Quantifying the amount of ice in cirrus clouds

How much cloud ice is in the atmosphere, globally? How does one begin to answer this question? Clouds are currently the largest source of uncertainty in climate models, and the ice water content (IWC) of cold cirrus clouds is needed to understand the total water and radiation budgets of the upper troposphere and lower stratosphere (UT/LS). The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, originally a "pathfinder" mission only expected to last for three years, has now been operational for almost eight years. Lidar data from CALIPSO can provide information about how IWC is vertically distributed in the UT/LS, and about inter-annual variability and seasonal changes in cloud ice. However, cloud IWC is difficult to measure accurately with either remote or in situ instruments because IWC from cold cirrus clouds is derived from other measured quantities, such as the particle cross-sectional area or visible extinction coefficient. Assumptions must then be made about the relationship between the area and the volume and density of ice particles with various crystal habits and formation histories to derive the ice water content. Recently there have been numerous aircraft field campaigns providing detailed information about cirrus ice water content from cloud probes. This presentation will evaluate the assumptions made when creating the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) global IWC data set, using an ensemble of recent aircraft particle probe measurements from various cirrus cloud types and locations. Among these is the assumption that all randomly-oriented ice clouds have the same extinction to backscatter coefficient ratio. Additionally, several empirical parameterizations of ice water content from retrieved visible extinction coefficients will be evaluated using independent probe data from recent field campaigns. Using this information, estimates of regional uncertainties in the global UT/LS IWC will be provided in addition to IWC vertical extent and variability observed by CALIOP.