Dissertation

Heavy Rainfall Enhanced by Warm Season Fronts and Orography in Western North Carolina: Synoptic Classification and Physical Drivers

Mackey, Brian Patrick

2020

Overview

Physical processes that enhance heavy rainfall in association with warm season (April-September) fronts are investigated over western North Carolina. In this region of complex terrain encompassing the basins of the Upper Catawba River, the South Yadkin River, and the Upper Yadkin River, quantitative precipitation estimates and forecasts exhibit known biases, and a variety of large-scale atmospheric patterns can lead to heavy rainfall and flash flooding. The focus is on events with space-time dimensions on the meso-β scale (horizontally up to 200 km and temporally up to about 12–18 hours). The most frequent internally forced mesoscale weather features that produce such heavy rainfall episodes in the region are mid-latitude fronts. External mechanical forcing due to the orography of the southern Appalachians also plays an important role in shaping the rainfall intensity and distribution.A 17-year climatology comprised of 98 heavy rainfall cases is constructed using daily 4-km stage IV precipitation analyses. Cases are categorized according to the type of front (cold or stationary), front location (if stationary) relative to the study area (north, south, or over the basins), time of year (April and September vs. May–August), and time of day of peak rainfall. Classical warm front cases are too few to be included in this study. Results show that the majority of warm season heavy rainfall episodes tend to peak in the afternoon or evening, but there is a notable exception regarding events associated with stationary fronts located south of the basins. These episodes have a nocturnal maximum in rainfall in the foothills of the Blue Ridge Mountains, suggesting an interaction between the small-scale mountain-valley breezes and a mesoscale easterly low-level jet.The front type clusters are further evaluated utilizing the latest fifth-generation reanalysis (ERA5) produced by the European Centre for Medium-Range Weather Forecasts. Through the use of percentile rankings, the raw values of precipitable water and convective available potential energy are transformed into more meaningful and useful quantities that show well-defined maxima in and around the study region. For cold fronts as well as April and September cases of stationary fronts south of the basins, correlation analysis shows that the presence/strength of an atmospheric river plays a key role in determining the amount and areal extent of heavy rainfall. Also quantified is the low-level upslope flow, which helps regulate the spatial and temporal variability of heavy rainfall for nearly all frontal regimes in the study. In addition, rainfall events associated with stationary fronts to the north of the basins are heaviest when those fronts retreat farther north and west, coincident with a stronger Atlantic high pressure cell which increases the low-level moisture transport up the slopes of the Blue Ridge Mountains. For stationary fronts over the basins, the cases with the heaviest, most widespread rainfall are associated with a sharper west-east moisture gradient from the mountain ridges to the North Carolina Piedmont.

• Publisher: ProQuest Dissertations & Theses
• Subject: Atmospheric sciences / Hydrologic sciences / Meteorology
• ISBN: 9798662465756