FARM: A Fast Retrieval Model for the Inversion of Surface, Atmospheric and Cloud properties from FIR and TIR measurements

In the frame of the study "FORUM-Scienza" (FORUM-Science), funded by the Italian Space Agency (ASI), a fast code to simulate top of the atmosphere Far InfraRed (FIR) and Thermal InfraRed (TIR) spectral radiances has been developed. The code, named ?-FORUM, is an upgraded version of the fast radiative transfer model developed for IASI (?-IASI [1,2]). It enables the simulation of both clear and cloudy sky monochromatic radiances, simultaneously computing their derivatives with respect to the injected atmospheric and surface state. ?-FORUM has been embedded in a Bayesian retrieval code (named FAst Retrieval Model - FARM) that enables to invert measured radiances to obtain surface emissivity, surface temperature, and vertical distributions of temperature, trace gases and cloud properties. FARM can be applied to any broadband, spectrally resolved ground-based, airborne or satellite measurement in the FIR and TIR frequency regions (from 50 to 2600 cm-1). Optionally, the code can also operate the synergistic retrieval from co-located measurements of two different sounders, like the ones anticipated for FORUM and IASI-NG. In this work, we report the structure of FARM, its performance on FORUM simulated measurements, and some results obtained from the analysis of FORUM-like measurements acquired with two FORUM demonstrators, specifically, the REFIR-PAD measurements collected in a stratospheric balloon flight campaign at Teresina (Nord-East Brazil) and FIRMOS measurements of the downwelling longwave radiation acquired from the observatory of Zugspitze (Germany) at about 3000 m a.m.s.l.