Impact of large-scale meteorology on the analysis of satellite measurements of aerosol properties near clouds and far from clouds

Aerosol measurements in the vicinity of clouds play a crucial role in understanding aerosol-cloud interactions and assessing aerosol radiative effects. Numerous analyses of remote sensing observations have shown that aerosol particle populations are quite different in clear areas near clouds and in areas lying far from clouds. These studies suggested that much of the differences come from local changes in a transition zone between cloudy and clear air. This transition zone is created by aerosol humidification as well as by processes such as in-cloud aerosol generation or the detrainment of small evaporating cloud elements.

This presentation explores the possibility that some differences between aerosol statistics near clouds and far from clouds does not come from local changes within individual cloud fields but rather from building statistics by sampling systematically different large-scale meteorological conditions near clouds and far from clouds. The statistical analysis of large MODIS and CALIOP datasets shows that both local changes and differences between the sampled large-scale meteorological conditions contribute significantly to the overall statistical differences between aerosol properties observed near clouds and far from clouds. The presentation will discuss how the sampling issues in the statistical analysis can lead to misinterpretation of the aerosol changes in the vicinity of clouds as aerosol-cloud interaction and how these issues can be removed by properly stratifying the data by large-scale meteorology.