Orographic Effects on Tropical Cyclogenesis over Eastern Pacific Ocean

Observational evidence shows that the Eastern Pacific Ocean is the most active region of tropical cyclone genesis in the world. In this study, we perform nested numerical experiments using the Advanced Research Weather Forecast and Research (WRF ARW) model at resolutions of 27 km, 9 km, and 3 km to investigate the orographic effects on the genesis of tropical cyclones in the Eastern Pacific Ocean. In particular, we test the hypothesis that the formation of hurricanes is due to the merging of the orographically modified African easterly waves (AEWs) by Central American Mountains, mesoscale convective systems (MCSs) embedded within the AEWs, and the MCSs induced by diurnal heating over the mountains. Effects of orography and moisture are studied by performing sensitivity experiments using WRF with the mountains removed and moisture reduced, respectively. Fundamental understanding of tropical cyclogenesis of Hurricanes Javier (2004) and Fausto (2008) over the Eastern Pacific Ocean will be presented by comparing the results from the control experiments and sensitivity experiments with the satellite imagery. In this study, it is found that the convection preceding the tropical cyclogenesis came from the Central American mountains due to diurnal heating, while the vorticity preceding the tropical cyclogenesis came from the interaction of the preexisting African easterly wave and the Central American mountains. The orographically modified AEW then help get the mesoscale convective elements organized into MCSs. The coupled AEW-MCS then developed into a tropical cyclone over an area with high sea surface temperature over the eastern Pacific Ocean.