Temperature dependence of cirrus extinction: Implications for climate feedback

Abstract

The measured temperature dependence of infrared absorption in cirrus cloud obtained from previous extensive lidar and radiometer (LIRAD) observations of cirrus is used to investigate the sensitivities of changes in cirrus optical properties to changes in global temperature. Values of infrared absorption and ice water content calculated previously from observations of cirrus cloud microphysics are also used to investigate climate sensitivities. The values of calculated mean infrared absorption give a very similar temperature dependence, and thus climate sensitivity, to the LIRAD results. Values of mean ice water content calculated from the observations of cloud microphysics are compared with the available ice water content calculated from moist adiabatic ascent through a vertical cloud depth of 300 m, and a similar temperature dependence is found in both cases. However, taking into account the observed cloud depths of cirrus, considerable dilution of ice water content below adiabatic values is obviously occurring. The temperature sensitivities of extinction (or absorption) coefficients (calculated from a combination of LIRAD and microphysics results) are found to be generally less than the temperature sensitivities of ice water contents, due to an increase in cloud particle mode radius with temperature. The temperature sensitivity of cirrus extinction is found to vary from 0.033°C−1 at −22.5°C to 0.200°C−1 at −72.5°C. Equivalent sensitivities for cirrus optical depth are influenced by observed changes in cirrus cloud depth with temperature and vary from 0.003°C−1 at −22.5°C to 0.241°C−1 at −72.5°C. As high cold cirrus clouds are considered to cause a positive feedback with global temperature change, such a feedback is seen to become particularly strong at the lowest temperatures. A simple method of parameterization of cirrus visible and infrared optical properties in terms of cloud temperature is presented, thus making it possible to introduce cirrus optical properties into a numerical model without requiring knowledge of the ice water content.