Observational and modeling study of ice hydrometeor radar dual-wavelength ratios Journal of Applied Meteorology and Climatology

The influence of ice hydrometeor shape on the dual-wavelength ratio (DWR) of radar reflectivities at millimeter-wavelength frequencies is studied theoretically and on the basis of observations. Data from dual-frequency (Ka–W bands) radar show that, for vertically pointing measurements, DWR increasing trends with reflectivity Ze are very pronounced when Ka-band Ze is greater than about 0 dBZ and that DWR and Ze values are usually well correlated. This correlation is explained by strong relations between hydrometeor characteristic size and both of these radar variables. The observed DWR variability for a given level of reflectivity is as large as 8 dB, which is in part due to changes in mean hydrometeor shape as expressed in terms of the particle aspect ratio. Hydrometeors with a higher degree of nonsphericity exhibit lower DWR values when compared with quasi-spherical particles because of near-zenith reflectivity enhancements for particles outside the Rayleigh-scattering regime. When particle mass–size relations do not change significantly (e.g., for low-rime conditions), DWR can be used to differentiate between quasi-spherical and highly nonspherical hydrometeors because (for a given reflectivity value) DWR tends to increase as particles become more spherical. Another approach for differentiating among different degrees of nonsphericity for larger scatterers is based on analyzing DWR changes as a function of radar elevation angle. These changes are more pronounced for highly nonspherical particles and can exceed 10 dB. Measurements of snowfall spatiotemporally collocated with spaceborne CloudSat W-band radar and ground-based S-band operational weather radars also indicate that DWR values are generally smaller for ice hydrometeors with higher degrees of nonsphericity, which, for the same level of S-band reflectivity, exhibit greater differential reflectivity values.

本研究从理论与观测结合的角度，探究了冰相水凝物（ice hydrometeor）形状对毫米波频段雷达反射率双波长比（dual-wavelength ratio, DWR）的影响。双频段（Ka波段与W波段）雷达的观测数据表明，在垂直指向观测场景中，当Ka波段反射率因子（Ze）大于约0 dBZ时，双波长比随反射率因子的增长趋势极为显著，且双波长比与反射率因子通常具备良好的相关性。该相关性可通过水凝物特征尺度与这两类雷达变量间的强关联得到合理解释。在相同反射率因子水平下，观测到的双波长比变异幅度可达8 dB，这一现象在一定程度上源于以粒子轴比（aspect ratio）表征的平均水凝物形状变化。相较于准球形粒子，非球形度更高的冰相水凝物的双波长比更低，这是因为处于瑞利散射（Rayleigh-scattering）区外的粒子会出现近天顶方向反射率增强效应。当粒子质量-尺度关系无显著变化时（例如低结霜条件下），双波长比可用于区分准球形与高度非球形的冰相水凝物：在给定反射率因子的前提下，粒子越接近球形，双波长比越高。针对大散射体的非球形度差异区分，另一种方法是分析双波长比随雷达仰角的变化特征。高度非球形粒子的该变化更为显著，幅度可超过10 dB。与星载CloudSat W波段雷达和地面S波段业务天气雷达时空匹配的降雪观测数据同样显示，非球形度更高的冰相水凝物的双波长比通常更低；在相同S波段反射率因子水平下，这类水凝物的差分反射率（differential reflectivity）更高。