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Volume 17, issue 4 | Copyright
Atmos. Chem. Phys., 17, 2741-2757, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 23 Feb 2017

Research article | 23 Feb 2017

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

Jie Gong1,2 and Dong L. Wu2 Jie Gong and Dong L. Wu
  • 1Universities Space Research Association, Columbia, MD, USA
  • 2Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

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 166GHz 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±3GHz channel brightness temperature (Tb), we find that the scattering by frozen particles is highly polarized, with V–H polarimetric differences (PDs) being positive throughout the tropics and the winter hemisphere mid-latitude jet regions, including PDs from the GMI 89 and 166GHz TBs, as well as the PD at 640GHz from the ER-2 Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) during the TC4 campaign. Large polarization dominantly occurs mostly near convective outflow regions (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–TBV relationship, where the PD amplitude peaks at  ∼ 10K for all three channels in the tropics and increases slightly with latitude (2–4K). Moreover, the 166GHz PD tends to increase in the case where a melting layer is beneath the frozen particles aloft in the atmosphere, while 89GHz PD is less sensitive than 166GHz 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 that works effectively in reducing the PD. The current GMI polarimetric measurements themselves cannot fully disentangle the possible mechanisms.

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Short summary
Under certain temperature or aerodynamic conditions, ice crystals prefer to orient along certain directions. The preferred orientation direction of non-spherical ice particles would result in a difference in the satellite remote sensing using different polarized channels. This paper studies this polarization difference using the Global Precipitation Measurement Microwave Imager, where we can infer the dominant ice particle orientation and shape factors from passive remote sensing measures.
Under certain temperature or aerodynamic conditions, ice crystals prefer to orient along certain...