Characterizing Near-Surface Moisture in Marine Planetary Boundary Layer Using GNSS-RO Grazing Reflection Observations

Due to its important role that controls the exchange of mass, momentum, and energy between the surface and the free troposphere, planetary boundary layer (PBL) was listed as one of the most important observations in the 2017 Earth Science Decadal Survey. Global Navigation Satellite System (GNSS) radio occultation (RO) is one of an emerging remote sensing techniques for PBL probing because of its high vertical resolution and superior cloud-penetrating capability. Traditinally we use only direct GNSS-RO ray path observations, despite the fact that the portion of the signal being reflected by the Earth’s surface also contains PBL information. Due to their lower signal quality at higher elevation angle and complicated extraction process from the direct signal measurements, the potential of grazing reflected GNSS-RO signals has not been fully exploited.

Here we utilize the coherent grazing surface reflection signal phase observation to characterize the marine PBL near-surface moisture and complement the traditional RO measurements. Unlike the direct GNSS-RO signals, the grazing/reflected RO measurements can be related to the temperature and moisture at all altitudes within the PBL, and its information content is heavily weighted near the surface. A sensitivity study is conducted to show that the phase of the RO grazing reflection signals is sensitive to the near-surface moisture and refractivity. By combining the model we further retrieve the near-surface moisture for the first time leveraging the COSMIC-2 and Spire CSDA products without the assistance from GNSS-RO direct signal observations. The preliminary statistics suggests that the novel observation can provide accurate and unbiased global near-surface moisture products for marine PBL studies and numerical weather prediction (NWP) applications, and its impact can be amplified when tracked independently along with corresponding direct RO signals.