Tropical Cyclone Intensification in the Vicinity of Land–Sea Roughness Discontinuity

Upon approaching the land, tropical cyclones (TCs) experience a discontinuity in surface roughness along the coast due to different surface properties. This discontinuity, manifesting itself as differential frictional drag in the lower planetary boundary layer (PBL), is hypothesized to be aiding TC intensification near the coast in a way resembling an Ekman Pump. Taking Hurricane Karl (2010) as a prototype, a series of numerical simulations using the Advanced Weather Research and Forecasting model (WRF-ARW) were carried out to examine TC structural and intensity change before making landfall. The effect of two major dynamical factors, elevated terrain and increased roughness over land, were isolated by manipulating bottom boundary input into the model. It was found that with the presence of increased roughness over a flat land, simulated TC gained stronger boundary layer inflow, exhibited more vigorous inner-core convection, and attained higher intensity prior to making landfall. This positive contribution could be counteracted by terrain over land, and could also be sensitive to model parameterization schemes such as the PBL scheme. Idealized simulations are currently being utilized to better illustrate some of the key findings from this case study.