Improved Understanding of the Moist Dynamics of the Extratropical Storm Tracks and Their Response to Climate Change

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At a fundamental level the frontal weather systems we see on maps can be explained by the theory of baroclinic instability, in which small disturbances grow by extracting energy from the temperature contrast between the warm tropics and the cold poles. The theory works but it neglects the enormous heat release that occurs when water vapor condenses to form clouds and precipitation. It is reasonable to assume that condensational heating is destabilizing but our understanding of its effects on the growth rate, propagation speed, precipitation intensity, and other aspects of weather systems is quite limited. The role of condensational heating is a practical concern as the amount of water vapor in the atmosphere increases with temperature, thus one might expect more intense precipitation and stronger storms as climate warms.Work under this award seeks to develop a theoretical basis for the effects of condensational heating on baroclinic instability and the dependence of these effects on global temperature. One target of the study is diabatic Rossby vortices (DRVs, also called diabatic Rossby waves), which are relatively small-scale weather systems fueled by condensational heating which propagate quickly and can intensify rapidly into powerful storms. DRVs were first identified in simplified model simulations but have now been implicated in the development of severe storms like winter storm Lothar in 1999, among the most destructive to hit western Europe in several decades. A more theoretical concern is the effect of condensational heating on the most unstable baroclinic mode, meaning the pattern of troughs and ridges that grows most rapidly for a given middle-latitude temperature contrast and its accompanying upper-level jet stream. Earlier work by the Principal Investigator and others shows that condensational heating causes the regions of upward motion in the mode to contract and become more intense relative to the regions of downward motion. The narrowing and intensification of the updraft regions is important as it increases the likelihood of extreme precipitation.The work has societal relevance due to its focus on the strength of weather systems, the intensity of precipitation they generate, and the extent to which warmer temperatures lead to more destructive storms. The project also provides support and training to a graduate student, thereby contributing to the future workforce in this research area. In addition, the Principal Investigator serves as a faculty mentor in the MIT Summer Research Program, which seeks to increase the engagement of undergraduate students from underserved minority groups in science and engineering.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.