Spatial structure and evolution of low-level winds in modeled hurricanes at landfall

As was made clear by recent U.S. landfalling hurricanes, the distribution of low-level windspeed varies greatly from case to case. The factors controlling these differences are likely to be very complex and inter-dependent. This study will simplify the problem by investigating just the impacts of the land surface. Six identical, idealized hurricanes, are forced to make landfall on a straight, east-west oriented, flat coast, with a different combination of roughness length and moisture availability in each case. The results are compared to a control experiment without land.

The storms are initially located 400 km south of the straight coastline and move at around 4 m s-1 in a northeasterly direction in response to the evolving environmental steering flow. At t = 15 h into the simulation, the storm centers cross the coastline about 400 km east of the original location. The storms fill slowly as they approach land and continue to do so after the centers cross the coastline. The 10m windspeed drops off abruptly at the centers cross the coastline. Cases with greater roughness length and lower moisture availability weaken more, as expected.

The maximum 10m windspeed occurs over water during the entire simulation and begins to reduce in magnitude about 4 hours prior to landfall. At this point, the outer edge of the eyewall has reached land and friction begins to play a role. After landfall, the 10m winds drop to tropical storm (TS) force in all cases. Cases with a larger surface roughness display a stronger weakening of the 10m winds. Storm with a larger surface roughness length show a smaller areal coverage of TS force winds over land and areal coverage reduces 1 hour after landfall. The smaller roughness length cases display a larger areal coverage of TS force winds over land and coverage reduces 6-7 hours after landfall.

At the conference, the structure and evolution of the 10m wind will be presented and its relationship to the central surface pressure, pressure gradient, surface fluxes, and warm core will be explored. Differences between cases will be presented in the hope of ultimately improving understanding and prediction of storm wind speed evolution at landfall.