Florida Coastal Monitoring Program Research Articles

A New Framework for Evaluating Model Simulated Inland Tropical Cyclone Wind Fields

AbstractThough tropical cyclone (TC) models have been routinely evaluated against track and intensity observations, little work has been performed to validate modeled TC wind fields over land. In this paper, we present a simple framework for evaluating simulated low‐level inland winds with in‐situ observations and existing TC structure theory. The Automated Surface Observing Systems, Florida Coastal Monitoring Program, and best track data are used to generate a theory‐predicted wind profile that reasonably represents the observed radial distribution of TC wind speeds. We quantitatively and qualitatively evaluated the modeled inland TC wind fields, and described the model performance with a set of simple indicators. The framework was used to examine the performance of a high‐resolution two‐way nested Geophysical Fluid Dynamics Laboratory model on recent U.S. landfalling TCs. Results demonstrate the capacity of using this framework to assess the modeled TC low‐level wind field in the absence of dense inland observations.

Influences on Observed Near-Surface Gust Factors in Landfalling U.S. Gulf Coast Hurricanes: 2004–08

AbstractThe deployment of ruggedized surface observing platforms by university research programs in the path of landfalling tropical cyclones has yielded a wealth of information regarding the near-surface wind flow characteristics. Data records collected by Texas Tech University’s Wind Engineering Mobile Instrument Tower Experiment and StickNet probes and by the Florida Coastal Monitoring Program along the Gulf Coast of the United States from 2004 to 2008 were compiled to examine influences on near-surface gust factors. Archived composite reflectivity data from coastal WSR-88D instruments were also merged with the tower records to investigate the influence of precipitation structure. Wind records were partitioned into 10-min segments, and the ratio of the peak moving-average 3-s-gust wind speed to the segment mean was used to define a gust factor. Observations were objectively stratified into terrain exposure categories to determine if factors beyond those associated with surface frictional effects can be extracted from the observations. Wind flow characteristics within exposure classes were weakly influenced by storm-relative position and precipitation structure. Eyewall observations showed little difference in mean gust factors when compared with other regions. In convective precipitation, only peak gust factors were slightly larger than those found in stratiform conditions, with little differences in the mean. Gust factors decreased slightly with decreasing radial distance in rougher terrain exposures and did not respond to radar-observed changes in precipitation structure. In two limited comparisons, near-surface gusts did not exceed the magnitude of the wind maximum aloft detected through wind profiles that were derived from WSR-88D velocity–azimuth displays.

Mapping of Near-Surface Winds in Hurricane Rita Using Finescale Radar, Anemometer, and Land-Use Data

Abstract Data collected from a Doppler on Wheels (DOW) mobile radar deployed in Port Arthur, Texas, near the point of landfall of Hurricane Rita (2005) and from two Florida Coastal Monitoring Program 10-m weather stations (FCMP-WSs) are used to characterize wind field variability, including hurricane boundary layer (HBL) streaks/rolls, during the hurricane's passage. DOW data, validated against nearby weather station data, are combined with surface roughness fields derived from land-use mapping to produce fine spatial scale, two-dimensional maps of the 10 m above ground level (AGL) open-terrain exposure and exposure-influenced winds over Port Arthur. The DOW collected ~3000 low-elevation radar sweeps at 12-s intervals for >10 h during the passage of the hurricane. This study focuses on the 2–3-h period when the western eyewall passed over Port Arthur. Finescale HBL wind streaks are observed to have length scales of O(300 m), smaller than previously identified in other HBL studies. The HBL streaks are tracked as they pass over an FCMP-WS located in flat, open terrain and another FCMP-WS located near a subdivision. DOW data collected over the FCMP-WS are reduced to anemometer height, using roughness lengths calculated from DOW and FCMP-WS data. Variations in the radar-observed winds directly over the FCMP-WS are very well correlated, both in their timing and magnitude, with wind gusts observed by the weather stations, revealing directly for the first time the surface manifestation of these wind streaks that are observed frequently by radar >100 m AGL. This allows for the generation of spatially filled maps of small-scale wind fluctuations over Port Arthur during the hurricane eyewall's passage using DOW-measured winds.

