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

In this study, we hypothesize that the track continuity for hurricanes passing over Appalachian Mountains, as found by Lin et al. (2005 JAS – L05), is mainly controlled by the vortex Froude number Vmax/Nh and less sensitive to the basic flow Froude number U/Nh, where Vmax is the maximum tangential wind of the tropical cyclone (TC) vortex, U is the basic or steering flow speed, N the Brunt-Vaisala frequency, h the mountain height, and the track becomes continuous (discontinuous) with larger (smaller) values of these parameters and a threshold of Vmax/Nh of 1.5. The hypothesis is tested by conducting a climatology study. Through a climatology study of all TC cases passing over the Appalachians from 1850-2010, it is found that 34 cases were found to be impinging from the east and 8 cases impinging from the west. Both Froude numbers were estimated from the HURDAT data for all 42 cases. All cases were found continuous and have Vmax/Nh greater than 1.5 indicating the same threshold as found in L05 and that the track discontinuity is also less sensitive to U/Nh. The lack of track discontinuity for TCs passing over Appalachians is explained by the reduction of the strength of the TC by friction and lack of moisture supply over the land, thus led to larger Froude numbers.

In order to critically test L05's theory, we hypothesize that an extratropical cyclone (ETC) over Applachians will experience track discontinuity because their vortex Froude numbers (Vmax/Nh) is smaller due to weaker tangential wind leading to stronger orographic blocking. This hypothesis is then tested by investigating 13 cases of observed track discontinuity of the 30 heaviest snowstorms affecting the Northeast U.S. from 1950-2003. Two approaches were used to estimate the Vmax and results showed both Vmax/Nh values fell under 1.5, identical to the threshold found by L05. To further test the hypothesis, the 2-5 February 1995 storm was chosen as a typical discontinuous case and simulated by the WRF model. Results for Vmax/Nh using the two approaches were 1.06 and 1.25, respectively; both are below the threshold of 1.5. Thus we can conclude that any storm whether a TC or an extratropical, its track continuity will be controlled mainly by the vortex Froude number Vmax/Nh, having a threshold of 1.5 below which the track becomes discontinuous.