This study will examine the processes and predictability associated with midlatitude warm-season extreme precipitation on multiple spatial and temporal scales. Despite the great need for accurate precipitation forecasts and flood warnings, the prediction of warm-season heavy rainfall continues to be one of the greatest challenges in weather forecasting. Investigating the weather systems that produce extreme rainfall and deadly flash flooding will provide us with basic understanding of these systems.
The ultimate goal of this research is to improve prediction of the weather systems that lead to extreme warm-season precipitation. This research will identify the mechanisms that enhance or limit predictability in warm-season convective systems.
Intellectual merit. One focus of this research will be elevated, linear mesoscale convective systems (MCSs) that have been observed to produce extreme local rainfall and flash flooding. The lack of knowledge about elevated, organized convection limits the ability to produce accurate forecasts and warnings of potential flash-flood situations. To address this issue, numerical simulations will be conducted to quantify the large-scale controls on the initiation, organization, and rainfall production of elevated linear MCSs, particularly the respective roles of tropical, subtropical, and midlatitude influences. These simulations will also be combined with observations to quantify the interactions between frontogenetical circulations and convective-scale processes in the evolution of these MCSs.
At larger scales, widespread, multiple-day heavy rainfall events, such as those that affected the Southern Plains of the United States in 2007 and the Midwest in 2008, have far-reaching impacts. In some cases, the heavy rainfall occurs in association with a series of feedbacks between deep convective activity and the development and maintenance of long-lived, slow-moving circulations at the mid-levels of the atmosphere. Preliminary research has shown that events such as this have low predictability at the medium range. This research will create an observational climatology of heavy rainfall associated with such mesoscale circulations. Then, ensemble forecast data will be used to assess the predictability of these events, the processes that lead to their development and to the success or failure of numerical forecasts, and their larger-scale effects.
Broader impacts. Extreme precipitation, and uncertainties in predicting it, can also have considerable impacts on society. This study integrates research and education by introducing students in the atmospheric sciences to research methods that explore these impacts. Graduate students will be trained in established social-science methods, and a multidisciplinary undergraduate course will be developed to provide the basis for discussions and inquiry into the relationship between weather and society. The meteorological research on extreme rainfall described above will be enhanced by performing student-designed societal analyses of local flood and flash-flood events. A regional workshop will bring together students, researchers, practitioners, and stakeholders to forge new collaborations and to address the complex challenges associated with extreme rainfall. Furthermore, integrated analyses of weather events will engage the public, and will reveal areas where forecasts and warnings can be improved from both the meteorological and societal perspective.
