Retrieval of cloud properties from ground-based and airborne FTIR spectrometer measurements

Clouds alter the incoming solar and outgoing thermal radiation, and thus strongly influence the Earth´s energy budget. To improve understanding of the role of clouds in climate, knowledge about the distribution in time and space of the cloud liquid water path, LWP=∫w(z)dz, and the effective radius of the cloud droplets, r_eff=∫πr³n(r)dr/∫πr² n(r)dr, is required. These equations involve the liquid water content and cloud base and top altitudes. n(r) is the number density of droplets with radii between r and l+dr (the droplet size distribution). LWP and r_eff can be obtained from the inversion of spectral observations of the radiation emitted by clouds in the thermal infrared atmospheric window region (800-1200 cm^-1). The use of Fourier Transform InfraRed (FTIR) spectrometers for thermal emission measurements allows, through their higher spectral resolution, to analyze cloud spectra inbetween absorption lines and to improve the retrievals. The authors have developed a particular retrieval algorithm for LWP and r_eff from high spectral resolution measurements and in this paper the authors demonstrate its usefulness by applying it to airborne and ground-based cloud observations made with FTIR spectrometers