Simultaneous Retrieval of the Water Vapour and Temperature Profiles, Surface Temperature and Cloud Properties from FORUM Spectral Radiance Simulated Observations

The Simultaneous Atmospheric and Cloud Retrieval (SACR) code is an algorithm comprising a forward and a retrieval model capable to simultaneously retrieve the main surface and atmospheric variables and of the cloud optical and microphysical properties, from infrared spectral radiance measurements. These atmospheric parameters are relevant in climate change studies. In particular, the characterization of cirrus cloud radiative properties represents a paramount goal, since this type of clouds modulates the incoming solar radiation and the thermal outgoing emission from the ground. Furthermore, cirrus clouds cover permanently more than 30% of the whole planet surface reaching 70% in the tropical regions. Despite their importance in the Earth Radiation Budget (ERB), cirri are difficult to detect, especially in polar regions, and their properties are still affected by large uncertainties. It has been recently recognized that the spectral region between 100 and 1600 cm -1 (6.2-100 ?m), and in particular the far infrared (FIR) region below 600 cm -1 , contains key information about the atmospheric water vapour and ice clouds. Despite this, there is a lack of spectrally resolved measurements covering the FIR region, either from air/space borne and from ground-based sensors. To cover this observational gap, the Far infrared Outgoing Radiation for Understanding and Monitoring (FORUM) mission was selected by the European Space Agency (ESA) as the 9th Earth Explorer (EE-9). For the first time from space, FORUM will acquire spectrally resolved measurements of the Earth emission. The procedure to retrieve relevant atmospheric parameters from FIR and MIR simulated spectral radiances (as those that will be measured by FORUM) is discussed in this work. Specifically, the parameters considered in the retrieval are: the water vapour and temperature profiles, the cloud optical depth, effective diameters of the particle distributions, the ice fraction in the presence of mixed phase clouds, the cloud position and the surface temperature. The performance of the retrieval is examined on the basis of a few test cases.