PNNL

The Science

Mesoscale convective systems (MCSs)—intermediate-scale thunderstorm clusters lasting up to about 24 hours—are important precipitation producers. They account for 30-70 percent of warm-season (April to August) rainfall between the Rocky Mountains and Mississippi River, and 50-60 percent of tropical rainfall.

The increasing frequency of long-lived MCSs in the past 35 years across the U.S. Great Plains motivates the need to understand the environments that favor their development.

Analyzing realistic simulations of these systems, researchers at the U.S. Department of Energy's Pacific Northwest National Laboratory found that MCSs lasting nine hours or more strengthen the cyclonic (counterclockwise) circulation that feeds dry, cool air into the rear of the MCS region. This process increases evaporative cooling and helps maintain the MCS.

The Impact

MCSs tend to not only produce floods, but they carry with them a variety of severe weather phenomena. The finding that MCSs can be "self-sustaining" suggests that model errors in the large-scale environment can greatly limit their ability to simulate long-lived MCSs.

Furthermore, small changes in the large-scale environment may result in large changes in the frequency of long-lived MCSs because the changes can be amplified through interactions between the MCSs and their large-scale environment. Hence, understanding how the large-scale environment may change in the future has important implications for predicting future changes in floods and severe weather in the United States.

Summary

Recent research shows that long-lived MCSs over the U.S. Great Plains have become more frequent and produce more extreme rainfall compared to 35 years ago. To better understand interactions between the large-scale environments and MCSs, researchers performed continental-scale, convection-permitting simulations of the 2011 and 2012 warm seasons for analysis. These simulations—conducted using the Weather Research and Forecasting model—realistically reproduced the structure, lifetime, and mean precipitation of MCSs over the central United States.

Researchers analyzed the simulations to determine the environmental conditions conducive to generating long-lived MCSs. The simulations showed that MCSs systematically formed over the central Great Plains ahead of a trough in the upper-level westerlies in combination with an enhanced low-level jet bringing moisture from the Gulf of Mexico. These environmental properties at the time of storm initiation were most prominent for the MCSs that persisted at least nine hours. Those MCSs exhibited the strongest feedback to the environment through diabatic heating produced by condensation and precipitation from the MCSs.

The feedback produced a midlevel cyclonic circulation near the trailing portion of the MCS. Researchers found that the mesoscale low-pressure center fed dry, cool air into the environment at the rear of the MCS region, increasing evaporative cooling and helping to maintain the MCS.

Acknowledgments

Sponsors: The U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research supported this study as part of the Regional and Global Climate Modeling program through the Water Cycle and Climate Extremes Modeling (WACCEM) Scientific Focus Area. Professor Houze's University of Washington participation is supported by Pacific Northwest National Laboratory under Task Order 292896 (WACCEM) of Master Agreement 243766.

Facilities: This research used computational resources from the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility. Sounding data were obtained from the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a DOE Office of Science user facility sponsored by the Office of Biological and Environmental Research.

Reference: Q. Yang, R.A. Houze Jr., L.R. Leung, Z. Feng, "Environments of Long-Lived Mesoscale Convective Systems Over the Central United States in Convection Permitting Climate Simulations." Journal of Geophysical Research: Atmospheres 122, 13,288-13,307 (2017). [DOI: 10.1002/2017JD027033]