GNSS-RO Deep Refraction Signals for Sensing PBL Water Vapor

Strongly impacted by a moisture atmospheric planetary boundary layer (PBL), the amplitude of GNSS-RO (radio occultation) signals can fluctuate substantially at the straight-line height (HSL) below -80 km. In this study we analyze the SNR (signal-to-noise ratio) of these deeply-refracted signals from COSMIC-1 GNSS-RO, and find a significant correlation between marine PBL water vapor (WV) at ~950 hPa (~400 m) and the normalized RO amplitude at HSL= -100 km or SNR/SNR0 (SNR0 is SNR in the free atmosphere). This correlation with RO radiometric amplitude is particularly strong in the tropics and subtropics where specific humidity is greater than 4 g/kg. These deep-refraction signals can be attributed to the uniform PBL moisture layers over a horizontally-stratiform and dynamically-quiet domain. Unlike the bending angle derived from RO access phase measurements, the deep-refraction SNR is based on the RO radiometry at a grazing angle to infer the amount of PBL WV. The RO radiometry-based PBL WV retrieval can be further improved with auxiliary data and machine learning (ML) techniques.