Characterization of Super‐Cooled Liquid Water Clouds Using the Research Scanning Polarimeter Measurements

Abstract:

Super‐cooled liquid water (SCW) clouds, where liquid droplets exist at temperatures below 0^oC, impact both the radiative budget and the development of precipitation. They also present an aviation hazard due to their role in aircraft icing. The two 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) provided a unique opportunity to observe SCW clouds from the high-altitude airborne platform of NASA's ER-2 aircraft. We present an analysis of the measurements made by the Research Scanning Polarimeter (RSP) during these experiments. This instrument measures both polarized and total reflectance in 9 spectral channels with central wavelengths of 410, 470, 555, 670, 865, 960, 1590, 1880 and 2250 nm. The RSP is a scanning sensor taking samples at 0.8^o intervals within 60^o from nadir in both forward and backward directions. This unique high angular resolution allows for characterization of liquid water droplet size using the rainbow structure observed in the polarized reflectances in the scattering angle range between 135^o and 165^o for every pixel independently. Simple parametric fitting algorithms applied to the polarized reflectance provide retrievals of the droplet effective radius and variance assuming a prescribed size distribution shape (gamma distribution). In addition to this, we use a non-parametric method, Rainbow Fourier Transform (RFT), which allows us to retrieve the droplet size distribution itself. The latter is important in the case of SCW clouds, which often have complex spatial and microphysical structure. For example the measurements made on 22 September 2013 during SEAC4RS indicate a cloud that alternates between being in glaciated and liquid phases, with super-cooled liquid drops at altitudes as high as 10 km, which correspond to temperatures close to the homogeneous freezing temperature of pure water drops (about -38^oC). The multi-modal droplet size distributions retrieved from RSP data in this case are consistent with the multi-layer cloud structure observed by correlative Cloud Profiling Lidar measurements.