Marine boundary layer albedo continuum investigations utilizing active and passive remote sensing data

Planetary albedo is a key parameter in determining the Earth's radiation balance. Top-of-atmosphere (TOA) albedo calculations are based upon reflectivity contributions from clear and cloudy atmospheric states. However, a recent study presented regional distributions of a TOA reflectivity metric, which revealed a non-zero minimum between the clear and cloudy modes of the distribution. The study also found this minimum, or continuum region, comprised mainly optically thin clouds. Results from space-borne passive remote sensing data suggest that approximately one-third of marine low-clouds between ±45° have optical depths less than 3, and contribute approximately 10% to cloud albedo. Modeling studies show that the albedo of these thin clouds is more susceptible to microphysical and macrophysical changes in their environment, than the albedo of optically thicker clouds. Although some space-borne passive remote sensors (e.g. the Moderate Resolution Imaging Spectroradiometer (MODIS)) have the sensitivity to detect optically thin clouds, very little is currently known about the spatiotemporal distribution of the continuum region features.

The Cloud-Aerosol Lidar with Infrared Satellite Observation (CALIPSO) satellite was launched in June 2006, carrying the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. CALIOP non-polar ocean data will be used for this work, providing high vertical and horizontal resolution TOA 180° backscatter data, with no contamination from surface return. Vertically resolved information is key to understanding not only the vertical extent of these continuum region features, but also their vertical location relative to adjacent aerosol layers. The aerosol are potential sources of cloud condensation nuclei. Air-borne High Spectral Resolution Lidar data will also be utilized to investigate continuum region features at the higher horizontal resolution of, approximately, 100 m compared to 330 m for CALIOP. The CALIOP footprint is 70 m in diameter, therefore, MODIS data will be used to provide a broader geographical context for the analysis, e.g., to establish whether continuum region features are typically associated with shallow cumulus clouds or expansive stratus decks.

The primary tool for the analysis is the vertically integrated attenuated backscatter (IABS) signal. Initial results indicate a continuum region is clearly evident in CALIOP IABS distributions, on both global, and regional scales.