Philip Stier

Abstract:

Previous global satellite studies into the indirect aerosol effect have relied on determining the sensitivity of derived Cloud Droplet Number Concentration (Nd) to co-located Aerosol Optical Depth (AOD). These studies generally find a positive Nd sensitivity to AOD changes over ocean, but some find a negative sensitivity over land, in contrast to that predicted by models and theory. Here we investigate the Nd sensitivity to AOD in different cloud regimes, determined using a k-means clustering process on retrieved cloud properties. We find the strongest positive Nd sensitivity in the stratiform regimes over both land and ocean, providing the majority of the total sensitivity. The negative sensitivity previously observed over land is generated by the low cloud fraction regimes, suggesting that it is due to the difficulty of retrieving Nd at low cloud fractions. When considering a mean sensitivity, weighted by liquid cloud fraction to account for sampling biases, we find an increased sensitivity over land, in some regions becoming positive. This highlights the importance of regime based analysis when studying aerosol indirect effects.

Abstract:

Scales of spatial variability of black carbon (BC) aerosol plumes are quantified during the HIPPO aircraft campaign, which flew multiple missions from pole-to-pole over the Pacific ocean. During the first three missions of HIPPO, over 400 vertical profiles of BC concentrations were obtained using a Single Particle Soot Photometer (SP2). In this work, a total of 100 plumes are identified and analyzed. Due to the nature of the HIPPO flight track, the plume length scale is defined along the slanted flight track, having both vertical and horizontal components. These plumes comprise 57% of the total BC mass measured and have a median scale of 113 km. An analysis of BC variability based on autocorrelation functions confirms that most of BC's variability exists on scales similar to the majority of measured plume scales, with a range of 85-155 km. The plume scales are compared to an effective along-track global circulation model (GCM) resolution, which ranges from 20 km for low altitudes and steep ascents to 230km for high altitudes and shallower ascents. The results suggest that plumes characterized predominantly by their horizontal variation at these scales are too small to be captured by GCMs running at resolutions currently suitable for climate simulations. © 2012. American Geophysical Union. All Rights Reserved.