Modification of local roughness length by advancing storm surge in landfalling tropical cyclones

Introduction The effect of the transition from marine to land exposure on a hurricane's near-surface turbulence structure has fostered considerable study and debate, both on the landward and the seaward side of the interface (e.g. Schroeder et al., 2009; Vickery et al., 2010; Kennedy, 2011). Near-shore ocean conditions have been shown to vary with changing wind speeds, which in turn influences the surface roughness (zo) felt by the wind approaching the coastline. Once the wind has moved over land its turbulent structure will be governed by the marine-land transition and then the local surface zo (Paulsen and Schroeder, 2005). When storm surge inundates a coastal region a third regime is possible in which underlying surface conditions are obscured by water, leading to an aerodynamically smoother exposure characterized by lower zos. The American Society of Civil Engineers (ASCE) currently classifies costal building design using roughness categories according to the conditions of a non-storm environment. Evidence from data collected by Texas Tech University (TTU) in Hurricane Ike suggests that a tropical cyclone's storm surge can lead to a decrease in an inland region's aerodynamic surface roughness. Data An array of 2.25 m observation probes (termed “StickNet” probes) were deployed around the Galveston Bay and Bolivar Peninsula by TTU students to collect wind speed and direction data for the landfall of Hurricane Ike. Details of the StickNet platform are available in Weiss and Schroeder (2008). The probes collected data continuously at a rate of 1-10 Hz throughout the passage of the center of circulation. Coastal topography and bathymetry, along with Hurricane Ike's large size, led to a significant storm surge which inundated several of the StickNet probes. The StickNets continued to record wind speed and direction data, which led to a unique dataset in which the effect of storm surge inundation on surface roughness can be evaluated. Analysis Method Prior to analysis, StickNet data were adjusted from an observation height of 2.25 m to a height of 10 m using the log law, roughness length (zo), and friction velocity. The data were divided into consecutive 10-minute segments then run through a set of Matlab algorithms which calculated the wind speed, direction, and zo. The NOAA SLOSH Model was used to estimate the timing of the advance of Hurricane Ike's storm surge and determine the approximate times at which each of the probes were inundated. WSR-88D radar center fixes were plotted in ArcMap and used along with the SLOSH data to identify a likely time of inundation for each probe of interest. Once an estimated time of inundation was determined for each probe, time histories of wind speed and direction were used to evaluate zo behavior before and after the estimated time of inundation. To ensure that any identified changes in zo in each probe's time history were due to inundation by storm surge and not changes in zo, inherent to the local environment, Google Earth satellite imagery was used to qualitatively estimate the ASCE surface roughness category for eight 45 degree sectors surrounding each probe's location. Preliminary Results and Discussion Probe 110A was located at Fort Travis Park on the western end of the Bolivar Peninsula. The site experienced inundation by storm surge on the order of 0.6 m (2 ft), but the probe remained operational throughout the passage of the storm. The site was characterized by largely open terrain with the occasional tree and low park building. Time histories of wind speed and direction indicate that the eye of Hurricane Ike passed over the probe just after 5:30 on the morning of 13 September, 2008. Prior to passage of the eyewall, the wind direction was easterly, turning south-southwesterly after passage of the eyewall. Despite the uniform terrain surrounding Probe 110A, the zo time history indicates a reduction in zo just after the second passage of the eyewall (Figure 1). Based upon the timing of the surge as indicated by radar imagery and storm surge observations and models, it is thought that this reduction in zo is due to inundation by storm surge. References Kennedy, A. B., U. Gravois, and B. Zachry, 2011: “Observations of Landfalling Wave Spectra during Hurricane Ike,” Journal of Waterway, Port, Coastal, and Ocean Engineering, 137, 142-145. Paulsen, B. M., and J. L. Schroeder, 2005: “An Examination of Tropical and Extratropical Gust Factors and the Associated Wind Speed Histograms,” Journal of Applied Meteorology, 44, 270-280. Schroeder, J. L., B. P. Edwards, and I. M. Giammanco, 2009: “Observed tropical cyclone wind flow characteristics,” Wind and Structures, 4, 1-33. Weiss, C. C., and J. L. Schroeder, 2008: “StickNet: A New Portable Rapidly Deployable Surface Observations System,” Bulletin of the American Meteorological Society, 89, 1502-1503. Vickery, P. J., D. Wadhera, J. Galsworthy, J. A. Peterka, P. A. Irwin, and L. A. Griffis, 2010: “Ultimate Wind Load Design Gust Wind Speeds in the United States for Use in ASCE-7,” Journal of Structural Engineering, 136, 613-625.