Using radiance and polarization measurements from the Research Scanning Polarimeter (RSP) to evaluate ice crystal sizes in CRM simulations of anvil outflow cirrus

Measurements of the Aerosol Polarimetry Sensor (APS), scheduled for launch on the Glory mission at the end of 2010, are expected to significantly improve our knowledge of ice crystals sizes and shapes in ice clouds. APS will provide unique, global measurements of total and polarized reflectance in the visible to infrared wavelength range at over 200 viewing angles per footprint, which contain not only information about cloud optical thickness and ice effective radius, such as retrieved by MODIS, but also significant information about ice crystal shape and roughness, which is crucial for accurately determining radiative properties of ice clouds. An airborne version of APS, the Research Scanning Polarimeter (RSP) has collected measurements in a number of field campaigns, but mainly above clear skies (focusing on aerosols) or warm clouds. Here we analyze the only RSP measurements available over thick anvil clouds, obtained during the 2002 CRYSTAL-FACE campaign. These data are used to evaluate ice properties in subtropical anvil outflow cirrus simulated by a 3D cloud-resolving model (CRM) with size-resolved microphysics, driven with realistic meteorology and using observed background aerosol profiles. Of particular concern in this study are ice crystal sizes produced in fresh anvil outflow and how these sizes vary as a function of distance from the convective core. To ensure the analyses of the RSP data are as consistent as possible with the CRM microphysics and output, we use a forward model to simulate the RSP data based on the CRM in- and output and compare the simulated and real measurements directly in a statistical manner.