Water vapor and climate change
The amount of water vapor in the atmosphere responds sensitively to changes in temperature; it increases by over 20% for a 3K rise in temperature if the relative humidity remains approximately constant. Climate model simulations do predict changes in the distribution of relative humidity, but the overall change is relatively small. This has implications for many aspects of atmospheric dynamics when considering global warming or very warm past climates. An outstanding challenge involves fully incorporating the effect of water vapor and latent heat release into theories of how, for example, the extent of the Hadley cell, the extratropical storm track position, or the strength of extratropical storms change as climate changes. One promising approach for problems involving large-scale eddies is to use an effective static stability that accounts for the asymmetry in latent heating between downward and upward motions.

Precipitation and its extremes
Given the large increase in atmospheric water vapor with increasing temperature, it is natural to ask how precipitation changes. Climate models suggest that both global mean precipitation and the intensity of precipitation extremes will increase in a warmer climate. Local decreases in precipitation are also expected in already dry parts of the world. Quantifying the expected changes in precipitation and in intense precipitation events is a major challenge for climate modeling. We are endeavoring to understand the physical basis for changes in precipitation and its extremes (including both liquid and solid phases) using simulations, theory, and observations.

Extratropical storms
Global warming is expected to cause a poleward shift in extratropical storm tracks. The change in the intensities of extratropical storms is less clear, and likely depends on the season or hemisphere under consideration. The kinetic energy of transient eddies is found to scale with the available potential energy of the mean state in simulations with idealized and comprehensive climate models. Increased latent heating in warmer climates is taken into account by using a moist available potential energy. We seek to further quantify changes in storm tracks arising from influences of changing greenhouses gases and sea-ice, and to understand if abrupt changes in behavior can occur.