Quantifying the Impact of Land Surface Roughness on the Hurricane Wind Structure Before Landfall

Hurricane landfall has been an important research problem because of the potential destruction of lives and properties when severe winds and storm surges batter coastal regions. Previous observational and modeling studies have shown significant changes in the surface wind structure over water prior to hurricane landfall. One notable feature is the amplification of offshore winds in the left-front quadrant of the storm (relative to the direction facing land in the Northern Hemisphere). This study aims to advance our understanding of the physical mechanisms responsible for the changes in the surface wind field and quantify their effects as a hurricane makes landfall.

Idealized numerical experiments are conducted using a three-dimensional hurricane boundary layer wind model that simulates hurricane landfall scenarios by including a land-sea interface and applying a constant roughness length over land. These simulations are compared with ocean-only scenarios where land effects are excluded. Sensitivity experiments with varying hurricane sizes, translation speeds, and land surface roughness lengths are also performed.

The surface wind simulations reveal a developing asymmetry in the offshore flow component of the radial wind that first appears in the front-left quadrant when the hurricane center is approximately 100 - 200 km from the coastline. This distance varies depending on the storm's size, translation speed, and land surface roughness. As the storm progresses towards the coast, the tangential wind component intensifies in the rear-left quadrant near the radius of maximum winds, shifting its maximum toward the rear side behind the storm center.

Analysis of the radial and tangential momentum budgets indicates that these changes are first triggered by the reduction of the tangential wind due to land-induced friction in the frontal region of the hurricane, which is advected offshore by the cyclonic circulation. This results in a reduced tangential wind over the water on the storm's left side, causing an imbalance between the pressure gradient force and the Coriolis & centrifugal forces. This imbalance leads to the acceleration of the radial wind on the storm's left side, subsequently enhancing the advection of angular momentum. The latter triggers an increase in the tangential wind in the rear-left quadrant of the hurricane. Velocity advection in both radial and tangential directions also plays an important role in the evolution of the surface wind structure as the hurricane approaches the shore.

While this study employs idealized hurricane simulations, similar offshore wind asymmetries have been observed during the landfall of actual hurricanes, such as Hurricane Frederic (1979), Mitch (1998), Wilma (2005), and Michael (2018). Understanding the underlying physical mechanisms and qualifying these changes in surface winds is crucial, given that landfalling hurricanes have significant implications for coastal communities, infrastructure, and economies.