Mesoscale Convection Changing Spring Rain in the Great Plains States
Mesoscale convective systems (MCSs)—a collection of strong convective storms that travel together—are key contributors to the warm season rainfall in the U.S. Great Plains, and many regions worldwide. As the largest type of convective storms, it is important to understand their changes over time and in the future to understand regional trends in heavy precipitation and flooding. Researchers at Pacific Northwest National Laboratory found that in recent decades, the increased frequency and intensity of long-lasting MCSs have dominated the observed increases in both the total and extreme rainfall during springtime in the central United States, as contributions from isolated storms has coincidentally declined.
Despite their significant role in regional precipitation and the water cycle, how MCSs may respond to a warming climate is not known partly because climate models do not yet adequately represent MCSs and the precipitation they produce. The team led by Pacific Northwest National Laboratory developed new algorithms to detect MCSs based on their precipitation features and showed that MCS frequency, intensity, and lifetime has increased and dominated the trend from observed data in springtime total and extreme rainfall in the central United States. The increase in MCS activity is consistent with large-scale circulation changes associated with a warming climate. This motivates the need to understand the interactions of MCSs with large-scale environments and to improve how they are represented in climate models to produce more robust projections of their changes in the future.
Rainfall extremes associated with convection are increasing faster than the rate of change in atmospheric precipitable water with warming temperatures. However, the response of extreme precipitation to a warmer climate depends on the type of storm supported by the atmospheric environment. In this research, the team used long-term satellite, surface radar, and rain-gauge network data and atmospheric reanalysis, and a new technique that detects and tracks MCSs to show that the observed increases in springtime total and extreme rainfall in the central United States are dominated by MCSs. As the largest type of convective storm, long-lasting MCSs showed an increased frequency and intensity. Surface warming over the Rocky Mountains increased the pressure gradient across the central United States. This enhanced pressure gradient strengthens the southerly low-level jet and its associated moisture transport from the Gulf of Mexico, favoring more frequent, intense, and long-lived MCSs.