Microphysical properties of frozen particles inferred from Global Precipitation Measurement (GPM) Microwave Imager (GMI) polarimetric measurements

摘要

Scattering differences induced by frozen particlemicrophysical properties are investigated, using the vertically (V) andhorizontally (H) polarized radiances from the Global PrecipitationMeasurement (GPM) Microwave Imager (GMI) 89 and 166 GHz channels. It is thefirst study on frozen particle microphysical properties on a global scalethat uses the dual-frequency microwave polarimetric signals.From the ice cloud scenes identified by the 183.3 ± 3 GHz channelbrightness temperature (), we find that the scattering by frozen particles is highlypolarized, with V–H polarimetric differences (PDs) being positive throughoutthe tropics and the winter hemisphere mid-latitude jet regions, including PDsfrom the GMI 89 and 166 GHz TBs, as well as the PD at 640 GHz from the ER-2Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) during theTC4 campaign. Large polarization dominantly occurs mostly near convectiveoutflow regions (i.e., anvils or stratiform precipitation), while thepolarization signal is small inside deep convective cores as well as at theremote cirrus region. Neglecting the polarimetric signal would easily resultin as large as 30 % error in ice water path retrievals. There is auniversal in the PD–TB relationship, where the PD amplitudepeaks at  ∼  10 K for all three channels in the tropics and increasesslightly with latitude (2–4 K). Moreover, the 166 GHz PD tends to increasein the case where a melting layer is beneath the frozen particles aloft inthe atmosphere, while 89 GHz PD is less sensitive than 166 GHz to themelting layer. This property creates a unique PD feature for theidentification of the melting layer and stratiform rain with passive sensors.Horizontally oriented non-spherical frozen particles are thought to producethe observed PD because of different ice scattering properties in the V and Hpolarizations. On the other hand, turbulent mixing within deep convectivecores inevitably promotes the random orientation of these particles, amechanism that works effectively in reducing the PD. The current GMIpolarimetric measurements themselves cannot fully disentangle the possiblemechanisms.