Directional Wave Spectra Observed During Intense Tropical Cyclones

Abstract

Two deep‐sea moorings were deployed 780 km off the coast of southern Taiwan for 4–5 months during the 2010 typhoon season. Directional wave spectra, wind speed and direction, and momentum fluxes were recorded on two Extreme Air‐Sea Interaction buoys during the close passage of Severe Tropical Storm Dianmu and three tropical cyclones (TCs): Typhoon Fanapi, Super Typhoon Megi, and Typhoon Chaba. Conditions sampled include significant wave heights up to 11 m and wind speeds up to 26 m s−1. Details varied for large‐scale spectral structure in frequency and direction but were mostly bimodal. The modes were generally composed of a swell system emanating from the most intense storm region and local wind‐seas. The peak systems were consistently young, meaning actively forced by winds, when the storms were close. During the peaks of the most intense passages—Chaba at the northern mooring and Megi at the southern—the bimodal seas coalesced. During Chaba, the swell and wind‐sea coupling directed the high frequency waves and the wind stress away from the wind direction. A spectral wave model was able reproduce many of the macrofeatures of the directional spectra.

Typhoon/Hurricane-Generated Wind Waves Inferred from SAR Imagery

“…For instance, this is the case in the cyclone's left forward quadrant where the direction of wind deviates considerably from the wave direction [31]. As shown in a previous study [11], the analysis of directional spectra observed by Extreme Air-Sea Interaction buoys shows that a variety of spectral geometries can exist close to the eyes of typhoons. Thus, the effectiveness of the simple fetch-limited parametric models should be further discussed, in terms of the cyclone quadrant under consideration, and the rate of development or change the intensity of the storm.…”

“…Moreover, many studies have shown that most of the spectra are monomodal under extreme conditions, similar to the spectra generated under fetch-limited, steady wind conditions. However, bi-modal spectra are also found in both measurements and model results under intense cyclone conditions [11,29,31], in which case the accuracy of the parametric models used in this study can be degraded. For instance, this is the case in the cyclone's left forward quadrant where the direction of wind deviates considerably from the wave direction [31].…”

“…A well-known example is the Perfect Storm in October 1991, which is sometimes known by wave researchers as the Halloween Storm, which generated maximum waves in excess of 30 m [6]. The development of remote sensing has greatly motivated the studies of large storm waves [7][8][9], as in situ observations are spatially sparse and expensive undertakings [10][11][12][13]. Because of its ability to make measurements under almost all weather conditions, day or night, synthetic aperture radar (SAR) is an important way to monitor marine winds and waves.…”

“…Young [29] suggested that the nonlinear wave-wave interactions play a central role in stabilizing the shape of unimodal spectrum [33,34]. This is achieved by the transfer of wave energy within the spectrum, from high frequencies to low frequencies, and the other way around, from low to high frequencies [35,36], although the wind input and dissipation by breaking may also be important at this stage of wave growth [11,31,37]. Many parametric models were built on the assumption of the fetch-limited condition, particularly in the 1970s and 1980s [38], providing a rapid means to explore the general characteristics of waves generated by storms, especially the asymmetry of the wave field [39][40][41] and energy [32] inside the storms.…”

Introduction to Special Section on Oceanic Responses and Feedbacks to Tropical Cyclones

“…New techniques and algorithms are continuously proposed to improve the retrieval of TC properties from Earth orbiting satellites; for example, Shao et al () improved estimation of TC wind speeds by exploiting wind‐generated waves extracted from spaceborne synthetic aperture radar (SAR). Intensive and well‐organized projects are usually an auspice of significant progress in the ocean‐TC studies, such as the Coupled Boundary Layer Air‐Sea Transfer (CBLAST) experiment (Black et al, ) and the Impact of Typhoons on the Ocean in the Pacific (ITOP) project (Collins et al, ; D'Asaro et al, ; Potter et al, ). Recently, in order to collect a chronic and three‐dimensional perspective on changes to the water column during TCs, a series of mooring/buoy arrays are deployed in the northern South China Sea (Zhang et al, ) where historical statistics show that typhoon has the highest occurrence probability.…”