Mesoscale Convective Systems Crossing the Appalachian Mountains

mesoscale convective systems;orography;idealized modeling

Letkewicz, Casey Elizabeth ; Sandra Yuter, Committee Member,Gary Lackmann, Committee Member,Matthew D. Parker, Committee Chair,Letkewicz, Casey Elizabeth ; Sandra Yuter ; Committee Member ; Gary Lackmann ; Committee Member ; Matthew D. Parker ; Committee Chair

University:North Carolina State University

关键词: mesoscale convective systems;    orography;    idealized modeling;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/2464/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

Forecasting the maintenance of mesoscale convective systems (MCSs) is a uniqueproblem 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 thelee, 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 theenvironment east of the mountains best separated the cases. The thermodynamic andkinematic variables which had the most discriminatory power included those associatedwith instability, several different shear vector magnitudes, and also the mean troposphericwind. Crossing cases were unsurprisingly characterized by higher instability; however,these cases unexpectedly also contained weaker shear and a smaller mean wind. Idealizedsimulations using a thermodynamic profile favorable for convection revealed that the windprofile is indeed an important factor, but does not uniquely determine whether systemshave a successful crossing. All simulated convective systems underwent a cycle orographicenhancement, suppression, and subsequent reinvigoration, the magnitude of which wassensitive 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 ofthe mountain, respectively. The strength of the mean wind also influenced the scale ofterrain-induced gravity waves which played a significant role in the reintensication of theconvection, along with a hydraulic jump of the cold pool at the base of the mountain inthe lee. Variations in low-level shear impacted the intensity of the MCS, yet the simulatedsystems were always able to successively traverse the barrier due to the influence of thehydraulic jump and mountain waves. Simulations utilizing crossing and noncrossingobserved wind profiles suggested that the mean wind exerts a stronger influence thanthe shear. Despite the differing impacts of the wind profile, the availability of instabilityappears to be the most important factor to consider when predicting the maintenance ofconvective systems crossing mountain ridges.