Orographic Precipitation from Tropical Cyclones in a Changing Climate: Hurricane Ivan (2004)

Western North Carolina has experienced many cases of extreme precipitation from hurricanes traversing the Appalachian Mountains. This often results in flooding, landslides, infrastructure damage, and loss of life. Hurricane Ivan (2004) is one of the most notable cases. Heavy rainfall from Hurricane Ivan helped trigger over 150 landslides in one western North Carolina county alone (Wooten et al. 2008), causing numerous adverse impacts. Knowing that we live in a warming climate, it is imperative to understand: 1) How will orographically-forced rainfall from storms like Hurricane Ivan change in a future, warmer climate? 2) Why, meteorologically, do these changes occur?

This study uses high-resolution Weather Research and Forecasting (WRF) model simulations of Hurricane Ivan (2004). The Pseudo-Global Warming (PGW) method is employed to simulate a version of Hurricane Ivan that might occur in a future, warmer climate. We have found that the use of high-resolution simulations and model terrain is crucial to represent the fine-scale orographic forcing that leads to strong vertical motion and intense rain rates. Using 30 Arc-Second (approximately a 1 km grid) terrain and a triple-nested model domain setup, we analyze Ivan’s extreme precipitation with a focus on the fine-scale orographic forcing that leads to local rainfall maxima. It has been shown that future hurricanes in the coastal plain will produce more total rainfall, and more intense rainfall. We investigate if this same trend is seen in the western North Carolina mountains, and if the orographic nature of the rainfall causes a more drastic or muted change in the future. In a warmer climate, there will be a higher freezing level, thus warm clouds will extend higher into the atmosphere. Since orographic rainfall involves warm rain processes, we suspect that this, coupled with the forced ascent due to orographic lift, could result in differences in future hurricane rainfall in the mountains versus in the coastal plain.