Analysis of hurricane wind loads on low-rise structures

Time series of pressure coefficients collected on the roof of a house by the Florida Coastal Monitoring Program during landfall of Hurricane Ivan on the Florida panhandle in 2004 are analyzed. Rather than using peak values, which could vary due to the stochastic nature of the data, a probabilistic analysis is performed to characterize extreme values of pressure coefficients and associated wind loads. It is shown that the pressure coefficient time series follows a three parameter Gamma distribution, while the peak pressure follows a two-parameter Gumbel distribution. The analysis yields a probability of non-exceedance of a given threshold of the pressure or load coefficients. For this specific house and specific storm, the 80 percentile load coefficient value of the probability of non-exceedance is −1.7. This is discussed in the context of ASCE 7 GCp values.

The Florida Coastal Monitoring Program (FCMP): A review

The Florida Coastal Monitoring Program (FCMP) is a multi-university field research program that was founded in 1998 to study the near-surface wind and rain characteristics of Atlantic hurricanes and their effects on coastal infrastructure. The FCMP has three research thrusts: (1) characterization of surface wind and wind-driven rain conditions, (2) quantification of wind induced component and cladding pressures, and (3) assessment of damage to evaluate the performance of single family homes and the building codes and standards that guided their construction. This paper presents an overview of the program, including the motivation for the field research, a review of the program's scientific objectives, the history of the program from 1999–2010, and descriptions of the instrument systems and supporting infrastructure. A case study of Hurricane Frances (2004) is presented, and the current and potential uses of the collected wind field data are discussed.

On Momentum Transport and Dissipative Heating during Hurricane Landfalls

Abstract Momentum transport and dissipative heating are investigated using the high-resolution (10 Hz) wind data collected by Florida Coastal Monitoring Program portable weather stations in the surface layer of three landfalling hurricanes. The momentum flux is calculated using the eddy correlation method. The drag coefficient is determined from the momentum flux and surface wind speed. The values of the momentum flux and drag coefficient are found to be generally larger than those observed over the ocean at similar wind speeds up to near hurricane strength. The rate of dissipation is determined from the wind velocity spectra. The dissipative heating is estimated using two different methods: 1) integrating the rate of dissipation in the surface layer and 2) multiplying the drag coefficient by the cubic of the surface wind speed. It is found that the second method, which has been widely used in previous theoretical and numerical studies, significantly overestimates the magnitude of dissipative heating. This finding is consistent with a recent study on estimation of the dissipative heating over the ocean using in situ aircraft observations. This study is a first attempt at estimating the magnitude of dissipative heating in landfalling hurricanes using in situ observations. The results are believed to offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity.

On momentum transport and dissipative heating during hurricane landfalls

Momentum transport and dissipative heating are investigated using the high-resolution (10 Hz) wind data collected by Florida Coastal Monitoring Program portable weather stations in the surface layer of three landfalling hurricanes. The momentum flux is calculated using the eddy correlation method. The drag coefficient is determined from the momentum flux and surface wind speed. The values of the momentum flux and drag coefficient are found to be generally larger than those observed over the ocean at similar wind speeds up to near hurricane strength. The rate of dissipation is determined from the wind velocity spectra. The dissipative heating is estimated using two different methods: 1) integrating the rate of dissipation in the surface layer and 2) multiplying the drag coefficient by the cubic of the surface wind speed. It is found that the second method, which has been widely used in previous theoretical and numerical studies, significantly overestimates the magnitude of dissipative heating. This finding is consistent with a recent study on estimation of the dissipative heating over the ocean using in situ aircraft observations. This study is a first attempt at estimating the magnitude of dissipative heating in landfalling hurricanes using in situ observations. The results are believed to offer useful guidance in numerical weather prediction efforts aimed at improving the forecast of hurricane intensity.

Surface wind measurements in three Gulf Coast hurricanes of 2005

Detailed observations from surface layer experiments in the path of Atlantic hurricanes are presented in this paper. The purpose is to obtain information that will aid in the reduction of hurricane wind damage to residential structures by providing input for wind tunnel and full-scale flow simulation experiments that assess wind loads on these structures under hurricane conditions. The contents of the paper document the mean flow and turbulence characteristics from data recorded by nine mobile instrumented towers deployed at coastal locations near the anticipated path of each of the three hurricanes (Katrina, Rita, and Wilma) that made landfall on the US coast along the Gulf of Mexico in 2005. Wind data were collected at two elevations (5 and 10 m) from these towers by the Florida Coastal Monitoring Program, a joint research program led by the University of Florida and the Institute for Business and Home Safety.

