Improved Retrieval of Mid-upper Tropospheric Water Vapour by Measurement of Far-infrared Radiance – a Case Study

Abstract:

This paper describes studies undertaken in support of the Far infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, designed to investigate whether observations of the far-ir spectrum from aircraft can be inverted to provide beneficial information on mid and upper tropospheric water vapour concentrations. We outline the aircraft campaign and relevant instrumentation, provide examples of the radiative observations and associated interpretive forward modelling using in-situ, NWP forecast and satellite derived humidity and temperature information, and detail the retrieval approach and results.

We focus particularly on a flight of the Facility of Atmospheric Airborne Measurements (FAAM) aircraft over the North Sea in March 2019. During the high-level (~ 9 km) straight and level clear-sky runs over the ocean, multiple dropsondes were released to characterise the underlying atmosphere while in-situ radiance observations were made across the infrared by the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) and Airborne Infrared Research Evaluation System (ARIES) instruments. Although the airborne instrument characteristics are not the same as those proposed for FORUM they provide a first, observational indication of the likely benefit of the proposed far-infrared measurements for water vapour retrievals.

Our retrieval framework makes use of the Met Office 1D-var scheme, using the nearest temperature and humidity profiles in time and space as obtained from the global forecast of the Met Office Unified Model as a-priori estimates of the atmospheric state. We show that the retrievals using either the mid or far-ir radiance measurements alone shift the a-priori towards the generally drier dropsonde measurements at altitudes above 4 km. However, the level of agreement between the retrievals and the dropsonde measurements, particularly between 4-7 km, is markedly improved in the far-ir case. Combining the far-ir and mid-ir radiances has a small additional benefit for the humidity retrievals but improves the estimation of the temperature profile and surface temperature. This is confirmed by the total information content analysis for all three variables which gives values of: 4.1 (ARIES alone); 6.0 (TAFTS alone); 7.2 (ARIES and TAFTS combined).