Research Overview

Precipitation and its extremes
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. We are endeavoring to understand the physical basis for changes in precipitation and its extremes (including rainfall and snowfall) using simulations, theory, and observations. Our recent results highlight the need to better understand changes in preciptiation extremes in the tropics (Fig. 1) and changes in mean precipitation over land.
Fig. 1 Sensitivity of the 99.9th percentile of daily precipitation to global mean surface temperature under climate change in CMIP5 global climate-model simulations. Shown are the multimodel median (green line with circles) and the full model range (dotted lines). Also shown are sensitivities inferred by constraining the model sensitivities using observations of tropical variability (black line) with a 90 % confidence interval obtained by bootstrapping. See this review paper for details.

The general circulation and water vapor
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.

Moist convection
Moist convection plays a key role in helping to set the mean state of the tropical troposphere, and it is also a basic aspect of weather in both the tropics and midlatitudes. Climate models suggest that the convective available potential energy (CAPE) increases with warming, with implications for convective updraft velocities and severe weather. Recent studies with cloud-system resolving models show that CAPE also increases with warming in the idealized setting of radiative convective equilibrium (Fig. 2). Visualizations of our simulations illustrate that moist convection is deeper and faster at higher temperatures. We are interested in better understanding the increases in CAPE, their effect on moist convection, and the behavior of moist convection in different climates more generally.
Fig. 2 Convective available potential energy (CAPE) versus sea surface temperature (SST) in simulations of radiative convective equilibrium. Also shown (dashed line) is the prediction from a simple theory that assumes the atmosphere remains neutrally stratified with respect to an entraining plume. CAPE is calculated using a vertical integral to the anvil detrainment level. See Singh and O'Gorman 2013 for details.