Spectral Flux Calculation with FORUM-like Measurements Acquired from a Stratospheric Balloon

The Earth's Outgoing Longwave Radiation (OLR) spectrum from 100 to 1600 cm-1 that will be measured by FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring), the ninth Earth Explorer mission of ESA, planned for the launch in 2026, can be exploited to retrieve the longwave spectral fluxes. In this study, we consider the approach that calculates the spectral flux from the atmospheric state retrieved from the spectral radiance measurements. The atmospheric state (mainly, vertical profiles of humidity and temperature, and ozone column), cloud and surface properties are retrieved using an inversion model based on a line-by-line radiative transfer code with the capability of simulating the multiple scattering. The retrieved parameters are then used to simulate the hemispherical spectral emission at the top-of-atmosphere that can be angularly integrated to give the spectral flux, potentially extending the spectral range to include the whole OLR. The radiance is assumed to be homogeneous in azimuth and the integral over the zenith angle is solved using a Gaussian quadrature. If the spectral radiance measurement contains most of the emitted energy, as in the case of FORUM, the error due to the conversion of radiance to flux can be very small.We apply this approach to one of the few existing set of measurements covering the wide-spectral range of FORUM in conditions similar to space observations, i.e. from a stratospheric balloon platform. This measurement was performed in 2005 from Teresina, Brazil, by the Radiation Explorer in the Far InfraRed - Prototype for Applications and Development (REFIR-PAD) instrument that was operated to measure at nadir the spectral radiance from 100 to 1400 cm-1 with a spectral resolution of 0.5 cm-1. The flight lasted approximately 8 hours acquiring measurements from an average floating altitude of 34 km in clear sky conditions.From these measurements, the vertical profiles of humidity and temperature and the surface temperature were retrieved using a few inversion codes based on forward models developed at our own premises (KLIMA, SACR, and ?-FORUM). Spectral fluxes were then computed via a quadrature approach. From spectral fluxes, band integrated fluxes can be finally calculated and compared with the results from the Rapid Radiative Transfer Model (RRTM).We discuss the advantages and the disadvantages of these methods and analyse the limits of their applicability.