06 May 2021

06 May 2021

Review status: a revised version of this preprint is currently under review for the journal AMT.

Are elevated moist layers a blind spot for hyperspectral infrared sounders? – A model study

Marc Prange, Manfred Brath, and Stefan Alexander Buehler Marc Prange et al.
  • Universität Hamburg, Meteorologisches Institut, Bundesstraße 55, 22525 Hamburg, Germany

Abstract. The ability of the hyperspectral satellite based passive infrared instrument IASI to resolve Elevated Moist Layers (EMLs) within the free troposphere is investigated. EMLs are strong moisture anomalies with significant impact on the radiative heating rate profile and are thought to be coupled to freezing level detrainment of convective cells in the tropics. Based on an exemplary EML testcase and forward modelled IASI observations, it is shown that if sufficient independent humidity and temperature information is available, EMLs do not pose a blind spot for passive satellite observations, contrary to what results of Stevens et al. (2017) have indicated. To further quantify the retrieval’s ability to capture moisture anomalies, a statistical evaluation of synthetic retrievals of 1438 clear sky tropical ocean short-range forecast model atmospheres is conducted. For this purpose, a framework for the identification and characterisation of moisture anomalies, a subset of which are EMLs, is introduced. The statistical evaluation shows that retrieved moisture anomalies are on average 37 % weaker and 28 % thicker than their true counterparts, which can be attributed to the retrieval smoothing error and the fact that rather weak and narrow moisture anomalies are most frequently missed by the retrieval. Smoothing is found to also constrain the magnitude of local heating rate extremes associated with moisture anomalies, particularly for the strongest anomalies that are found in the lower to mid troposphere. In total, about 72 % of moisture anomalies in the reference dataset are found by the retrieval. Below 5 km altitude, this fraction is only on the order of 30 %, which can be attributed to the fact that lower tropospheric moisture anomalies are typically more narrow and therefore tougher to retrieve than anomalies aloft. We conclude that the retrieval of lower to mid tropospheric moisture anomalies, in particular of EMLs, is possible when the anomaly is sufficiently strong and its thickness is at least on the order of about 1.5 km. This study sets the methodological basis, from a retrieval setup and evaluation perspective, to investigate real world EMLs in IASI observations in the future.

Marc Prange et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-48', Anonymous Referee #1, 20 May 2021
    • AC1: 'Reply on RC1', Marc Prange, 01 Jun 2021
  • RC2: 'Comment on amt-2021-48', Anonymous Referee #3, 08 Jun 2021
    • AC2: 'Reply on RC2', Marc Prange, 11 Jun 2021
      • RC3: 'Reply on AC2', Anonymous Referee #3, 23 Jun 2021
        • AC3: 'Reply on RC3', Marc Prange, 30 Jun 2021

Marc Prange et al.

Data sets

Supplementary data for "Are elevated moist layers a blind spot for hyperspectral infrared sounders? - A model study" Marc Prange, Manfred Brath, Stefan A. Buehler

Model code and software

Atmospheric Radiative Transfer Simulator Stefan Buehler et al.

Marc Prange et al.


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Short summary
We investigate the ability of the hyperspectral infrared satellite instrument IASI to resolve moist layers in the tropical free troposphere in a model framework. Previous observational results indicated major deficiencies of passive satellite instruments to resolve moist layers around the freezing level. We show that the humidity retrieval can be improved by better resolving the temperature structure and conclude that such moist layers do not pose an inherent satellite blind spot.