Abstract
The present study aims to quantify the potential of hyperspectral thermal infrared sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the future IASI next generation (IASI-NG) for retrieving the ice cloud layer altitude and thickness together with the ice water path. We employed the radiative transfer model Radiative Transfer for TOVS (RTTOV) to simulate cloudy radiances using parameterized ice cloud optical properties. The radiances have been computed from an ice cloud profile database coming from global operational short-range forecasts at the European Center for Medium-range Weather Forecasts (ECMWF) which encloses the normal conditions, typical variability, and extremes of the atmospheric properties over one year (Eresmaa and McNally (2014)). We performed an information content analysis based on Shannon’s formalism to determine the amount and spectral distribution of the information about ice cloud properties. Based on this analysis, a retrieval algorithm has been developed and tested on the profile database. We considered the signal-to-noise ratio of each specific instrument and the non-retrieved atmospheric and surface parameter errors. This study brings evidence that the observing system provides information on the ice water path (IWP) as well as on the layer altitude and thickness with a convergence rate up to 95% and expected errors that decrease with cloud opacity until the signal saturation is reached (satisfying retrievals are achieved for clouds whose IWP is between about 1 and 300 g/m2).
Highlights
Clouds are of major interest for the Earth’s radiation budget studies as further emphasized by the latest Intergovernmental Panel on Climate Change (IPCC) report [1]
We have added to these figures the goal and threshold requirements laid out by the Global Climate Observing System (GCOS) Implementation Plan and the Word Meteorological Organization (WMO) Observing Systems
In this paper, which deals with the capability of thermal hyperspectral measurements for the study of ice clouds, we have shown the potential of instruments like Infrared Atmospheric Sounding Interferometer (IASI) and IASI next generation (IASI-NG) to retrieve parameters such as the integrated ice water content, layer position, and vertical extent of these clouds
Summary
Clouds are of major interest for the Earth’s radiation budget studies as further emphasized by the latest Intergovernmental Panel on Climate Change (IPCC) report [1]. In the longwave part of the spectrum, they act as a greenhouse component by absorbing the emission from the lower atmosphere and surface and re-emitting at their own temperature The balance between those two antagonist effects depends on the nature and properties of clouds (e.g., layer altitude and thickness, layer temperature, condensed water concentration, particle size, shape, and roughness) and determines the net radiative impact of clouds in the global climate system. This is especially true for ice clouds, which can either be associated to a warming or cooling effect depending on their macro- and micro-physical properties [2].
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