Thermodynamic phase of maritime Antarctic clouds from FTIR and supplementary radiometric data

Abstract

A Fourier Transform Infrared (FTIR) spectroradiometer was deployed at Palmer Station, Antarctica, from 1 September to 17 November 1991. This instrument is similar to the Atmospheric Emitted Radiance Interferometer (AERI) deployed with the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program. The instrument measured downwelling zenith radiance in the spectral interval 400–2000 cm−1, at a resolution of 1 cm−1. The spectral radiance measurements, which can be expressed as spectral brightness temperature Tb(ν), contain information about cloud optical properties in the middle infrared window (800–1200 cm−1, 8.3–12.5 μm). In this study, this information is exploited to (1) provide additional characterization of Antarctic cloud radiative properties, and (2) demonstrate how multisensor analysis of ARM data can potentially retrieve cloud thermodynamic phase. Radiative transfer simulations demonstrate how Tb(ν) is a function of cloud optical depth τ, effective particle radius re, and thermodynamic phase. For typical values of τ and re, the effect of increasing the ice fraction of the total optical depth is to flatten the slope of Tb(ν) between 800–1000 cm−1. For optically thin clouds (τ ∼ 3) and larger ice particles (re (ice) > 50 μm) the behavior of Tb(ν) in this interval switches from a decrease with increasing wavenumber ν to an increase with ν, once the ice fraction of the total optical depth exceeds ∼0.7. The FTIR spectra alone cannot be interpreted to obtain thermodynamic phase, because a relatively small slope in Tb(ν) between 800–1000 cm−1 could represent either an optically thin cloud in the ice or mixed phase, or an optically thick cloud radiating as a blackbody. Sky observations and ancillary radiometric data are needed to sort the FTIR spectra into categories of small cloud optical depth, where the mid‐IR window data can be interpreted; and larger cloud optical depth, where the FTIR data contain information only about cloud base temperature. Spectral solar ultraviolet (UV) irradiance measurements from the U.S. National Science Foundation's UV Monitor at Palmer Station are used to estimate area‐averaged effective cloud optical depth τsw, and these estimates are used to sort the FTIR data. FTIR measurements with colocated τsw < 16 are interpreted to estimate cloud thermodynamic phase. Precipitating cloud decks generally show flatter slopes in Tb(ν), consistent with the ice or mixed phase. Altostratus decks show a larger range in Tb(ν) slope than low cloud decks, including increasing slopes with ν, suggesting a more likely occurrence of the ice phase. This study illustrates how cloud thermodynamic phase can be defensibly retrieved from FTIR data if high quality shortwave radiometric data are also available to sort the FTIR measurements by cloud opacity, and both data types are available at the ARM sites.