Warm Season Lee Troughs about the Southern Appalachians.

This dissertation describes the lower tropospheric conditions during lee trough episodes to the east of the southern Appalachians. The boundary layer winds are studied using 1956 pilot balloon data. These show a diurnally -forced circulation with convergence into the lee trough below mountain height (1 km above mean sea level) and a mainly zonal flow above mountain height which is dominated by anticyclonic shear. The nocturnal flow is marked by a pronounced shift to southerly winds below mountain height to the west of the mountains. An investigation of cloud-to-ground lightning flashes during lee trough episodes shows a bimodal distribution of flashes: one maximum over the eastern slopes of the mountains and one over the coastal plain. The strength of each signal depends on the strength of the synoptic -scale forcing, but the lee trough lies in a relative minimum of lightning activity. A case study performed during a nine-day lee trough episode further clarifies the conclusions from the lightning data. The study shows that the convective signal is strongly affected by the interaction of weak synoptic-scale of mesoscale features, which are most easily identified in the lower troposphere, with the diurnally-forced local circulations associated with both the mountains and the coastline.