The combustion of fossil fuels releases not only carbon dioxide, which warms the climate through the greenhouse effect, but also sulfur dioxide, which produces atmospheric aerosol that can cool the climate through two processes: First, by directly scattering solar radiation back to space, and second, by increasing cloud reflectivity through increased droplet concentrations. Because marine stratiform clouds cover a third of the oceans and reflect much more solar energy than the ocean surface, they play an important role in the global radiative heat budget. Therefore, reliable predictions of global climate change depend on understanding the effects of aerosol pollution on marine clouds.
Ship tracks, the long-lived cloud lines that form downwind of ships due to aerosols in ship exhaust, provide a graphic example of the effects of pollution on the reflectivity and lifetime of clouds. Figure 1 is a satellite image (provided by the Naval Postgraduate School) of a large number of ship tracks off the coast of Washington State (which appears in green). It has been generally accepted that aerosols from ships can produce more but smaller cloud drops, thereby enhancing cloud reflectivity, as depicted in Figure 2 . Although measurements of ship tracks have been made, theoretical understanding of them has been rather limited. We have developed a sophisticated computer model at NASA Ames Research Center to better understand the complex interactions between aerosols and clouds in the marine atmosphere.
Through simulations of observed cases, the model has revealed why there are two types of ship tracks. The more common type of ship track is evident only at near-infrared wavelengths, as seen in Figure 1 . Our model shows that this type of ship track results simply from increased concentrations of smaller cloud droplets, which agrees with the previously accepted hypothesis. However, another type of ship track is seen at visible wavelengths, appearing in regions where the surrounding cloud reflectivity is relatively low. Our model simulations show that this rarer type of ship track results from an interaction between droplet concentrations and vertical mixing in the marine atmosphere. We have recently discovered that at very low droplet concentrations a cloud layer can collapse to a shallow fog. This happens when the cloud layer cannot maintain the vertical mixing (due to infrared cooling at cloud top) that supplies its moisture from the ocean surface. The addition of aerosols from ship exhaust into such a fog can lead to the formation of a visible cloud line downwind of a ship. The incorporation of such interactions between aerosols and clouds into large-scale atmospheric models should improve marine weather forecasts and global climate predictions.
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