GNSS Polarimetric Radio Occultations: Thermodynamical Structure Within Precipitation

The temperature and moisture structure of the surrounding environment is the main control on the thermodynamical processes leading to the development of precipitation.   The surrounding environmental state acts as the broader sink and source for moisture exchange between clouds and their surroundings, the vertical air motion associated with convective cloud growth transports lower-level moisture to higher levels, influencing precipitation intensity and duration.  As precipitation develops, the condensation of water vapor leads to an evolving three-dimensional temperature and moisture structure in and near clouds that differs from the larger scale structure of the surrounding environment.  The representation of the processes that link the condensation of water vapor and the growth and invigoration of convective precipitation are the processes that appear to produce the greatest disparities between cloud resolving models and current observations of convective cloud systems.  Yet there is a gap in existing space-based observations since conventional IR and microwave sounding data are degraded in the presence of clouds and precipitation.

Global Navigation Satellite System (GNSS) radio occultations (RO) have emerged as a low-cost approach for sounding the global atmosphere with high precision, accuracy and vertical resolution, through clouds and across land-ocean boundaries.   The GPS system is an integral part of the US infrastructure as are similar systems in Europe and elsewhere, providing reliable, sustained signal sources.  However, current RO data are limited insofar that the received signal provides no direct information on the associated precipitation state.  To characterize the moist thermodynamic state within precipitating systems, a recently studied concept of polarimetric RO (PRO) measurements has been proposed.  PRO is predicated on the fact that since precipitation-sized hydrometeors are non-spherically shaped, a cross-polarized component is induced during propagation through clouds associated with heavy precipitation, recorded by a dual-channel RO receiver as a differential phase shift.  Theoretical analysis performed by the authors using coincident TRMM Precipitation Radar and COSMIC observations has shown that the polarimetric phase shift is sensitive to the path-integrated rain rate with a precision depending on the integration time used to smooth the phase measurements, and represents a tradeoff between sensitivity and vertical resolution.  Based on the expected signal-to-noise ratio (SNR) of simulated PRO measurements, the precision of the differential phase signal averaged over 1-sec has been estimated to be greater than 1.5 mm, with rain rates exceeding 5 mm/hr detectable above the instrument noise level 90% of the time.