Reconstruction of Hurricane Katrina's wind fields for storm surge and wave hindcasting

As the most costly US natural disaster in history, Hurricane Katrina fostered the IPET forensic study to better understand the event. All available observations from several hundred space-, land-, sea-, and aircraft-based measurement platforms were gathered and processed to a common framework for height, exposure, and averaging time, to produce a series of wind field snapshots at 3 h intervals to depict the wind structure of Katrina when in the Gulf of Mexico. The stepped-frequency microwave radiometer was calibrated against GPS sondes to establish the upper range of the instrument and then used to determine the wind field in the storm's core region in concert with airborne Doppler radar winds adjusted to the surface from near the top of the PBL (500 m). The SFMR data were used to develop a method to estimate surface winds from 3 km level reconnaissance aircraft observations, taking into consideration the observed azimuthal variation of the reduction factor. The “SFMR method” was used to adjust reconnaissance flight-level measurements to the surface in the core region when SFMR and Doppler winds were not available. A variety of coastal and inland mesonet data were employed, including portable towers deployed by Texas Tech University, University of Louisiana at Monroe, and the Florida Coastal Monitoring Program, as well as fixed mesonet stations from Louisiana State Universities Marine Consortium, University of Southern Mississippi, and Agricultural Networks from Louisiana, Mississippi, and Alabama, and the Coastal Estuarine Network of Alabama and Mississippi. Also included were land- (WSR-88D VAD and GBVTD, ASOS, Metar, LLWAS, HANDAR), space- (QuikScat, GOES cloud drift winds, WindSat), and marine- (GPS sondes, Buoys, C-MAN, ships) platforms. The wind fields serve as an analysis of record and were used to provide forcing for wave and storm surge models to produce hindcasts of water levels in the vicinity of flood control structures.

Gust Factors and Turbulence Intensities for the Tropical Cyclone Environment

Abstract Gust factors are used to convert peak wind speeds averaged over a relatively short period (e.g., 3 s) to mean wind speeds averaged over a relatively long reference period (e.g., 1 h) or vice versa. Such conversions are needed for engineering, climatological, or forecasting purposes. In this paper, gust factors in tropical cyclone (TC) winds are estimated from Florida Coastal Monitoring Program (FCMP) observations of near-surface TC wind speeds representative of flow over the sea surface and over open flat terrain in coastal areas. Comparisons are made with gust factors in extratropical winds over open flat terrain that are available in the literature. According to the results of the study, for gust durations of less than 20 s, the Durst model underestimates, and the Krayer–Marshall model overestimates, gust factors of TC winds over surfaces with roughness specified in the American Society of Civil Engineers (ASCE) 7 Standard Commentary as typical of open terrain. Consideration should be given to these findings when updating the gust factors provided in the ASCE 7 Standard Commentary. The study also compares gust factors in TC winds obtained from FCMP data with gust factors in extratropical winds obtained from near-surface wind data collected at eight Automated Surface Observing System (ASOS) stations and concludes that, depending upon terrain roughness, gust factors in TC winds can be higher by about 10%–15% than gust factors in extratropical winds. The study also presents FCMP-based estimates of turbulence intensities and their variability and shows that turbulence intensities in TC winds increase as the terrain roughness increases. The longitudinal turbulence intensity can vary from storm to storm and can exceed its typical value by as much as 20%. It is recommended that future TC wind measurement campaigns obtain temperature data usable for stratification estimation purposes, as well as information on waves and storm surge upwind of the anemometer towers.

Hurricane Wind Power Spectra, Cospectra, and Integral Length Scales

Atmospheric turbulence is an important factor in the modelling of wind forces on structures and the losses they produce in extreme wind events. However, while turbulence in non-hurricane winds has been thoroughly researched, turbulence in tropical cyclones and hurricanes that affect the Gulf and Atlantic coasts has only recently been the object of systematic study. In this paper, Florida Coastal Monitoring Program surface wind measurements over the sea surface and open flat terrain are used to estimate tropical cyclone and hurricane wind spectra and cospectra as well as integral length scales. From the analyses of wind speeds obtained from five towers in four hurricanes it can be concluded with high confidence that the turbulent energy at lower frequencies is considerably higher in hurricane than in non-hurricane winds. Estimates of turbulence spectra, cospectra, and integral turbulence scales presented can be used for the development in experimental facilities of hurricane wind flows and the forces they induce on structures.