Monsoon Extremes: Impacts, Metrics, and Synoptic-Scale Drivers
Much of the water supply for low-latitude land regions, including the southwestern U.S., is delivered by propagating vortices and waves embedded within seasonal-mean, continental-scale monsoon circulations. These propagating disturbances---which include monsoon depressions, easterly waves, and orographically trapped moisture surges---frequently produce extreme precipitation that challenges water management networks, hydropower generation, and natural ecosystems. Despite the importance of this monsoon rainfall, its sensitivity to greenhouse gas emissions, atmospheric aerosols, and other products of energy generation remains poorly understood, in part because the global models and data used to study planetary-scale monsoon flow are only now beginning to achieve resolutions needed to represent the transient disturbances that produce much of the mean and extreme monsoon rainfall. This project has improved understanding of a variety of transient atmospheric disturbances in monsoon regions, the precipitation extremes they produce, and their interactions with larger-scale monsoon winds. Specifically, the project discovered new mechanisms controlling wind and precipitation variability on synoptic (2-12 day) time scales, which include important events such as monsoon depressions, upper-tropospheric troughs, and Gulf of California moisture surges. The project also discovered new mechanisms that govern the North American monsoon, and precipitation over low-latitude mountains in general. Metrics and statistical models were developed for multiple monsoon regions, including those in South Asia and North America. The main research tasks involved (1) developing feature-tracking algorithms to identify synoptic-scale disturbances in large ensembles of observational data and numerical model output, (2) estimating the multidimensional sensitivities of disturbance genesis, growth, and rain rate to properties of the larger-scale atmospheric state in these ensembles, and (3) constructing physically based models for disturbance genesis, amplification rate, propagation speed, and precipitation rate. The project provided process-based projections of changes in precipitation extremes together with process-based assessment of model bias. Project benefits and outcomes included improved understanding of the physical processes responsible for variations in extreme rainfall in the southwestern U.S. and other monsoon regions, characterization of model bias in simulating the full rainfall distribution in the atmospheric vortices that produce much extreme rainfall in monsoon regions, and creation of dynamically based metrics for extreme monsoonal rainfall. The project trained three graduate students and four postdoctoral scientists.
- Research Organization:
- Univ. of California, Oakland, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0019367
- OSTI ID:
- 2301565
- Report Number(s):
- DOE-BERK-0019367
- Country of Publication:
- United States
- Language:
- English
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