Abstract
Relative contributions of common ingredients to heavy orographic rainfall associated with the passage of Hurricanes Hugo (1989) and Isabel (2003) over the Appalachian Mountains are examined using a numerical weather prediction model. It is found that the key ingredients for producing local heavy orographic rainfall were: high precipitation efficiency, strong low-level flow, strong orographically forced upward motion associated with strong low-level flow over relatively gentle upslope, concave geometry providing local areas of convergence, high moist flow upstream, a relatively large convective system associated with both tropical cyclones (TCs), and relatively slower movement. In addition, neither conditional instability nor potential (convective) instability is found to play essential roles in producing strong upward motion leading to heavy orographic TC rain. A modified Orographic Rain Index (ORI) is proposed as a predictor for heavy orographic TC precipitation, which includes the upstream incoming horizontal wind speed normal to the local orography, the steepness of the mountain, the relative humidity, the TC moving speed, and the horizontal scale of the TC. It is found that the ORI estimated in regions of local maximum rainfall by using fine-resolution numerically simulated results correlate well with rainfall rates for both hurricanes, indicating that it may serve as a predictor for heavy orographic TC rainfall.
Highlights
When a tropical cyclone (TC) passes over a mesoscale mountain, its cyclonic circulation may generate strong upward motion when it impinges on a mesoscale mountain, which tends to initiate or enhance heavy rainfall leading to flash flooding and debris flow
It is found that the key ingredients for producing local heavy orographic rainfall were: high precipitation efficiency, strong low-level flow, strong orographically forced upward motion associated with strong low-level flow over relatively gentle upslope, concave geometry providing local areas of convergence, high moist flow upstream, a relatively large convective system associated with both tropical cyclones (TCs), and relatively slower movement
It is found that the Orographic Rain Index (ORI) estimated in regions of local maximum rainfall by using fine-resolution numerically simulated results correlate well with rainfall rates for both hurricanes, indicating that it may serve as a predictor for heavy orographic TC rainfall
Summary
When a tropical cyclone (TC) passes over a mesoscale mountain, its cyclonic circulation may generate strong upward motion when it impinges on a mesoscale mountain, which tends to initiate or enhance heavy rainfall leading to flash flooding and debris flow. In order to examine the relative contributions of the common ingredients of the proposed ORI associated with the orographic TC rain, we select two hurricanes, Hugo (1989) and Isabel (2003), due to their similarity in impinging angle on the Appalachian Mountains and very different orographic rainfall. Hugo (1989) and Isabel (2003) were chosen for this study because both hurricanes belonged to Type AB2 (Liu et al 2016) and impinged on the Appalachian Mountains at nearby locations from a similar angle, but produced significantly different rainfall amounts over the mountains, as revealed in the rainfall maps (WPC 2016) This makes it easier to compare the relative contribution from individual key ingredients for producing heavy orographic precipitation. Hurricane Isabel created about 3.6 billion dollars in damage (Lawrence et al 2005) and caused 34 fatalities in North Carolina, Virginia and Maryland (Nolan et al 2009)
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