两种相态降水粒子对微波、毫米波衰减特性的影响分析

Effects Analysis of Two-phase Precipitations on Microwave and Millimeter-wave Attenuation Characteristics

厘米波雷达和毫米波雷达已广泛应用于云以及降水的观测,实际观测时会遇到不同相态(液相或者冰相)的降水粒子,研究气象雷达电磁波探测两种相态降水粒子时衰减特性的差异有助于更好地利用雷达回波信息反演降水粒子的微物理参数。假设气象雷达发射的频率分别为3、6、10GHz(厘米波雷达)以及35、94、140、220GHz(毫米波雷达),研究两种相态球形降水粒子的复介电常数随温度以及频率的变化,利用Lorenz-Mie理论研究单个降水粒子的衰减效率后发现,冰相降水粒子的振荡幅度大于液相降水粒子的幅度,虽然液相降水粒子的介电常数值大于冰相降水粒子的介电常数值。假设降水粒子谱满足M—P谱分布,计算降水粒子群的衰减系数随降雨率以及温度的变化,结果表明:当雷达发射频率≤35GHz时,液相降水粒子群的衰减系数大于冰相降水粒子群的衰减系数,但当雷达发射频率≥94GHz时,冰相降水粒子群的衰减系数大于液相降水粒子群的结果,原因在于此时冰相降水粒子衰减效率的振荡幅度及频次均大于液相降水粒子的情况。无论是冰相还是液相降水粒子群,其衰减系数随温度变化的影响较小,最后分别建立两种相态降水粒子的衰减系数与降雨率及温度的经验关系式。

Centimeter wave radar and millimeter wave radar have been widely used to detect cloud and pre- cipitation, two-phase precipitations may be detected when radars detect meteorological targets and the stud-y on the attenuation characteristics of two-phase precipitations can help us retrieve the micro-physical pa- rameters of precipitation particles using radar echoes.Assuming precipitation meets M--P distribution, var- iations in the attenuation coefficient of precipitation as functions of rainfall rate and temperature, the re- suits showed when the transmitting frequency of radar is less than or equal to 35 GHz,the attenuation co- efficient of liquid-phase precipitation is larger than that of ice-phase precipitation, however this case is re- verse when the transmitting frequency of radar is greater than 94 GHz,the reason is oscillating amplitude and times of the attenuation efficiency in the case of ice-phase precipitation particle are larger than that of liquid-phase precipitation particle.Whether ice-phase precipitation particle or liquid-phase precipitation, the attenuation coefficients are less affected by temperature changes, At last ,the empirical formulas were estab- lished to describe the relationships amongst attenuation coefficient ,rainfall rate and temperature.Assuming the frequencies of radars are 3 GHz,6 GHz,10 GHz（Centimeter-wave）and 35 GHz,94 GHz,140 GHz,220 GHz（millimeter-wave）,variations in the complex dielectric constant of two-phase spherical precipitations as functions of temperature and frequency would be studied, the attenuation efficiency of single precipitati- on particle can be computed by Lorenz-Mie theory and the result showed that the oscillating amplitude of ice-phase precipitation particle is larger than that of liquid-phase precipitation particle though the permit- tivity of liquid-phase precipitation particle is larger than that of ice-phase precipitation particle.