Passive determination of cloud physical thickness and droplet number concentration

A particularly important aspect of the remote sensing of clouds is determining the number concentration of cloud droplets, which provides a more robust diagnostic of the indirect effects of aerosols on clouds than droplet size itself. Active remote sensing by radar provides the most direct and generally applicable approach to estimating the number concentration of cloud drops. Alternatively lidar can be used to determine the extinction coefficient near cloud top as a first step in determining droplet number concentration. In either case other observations (e.g. near infrared radiances or polarization observations in the rainbow) are required to constrain the size of the cloud droplets, if the number concentration is to be estimated. A class of cloud for which the indirect effects of aerosols are of particular interest is stratocumulus clouds. Unfortunately space-borne cloud radars have problems detecting the small droplets near cloud base, particularly given how close the clouds are to the surface and the tops of stratocumulus clouds are strongly affected by the entrainment of free tropospheric air raising concerns regarding the relevance of a cloud top estimate of number concentration from lidar to the bulk of the cloud. For these reasons alternative approaches to determining cloud droplet number concentration from remote sensing observations merit consideration. In this paper we describe a method for determining the amount of gaseous absorption above and within clouds separately through the use of polarized and unpolarized radiance observations in the vicinity of the rainbow. Since the absorption optical depth within the cloud is proportional to the pressure thickness of the cloud this method allows us to determine the physical thickness of the cloud. By combining the physical thickness with droplet size and cloud optical depth estimates the number concentration of cloud droplets can then be estimated. Here we discuss the uncertainties and limitations of this approach to the determination of cloud physical thickness and droplet number concentration and present some sample results from the Coastal Stratocumulus Imposed Perturbation Experiment that use observations made by the Research Scanning Polarimeter (RSP) in the water vapor absorption band at 960 nm. The RSP serves as an airborne simulator for the Aerosol Polarimetry Sensor on the upcoming NASA Glory mission that will make similar observations at 910 nm that will be used to provide cloud pressure thickness and droplet number concentration estimates.