Retrieval of Cloud Optical Depth and Effective Radius from SSFR Irradiance Measurements in the Presence of Overlying Aerosol Layers During NEAQS-ITCT and Comparison to MODIS Cloud Product

Aerosol effects on cloud properties, the so-called aerosol indirect effect, remain highly uncertain. Theoretical calculations by Haywood et al. [2004] examine the effect of overlying absorbing aerosol layers on cloud retrievals from satellite-measured reflectance and suggest that retrieved cloud optical depth and liquid water path are subject to systematic low bias when aerosol influences are excluded. Retrievals of cloud effective radius were found to be subject to either high or low bias, depending upon the radiance wavelength pair used in the retrieval, and the type of aerosol layer (biomass burning aerosol, or mineral dust) overlying the cloud. This study, although specific to satellites, raises the question that an understanding of how aerosols affect cloud retrievals of effective radius and optical depth from any remote sensing instrument is important when quantifying aerosol indirect effects. We made spectral measurements of upward and downward irradiance with the Solar Spectral Flux Radiometer (SSFR) during the New England Air Quality Study - Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) onboard the Sky Research Jetstream (J-31) aircraft. These irradiance measurements are used in conjunction with a SSFR-specific radiative transfer model to retrieve cloud effective radius and optical depth in the presence and absence of overlying aerosol layers, as measured by the Ames Airborne Tracking Sunphotometer (AATS-14) integrated onboard the same aircraft. We show cloud retrieval results for two days: 15th July, and 20th July 2004, where SSFR measurements were made between cloud and aerosol layers, and then above both the cloud and aerosol layers. Comparisons of retrieved cloud optical depth and effective radius are shown between the SSFR and coincident overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the NASA-EOS Terra and Aqua satellites. We examine the effects of aerosols on SSFR cloud retrievals by including aerosols in case-specific forward models. For these two dates, neglecting aerosols in the SSFR forward model led to negligible difference in retrieved cloud optical depth, but cloud effective radius was larger than when aerosol particles were included. We also examine the sensitivity of the cloud retrievals to assumptions made in aerosol single scattering albedo.