Mesoscale Convective Systems Crossing the Appalachian Mountains

Abstract

Forecasting the maintenance of mesoscale convective systems (MCSs) is a unique problem in the eastern United States due to the influence of the Appalachian Mountains. At times these systems are able to traverse the terrain and produce severe weather in the lee, while at other times they instead dissipate upon encountering the mountains. Thus, there exists a need to differentiate between crossing and noncrossing MCS environments. Examination of twenty crossing and twenty noncrossing MCS cases revealed that the environment east of the mountains best separated the cases. The thermodynamic and kinematic variables which had the most discriminatory power included those associated with instability, several different shear vector magnitudes, and also the mean tropospheric wind. Crossing cases were unsurprisingly characterized by higher instability; however, these cases unexpectedly also contained weaker shear and a smaller mean wind. Idealized simulations using a thermodynamic profile favorable for convection revealed that the wind profile is indeed an important factor, but does not uniquely determine whether systems have a successful crossing. All simulated convective systems underwent a cycle orographic enhancement, suppression, and subsequent reinvigoration, the magnitude of which was sensitive to the wind profile. Increasing (decreasing) the mean wind led to greater (less) enhancement and suppression of vertical velocities on the windward and lee sides of the mountain, respectively. The strength of the mean wind also influenced the scale of terrain-induced gravity waves which played a significant role in the reintensication of the convection, along with a hydraulic jump of the cold pool at the base of the mountain in the lee. Variations in low-level shear impacted the intensity of the MCS, yet the simulated systems were always able to successively traverse the barrier due to the influence of the hydraulic jump and mountain waves. Simulations utilizing crossing and noncrossing observed wind profiles suggested that the mean wind exerts a stronger influence than the shear. Despite the differing impacts of the wind profile, the availability of instability appears to be the most important factor to consider when predicting the maintenance of convective systems crossing mountain ridges.