Our ability to assess aerosol radiative effects using remote sensing data depends on the discrimination between cloudy and cloud-free viewing elements. In the case of the direct aerosol radiative forcing of climate it is essential to avoid sub-pixel cloud contamination of pixels used for aerosol clear-sky retrievals, while including areas of increased aerosol concentration in the vicinity of clouds. In the case of the indirect and semi-direct aerosol effects on climate it is also mandatory to distinguish cloudy from cloud-free pixels, while ensuring spatial correspondence and hence interaction of aerosol and cloud airmasses studied. Therefore, studying the spatial variability of aerosol properties in the vicinity of clouds is essential for our ability to separate aerosol and cloudy pixels in remotely sensed data and hence to determine aerosol effects on climate at different spatial scales.

The current generation of satellite aerosol sensors, such as MODIS [Kaufman et al., 1997] and MISR [Kahn et al., 2001] aboard the Terra satellite, MODIS aboard Aqua, and OMI aboard Aura [Levelt et al., 2006] are much more capable of detailed global aerosol observations than the previous generation, including TOMS and AVHRR. The advantages of the new sensors include their improved spectral coverage, narrower bandwidth of the individual channels and improved spatial resolution. Spatial variability in radiance fields observed by passive satellite sensors is frequently used in the separation of cloud and aerosol signals, under the assumption that most clouds exhibit larger spatial variability than aerosols. However, only a small number of studies have been directed at assessing the variability of aerosol optical properties in the vicinity of clouds [e.g., Redemann et al., 2006], because only a small number of suitable instruments exists.

In the last two years of his life, Yoram Kaufman had a renewed interest in the issue of aerosol-cloud separation in remote sensing, as evidenced by his 2005 and 2006 publication records. For example, in Kaufman et al. [2006] he pointed out that aerosol and cloud fields are often spatially correlated and that therefore, “rigorous cloud screening can systematically bias toward less cloudy and drier conditions, underestimating the average aerosol optical thickness (AOT)”. Also, several studies [e.g. Kaufman et al., 2005; Redemann et al., 2006], found that differences between MODIS and suborbital AOD measurements were correlated with MODIS-derived cloud fraction, indicating the need for a renewed focus on cloud screening.

There are many pitfalls to remote sensing studies of aerosols in the vicinity of clouds. For example, Wen et al. [2006] found that 3-D radiative transfer effects in a broken Cumulus cloud field embedded in background biomass burning aerosol with an AOD of 0.1 could reach as far as 3 km away from clouds and cause an overestimate of the apparent clear sky reflectance that would lead to an overestimate in AOD by 40%.

In this paper we will describe our recent efforts in studying the aerosol-cloud boundary using combined suborbital and satellite observations. In a number of recent field campaigns, the NASA Ames Airborne Tracking Sunphotometer, AATS-14, has observed strong gradients in AOD in the immediate vicinity of clouds. For example, in a preliminary study of AOD data near stratus cloud edges collected in the EVE (Extended-MODIS-λ Validation Experiment) experiment off the Northern CA coast, we found that in 75% of the cases there was an increase of 5-25% in AOD in the closest 2 km near the clouds. Interestingly, the AOD increases at the cloud edges all occurred without increases in particle size (as inferred from AOD wavelength dependence). Concurrently, the MODIS-observed mid-visible reflectances in the vicinity of the suborbital cloud observations also show an increase with decreasing distance to cloud edge. Possible causes include 3-D radiative effects [Wen et al., 2006], but also the increased variability in the aerosol fields near clouds as indicated by the suborbital observations. The central issues in this paper are the detailed description of the suborbital observations and the discussion of the possible causes of the increased variability in the MODIS reflectances near clouds.

References

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