Profiling the Planetary Boundary Layer with COSMIC-2 GNSS Observations Under Ducting Condition

The GNSS radio occultation (RO) technique is highly valuable for sensing the planetary boundary layer (PBL) due to its high vertical resolution, global coverage, and all-weather capabilities. However, within the moist PBL, a sharp moisture gradient trapped below a strong temperature inversion often causes ducting at the PBL top. This elevated ducting layer leads to systematically negative biases in RO refractivity retrievals due to a non-unique inversion problem. These sharp refractivity gradients cause significant bending, producing deep RO signals detectable by the receiver with high signal-to-noise ratio.

This study explores a robust method to detect ducting with deep signals in the northeastern Pacific Ocean, where ducting frequently occurs. We also investigate COSMIC-2 grazing reflection measurements, commonly observed over oceans, as an external constraint for RO refractivity reconstruction in ducting conditions. A sensitivity analysis of the reconstruction technique will be presented, comparing the reconstructed (bias-free) RO profiles with colocated radiosondes and the ECMWF reanalysis version 5 (ERA5) data. The synergy between COSMIC-2 RO and near-coincident grazing reflection measurements offers a model-independent measurement of PBL thermodynamic structures, which is vital for understanding complex boundary layer and low cloud dynamics in global weather and climate models.