Airborne and satellite remote sensing of the mid-infrared water vapour continuum

Abstract

Remote sensing of the atmosphere from space plays an increasingly important role in weather forecasting. Exploiting observations from the latest generation of weather satellites relies on an accurate knowledge of fundamental spectroscopy, including the water vapour continuum absorption. Field campaigns involving the Facility for Airborne Atmospheric Measurements research aircraft have collected a comprehensive dataset, comprising remotely sensed infrared radiance observations collocated with accurate measurements of the temperature and humidity structure of the atmosphere. These field measurements have been used to validate the strength of the infrared water vapour continuum in comparison with the latest laboratory measurements. The recent substantial changes to self-continuum coefficients in the widely used MT_CKD (MlawerTobin-Clough-Kneizys-Davies) model between 2400 and 3200 cm −1 are shown to be appropriate and in agreement with field measurements. Results for the foreign continuum in the 1300-2000 cm −1 band suggest a weak temperature dependence that is not currently included in atmospheric models. A one-dimensional variational retrieval experiment is performed that shows a small positive benefit from using new laboratory-derived continuum coefficients for humidity retrievals.

Accurate laboratory determination of the near‐infrared water vapor self‐continuum: A test of the MT_CKD model

“…This model is a semi‐empirical formulation which includes ad hoc line profile parameters adjusted to account for experimental constraints. As consequence of the difficulty to measure weak broadband absorption signals, the available experimental information is scarce and consists mainly of laboratory [ Burch , ; Burch and Alt , ; Burch , ; Cormier et al ., ; Baranov et al , ] and field measurements in the microwave and midinfrared regions [ Newman et al ., ] which have the strongest impact on the radiative fluxes [ Clough et al ., ; Costa and Shine , ]. The major constraint of the MT_CKD self‐continuum in the 10 µm window is an analysis of field measurements [e.g., Turner et al ., ].…”

The Water Vapour Continuum: Brief History and Recent Developments

“…Another far-wing theory, based on asymptotic semi-classical approach (Nesmelova et al 1986;Bogdanova Ju and Rodimova 2010;Klimeshina et al 2011), shows a much better agreement with experimental data in the 4-lm window at high temperatures (*350 K) than Tipping and Ma (1995). However, at room temperature, this model also strongly underestimates the self-continuum in this window (Klimeshina et al 2011).…”

“…An integral representation of the absorption coefficient of spectral lines, developed earlier by adopting a quantum-mechanical formalism in the works of Anderson (1949) and Tsao and Curnutte (1962), was applied to specifically develop a far-wing approximation within a statistical approach in Fomin and Tvorogov (1973). Tvorogov and Nesmelova (1976) suggested an alternative far-wing model based on the semi-classical asymptotic approach, fully developed later by Nesmelova et al (1986). In general, theoretical far-wing models of the continuum developed in that period were characterised by a number of adjustable parameters fit to experimental data, which, however, did not always guarantee that they would agree with experimental data at different temperatures or wavenumbers.…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…It is a semi-empirical formulation based on ad hoc (line-profile) parameters adjusted to measured spectra. The latter consist mainly of laboratory (eg [361][362][363][364], see [357]) and field (eg [365]) measurements in spectral regions that have [366,367] the strongest impact on the radiative fluxes in the Earth atmosphere. The 10 µm window has been studied for decades (eg [357,362,[368][369][370]) and a strong constraint of the MT_CKD self continuum (ie pure H 2 O) in this region comes from field measurements (eg [371]).…”