ACCOUNTING FOR MOLECULAR ABSORPTION WITHIN THE SPECTRAL RANGE OF THE CERES WINDOW CHANNEL

Infrared active molecular species residing within the atmosphere cause the emerging thermal infrared spectrum of the Earth to be characteriz ed by both line and continuum absorption (emission). Accounting for th e molecular absorption within the atmosphere is critical for the prope r interpretation of the satellite measured radiances. Thus, correlated k-distribution procedures have been created to account for the molecu lar line absorption located within the spectral range of the Clouds an d the Earth's Radiant Energy System (CERES) infrared window channel (8 -12 mu m); The derivation of the correlated k-distributions is based u pon an exponential sum fitting of transmissions (ESFT) procedure that has been applied to monochromatic calculations at predetermined refere nce pressure and temperature conditions. In addition, an empirically d erived, yet highly accurate parameterization of the CKD-2.1 code has b een developed to calculate the atmospheric absorption attributed to th e water vapor continuum located within the spectral range of the CERES infrared window channel. The multiplication transmissivity approximat ion has been employed to account for the overlap of the spectral featu res of different molecular species. The accuracy of the radiative tran sfer procedures incorporating the correlated k-distribution routines a nd the parameterized CKD-2.1 continuum routines has been established t hrough comparisons with the reference monochromatic procedures. The co rrelated k-distribution yields an upwelling top of atmosphere (TOA) fl ux for the midlatitude summer (MLS)atmosphere that is within 0.1% of t he monochromatic procedures for the CERES window channel. Neglecting t he contributions from all the molecular species in the correlated k-di stribution except H2O and O-3 yields an upwelling TOA flux for the MLS atmosphere with a 1.5% overestimation. Under circumstances where rapi d processing is extremely critical, an error of this magnitude may be deemed acceptable; Neglecting the contributions from all of the molecu lar species yields an upwelling TOA flux for the MLS atmosphere with a 17.7% overestimation. An error of this magnitude is certainly not acc eptable but does emphasize the need to account for the molecular absor ption within the spectral range of the CERES window channel instrument . Published by Elsevier Science Ltd.