Minimum Local Emissivity Variance Retrieval of Cloud Altitude and Effective Spectral Emissivity—Simulation and Initial Verification

• Published in: Journal of applied meteorology (1988) 2004-05, Vol.43 (5), p.795-809
• Main Authors: Huang, Hung-Lung, Smith, William L., Li, Jun, Antonelli, Paolo, Wu, Xiangqian, Knuteson, Robert O., Huang, Bormin, Osborne, Brian J.
• Format: Article
• Language: English
• Subjects: Altitude; Clouds; Earth, ocean, space; Emission spectra; Emissivity; Exact sciences and technology; External geophysics; Infrared spectrum; Meteorological applications; Meteorological satellites; Meteorology; Pressure; Radiance; Spectral resolution; Temperate regions
• ISSN: 0894-8763; 1520-0450
• DOI: 10.1175/2090.1

This paper describes the theory and application of the minimum local emissivity variance (MLEV) technique for simultaneous retrieval of cloud pressure level and effective spectral emissivity from high-spectral-resolution radiances, for the case of single-layer clouds. This technique, which has become feasible only with the recent development of high-spectral-resolution satellite and airborne instruments, is shown to provide reliable cloud spectral emissivity and pressure level under a wide range of atmospheric conditions. The MLEV algorithm uses a physical approach in which the local variances of spectral cloud emissivity are calculated for a number of assumed or first-guess cloud pressure levels. The optimal solution for the single-layer cloud emissivity spectrum is that having the ‘‘minimum local emissivity variance’’ among the retrieved emissivity spectra associated with different first-guess cloud pressure levels. This is due to the fact that the absorption, reflection, and scattering processes of clouds exhibit relatively limited localized spectral emissivity structure in the infrared 10–15-μm longwave region. In this simulation study it is shown that the MLEV cloud pressure root-mean-square errors for a single level with effective cloud emissivity greater than 0.1 are ∼30, ∼10, and ∼50 hPa, for high (200–300 hPa), middle (500 hPa), and low (850 hPa) clouds, respectively. The associated cloud emissivity root-mean-square errors in the 900 cm−1spectral channel are less than 0.05, 0.04, and 0.25 for high, middle, and low clouds, respectively.