Is Intense Summertime Rainfall Fated in the Winds?

The Science

Large clusters of organized thunderstorms known as mesoscale convective systems (or MCSs) account for roughly half of summer rainfall in the central and eastern United States. Mid-level perturbations (MPs) are weather patterns within mid-levels of the troposphere which can influence the formation of MCSs. However, the control that MPs exert on the potential of a MCS to produce heavy rainfall remains poorly understood. This study tracked 4,100 MPs in May – August from 2004 – 2021. Most MCSs did not require an existing MP to form, but MCSs tended to produce more rain and were wider and faster moving when there was a co-existing MP during their formation. Heavier amounts of rain were linked to MPs, which brought increased vertical motion and atmospheric conditions that are more favorable for thunderstorms.

The Impact

Large, organized thunderstorms frequently occur in the central United States during the summer months. However, weather models often fail to predict these storm systems accurately. This can result in poor rainfall forecasts that are critical for both agriculture and warning the public about possible severe weather and flooding events. This research compares existing weather conditions before the formation of weak and strong thunderstorm systems. Understanding these conditions identifies what weather patterns must be included in weather models to best predict flooding and critical rainfall forecasts.

Summary

Significant thunderstorms may form in the summer over the central United States within certain background airflow conditions that are otherwise adverse for storms. This happens when MPs—migratory vertical vorticity perturbations in the middle troposphere—lift and destabilize the atmosphere to make it conducive for storms. However, it is not clear how often such MPs promote storms across a wide variety of other summertime meteorological conditions. This study tracked 250 – 1250 km-wide MPs and mapped them relative to radar tracked mesoscale convective systems in the central United States over 18 consecutive spring-summer seasons. Rainfall characteristics of the convective systems colocated with MPs of various size and intensity were compared with those that were not.

Only 20 percent of convective systems formed while colocated with MPs. However, this subset of storms produced greater lifetime rainfall and coverage area than storms that form in the absence of an MP. Further, half of the tracked storms were colocated with an MP at some point in their lifetime and were likely amplified due to this colocation. The impact on convective system rainfall was generally proportional to a colocated MP’s size and the magnitude of ascent of the air within it. This work emphasizes the need to improve representation of possibly subtle mesoscale flow anomalies in forecast models that presently inaccurately predict summertime precipitation events. Results also indicate the importance of further investigation into the effects of topography on the formation of MPs as most of the events in this study were first detected over the high plains near the central Rockies.

Contact

L. Ruby Leung, ruby.leung@pnnl.gov

Funding

This work was supported by the Department of Energy’s Office of Science, Biological and Environmental Research program as part of the Regional and Global Model Analysis program area through the Water cycle: Modeling of Circulation, Convection, and Earth system Mechanisms scientific focus area. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute.