How Do Shallow Cumulus Respond to Highly Variable Natural Aerosol? - An Analysis of A-Train Data Downstream of Kilauea

Abstract:

It is commonly accepted that increases in sulfate aerosol over what is typical of the background state causes shallow boundary layer clouds to become brighter and precipitate less efficiently. However, it has been difficult to separate these effects from other mitigating factors that may control both cloud properties and aerosol simultaneously. Mount Kilauea on the island of Hawaii began erupting episodically in 2008 injecting large amounts of sulfur dioxide into the marine boundary layer. Typical peaks in sulfur dioxide injection lasted a few days followed by periods of relative quiet. This somewhat random injection of sulfur dioxide was, of course, independent of changes in the meteorology creating an excellent natural laboratory to investigate the influence of sulfate aerosols on warm shallow marine clouds. Dividing the SO₂ injection rate into 3-day lagged quartiles, we conduct a statistical analysis of A-Train data to examine the effects of SO₂ on cloud properties. As expected, the clouds brighten in the visible and show measurable decreases in column integrated radar reflectivity. However, these changes were subtle. Surprisingly, we find the strongest response in cloud depth. From the lowest SO₂ quartile to the highest, we find that cloud top decreases from 1.75 km to 1.5 km on average. This change in cloud top is accompanied by near constant microwave brightness temperature and a decrease in cloud fraction. We speculate that the decrease in cloud top is the result of enhanced entrainment and/or a reduction in updraft speed due to fewer and weaker precipitation induced cold pools. In any case, it is clear that the response of these clouds to variable sulfate aerosol is more complicated than the simple aerosol indirect effects commonly invoked.