Multi-satellite aerosol observations in the vicinity of clouds

Improved characterization of aerosol properties in the vicinity of clouds is important for better understanding two critical aspects of climate: aerosol-cloud interactions and the direct radiative effect of aerosols. Satellite measurements have provided important insights into aerosol properties near clouds, but also suggested that the observations can be affected by 3D radiative processes and instrument blurring not considered in current data interpretation methods. This presentation will examine systematic changes in particle properties and radiation fields that influence satellite measurements of aerosols in the vicinity of clouds. For this, we will present a statistical analysis of a yearlong global dataset of co-located MODIS and CALIOP observations and theoretical simulations. The results reveal that CALIOP-observed aerosol particle size and optical thickness, and MODIS-observed solar reflectance increase systematically in a wide transition zone around clouds. It is estimated that near-cloud changes in particle populations—including both aerosols and undetected cloud particles—are responsible for roughly two thirds of the observed increase in 0.55 µm MODIS reflectance. The results also indicate that 3D radiative processes significantly contribute to near-cloud reflectance enhancements, while instrument blurring does not.

We will also discuss a simple model to compute clear-sky radiance enhancement due to radiative interaction between boundary layer clouds and the molecular layer above. For the first time, the model has been applied to a full MODIS granule. The process of the correction includes converting CERES broadband flux to visible narrowband flux, computing the clear-sky radiance enhancement, and aerosol retrieval. It was found that correction leads to smaller values in aerosol optical thickness (AOT), the Ĺngström exponent, and the small mode fraction of AOT. The presentation will discuss the first results of implementation of the correction algorithm to MODIS aerosol retrievals near clouds.