Tomographic Limb-Sounding of the Upper Troposphere and Lower Stratosphere

Abstract

The UTLS region is characterised by large vertical and horizontal gradients in trace gases such as O3 and H2O. Limb-sounding naturally provides products with high vertical resolution, but is sometimes criticised for its lack of horizontal resolution. Traditionally, limb-sounding retrievals assume the atmosphere to be spherically-symmetric so, at best, the retrieved profiles represent line-of-sight averages over horizontal structure in the true field. For 1-2 km thick layers, this averaging distance is ~200-300 km, which compares unfavourably with the horizontal resolution of nadir-sounding and does not satisfy contemporary requirements for UTLS research (eg stratosphere-troposphere exchange) or numerical weather prediction. The MASTER (Millimetre-wave Acquisitions for Stratosphere-Troposphere Exchange Research) instrument concept, developed by ESA, is designed to specifically address this limitation by viewing along-track and at a limb-scan spacing small enough for the horizontal structure wihin each tangent-layer to be over-sampled and each air-mass to be viewed from many different directions. A tomographic approach to obtain improved horizontal resolution is then feasible, provided that radiative transfer can be modelled accurately and efficiently in 2-D and that multiple limb-scans can be employed simultaneously to retrieve 2-D fields. In order to examine this approach, a state-of-the-art 2-D radiative transfer model and retrieval model have been developed and used in realistic simulation experiments for MASTER. This paper outlines the methodology and presents preliminary results for water vapour, ozone and other trace gases.