A study of the impact of the instrument characteristics and of the level 2 approximations on the FORUM level 1 and level 2 products

Abstract:

In Sept. 2019 the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has been selected to be the ESA 9^th Earth Explorer mission. Scheduled for launch in the 2026 time-frame, FORUM will measure the Earth’s Outgoing Longwave Radiation (OLR) at nadir in a wide spectral range covering the (Far-Infrared (FIR) and the Middle Infrared (MIR) regions from 100 cm^-1 to 1600 cm^-1. The FORUM payload is composed by a Fourier Transform Interferometer, measuring the OLR with a nominal spectral resolution of about 0.5 cm^-1, an absolute radiometric accuracy of 0.1 K, and a circular footprint of 15 km radius, and an Embedded Imager used to characterize the homogeneity of FORUM footprint. The primary goal of the FORUM mission is the characterization of the OLR in the FIR and in the MIR regions. However, the measurement of the spectrally resolved emission of the Earth may be exploited to measure the vertical distribution of several trace gases (i.e. water, ozone), the clouds properties and the surface spectral properties in the FIR over the polar regions, where the relatively dry conditions enable to directly observe the Earth surface.

During the FORUM Phase A/B1, several studies have been performed to address the capability of the proposed instrument to provide information on important climate variables like surface temperature and emissivity, vertical distributions of humidity, Temperature, trace gases, and ice cloud parameters. Here we will summarize the results of the study, performed in the frame of the ESA funded FORUMreq project, on the impact of the instrumental characteristics and of the approximations used in the level 2 analyses on FORUM measurements. In particular we will discuss the impact of the instrumental solutions, studied to enable the measurement of the OLR with the required accuracy in the challenging spectral region of the FIR, on the measured spectra and the accuracy with which FORUM will measure the Upper Troposphere and Lower Stratosphere (UTLS) distribution of trace gases and the polar surface properties, investigating the sensitivity of these products to both the instrumental characteristics and the level 2 approximations.