Using A-Train satellite data to investigate the relationship between cloud ice water path and cloud radiative effects

The active remote sensors in the NASA A-Train offer vertically resolved observations of hydrometeors. We obtain ice microphysical properties from the CloudSat 2C-ICE dataset, which combines information from the CloudSat radar and CALIPSO lidar to retrieve the ice water content. We find that the distribution of ice water path (IWP) is highly non-Gaussian. Because of this highly skewed distribution, the cirrus that contribute most to the mean radiative forcing have little to do with the mean of the IWP distribution. Therefore using the mean IWP as a value for comparison with and among global climate models is not a useful evaluation with respect to the radiative effects of cirrus. We also investigate the influence of each instrument (radar and lidar) to the total (combined radar and lidar) IWP distribution, highlighting the IWP sensitivity thresholds for each instrument. In this study we explore the physical relationship of cloud radiative effect (CRE) as a function of the cloud IWP. We find that the net CRE at the top of the atmosphere (TOA) increases with increasing cloud IWP up to 40 g/m^2, then begins to decrease and eventually becomes negative at 200 g/m^2. It remains to be determined whether global climate models show a similar physical relationship between the microphysical quantity of cloud IWP and the net CRE at the TOA. We also find that the solar and infrared CREs are highly dependent on the IWP, changing at different rates with increasing IWP. To evaluate which high clouds are most important in terms of warming the atmosphere, we consider their frequency of occurrence and net CRE at the TOA. We find that cirrus with an IWP around 20 g/m^2 contribute most to the heating of the upper troposphere. This IWP that is most significant to the cloud radiative effects is to be contrasted with the mean of the IWP distribution near 300 g/m^2. While cirrus clouds with IWP near 20 g/m^2 have a small CRE, their frequency of occurrence results in a significant radiative impact. This result highlights the importance of the lidar measurements for observing the full spectrum of ice clouds that are important to the radiation balance.