Parameterization of Outgoing Infrared Radiation Derived from Detailed Radiative Calculations

Abstract

State-of-the-art radiative transfer models can calculate outgoing infrared (IR) irradiance at the top of the atmosphere (F) to an accuracy suitable for climate modeling given the proper atmospheric profiles of temperature and absorbing gases and aerosols. However, such sophisticated methods are computationally time consuming and ill-suited for simple vertically-averaged models or diagnostic studies. The alternative of empirical expressions for F is plagued by observational uncertainty which forces the functional forms to be very simple. We develop, a parameterization of climatological F by curve-fitting the results of a detailed radiative transfer model. The parameterization comprises clear-sky and cloudy-sky terms. Only two parameters are used to predict clear-sky outgoing IR irradiance: surface air temperature (T_s) and 0–12 km height-mean relative humidity (ˆRH). With this choice of parameters (in particular, the use of ˆRH instead of precipitable water) the outgoing IR irradiance can be estimated without knowledge of the detailed temperature profile or average lapse rate. Comparisons between the clear-sky parameterization and detailed model show maximum errors of ∼10 W m⁻² with average errors of only a few watts per square meter. Single-layer “black” clouds are found to reduce the outgoing IR irradiance (relative to clear-sky values) as a function of T_s − T_c, T_c and ˆRH, where T_c is the cloud-top temperature. Errors in the parameterization of the cloudy-sky term are comparable to those of the clear-sky term.