1. Retrieval of Tropospheric Water Vapor From Airborne Far‐Infrared Measurements: A Case Study.
- Author
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Warwick, L., Brindley, H., Di Roma, A., Fox, S., Havemann, S., Murray, J., Oetjen, H., Price, H. C., Schüttemeyer, D., Sgheri, L., and Tiddeman, D. A.
- Subjects
WATER vapor ,ATMOSPHERIC water vapor ,FOURIER transform spectrometers ,RADIATION measurements ,HUMIDITY ,WATER temperature - Abstract
We describe studies undertaken in support of the Far‐infrared Outgoing Radiation Understanding and Monitoring mission, European Space Agency's ninth Earth Explorer, designed to investigate whether airborne observations of far‐infrared radiances can provide beneficial information on mid and upper tropospheric water vapor concentrations. Initially we perform a joint temperature and water vapor retrieval and show that the water vapor retrieval exploiting far‐infrared measurements from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) shows improvement over the a‐priori Unified Model global forecast when compared to in situ dropsonde measurements. For this case the improvement is particularly noticeable in the mid‐upper troposphere. Equivalent retrievals using mid‐infrared radiances measured by the Airborne Research Interferometer Evaluation System (ARIES) show much reduced performance, with the degrees of freedom for signal (DFS), reduced by a factor of almost 2. Further sensitivity studies show that this advantage is decreased, but still present when the spectral resolution of the TAFTS measurements is reduced to match that of ARIES. The beneficial role of the far infrared for this case is further confirmed by performing water vapor only retrievals using ARIES and TAFTS individually, and then in combination. We find that the combined retrieval has a DFS value of 6.7 for water vapor, marginally larger than that obtained for the TAFTS retrieval and almost twice as large as that obtained for ARIES. These results provide observational support of theoretical studies highlighting the potential improvement that far‐infrared observations could bring for the retrieval of tropospheric water vapor. Plain Language Summary: The Far‐infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected as the European Space Agency's ninth Earth Explorer. Far‐infrared Outgoing Radiation Understanding and Monitoring will be the first mission to make high spectral resolution measurements of the Earth's outgoing energy in a region of the spectrum known as the far‐infrared. These observations will complement our existing knowledge of the Earth's outgoing spectrum in the mid‐infrared, where measurements have been made for many years. This paper describes a study in support of FORUM, using unique aircraft‐based observations of far‐ and mid‐infrared spectrally resolved radiation, complemented by measurements of the vertical profile of water vapor and temperature below the aircraft from dropsonde. We infer the humidity and temperature profile from the radiation observations and show that using the far‐infrared observations improves our ability to properly capture the observed dropsonde behavior compared to using the mid‐infrared measurements alone. This is the first time that far‐infrared aircraft observations have been used to demonstrate an improvement in our ability to characterize atmospheric humidity. It suggests that FORUM measurements may be able to enhance our understanding of atmospheric water vapor, with associated benefits for climate prediction. Key Points: Co‐incident mid‐ and far‐infrared upwelling radiance spectra have been observed from aircraft under clear sky conditionsHumidity and temperature retrievals show improved information content for the far‐infrared over the mid‐infrared for these instrumentsThis advantage is retained when water vapor only retrievals are performed [ABSTRACT FROM AUTHOR]
- Published
- 2022
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