Planetary Boundary Layer Retrieval Using a Combination of GPS Radio Occultation and Microwave Radiometer Measurements

The planetary boundary layer (PBL) plays a significant role in the Earth’s climate system through their influence on cloud distribution and surface radiative fluxes. Characterizing the structure and properties of the PBL is an important but challenging task with current remote sensing technology. Global Positioning System (GPS) radio occultation (RO) has proven to be an effective technique to probe the PBL due to its high vertical resolution and cloud-penetrating capability. However, at low altitudes super-refraction (SR) can be caused by extreme refractivity gradients that create a ducting layer which violates the uniqueness condition to perform traditional inverse Abel transform RO retrievals. The resulting Abel solution is the minimum profile among an infinite numbers of potential refractivity solutions which correspond to the same observed bending angle profile. Consequently, the high resolution RO refractivity measurements will be negatively biased under ducting layers, which can be observed frequently in most cases over the subtropical oceans. This research incorporates optimization techniques into low altitude RO retrievals constrained by collocated precipitable water (PW) measurements, which are widely available from passive microwave radiometers such as AMSR-E. Our results suggest that this new approach drastically reduces the negative bias in refractivity measurements and improve RO data quality under the ducting layer. In addition, the results also show that GPS-RO can capture the sharpness and height for the transition from the PBL to the free troposphere.