微波成像仪通道对降水云参数响应的数值模拟研究

Responses of TMI channels to precipitation cloud parameters as suggested by numerical simulation.

由于降水云类型和结构等的差异,将导致微波信号出现不同的响应,因此基于星载微波仪器观测结果对降水云参数进行反演仍是目前国际上的难题之一。为了解层云和对流云等两类降水云的TMI微波信号特点,本文以相应的结构模型为基础,利用微波辐射传输模式MWRT分别模拟了不同下垫面和降水云参数条件下的TMI各通道微波亮温,并分析了各种组合通道信号,包括反映发射信息的极化差D(洋面)及反映散射信息的高频极化校正亮温PCT85(洋面)和高低频率垂直极化差VFD(陆面)的变化情况,以此从理论上探讨可能的降水反演方法。研究结果表明:在洋面,D和PCT85均随降水率的升高而减小,其中D_(10)和PCT85都能很好地反映降水率的变化,但分别受地表状况和云体性质(冻结层高度和霰粒大小)的影响较大;D_(19)和D_(37)虽然对下垫面状况和冻结层高度都不甚敏感,但随降水率的增加,存在极化丧失现象,因此不适合反演降水。在陆面,较高频取85 GHz的VFD与降水率有很好的对应关系,随降水率的增长而增大,其中VFD37-85受地表状况影响最小,但受云体性质影响较大;37 GHz对液态水变化敏感,其发射效应易混淆散射信号,故较高频取37 GHz的VFD10-37和VFD19-37不适合研究降水与微波信号之间的关系;层云与对流云存在有无霰粒的差异,会对微波散射性质造成影响,导致PCT85和VED对层云降水率的响应要高于对流云。

Since microwave signals are very sensitive to the variability in the type and structure of precipitation clouds,retrieving the precipitation cloud parameters based on the satellite-borne microwave observations is still one of the most challengeable problems.In order to investigate the characteristics of microwave signals on TMI channels in responding to two precipitation cloud types of stratus and convective clouds,a radiative transfer model（MWRT） with a better treatment of the solid precipition particles is used in this paper to simulate microwave brightness temperatures（TB） under the condition of different surface and precipitation cloud parameters.In addition,variations of combined TB from multiple channels are also analyzed which include the polarization difference（D） as a mirror of emission signals（for ocean） as well as the 85 GHz polarization corrected temperature （PCT85） for ocean and the vertical frequency polarization difference（VFD） for land as functions of scattering signals,so as to theoretically explore the precipitation retrieval method.The results show that both of D and PCT85 decrease as increasing rain rate over ocean,among which D_（10） and PCT85 are able to reflect well the variation of rain rate but with the former in a greater extent affected by the surface conditions and the latter by the cloud properties such as the height of the freezing level and the size of graupel.On the other hand,though D_（19） and D_（37） are insensitive to both surface conditions and the height of the freezing level they show no polarization difference in the case of high rain rate,implying that they are unsuitalbe to vetrieving rain rate.Over land,the VFD with the higher frequency as 85 GHz shows a good response to rain rate and augments with increasing rain rate,among which VFD37-85 is hardly influenced by surface conditions but strongly affected by cloud properties.Especially, for the channel of 37 GHz,owing its high sensitivity to variations in liquid water so that scattering signal is easily confused by emission signal,it is difficult that the VFDs with the higher frequency like 37GHz are applied to investigate the relationship between rain rate and microwave signals.Besides,since the existence of graupel in convection clouds would have complicated the characteristics of scattering signals,PCT85 and VFD would worse respond to rain rate for convection clouds than for stratus clouds.Based on the results above,a theoretical approach to retrieving rain rate can be suggested as follow：over ocean it is preferred to use D_（10） with proper surface conditions or PCT85 with proper height of freezing level,and over land use VFD37-85 with proper type（stratus or convection） of precipitation clouds.