Information Embedded in the Finely Resolved UWBRAD Tb Spectra of Polar Ice Sheets

The Ultra-Wideband Software Defined Microwave Radiometer (UWBRAD) is a system recently developed to measure brightness temperatures (Tb) of polar ice sheets from 0.5 to 2.0 GHz. This is accomplished by sampling the microwave emission in 12 channels, each spanning 88MHz, in the time domain, followed by analyzing their spectra. The tempo-spectra are then cleaned to filter radio-frequency interference (RFI) in the unprotected spectrum. Two unprecedented capabilities are achieved: (i) UWBRAD observes wide band emissions from the ice sheet, which include information about the ice sheet vertical temperature profile; (ii) UWBRAD acquires finely resolved Tb spectra comprised of 512 subchannels in the 125MHz sampled bandwidth of each channel. A partially coherent model has been developed as the baseline physical model to support temperature retrieval; the implications of the second capability have yet to be examined.

Traditional usage of the Tb spectra is through averaging over each of the 12 channels. This smooths out the fluctuations within each channel that correspond to potential coherent wave effects not completely eliminated in the spatial average over the radiometer footprint of 1 km diameter. This process therefore ignores any potential information carried by the finely resolved spectra. In this paper, we model the physical origin of Tb fluctuations on fine frequency scales and link the width of the frequency correlation function to the effective total thickness of the ice sheet top layers. We also analyze whether such signatures exist for other geophysical targets.

Initial analysis of UWBRAD Tb spectra collected in the 2016 and 2017 campaigns over Greenland shows that near surface density fluctuations create reflections that modulate the upward emission from the ice sheet body and complicate the temperature retrieval. We have modeled these density fluctuations using a random process with a two-scale Gaussian correlation function. Emission through the partially coherent model successfully explains the observed Tb spectra. However, in the temperature retrieval, little is known about these density fluctuations. Our recent work shows that incorporating information embedded in the finely resolved Tb spectra help confine some of these nuisance parameters and reduce uncertainty in temperature retrievals.