Abstract
Scattering differences induced by frozen particle microphysical properties are investigated, using the vertically (V) and horizontally (H) polarized radiances from the Global Precipitation Measurement (GPM) Microwave Imager (GMI) 89 and 166 GHz channels. It is the first study on frozen particle microphysical properties on a global scale that uses the dual-frequency microwave polarimetric signals. From the ice cloud scenes identified by the 183.3±3 GHz channel brightness temperature (TB), we find that the scattering by frozen particles is highly polarized with V-H polarimetric differences (PD) being positive throughout the tropics and the winter hemisphere mid-latitude jet regions, including PDs from the GMI 89 and 166 GHz TBs, as well as the PD at 640 GHz from the ER-2 Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) during the TC4 campaign. Large polarization dominantly occurs mostly near convective outflow region (i.e., anvils or stratiform precipitation), while the polarization signal is small inside deep convective cores as well as at the remote cirrus region. Neglecting the polarimetric signal would easily result in as large as 30% error in ice water path retrievals. There is a universal "bell-curve" in the PD - TB relationship, where the PD amplitude peaks at ~ 10 K for all three channels in the tropics and increases slightly with latitude (2-4 K). Moreover, the 166 GHz PD tends to increase in the case where a melting layer is beneath the frozen particles aloft in the atmosphere, while 89 GHz PD is less sensitive than 166 GHz to the melting layer. This property creates a unique PD feature for the identification of the melting layer and stratiform rain with passive sensors. Horizontally oriented non-spherical frozen particles are thought to produce the observed PD because of different ice scattering properties in the V and H polarizations. On the other hand, turbulent mixing within deep convective cores inevitably promotes the random orientation of these particles, a mechanism works effectively on reducing the PD. The current GMI polarimetric measurements themselves cannot fully disentangle the possible mechanisms.
Highlights
Cloud processes play an instrumental role in determining the surface precipitation characteristics (Rutledge and Hobbs, 1983; Gedzelman and Arnold, 1994; Igel et al, 2013; Tao et al, 2013)
To evaluate impacts of frozen particle orientation and ice water content (IWC) on the Tb and polarimetric differences (PDs)–TBV relationship, two numerical Radiative transfer models (RTMs) are employed in this study
All three frequency channels show a maximum PD amplitude of ∼ 10 K, occurring around Tb = 200–220 K. Such a PD–TBV relationship is robust for almost all frequencies and latitudes
Summary
Cloud processes play an instrumental role in determining the surface precipitation characteristics (Rutledge and Hobbs, 1983; Gedzelman and Arnold, 1994; Igel et al, 2013; Tao et al, 2013). While ice microphysical processes themselves cannot be readily observed from space, the integrated effects of these processes (e.g., cloud and precipitation structures, microphysical/macrophysical properties) can be inferred using remote sensing techniques (e.g., passive, active, multiple instruments). Remote sensing of microphysical properties in frozen precipitation and their connection to ice clouds above and surface precipitation, like this work, will provide a valuable surrogate for climate models in representing cloudprecipitation processes. For spaceborne remote sensing, Prigent et al (2001, 2005) analyzed the polarimetric observations at 37 and 85 GHz from SSMI/S (Special Sensor Microwave Imager Sounder) and 85 GHz from TRMM-TMI (Tropical Rainfall Measurement Mission Microwave Imager), where they attributed significant polarimetric differences to horizontally oriented non-spherical liquid or frozen precipitating particles (Prigent et al, 2001, 2005). The working hypotheses on the observed polarized signals are discussed in the subsequent section, with conclusions and future directions in the end
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