值体积尺寸之间的关系。检索表明 A 的范围从 ∼250 到几乎 4000（cgs 单位），与理论计算发现的范围相似。系数A趋于随着特征颗粒尺寸(例如，中值尺寸)增大而减小。作为气候建模工作的简化，我们提出了中值大小和 A 之间的经验关系，尽管这种关系存在相当大的可变性。利用多普勒测量和反演数据，我们还推导了质量加权终端速度与云冰水含量之间的关系。这种关系对于表示气候和云解析模型中冰粒的沉降很有用。
Knowledge of ice crystal terminal velocities, both for individual crystals and for size distributions, is important for an adequate representation of ice particle sedimentation in climate models. While the terminal velocities (υt) of individual crystals of simple shapes have been measured, theoretical relations of the form υt = ADB (where D is the maximum particle dimension), obtained using expressions for the aerodynamic drag force, are often more useful because they can be applied to a wide range of particle sizes and heights and temperatures in the atmosphere. For high tropospheric ice clouds the coefficient A has been found to vary over 1 order of magnitude; the exponent B is generally within the range 0.7–1.4. Aerodynamic drag force calculations show that A and B are related. A and B can also be used to characterize terminal-velocity-particle characteristic size relations for size distributions. In this study we use collocated, vertically pointing measurements of ice cloud radar reflectivity, Doppler velocity, and IR brightness temperatures to estimate the vertical profiles of cloud particle characteristic size, cloud ice water content, and vertically averaged value of the coefficient A, emphasizing cirrus clouds. We analyze variations in terminal-velocity-size relations for individual particles and corresponding variations for ensembles of particles: for example, in relations between the reflectivity-weighted terminal velocity and the median volume size and between the mass-weighted terminal velocity and the median volume size. The retrievals indicate that A ranges from ∼250 to almost 4000 (cgs units), similar to the range found from the theoretical calculations. The coefficient A tends to decrease as a characteristic particle size (e.g., median size) increases. As a simplification for climate modeling efforts, we present an empirical relation between median size and A, although there is a fair amount of variability about this relation. Using the Doppler measurements and retrieval data, we also derive relations between the mass-weighted terminal velocity and cloud ice water content. Such relations are useful for representing fallout of ice particles in climate and cloud-resolving models.