The aerodynamic friction between air and sea is an important part of the momentum balance in the development of tropical cyclones. Measurements of the drag coefficient, relating the tangential stress ...(frictional drag) between wind and water to the wind speed and air density, have yielded reliable information in wind speeds less than 20 m/s (about 39 knots). In these moderate conditions it is generally accepted that the drag coefficient (or equivalently, the “aerodynamic roughness”) increases with the wind speed. Can one merely extrapolate this wind speed tendency to describe the aerodynamic roughness of the ocean in the extreme wind speeds that occur in hurricanes (wind speeds greater than 30 m/s)? This paper attempts to answer this question, guided by laboratory extreme wind experiments, and concludes that the aerodynamic roughness approaches a limiting value in high winds. A fluid mechanical explanation of this phenomenon is given.
IMPACT OF TYPHOONS ON THE OCEAN IN THE PACIFIC D’Asaro, E.A.; Black, P. G.; Centurioni, L. R. ...
Bulletin of the American Meteorological Society,
09/2014, Letnik:
95, Številka:
9
Journal Article
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Odprti dostop
Tropical cyclones (TCs) change the ocean by mixing deeper water into the surface layers, by the direct air–sea exchange of moisture and heat from the sea surface, and by inducing currents, surface ...waves, and waves internal to the ocean. In turn, the changed ocean influences the intensity of the TC, primarily through the action of surface waves and of cooler surface temperatures that modify the air–sea fluxes. The Impact of Typhoons on the Ocean in the Pacific (ITOP) program made detailed measurements of three different TCs (i.e., typhoons) and their interaction with the ocean in the western Pacific. ITOP coordinated meteorological and oceanic observations from aircraft and satellites with deployments of autonomous oceanographic instruments from the aircraft and from ships. These platforms and instruments measured typhoon intensity and structure, the underlying ocean structure, and the long-term recovery of the ocean from the storms' effects with a particular emphasis on the cooling of the ocean beneath the storm and the resulting cold wake. Initial results show how different TCs create very different wakes, whose strength and properties depend most heavily on the nondimensional storm speed. The degree to which air–sea fluxes in the TC core were reduced by ocean cooling varied greatly. A warm layer formed over and capped the cold wakes within a few days, but a residual cold subsurface layer persisted for 10–30 days.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Data from five recent field campaigns are selected for pure wind sea, deep water, and fully rough flow conditions. The combined data set includes a wide range of wave ages, with high variability in ...both friction velocity and wave phase speed. These data, which are expected to follow Monin‐Obukhov similarity scaling, are used to investigate the influence of wave age on wind stress. The relationship between the dimensionless roughness and inverse wave age is found to be zo/σ = 13.4 (u*/cp)3.4, where zo is the surface roughness length, σ is the standard deviation of the surface elevation, u* is the friction velocity, and cp is the wave phase speed at the peak of the spectrum. This relationship, which represents a significant dependence of roughness on wave age, was obtained using a procedure that minimizes the effects of spurious correlation in u*. It is also shown to be consistent with the wave age relationship derived using an alternate form of the dimensionless roughness, namely, the Charnock parameter zog/u*2, where g is the gravitational constant.
High-temperature superconducting (HTS) direct current (dc) power cables allow high levels of power transmission and distribution at low loss and can be tailored to effectively limit fault currents. ...HTS Conductor on Round Core (CORC ) power transmission cables offer additional benefits over other HTS cable designs, including a much higher current density and a higher degree of flexibility. These benefits make CORC cables most suitable for applications in confined spaces where tight bends are required, such as onboard naval ships and in data centers. The development of CORC power transmission cables for operation in pressurized helium gas is described, including their ability to act as fault current limiting cables. The 10 m long bipolar dc CORC power transmission cable system is designed to operate at a current of 4000 A per pole at 50 K in pressurized helium gas. The test results at temperatures between 60 K-74 K in helium gas at a pressure of 1.7 MPa are described both during normal operation and during an overcurrent event. The results demonstrate the potential of CORC cables to operate at currents exceeding 10 000 A per pole at 50 K at current densities of more than 200 A mm−2, resulting in the most energy dense superconducting power transmission cable to date. The successful operation during an overcurrent event also shows the added benefits of the high level of current sharing between tapes in CORC cables that allow them to be operated as FCL cables without the need to incorporate a substantial amount of stabilizer. The successful test is a major milestone towards reliable high energy density power transmission in helium gas cooled superconducting power systems based on CORC cables.
