Control Parameters for Track Continuity of Cyclones Passing over the South-Central Appalachian Mountains

Abstract

he tracks of all 42 tropical cyclones (TCs) passing over the south-central Appalachians from the east or west during 1850?2011 are found to be continuous. By estimating the basic-flow and vortex Froude numbers, and the basic-flow and vortex Rossby numbers (U/Nh, Vmax/Nh, U/fLx, and Vmax/fR, respectively; where U is the basic-flow speed, Vmax is the maximum tangential wind speed, N is the buoyancy frequency, h is the mountain height, f is the Coriolis parameter, R is the radius of cyclone center to Vmax, and Lx is the mountain width), from the hurricane reanalysis data, the lack of track discontinuity is explained by weaker blocking associated with lower mountains. The track discontinuity is found to be mainly controlled by Vmax/Nh and Vmax/fR (greater than 1.5 and 4.0, respectively), and less sensitive to U/Nh and U/fLx, consistent with a previous study. It is hypothesized that stronger blocking associated with weaker near-surface tangential winds of extratropical cyclones tends to make their tracks across the Appalachians discontinuous. This hypothesis is verified by investigating 13 heavy snowstorms with discontinuous tracks during 1950?2003. The present results show that all Vmax/Nh fall below 1.5 and all Vmax/fR fall below 4.0. To help understand the dynamics associated with orographic blocking, the 2?5 February 1995 snowstorm is simulated by a numerical model. The simulated results indicate that the upstream Vmax/Nh and Vmax/fR are indeed less than 1.5 and 4.0, respectively. Therefore, it is concluded that track discontinuity of a cyclone passing over a mesoscale mountain is mainly controlled by strong orographic blocking as measured by lower vortex Froude number (i.e., Vmax/Nh) and/or lower vortex Rossby number (i.e., Vmax/fR).