Retrieving Water Vapor in the Marine Boundary Layer using GNSS Radio Occultation and Passive Microwave Radiance Sounding

Abstract:

Profiling the marine boundary layer by remote sensing is one of the great goals of present-day climate science, in large part because marine cumulus/stratocumulus are thought to contribute the leading term in uncertainty in knowledge of equilibrium climate sensitivity. Remote sensing in the boundary layer (BL) is difficult because radiance measurements of the BL are always complicated by a contribution from surface emission, the presence or absence of clouds, and the poor vertical resolution of most remote sensing techniques. Radio occultation (RO) using the transmitters of the Global Navigation Satellite Systems, however, is sensitive to water vapor in the BL with vertical scales approaching ~50 m. RO measurements themselves are complicated by the wet-dry ambiguity—its retrievable is constituted of both water vapor and temperature but does not provide enough information to distinguish between the two—and the frequent occurrence of super-refraction inevitably biases retrievals within the BL.

We attempt a retrieval of water vapor in the BL by fusing RO data with collocated microwave data, taking a systematic approach wherein strong assumption are gradually relaxed. First, a well-mixed moist, unsaturated adiabat with two degrees of freedom in the BL is assumed. The method is found to be robust and convergent but inefficient and uninformative: inefficient because the independent coordinates of the microwave and RO differ, imprecise because a well-mixed adiabat washes out interesting information. Second, a constant temperature lapse rate is assumed and high vertical resolution water vapor is retrieved using a strong reglurizer. This method shows immense promise: it can retrieve water vapor anomalies on vertical scales as small as 200 m with near-perfect accuracy; however, this method is unstable. In half of the cases, steep and unrealistic water vapor gradients are retrieved erroneously.

Our ongoing investigation into fusing passive microwave with GNSS RO will be presented.