Investigating the boundary layer wind structure in numerically simulated landfalling hurricanes

Recent studies using GPS drop-sondes have revealed detailed information about the structure of vertical profiles of the horizontal windspeed in hurricanes. This has raised questions about the true value of the ratio between flight level and surface windspeeds. Since research- and reconnaissance-aircraft do not usually fly over land, much uncertainty remains about the structure of low-level winds in tropical cyclones over land. Using a numerical model and the landfall of idealized hurricanes on hypothetical straight, east-west oriented, flat coasts with different land-uses (varying in terms of roughness length and moisture availability), the structure of the low-level horizontal windspeed is analyzed in detail.

Ratios of windspeeds over land to windspeeds over water, at the surface, during landfall, were derived for a case where the land surface had a roughness length of 15 cm and a moisture availability of 30%. In the eastern half of the storm the value of the ratio was 0.77, while in the western half it was 0.80. These values are well within the range of observationally derived ratios in previous studies. The reason the ratio is lower for the western half of the storm is probably because most of the winds measured over water in the southwestern quadrant just came offshore from the northwest quadrant and, as a result, were relatively slower than the winds over water in the south-eastern quadrant. Therefore, there was not quite as big of a difference between the winds over land and those over water in the western half as in the eastern half. At the conference, ratios for other land-uses will be presented and their differences will be addressed. Additionally, the ratios between flight-level and surface winds over water and land, before, during, and after landfall will be compared. Wind profiles in different regions of the storm and in different stages of its evolution will be examined.

The ultimate goal of this study is to derive a ratios of flight-level wind speeds off shore to surface winds over land and to examine how such ratios differ with land-use and storm-region (for instance eyewall versus outer core; left versus right half of the storm). The intention is to provide better guidance to forecasters by identifying key differences and trends, to be verified against real observations in the future.