The Microphysical Feedback of Cirrus Cloud in Climate Change

Abstract

The feedback effects of cirrus clouds in climate change are investigated, taking account of the temperature dependence of both ice water content and size distribution. The sensitivity parameter which is defined as a sum of partial derivatives is calculated with an accurate radiation code applying the 4-stream discrete-ordinates method and the method of exponential-sum fitting for gaseous transmission (ESFT). Microphysical properties of cirrus clouds are parameterized in terms of the cloud temperature from the data of aircraft observations. The value of ∂H/∂[IWC]×∂[IWC]/∂T, i. e., a product of partial derivatives associated with ice water content feedback, is estimated to be 1.3-2.3Wm^-2°C^-1, meaning a positive feedback effect, and the value of ∂H/∂[SD]×∂[SD]/∂T which is associated with size distribution feedback is estimated to be -0.1--1.8Wm^-2°C^-1, meaning a negative feedback effect. The positive ice water content feedback is significantly cancelled by the negative size distribution feedback especially at cloud temperature in the vicinity of -45°C. It is thus shown that the temperature dependence not only of ice water content but also of size distribution is important to estimate the microphysical feedback of cirrus cloud.