simultaneous measurements of U10, Hs, and Tp in four hurricane hunter missions during Bonnie 1998 and. Ivan 2004 [16-18]. These datasets are identified as.
COUPLED NATURE OF HURRICANE WIND AND WAVE PROPERTIES DERIVED FROM SIMULTANEOUS MEASUREMENTS IN HURRICANE HUNTER MISSIONS Paul A. Hwang,1 and Yalin Fan2 1 2
Remote Sensing Division, Naval Research Laboratory, Washington DC
Oceanography Division, Naval Research Laboratory, Stennis Space Center, MS
The close connection between wind and wave properties has been the foundation of ocean remote sensing using microwave frequencies. This wind-wave connection is employed either explicitly or implicitly in the scatterometer wind measurement: the touted capability of “all-weather” and “day-and-night” operation of microwave wind sensing means that the sensor sees through clouds and atmosphere, thus obvious it does not measure the motion of air mass or clouds to deduce wind speed. Instead of tracking the air motion, the algorithm of scatterometer wind retrieval is based on the assumption of a deterministic relationship between wind and ocean surface waves. Because the ocean surface roughness is contributed mainly by the short scale surface waves and surface roughness critically determines the backscattering of radar waves impinged on the ocean surface, the remotely sensed normalized radar cross section (NRCS or 0) is then converted to wind velocity through the empirically determined geophysical model function (GMF). At very high wind speed, say U10 exceeding about 25 m/s, the wind speed sensitivity of 0 decreases significantly. This becomes a serious limitation for monitoring natural hazards such as hurricanes from space using altimeter, scatterometer or synthetic aperture radar (SAR). To retrieve high wind speed from ocean remote sensing, we can explore again the wind-wave coupling relationship in the energetic scale of waves near the wave spectral peak. Unlike the surface roughness, which is contributed mainly by short waves similar in length to the radar waves, the wavelengths of energetic surface wave in high winds are O(100 m). The connection between wind speed U10, significant wave height Hs, and dominant wave period Tp is the wind wave growth functions established from many decades of wind wave research, e.g., [1-7].
The robust wind-wave growth functions have been shown to be applicable to waves generated by hurricane wind fields [8-15]. Here we present results of hurricane wind speed retrieval using the significant wave height. The data used in this study are the simultaneous measurements of U10, Hs, and Tp in four hurricane hunter missions during Bonnie 1998 and Ivan 2004 [16-18]. These datasets are identified as B24, I09, I12 and I14 in this paper. Fig. 1 show the radial and azimuthal distribution of these data in colorcoded scatter plot. The coordinates of the plots have been rotated such that the hurricane heading is toward the top of the page, the left-right and front-back distances from the hurricane center shown in the figure are in km. Some of the material presented here have been given in [15], which is in the press. Fig. 2 shows the similarity relation of the windwave triplets in terms of the dimensionless wave variance as a function of dimensionless frequency: 2 # # , where # rms g 2U104 , #=pU10g-1, and g is the gravitational acceleration; the root mean squares (rms) surface elevation rms is related to the significant wave height by Hs=4rms and p=2/Tp. Superimposed in the background for reference are the results from a dataset (BHDDB) combining five field experiments with quasi-steady winds and near-neutral stability conditions; the combined dataset covers a broad range of wave conditions, particularly the wave age (the inverse dimensionless frequency). The BHDDB dataset is used as the basis for establishing the firstand second-order fitted fetch- and duration-limited growth functions [5, 6]; the fitted growth functions are shown with dashed and solid curves in each panel. Also plotted in the background with light green color are hurricane data collected by directional wave buoys in northwest coast of Australia over a period of more than 20 years [9 10]; the growth curves used in Young’s [8-10] discussions of hurricane waves are
based on [2] and [3], which are also illustrated and labeled H73 and D85, respectively. Interestingly, the degrees of data scatter of the hurricane and steadywind datasets are not that different, and the growth curves derived from ideal (steady and homogeneous) wind wave generation are applicable to both hurricane and steady-wind data groups. There are systematic differences in the agreement between the growth curves and the wind wave measurements in different sectors of the hurricane coverage area. For convenience of comparison, the data are shown with 8 different symbols (2 for each quarter referenced to the hurricane heading, see Fig. 2 inset). As observed in [11] based on analyzing a subset of B24 (60 spectra) reported in [16]: “The most variable wave conditions are in the backside of the hurricane, spanning the approximate upper and lower bounds of “hyper” and “hypo” growth conditions compared to the reference growth curves; with more hyper cases in the present measurements. In contrast, the wave conditions tend to be average to hyper in the right hand sector and hypo in the left hand sector.” More extensive discussions are presented in the analysis of the full set of B24 data [12]. Similar conclusions on the fetch- and duration-limited nature of wave growth and azimuthal variation are applicable to the other three datasets. Data points showing large deviation from the growth curves are either very close or very far from the hurricane center, indicating severe swell contamination; these data points are marked with a + (for r220 km). Using the algorithm described in [14], the hurricane wind speed can be derived from the significant wave height using and wind wave growth functions. Fig. 3 shows the comparison of wave height retrieved wind speed and the hurricane hunter measurements for the four datasets. The agreement is generally very good except for the I09 dataset. As discussed in [15], the zig-zag pattern in the I09 flight tracks (Fig.1, 2nd row) suggest frequent aircraft maneuvering and the data quality may have suffered. For B24, I12 and I14 datasets, the rms differences between retrieved and measure wind speeds are 3.3, 4.4 and 4.3 m/s; the correlation coefficients are 0.87, 0.84, and 0.88, respectively. Fig. 4 shows the same comparison but presented in terms of the retrieved and measured wind speed ratio vs. the distance from the hurricane center, with data in 8 different hurricane sectors (inset of Fig. 2) given in different symbols. The vertical dotted line
indicates the radius of maximum wind speed rm. Except for I09, the results are very good from r to about 200 km, where r is the distance of wave measurement to the hurricane center. The wind speed range presented in this paper is from 22.4 to 65.4 m/s.
Fig. 1. (Left column) U10, (middle) Hs, (right) Tp, measured in 4 hurricane hunter missions, from top to bottom: B24, I09, I12 and I14.
Fig. 2. The wave growth function in terms of #(#) for the surface waves inside hurricanes: (a) B24, (b) I09, (c) I12, and (d) I14. The data are shown with different symbols in 8 pie-shaped slices: 2 slices in each of the four hurricane quarters (L/B/R/F) shown in the inset. Measurements with r220 km are marked with + and x, respectively. On the background with light blue are quasi-steady data BHDDB and with light green are hurricane data from directional buoy recording (Y88/06 [9][10]); the reference growth curves shown are: dotted (H73 [2]), dashed-dotted (D85 [3]); and solid and dashed for the second- and first-order fittings through the BHDDB quasisteady wind forcing data (H04 [5]).
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Fig. 3. Comparison of the wind speed retrieved from the significant wave height with the hurricane hunter measurement: (a) B24, (b) I09, (c) I12, and (d) I14.
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Fig. 4. The ratio of the wind speed retrieved from the significant wave height and the hurricane hunter measurement, shown as a function of the distance from the hurricane center: (a) B24, (b) I09, (c) I12, and (d) I14.
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