Wednesday, 17 January 2007
Radiative effects of tropical thin cirrus clouds
212B (Henry B. Gonzalez Convention Center)
It is important to identify the radiative impacts of thin cirrus cloud considering the high occurrence of this type of cloud. It is found that about 40% of the Moderate-Resolution Imaging Spectroraiometer (MODIS) pixels flagged as clear sky contain detectible thin cirrus clouds. The optical depth is retrieved using MODIS level 2 cirrus reflectance and averaged over 1oX1o boxes. To investigate the radiative effects of thin cirrus cloud we simulate outgoing longwave radiation (OLR), radiative flux, and heating rate using LibRadtran radiative transfer code. The background gaseous absorption properties are taken into account on the basis of the correlated k-distribution method. The single scattering properties of cirrus clouds including the extinction efficiency, single-scattering albedo and asymmetry factor are newly parameterized with respect to the effective particle size (2.0~3100 mm) and wavelength (0.2~100mm). In this model, thin cirrus clouds are inserted in layers from 10 to 12km. The radiative heating rate is calculated as a function of ice water content (IWC) and with respect to particle habit. The cloud radiative forcing is calculated at the top of atmosphere (TOA) and bottom of atmosphere (BOA) as a function of IWC. For the simulation of global distribution of cirrus radiative forcing, the retrieved optical depth of thin cirrus cloud and AIRS atmospheric profile are used.