| The goal of this research is to use the Advanced Research WRF (ARW) model to simulate Hurricane Ivan (2004) in order to improve quantitative precipitation forecast (QPF) and understanding of the dynamics of orographic rainfall associated with the passage of tropical cyclones over the Appalachian Mountains. The simulation results compared well with the observation in terms of track, intensity, and rainfall distribution, although the precipitation amount is overpredicted by the WRF model. Lin et al. (2001) and Lin (2007) proposed nine common ingredients that are responsible for producing heaving precipitation in a mountain region. From the WRF simulations heavy orographic rainfall production from Hurricane Ivan appeared to be dominated by the low level jet, high precipitation efficiency of the incoming airstream, steep mountain orography, orographically induced vertical motion, CAPE, presence of a large convective system, slow movement of the convective system and a very moist airstream impinging on the mountains. Two modified Orographic Rain Indexes (ORIPW and ORICAPE) are proposed: ORIPW = (VNDeltah / Delta x)PW(Ls / cs) and ORICAPE = (VNDeltah / Deltax)CAPE(Ls / cs), where VN is the total upstream horizontal wind normal to the mountain range, Deltah/Deltax is the mountain steepness, cs is the storm's propagation speed, Ls is the horizontal scale of the convective system, PW is the total water mixing ratio or precipitable water, and CAPE is the convective available potential energy. Both ORIPW and ORI CAPE are then tested for six local areas of the Appalachians and the results of the modified ORIPW and ORICAPE values for each local region correlate well with the rainfall rate. |
