Simultaneous retrieval of aerosols lofted above clouds during the MILAGRO field campaign

Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. Even the sign of the aerosol DCF in the presence of clouds has yet to be resolved. The Aerosol Polarimetry Sensor (APS), on the upcoming NASA Glory mission, has the potential to retrieve both cloud and aerosol properties because of its polarimetric, multiple view angle, and multi spectral observations. The APS airborne prototype is the Research Scanning Polarimeter (RSP), which has similar characteristics. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On March 13th, the RSP overflew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. In a cloud free area several minutes later (and about 100km North) the aircraft spiraled down through the aerosols to find their vertical distribution with an onboard sun photometer. Aerosols encountered during MILAGRO were a complicated mix including urban/industrial particles from Mexico City, smoke from local agricultural fires, and dust from high altitude arid regions. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the rainbow and other features in the polarized reflectance. The selected cloud is then used as a starting point for optimization using a multiple scattering radiative transfer model, which is optimized to find the best aerosol and cloud match. Results are discussed in terms of the suitability of such an approach in systematic APS measurements and an error analysis that investigates the role of model assumptions, such as cloud or aerosol layer height.