Variations in Flash Flood-Producing Storm Characteristics Associated with Changes in Vertical Velocity in a Future Climate in the Mississippi River Basin

Abstract:

The Mississippi River Basin (MRB) is a flash flood hotspot in the United States, receiving the most frequent floods and highest rainfall accumulations across the country. In a future warmer climate, this region displays some of the largest increases in rainfall specifically associated with storms that produce flash floods. To better understand why the MRB experiences some of the greatest future increases in rainfall, this study examines how 484 flash flood-producing storms in the MRB change in a future warmer climate using convection-permitting simulations under a pseudo-global warming framework. Since most of the flash flood-producing storms in the MRB are due to warm-season convection, future changes in rainfall as a function of storm vertical velocity are analyzed in order to understand the role of storm dynamics in modulating future rainfall changes.

Changes in future rainfall characteristics in flash flood-producing storms among three different vertical velocity categories–weak, moderate, and strong–are explored. While all categories show a future increase in flood-producing storm rainfall, the storms with strong vertical velocity exhibit the greatest future increase of 18.4%, while the storms with weak vertical velocity only increase by 13%. The greater future rainfall increase in storms with stronger vertical velocity, along with stronger updrafts in these storms, suggest that storm dynamics could modulate future changes in rainfall.

Motivated by these results, WRF simulations are performed for two storms where future rainfall increases, but one storm has weak vertical velocity and the other has strong vertical velocity. A pseudo global-warming experiment is run on both cases, along with a Clausius-Clapeyron experiment where moisture is increased by 7% per degree of warming, in order to understand how the mesoscale dynamical and thermodynamic changes contribute to future changes in rainfall in storms with different vertical velocities.