Retrieval of cloud column susceptibilities of water clouds using satellite data

This study presents observed results of enhanced cloud droplet concentration on water cloud albedo on a near-global scale using ISCCP data. A new parameter, cloud column susceptibility, is introduced so that no assumption about the value of the liquid water content, which varies in a wide range, is necessary in the retrieval using satellite data. This will greatly reduce the uncertainties. Two approaches are used in retrievals of the relative and absolute cloud susceptibilities. The first approach assumes contant liquid water content and the second approach uses regression technique to avoid this assumption. The retrieved cloud column susceptibilities are compared with case studies in previous investigations and show broad agreement. The results also show that the cloud susceptibility is larger over remote ocean areas than over continents. Actually, cloud column susceptibilities within most continents are already close to saturation so that, on average, cloud albedo is insensitive to changes in cloud droplet concentrations. The results using regression technique are larger than those based on the assumption of constant liquid water path. The derived relationships such as column susceptibility and column droplet concentration, column susceptibility and cloud albedo, and the distribution of column susceptibility can be used as basis for model studies and guidance for selection of future field campaigns.