Remote sensing of liquid water clouds using the Research Scanning Polarimeter

We describe algorithms for remote sensing of liquid water clouds using the Research Scanning Polarimeter (RSP) measurements and present the retrieval products including cloud optical thicknesses (COT), droplet size distributions and droplet number concentrations.

The RSP is an airborne prototype for the Aerosol Polarimetery Sensor (APS), which was built for the NASA Glory Mission project. This instrument measures both polarized and total reflectances in 9 spectral channels with central wavelengths of 410, 470, 555, 670, 865, 960, 1590, 1880 and 2250 nm. The RSP is a push broom scanner making samples at 0.8 degree intervals within 60 degrees from nadir in both forward and backward directions. The data from actual RSP scans is aggregated into "virtual" scans, each consisting of all reflectances (at a variety of scattering angles) from a single point on the ground or at the cloud top.

In the case of liquid water clouds the rainbow is observed in the polarized reflectances in the scattering angle range between 135 and 165 degrees. It has a unique signature that is being used to accurately determine the droplet size and is not affected by cloud morphology. Simple parametric fitting algorithm applied to the polarized reflectances provides retrievals of the droplet effective radius and variance assuming a prescribed size distribution shape (gamma distribution). Of particular interest is the information contained in droplet size distribution width, which is indicative of cloud life cycle. In addition to this, we use a non-parametric method, Rainbow Fourier Transform (RFT), which allows to retrieve the droplet size distribution without a parametric model. This possibility is important for studies of non-convective phenomena such as fogs or super-cooled liquid clouds, which may have multi-modal droplet size distributions. The absorbing band (Nakajima-King) method was also applied to RSP total reflectance observations and the results show good agreement with polarimetric retrievals for COT greater than 10. In addition to this, we use adiabaticity assumptions for boundary layer clouds in order to estimate the droplet number concentration, which is the physical quantity directly used in cloud models. These estimates can be verified by in situ measurements when available.

We illustrate our methods on examples from the recent NASA's field campaigns POlarimeter Definition EXperiment (PODEX, based in Palmdale, California, January - February 2013) and Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS, based in Houston, Texas in August - September 2013). During these campaigns the RSP was onboard the NASA's long-range high-altitude ER-2 aircraft together with an array of other remote sensing instrumentation. Correlative sampling measurements from another aircraft were also available. The data obtained during these campaigns provides an excellent opportunity to study cloud properties in variety of locations and atmospheric conditions. We present examples of boundary layer cumulus and stratocumulus clouds, liquid altostratus clouds, and fogs. In the latter two cases the droplet size distribution derived from RFT analysis exhibited multiple modes corresponding to different cloud layers, as supported by the correlative lidar atmospheric profiles.