Global Ice Cloud Climatological Properties and Radiative Effects Based on Cloudsat and Calipso Measurements and Radiative Transfer Modelling

Abstract:

Ice clouds play an important role in Earth radiation balance by reflecting solar and absorbing thermal radiation, which is expressed as albedo and greenhouse effects, which cause significant differential atmospheric heating and cooling in both horizontal and vertical directions. Two co-orbital sensors, the CloudSat radar and the CALIPSO lidar, provide global profiles for the first time of atmospheric ice clouds. A combination of radar and lidar is useful for examining clouds of varying optical depth, as the radar excels in probing optically thick clouds while the lidar is better suited to the thin clouds. With these joint measurements from the CloudSat radar and the CALIPSO lidar, ice cloud climotological properties are studied and radiative effects are calculated for with a wide range of optical depth using libRadtran, a model package for radiative transfer.

Ice cloud climatological studies show that the global mean optical depth and effective radius are around 4 and 48 μm, respectively. Mean ice water path is approximately 110 g m^-2for all measurements. Their occurrence frequencies and ice mass amount distributions do not just depend on seasons, but also rely on the optical depth values. Meanwhile, ice water content and effective radius show different temperature dependent relationships among the tropics, mid- and high-latitudes.

Geographical variations of ice cloud forcing are investigated to discover where warming/cooling by ice cloud occurs, and how these effects are modulated by seasonal and regional variations. Vertical heating rate profiles are also studied to reveal heating/cooling structures for cases such as mid-latitude summer. Meanwhile, ice cloud forcing is calculated as a function of optical depth. Past studies have established in general that thin and high cirrus warm the earth system due to the greenhouse effect, whereas thicker ice clouds are cooling because of stronger solar albedo effect. However, the whole spectrum of ice cloud forcing have yet to be studied quantitatively. This study attempts to find thresholds where ice clouds change from a net warming to a net cooling effect.