Cloud-Assisted Retrieval of Lower-Stratospheric Water Vapor from Nadir-View Satellite Measurements

Abstract

AbstractThis study examines the feasibility of retrieving lower-stratospheric water vapor using a nadir infrared hyperspectrometer, with the focus on the detectability of small-scale water vapor variability. The feasibility of the retrieval is examined using simulation experiments that model different instrument settings. These experiments show that the infrared spectra, measured with sufficient spectral coverage, resolution, and noise level, contain considerable information content that can be used to retrieve lower-stratospheric water vapor. Interestingly, it is found that the presence of an opaque cloud layer at the tropopause level can substantially improve the retrieval performance, as it helps remove the degeneracy in the retrieval problem. Under this condition, elevated lower-stratospheric water vapor concentration—for instance, caused by convective moistening—can be detected with an accuracy of 0.09 g m−2 using improved spaceborne hyperspectrometers. The cloud-assisted retrieval is tested using the measurements of the Atmospheric Infrared Sounder (AIRS). Validation against collocated aircraft data shows that the retrieval can detect the elevated water vapor concentration caused by convective moistening.