The Impact of Terrain Height on Precipitation Processes in a Landfalling Tropical Cyclone

Analysis of numerical simulations investigates how the height of a continental mountain range, which affects the rate of decay of a landfalling tropical cyclone, impacts the processes controlling precipitation formation. Simulations of Hurricane Karl (2010) are generated by the Weather Research and Forecasting (WRF) model, where the Mexican terrain is replaced with an idealized plateau. Two experiments, each comprised of control runs and nine additional ensemble members, are designed with plateau heights of 0.5 and 2.5 km altitude. The two experiments reveal that the differing rates of decay induce disparate evolutions of the kinematic and thermodynamic structures. When the terrain is shallow, the weak rate of decay enables the simulated storm to retain a deep warm core, moist neutral processes within the robust eyewall, and rainband for a longer period. When the terrain is tall, rapid decay causes the warm core to shrink and shallow buoyant motions are more frequent near the remnant eyewall, whereas widespread convection develops in the region of upslope flow. During a nine-hour period as the storms pass over the sloping edge of each plateau, the maximum precipitation values for each experiment are comparable between the two experiments, but the horizontal organization differs. From a microphysical standpoint, cloud water mixing ratios are greater during landfall over the sloping terrain in the tall plateau experiment. The enhanced cloud water results from a mix of gentle ascent and buoyant motions near the inner core and convective motions at larger radii. Enhanced cloud water is located near locally enhanced rain, indicating the role of warm rain processes in precipitation enhancement. However, rain mixing ratio values and the precipitation intensity are correlated with graupel mixing ratios aloft, which exceed observational values by an order of magnitude.