Wednesday, 9 July 2014
Marine stratocumulus clouds, common off the western coast of continents, are climatologically important because they reflect significant amounts of shortwave radiation back to space, effectively cooling the planet. Because the reflective properties of these clouds depend fundamentally on the distribution of liquid water drop sizes, it is important to have very precise information regarding how this distribution varies over space and time. As these clouds can cover thousands of square kilometers, it is difficult to get adequate sampling using in situ airborne probes. Here we will describe new, high-resolution measurements of marine stratocumulus clouds made by the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), which files on the NASA high-altitude ER-2 aircraft. AirMSPI is an 8-band imager sampling in the UV-Visible-Near Infrared spectral range, with polarimetric observations at 470, 660, and 865 nm. The gimbal mounting of AirMSPI provides different modes of observation that can sample view angles from ±67° relative to nadir. From the 20 km vantage point of the ER-2, AirMSPI can observe hundreds of square kilometers with spatial resolutions ranging from 10 to 25 m.
During a number of flights from Palmdale, California, AirMSPI made sequences of observations of marine stratocumulus, sampling the clouds both near the coast and farther offshore. These observations were made over the span of several hours, providing an opportunity to study temporal changes in the clouds as well. Information on the droplet size distribution at the cloud top is derived using polarimetric observations of the glory, cloudbow, and supernumerary bows in the manner pioneered by POLarization and Directionality of the Earth's Reflectances (POLDER) instrument scientists. Where possible we compare and contrast the AirMSPI retrievals with observations made by other sensors.
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