An Observational Study of Warm Season Southern Appalachian Lee Troughs. Part I: Boundary Layer Circulation

Abstract

This paper describes the lower tropospheric conditions during lee trough episodes to the east of the Appalachians. The boundary layer winds are studied using 1956 pilot balloon data averaged for 45 lee trough cases. Four features are noted in the circulation. Low-level blocking is seen to the west of the mountains, which is consistent with the works of Smith and Han et al. The blocking, which is most evident overnight, leads to convergent flow in eastern Kentucky. Deflection of air around the southern edge of the Appalachians results in divergent flow across northern Georgia. A low-level southerly flow to the cut of the lee trough causes a displacement of the cyclonic vorticity associated with the lee trough into central South Carolina during the day. At night, the strengthening and turning of the flow to a southwesterly direction leads to ascent in central Virginia and the Carolinas. The diurnal signal of this feature is consistent with a diabatically forced low-level jet. Above mountain level, the flow is characterized by anticyclonic curvature and shear that increases in curvature during the day.