Bringing together storm tracks and clouds
We often talk about future climate change in terms of "global warming." But when it comes to the impacts of global warming, regional changes in winds and precipitation are more relevant. The latter depend on how the circulation of the atmosphere responds to increasing CO2. A workshop held in August in Grindelwald, Switzerland, looked at one aspect of the atmospheric circulation that is particularly relevant to those that live in the mid-latitudes: the storm tracks. The coupling of storm tracks with clouds received particular attention.
The storm tracks literally are what the word says. The idea is to trace an individual storm on a weather map on which it appears as a region of low surface pressure, with counter-clockwise winds around the pressure minimum in the Northern hemisphere (in the Southern hemisphere the winds are clockwise). By tracing many storms over a long period and superimposing their tracks on the same map, one can identify the regions in which storm activity is particularly high, i.e., the storm tracks. In the Northern hemisphere, there are actually two storm tracks, one over the Pacific and one over the Atlantic. The storms that make up the storm tracks vary in their intensity. The strongest ones are often associated with extreme weather events and bring heavy rainfall, strong surface winds and storm surges. One example is the 1962 North Sea Flood, when the passage of an extreme storm led to dike failures, pushed water into the city of Hamburg, and caused 347 deaths. The Columbus Day Storm along the Pacific Northwest coast and the Ash Wednesday Storm along the mid-Atlantic coast of the Unites States in the same year are other examples.
So one pressing question in storm track research is whether such extreme storms are the harbinger of future climate change, and whether they will occur more often as the planet warms. And, how do clouds determine the answer?
To answer this question, however, it is important to establish a fundamental understanding of the storm tracks and the processes that control them. This was one of the workshop's main motivations. The workshop brought together 100 scientists from all over the world. Each day was devoted to one particular question, with the question of the day posed in the morning and an hour-long discussion in the afternoon. The questions touched upon the processes that lead to the formation of storm tracks, the role of tropical convection, sea ice and the stratosphere, the representation of storm tracks in climate models, certain and uncertain aspects of climate model simulations of future storm track changes, the link between storm tracks and extreme events, and the coupling of storm tracks with clouds.
I here want to focus on the topic of clouds, because it is very much related to my own work and because it emerged as a new and exciting topic. Several talks took a large-scale climate perspective to show that the radiative heating from clouds is crucial for the response of the storm tracks to climate change. Another set of talks took a small-scale weather perspective and discussed how tightly coupled clouds and winds are within an individual storm, and how both radiative heating and the latent heat that is released when water vapor condenses to form a cloud affect the storm intensity and structure.
My own talk adapted the large-scale perspective and was based on a paper that I published with Tiffany Shaw earlier this year in Nature Geoscience (Voigt and Shaw, 2015). In that paper we showed that clouds lead to a poleward shift of the storm tracks under global warming. In this talk, we further showed that this cloud impact stems from a rise in high-level tropical ice clouds, a rise and poleward shift of high-level mid-latitude clouds, and an increase in low-level polar clouds. Crucial aspects of these cloud responses remain uncertain, however, illustrating the need to better understand how clouds will change in the future.
At the same time, much of our understanding of storm tracks stems from "dry" concepts that were developed in a hypothetical atmosphere with no water (and hence no clouds) in it at all. Many talks, including my own, made reference to now classical papers on how the storm tracks respond to idealized climate forcings in a water-free atmosphere (e.g., Butler et al., 2010; Tandon et al., 2011). Indeed, this will be an important line of future research: How can we combine our dry concepts with our growing understanding of clouds, and how does that help us to better predict the response of storm tracks to global warming? While we are only beginning to appreciate the scientific richness and practical relevance of this problem, I think there is good reason to believe that the coming 10 years will see substantial progress in storm track research.
The World Climate Research Program recently established a Grand Challenge on "Clouds, Circulation and Climate Sensitivity" that, among other questions, aims to stimulate research on clouds and storm tracks (Bony et al., 2015). Next year will see a large field campaign, the North Atlantic Waveguide and Downstream Impact Experiment, that will provide climate scientists with unprecedented measurements of cloud processes within single storms by combining airplane and ground-based measurement with high-resolution models. And lastly, the weather and climate communities, which in the past were often disjunct, are now realizing that they can learn from each other. For example, climate models are now used in weather-prediction mode (Tranpose-AMIP; Williams et al., 2013), and the German weather service and the Max-Planck-Institute for Meterology have developed a model that at the same time serves weather and climate purposes (the ICON model).
"The Transpose-AMIP II Experiment and Its Application to the Understanding of Southern Ocean Cloud Biases in Climate Models." J. Climate, 26, 3258–3274. doi: dx.doi.org/10.1175/JCLI-D-12-00429.1