Speaker: Prof. Stephan Fueglistaler (Princeton)
Abstract: Water plays a key role for climate, yet our theoretical understanding of its abundance in the atmosphere - both as vapour and condensate (clouds) - remains incomplete. In this talk, I will focus on water entering the stratosphere. Although a very specific problem, I will show that water entering the stratosphere has a few properties that render it highly interesting and promising for theoretical progress:

(i) its abundance is controlled by properties in a relatively confined region around the tropical tropopause;

(ii) there is a clear and strong signal of variability on a range of timescales (seasonal to interannual); and

(iii) the range of dynamical variability is dominated by two processes that are to leading order zonally homogeneous (QBO and strength of residual circulation), and one process that is zonally asymmetric (ENSO). This palette of dynamical variability is much smaller than for the corresponding problem in the troposphere.

I will show modeling results based on trajectory calculations that successfully reproduce much of the observed variability, and will argue that the unexplained residual may be key to suspected long-term trends. Hence, these failures may allow for the first time to precisely pinpoint deficiencies in our understanding of the governing processes. Further, I will show that even when we have a good model for atmospheric water vapour and clouds that reproduces observations correctly, much work remains to be done to develop a theory with predictive skill.

Speaker's website: http://www.princeton.edu/aos/people/faculty/fueglistaler/