Microphysical properties of ice particles as revealed by satellite microwave polarimetric measurements and radiative transfer modeling

D. WANG; GALLIGANI, VICTORIA SOL; PRIGENT, CATHERINE

Saint-Sauveur

Conferencia; XXII International TOVS Study Conference (ITSC); 2019

The understanding of cloud microphysical processes and their representation in climate models needs to be urgently improved. Frozen and mixed phase cloud processes, in particular, are the most poorly understood. The measurement of these clouds are difficult to obtain owing to the challenges involved in remotely sensing ice water content (IWC) and its vertical profile, including complications associated with multi-level clouds, mixed phases and multiple hydrometeor types, the uncertainty in classifying ice particle size and shape for remote retrievals, the relatively small time and space scales associated with deep convection. Microwave radiometry has shown a promising ability in the characterization of frozen particles, as it is able to penetrate and provide insight into the vertical profiles of most clouds, in contrast to infrared and visible observations, which essentially sense cloud tops. Microwave observations, specially at the higher frequency channels (37GHz), are sensitive to cloud scattering signals. The higher the microwave frequency, the larger the scattering signal produced by the interaction of frozen habits with EM radiation. The knowledge regarding the microphysical properties of frozen habits responsible for such scattering (size, density, shape, orientation, composition) is key in radiative transfer and climate modelling and needs to be further discussed. The difference in the vertically and horizontally polarized microwave measurements (TBV-TBH) at cloud scattering sensitive frequencies have been shown to contain information on ice particle shape (aspect ratio) and orientation. The launch of the Global Precipitation Measurement (GPM) satellite in 2014, which hosts the GPM Microwave Imager (GMI), has extended the availability of microwave polarized observations at higher frequencies (166 GHz), previously only available up to 89 GHz in platforms such as AMSR-E or TMI. Scattering by frozen habits is highly (poorly) polarized in stratiform and anvil clouds (deep convection) resulting from the horizontal orientation of non-spherical frozen habits (random orientation due to turbulence and strong updrafts), as shown in a pre-GPM era by e.g., Prigent et al., 2005, Galligani et al., 2013, Defer et al. 2014, and confirmed by Gong and Wu 2017 who explored GPM?s novel 166 GHz polarized channels. In this study, we analyzed one year of GMI observations at two window channels (i.e., 89 and 166 GHz). Stratiform clouds show larger polarization (up to 10 K in average) due to the deposition and aggregation growth of snowflakes, while the convective regions show smaller polarization, as the graupel and/or hail become randomly (even vertically) oriented due to the strong upward air motion. A robust relationship has been found between the polarization difference and vertical polarized brightness temperature for both land and ocean surfaces, and is parameterized using Hermite cubic spline interpolation which can be easily incorporated into radiative transfer models. The regional and seasonal variability has also investigated between different cloud regimes. In order to support these statistics, sensitivity tests are performed using a radiative transfer (RT) robust modeling framework for a deep convection case study in the highly severe weather producer Southeastern South American region. The Atmospheric Radiative Transfer (ARTS) model is coupled with the Weather and Research Forecasting (WRF) model to explore the sensitivity of polarized signals to frozen habit microphysics parameters such as aspect ratio, orientation or density. The midlatitude deep convection case study exploits coincident GMI-DPR observations, as well as ground radar polarimetric data, and supports physically the relationships parametrized from GMI global observations.