HR: 16:30h
AN: A14A-03    [Abstracts]
TI: Impacts of 3-D radiative effects on satellite-based cloud screening and aerosol optical depth (AOD) retrieval and their consequences on comparing AOD form different satellite sensors
AU: * Di Girolamo, L
EM: larry@atmos.uiuc.edu
AF: University of Illinois at Urbana-Champaign, Deparment of Atmospheric Sciences 105 South Gregory Street, Urbana, IL 61801, United States
AU: Yang, Y
EM: yyang@climate.gsfc.nasa.gov
AF: NASA/GSFC and UMBC/GEST, Code 613.2, Greenbelt, MD 20771, United States
AB: Several recent studies comparing global and regional aerosol optical depths (AODs) derived from space-based sensors have revealed large differences amongst the different sensors. These studies all point to differences in cloud screening strategies as one of the main culprits behind these differences, but differences in strategy on how the remaining clear pixel radiances are sampled for reporting AODs over a domain (typically 10 to 100 km in scale) should also be recognized. Cloud climatologies reveal that these domains often contain clouds, especially over ocean (i.e., > 95% of the time for 10 km scale domain), implying that clear pixels (~ 1 km scale) identified within the domain over which the AOD is to be reported is often near (i.e., a few km) cloud. This proximity of clear pixels to clouds implies the need to examine the impact of 3-D radiative interaction between clear and cloudy regions on AOD retrievals. We present the first detailed examination on how 3-D radiative transfer impacts satellite cloud screening and the subsequent AOD retrieval applied to a sample of the remaining clear pixels. The 3-D radiative transfer through predefined heterogeneous boundary-layer cloud fields embedded in a range of horizontally homogeneous aerosol fields have been carried out to produce synthetic satellite images. Our simulations are numerous, but they are restricted to a wavelength of 0.67 μm, and the procedures of cloud screening and AOD retrieval are applied to these single channel synthetic images. While not representative of any real operational algorithm, physical insight on the connection between cloud screening and AOD retrievals caused by 3-D radiative effects was gained. We show that significant overlap between the radiance distribution of clear and cloudy sky exists, the degree to which depends on many factors (resolution, solar zenith angle, surface reflectance, aerosol optical depth, cloud top variability, etc.). The 3-D radiative pathways that lead to this overlap were examined, revealing that the darkening of clouds by the shadow and leakage pathways can cause clouds of high optical depths (up to 5 in our simulations) to fall within the overlap region, and that the surface-cloud interaction pathway plays an important role in the brightening of clear regions. Large (up to 100's %) systematic errors in AOD retrievals were observed that depended on the details of the cloud mask and the factors that influence the clear/cloud radiance overlap, especially the solar zenith angle. Different sampling strategies commonly employed by modern sensors in producing domain-averaged AOD were performed showing that AOD retrievals on the domain-averaged radiances from all clear pixels produced the smallest AOD biases with the weakest (but still large) dependence on solar zenith angle. AOD biases tended to peak in the solar zenith angle range of 30° to 50°, and remained large even when perfect cloud screening was specified. Our results suggest that the dependence of AOD bias with solar zenith angle may, in part, explain some of the differences observed in AOD climatologies derived from sensors in different orbits, as does the different sampling strategies employed by the different sensors in deriving domain-averaged AOD.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0321 Cloud/radiation interaction
DE: 0360 Radiation: transmission and scattering
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting