P2H.13 Estimates of turbulence dissipation rate from high frequency surface wind observations for landfalling tropical cyclones

Thursday, 1 May 2008
Palms ABCD (Wyndham Orlando Resort)
Paul Ruscher, Florida State Univ., Tallahassee, FL; and W. Maxham

This study examines the changes in the atmospheric surface layer and during landfalling tropical cyclones. Several storms, which made landfall near surface observation platforms capable of high-resolution data storage, were examined. These records were subjected to spectral methods to explore the characteristics of the changing boundary layer turbulence.

These results were compared to recent observations of boundary layer roll features noted in some landfalling storms. Spectra were also used for determining turbulence dissipation rates in the storms.

Although retrieved 1 min data from NWS ASOS stations were helpful in providing some information about surface variability, the data were insufficient to reveal the structure of dissipating turbulence in the surface layer. Specially-instrumented CMAN stations in the southeastern United States provided data at rates of up to 5 Hz which helped to reveal smaller scale structure, enabling the extraction of turbulence dissipation rates. The storms studied are Hurricanes Earl (1998), Dennis, Floyd, and Irene (1999).

Turbulence dissipation rates calculated from the highest resolution data (5 Hz) did compare favorably with other turbulence studies during times of robust turbulence.

However, they were slightly higher than the rates found in tropical cyclones by Merceret in the 1970s during boundary layer aircraft penetration studies. This study suggests that boundary layer features occurring in tropical cyclones do have an impact at the surface. High-resolution surface based data shows significant portions of the wind speed attributed to frequencies expected from the small features detected by advanced radar during hurricane landfalls. Characterizing the loss of momentum via this technique in different landfall situations can be used to improve the accuracy of physical schemes in hurricane modeling. Numerous interceptions of landfalling storms in the last five years have been made by teams from several institutions, consisting of rapidly-deployable surface weather stations and high-frequency radars, and our results will be compared to their published work.

Subsequent examinations of surface data in landfalling tropical cyclones should address the question of universality of the results presented in this study. More specifically, dividing turbulence dissipation rates calculated into categories describing different storm conditions can explore how variable the momentum loss is during landfall. For example, differences in fetch from overland versus over water could be large. Also, movement of the storm could be a significant factor in determining changes in momentum characteristics during landfall. Such division into categories could also be useful in answering questions regarding the differences in the shapes and amplitudes of spectra. A vastly greater number of landfalling storms would have to be sampled to examine these kinds of questions in more detail.

The spectra of wind behavior have broader implications than just the description of features of hurricane structure. The frequencies present in these high winds also have a great deal of value to structural engineers attempting to design building materials resistant to damage from landfalling hurricanes.

There is a direct relationship between natural resonance frequencies of materials such as windows and frequencies in the wind. If it can be demonstrated which frequencies are most likely to occur, and the length of time they occur, the designs can be altered to minimize damage.

Most work in the structural engineering literature relates to properties such as “gust factors” which describe the size of perturbations or gusts present in the wind. This work suggests that spectral analysis could be used to explore changes in the behavior of the wind at different times as a storm makes landfall. Momentum loss characteristics that would be likely to occur in different landfall situations could be different from location to location, and building designs could be altered accordingly.

Unfortunately, there were important data gaps due to communication or power failures in some of the most potentially interesting time periods. These problems need to be addressed, since it is the data from intense weather events that is most valuable. It is also apparent that the CMAN buoys need more reliable instrumentation to record moisture content. It was not clear what was causing the errors and unphysical reports, but reliable moisture content information could be very useful.

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