Case studies of aerosol remote sensing in the vicinity of clouds

Studying the spatial variability of aerosol properties in the vicinity of clouds is essential to our ability to determine aerosol direct and indirect effects on climate. In this paper, we describe aerosol observations collected near cloud edges by an airborne Sun photometer over dark ocean waters. Focusing on case studies of aerosol measurements near eight cloud edges within a dissipating stratiform cloud deck, we compare the airborne Sun photometer observations to retrievals of aerosol properties using the standard Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol algorithm applied to 500-m-resolution MODIS spectral reflectances. We find a persistent, spectrally neutral increase in the Sun photometer–derived aerosol optical depth (AOD) of up to 10% (0.015) in the 2-km distances closest to the edges of several distinct clouds. At midvisible wavelengths, the MODIS AOD retrievals show similar increases toward cloud edges, although a larger increase in AOD is found in the MODIS along-scan direction. At shortwave infrared (SWIR) wavelengths (1240–2130 nm), the MODIS-derived AOD increases near cloud edges are of the order of 0.03 and as such three times as large as the Sun photometer–derived values. Hence, in contrast to recently discussed ‘‘bluing’’ of aerosols near cloud edges, i.e., a preferential apparent increase in the visible reflectances of clearsky pixels due to 3-D radiative transfer effects in the vicinity of clouds, we find a ‘‘reddening’’ of aerosols in the MODIS 500-m-resolution aerosol retrievals near clouds. This ‘‘reddening’’ in our study can be traced to larger absolute increases in SWIR reflectances when compared to visible reflectances near clouds, which in turn seem to stem from larger electronic cross talk in the MODIS SWIR bands (5–7). We note that the lack of ‘‘bluing’’ in our MODIS observations is likely due to the small geometric and optical thicknesses of the clouds considered.