Mission Configuration and Retrieval Technique for Profiling Water Vapor in the Marine Boundary Layer by Collocated Radio Occultation and Passive Microwave Sounding

Profiling water vapor in the marine boundary layer (MBL) is critical to marine weather prediction, maritime communications, and understanding feedbacks relevant to multi-decadal climate prediction, yet profiling the MBL remotely has proven extraordinarily difficult because of the spatial scales involved and the proximity of the ocean surface. Collocated radio occultation (RO) and nadir passive microwave (MW) soundings can be combined in retrieval to profile water vapor with the vertical resolution of RO and with super-refraction and the wet-dry ambiguity inherent to RO resolved by the MW. We have constructed a retrieval technique that considers collocated RO and MW soundings that yields profiles of water vapor in the MBL with unprecedented precision, accuracy, and vertical resolution. We have also performed RO and MW collocation studies that consider many current RO missions and MW instruments. The joint RO+MW retrieval technique mines the information in MW soundings for an inference of the microwave refractivity in the MBL surface air, removes the biasing effect of super-refraction following the approach of Xie et al. (2006, doi:10.1175/JTECH1996.1), and resolves the wet-dry ambiguity inherent to RO using the MW sounding as a constraint or a weather forecast as a prior in 1DVAR. We constructed a simulation-retrieval demonstration system that uses a multi-phase screen propagator to simulate RO amplitude and phase and the Optimal Spectral Sampler (OSS) to simulate AMSU-A radiances. In its current state, the retrieval technique is capable of producing MBL water vapor profiles with 2% accuracy and 100-meter vertical resolution. Our collocation study shows that existing RO satellites and orbiting MW instruments achieve approximately 1,300 collocations daily, defined with a temporal window of 10 minutes. To facilitate this study, we have formulated a time-dependent rotational transformation that is applied to RO geolocations. It is three orders of magnitude more efficient than a brute force approach to finding collocations and is valid to 4% precision. We have found that the greatest yield for collocations in low latitudes would come from RO satellites that would fly in tandem with the TROPICS MW CubeSats, potentially producing 1,500 daily RO+MW collocations in the Tropics and Subtropics.