Global Ice Cloud Properties and Their Radiative Effects

Ice clouds play an important role in earth radiation balance by reflecting solar and absorbing thermal radiation, the so-called albedo versus greenhouse eects, which cause signicant dierential atmospheric heating and cooling in horizontal as well as vertical directions. Two active sensors onboard A-train satellites, CloudSat radar and CALIPSO lidar, for the rst time provide global proles of atmospheric ice clouds. A combination of radar and lidar becomes the state-of-the-art technique examining clouds of varying optical depth, as the former excels in probing thick clouds while the later is better suited to the thin ones. In this study, ice cloud properties derived by a combination of CloudSat and CALIPSO observations are adopted to character atmospheric ice clouds. Ice cloud climatological studies show that the global mean optical depth and eective radius are around 4 and 48 &mu m, respectively. Mean ice water path is approximately 110 g/m2 for all measurements and approximately 190 g/m2 for cloudy situations (conditional mean). Their occurrence frequencies and ice mass amount distributions do not just depend on their optical depth values, but also rely on seasons and day-night cycle. Meanwhile, ice water content and eective radius show dierent temperature dependent relationships among the tropics, mid- and high-latitudes. Ice cloud radiative eects are obtained by radiative transfer modelling. Simulations show global ice clouds net eects at the top of atmosphere (TOA) may slightly heat or cool the atmosphere-earth system depending on model parameterizations and allowing for uncertainties. Additionally, a cloud forcing spectrum over optical depth at the TOA shows that ice clouds with optical depth <5 display a positive net forcing on a global scale, inducing a warming eect, whereas ice clouds otherwise tend to be cooling. Regionally, ice clouds have a negative net forcing in the mid-latitude warm seasons due to a stronger solar albedo eect but a positive net forcing during cold seasons due to a stronger greenhouse eect. Moreover, ice cloud internal heating rate proles in the atmosphere indicate shortwave heating above but cooling below, whereas the longwave heating pattern is oppositive. This heating structure is regionally and seasonally dependent, and it is associated with optical depth values as well.