Paper on changes to the low-level wind field in Hurricane Ida at landfall released online in Geophysical Research Letters – Hurricane Research Division

Wind structure in the hurricane boundary layer shows pronounced asymmetries over water and at landfall. Aircraft observations have noted significant changes in the nature of these asymmetries prior to and during Hurricane Ida’s landfall. Such changes can have important implications for damage patterns during TC landfall, and understanding the physical processes underlying these changes can help assess and improve numerical models to better predict impacts.

The structure of a hurricane’s near-surface wind field (in what we call the boundary layer) when it makes landfall plays a significant role in determining where and how damage may occur. This study uses aircraft observations to document the changes that occur before and during landfall, and it confirms the results from previous studies using computer models.

Important Conclusions:

• Wind in the boundary layer shows significant asymmetries (variations on opposite sides of the TC) over water and at landfall. Forecasting these asymmetries is crucial in predicting and determining where damage is most likely to occur during landfall.
• Over open water, the asymmetries are mostly determined by the storm motion and change in wind velocity between the top and the bottom of the TC (vertical wind shear). 
• At landfall, the asymmetries are caused by the difference in friction between the smooth ocean and the rough land surfaces. Figure 1 shows analyses of the radial flow (flow into or away from the TC center) from Doppler radar on NOAA’s P-3 aircraft during the time when Ida was over open water and then again when Ida was making landfall. The large pink arrows show where the radial inflow is the strongest. When Ida was over open water, peak inflow was on the east side; during landfall (bottom image), friction slowed the wind causing more air to flow inward (radial inflow). This in turn increased the wind speed downstream (on the southeast side of Ida in this case, see Fig. 2) thus changing where the strongest winds existed relative to the center.

The paper can be found at https://essopenarchive.org/doi/full/10.22541/essoar.169111777.76406363/v1. For more information, contact aoml.communications@noaa.gov. This work was supported by NOAA Base Funds, ONR TCRI Award N00014-20-1-2071, NOAA Grants NA21OAR4590370, NA22OAR4590178, and NA22OAR4050669D, and National Science Foundation (NSF) Awards 2228299 and 2211308.