In-situ Observation Over Land

In addition to observations made over the sea, observations of hurricane winds taken over land are also an important data source of direct measurements used in studying the wind field of the HBL. Unlike measurements taken above the sea by reconnaissance aircraft or by Doppler Radars, the in-situ measurement taken over land can only cap- ture wind dynamics at several discrete points where the observational tower or mast stands. Thus, studies using the in-situ observation data concentrated on ”local” HBL

wind features.

As early as 1979, Sethraman (1979) tried to analyze characteristics of hurricane winds using the wind measurements taken by observational towers along the coast at the mo- ment a hurricane passes by. By investigating the mean and fluctuating wind in the surface layer at three locations in the Long Island, N.Y. during the landfall of Hurricane Belle (1976), he found some basic wind characteristics of hurricanes making landfall, such as the significant increase of wind shears, the rough range of the friction velocity and turbulent energy dissipation rate. Besides, this study also revealed that the wind speed measured at the beach is 3− 5 times of that measured in land indicating a sharp decrease of the surface wind speed due to land friction.

In a series of studies conducted by Schroeder et al. (1998), Schroeder and Smith (2003) and Schroeder et al. (2009), meteorological data gathered when hurricanes making land- fall by many mobile towers and masts deployed by several different institutes were ana- lyzed in order to obtain hurricane wind characteristics. They focused on ”local” turbu- lence features, such as the point gust factor, turbulence intensity and turbulent integral length scale. By analyzing these ”local” features in different measuring environments, they studied the variation of such turbulence characteristics with surface roughness, with mean wind speeds, and with relative positions where the measurement is taken. The major finding of these studies is that the turbulence of the HBL is noticeably different from the turbulence in the standard ABL. More specifically, a considerable amount of turbulent energy of hurricane winds is concentrated in the region with a frequency lower than that found in ordinary ABL winds. Moreover, they concluded such turbulence char- acteristics strongly depend on the surface roughness, or the surface exposure, although some secondary dependencies were also presented.

Yu et al. (2008) followed the trend of using in-situ observations over land to derive tur- bulence characteristics of the HBL. The analysis was conducted based on the calculated wind spectra Suu, co-spectraSuw and turbulent integral length scale from data measured in the Florida Coastal Monitoring Program (FCMP). Different from the study of Sethra-

man (1979), this study utilized data from five towers during four hurricane passages. In an agreement with Schroeder and Smith (2003), they concluded that the energy at lower frequency is considerably higher in the hurricane winds than that in non-hurricane winds. The wind spectra, co-spectra and length scale derived can be used as a validation criteria in designing the novel wind tunnel facility in which the hurricane wind needs to be reproduced.

Using the data from the same measurement project, i.e. the FCMP, Yu and Chowd- hury (2009) and Masters et al. (2010) extended the study of Yu et al. (2008). In addition to the spectral representation of hurricane winds, Yu and Chowdhury (2009) discussed the gust factor. Based on the FCMP data, they found that the current model is inade- quate to predict the gust factor of hurricane winds over land with the surface exposure categorized as open flat terrain according to the American Society of Civil Engineers

(ASCE) 7 Standard Commentary, and it is 10%− 15% higher than the gust factor found

in extratropical winds. Using nine mobile observation towers in the FCMP project, Mas- ters et al. (2010) focused on mean and turbulence characteristics of hurricane winds for three hurricanes in 2005, i.e. Hurricane Katrina, Hurricane Rita and Hurricane Wilma. Since the purpose of the study of Masters et al. (2010) was to provide inputs for struc- tural wind tunnel tests or full-scale assessments for wind loads on structures, it contained turbulence characteristics in addition to those summarized by Yu et al. (2008), which are the friction velocity,u_∗, aerodynamic roughness lengthz0 and the 3− sgust wind velocity.

Using wind statistics summarized by Masters et al. (2010), it is possible to reproduce a wind field statistically close to natural hurricane winds in a experimental environment.

It can be seen that all studies using data from in-situ observations over land are confined to analyze wind characteristics within the surface layer for only a few discrete points. It is impossible to derive the vertical structure of wind characteristics in the entire HBL from such observations. Taking into consideration that observations over land follow the conventional methodology and give sufficient measurements to derive turbulence characteristics, its result should be used to find the conventional turbulence

statistics of the hurricane winds at isolated points, but not the vertical structure of the entire HBL.