A little behind schedule, I finally found time to read the article in the July 2010 edition of Physics Today “Touring the atmosphere aboard the A-Train” by Tristan S. L’Ecuyer and Jonathan H. Jiang. I think this article is a worth-while read, telling a fascinating story about how new satellite missions lead to greater understanding of our climate system.
The so-called ‘A-train’ consists of an array of satellites that follow in close succession, as shown in the figure above, each in a Sun-synchronous orbit, 705 km above the ground. The clever thing about this is that they all carry different instruments with minimal interference between different types of instruments, and together they provide a platform for many diverse types remote sensing, and hence complement each other in terms of looking at different physical aspects.
Among the results from the A-Train, according to L’Ecuyer and Jiang, is that a 16% decrease in the summer-time cloudcover over the Arctic played a significant role the well-known September 2007 Arctic sea-ice minimum. This comes in addition to the anomalous winds, and was news to me. But it makes sense: less cloudiness has resulted in greater absorption of short-wave radiation (sunlight) at the surface, providing increased heat for ice melting.
Another important topic that the data from the A-train can elucidate is the relationship between clouds and aerosols. According to L’Ecuyer and Jiang, high concentrations of aerosols cause clouds, that are made of liquid drops, to rain less frequently, grow deeper, have smaller cloud drops, and appear brighter from above.
The data from the A-train has also provided more insight into the question about how areosols affect ice clouds, suggesting that polluted ice clouds generally contain smaller particles and produce less precipitation than ice clouds embedded in cleaner air.
Some of these dependencies between aerosols and clouds have been predicted by researchers previously, and these are now confirmed by the remote sensing data from the ‘A-train’. For instance, this type of predictions are inserted into so-called ‘cloud parameterisation’ of the global climate models that try to include so-called ‘indirect aerosol effects’, and have some effect on climate simulations. This demonstrates real value of both the data and theory.