Spatial structure and evolution of 10m winds and rainfall in modeled hurricanes at landfall

As was made clear by recent U.S. landfalling hurricanes, the distribution of low-level windspeed and rainfall 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 storm is initially located 400 km south of the straight coastline and moves at around 7 m s-1 in a northeasterly direction as a result of the environmental steering flow in combination with the Coriolis force. At t = 15 h into the simulation, the storm center crosses the coastline about 400 km east of its original location. The storms fill slowly as they approach land and continue to do so after the center crosses the coastline. Cases with greater roughness length and lower moisture availability weaken more, as expected. The differences are not large.

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. Maximum rainfall rates over land differ little between the different cases and remain 2 - 4 cm/h after landfall. Cases with larger surface moisture content display a larger areal coverage of rainfall over land, but similar rainfall rates are seen.

At the conference, further differences in 10m wind and rainfall distribution will be presented. Reasons for these differences will be highlighted in the hope of ultimately improving understanding and prediction of storm evolution at landfall.