Radiative impacts of tropical thin cirrus clouds

Tropical thin cirrus clouds are frequently observed. A previous study based on the MODIS band 1 (0.66μm) and band 26 (1.38μm) reflectances suggests that about 40% of the Moderate-Resolution Imaging Spectrometer (MODIS) pixels flagged as clear sky contain detectible thin cirrus clouds. Thin cirrus clouds absorb longwave radiation and emit radiation at very low temperatures. This property can lead to local heating and a net positive cloud forcing. Given the high occurrence of tropical thin cirrus clouds, the effect these clouds on radiation budget can be significant.

In this study, we simulate the outgoing longwave radiation (OLR), radiative flux, and heating rate at each atmospheric level using the 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 parameterized with respect to effective particle size and wavelength. In this model, the atmosphere is divided into 100 layers and a thin cirrus clouds layer is inserted as one layer. The radiative heating rate is calculated as a function of the optical depth of thin cirrus clouds. Furthermore, the relationship of OLR and optical depth is investigated.