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

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.