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.
Plain Language Summary
Wind blowing over the ocean generates waves. Tropical cyclones, with their cyclonic wind pattern and extremely high winds, generate some of the most dangerous and complex wave fields on the planet. Using buoy measurements, which survived the close passages three intense tropical cyclones off the coast of Taiwan in 2010, we look at the details of these complex wave fields. We found that the wave field is generally made up of a system of longer waves which emanate from the most intense region of the storm and a system of shorter waves which are produced by the local winds. These two wave systems often propagate in different directions. As a storm passes, sometimes these two wave systems overlap and combine and sometimes they remain separate. It is unclear exactly what determines whether or not they combine, but it is certainly a nonlinear phenomenon. During one passage, the short and long seas combined shifting the direction of the short waves, which in turn interacted with the atmosphere by shifting the direction of the wind stress. This result was surprising and may help to improve our ability to model tropical cyclone intensity.
Key Points
Directional wave spectra were observed under the close passage of intense tropical cyclones
Seas were bimodal but coalesced during the two most intense passages; however, seas were young, refuting the dominance of shape stability
During Chaba, swell and wind‐sea coupled and directed high frequency waves and wind stress away from the wind direction
Abstract
Wind-sea generation was observed during two experiments off the coast of North Carolina. One event with offshore winds of 9–11 m s−1 directed 20° from shore normal was observed with eight ...directional stations recording simultaneously and spanning a fetch from 4 to 83 km. An opposing swell of 1-m height and 10-s period was also present. The wind-sea part of the wave spectrum conforms to established growth curves for significant wave height and peak period, except at inner-shelf stations where a large alongshore wind-sea component was observed. At these short fetches, the mean wave direction θm was observed to change abruptly across the wind-sea spectral peak, from alongshore at lower frequencies to downwind at higher frequencies. Waves from another event with offshore winds of 6–14 m s−1 directed 20°–30° from shore normal were observed with two instrument arrays. A significant amount of low-frequency wave energy was observed to propagate alongshore from the region where the wind was strongest. These measurements are used to assess the performance of some widely used parameterizations in wave models. The modeled transition of θm across the wind-sea spectrum is smoother than that in the observations and is reproduced very differently by different parameterizations, giving insights into the appropriate level of dissipation. Calculations with the full Boltzmann integral of quartet wave–wave interactions reveal that the discrete interaction approximation parameterization for these interactions is reasonably accurate at the peak of the wind sea but overpredicts the directional spread at high frequencies. This error is well compensated by parameterizations of the wind input source term that have a narrow directional distribution. Observations also highlight deficiencies in some parameterizations of wave dissipation processes in mixed swell–wind-sea conditions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Combination of laser and radar aboard an aircraft is used to directly measure long gravity wave surface tilting simultaneously with nadir-viewing microwave backscatter from the sea surface. ...The presented dataset is extensive, encompassing varied wind conditions over coastal and open-ocean wave regimes. Laser-derived slope statistics and Ka-band (36 GHz) radar backscatter are detailed separately to document their respective variations versus near-surface wind speed. The slope statistics, measured for λ > 1–2 m, show good agreement with Cox and Munk's oil-slickened sea measurements. A notable exception is elevated distribution peakedness and an observed wind dependence in this likely proxy for nonlinear wave–wave interactions. Aircraft Ka-band radar data nearly mimic Ku-band satellite altimeter observations in their mean wind dependence. The present calibrated radar data, along with relevant observational and theoretical studies, suggest a large (−5 dB) bias in previous Ka-band results. Next, wave-diverse inland, coastal, and open-ocean observations are contrasted to show wind-independent long-wave slope variance changes of a factor of 2–3, always increasing as one heads to sea. Combined long-wave and radar data demonstrate that this long-wave tilt field variability is largely responsible for radar backscatter variations observed at a given wind speed, particularly at wind speeds below 5–7 m s−1. Results are consistent with, and provide quantititative support for, recent satellite altimeter studies eliciting signatures of long-wave impacts resident in the radar backscatter. Under a quasi-optical scattering assumption, the results illustrate long-wave control on the variance of the total mean square slope parameter due to changes in the directional long-wave spectrum, with high-wavenumbers being relatively unaffected in a mean sense. However, further analysis suggests that for winds above 7 m s−1 the high-wavenumber subrange also varies with change in the longer wave field slope and/or energy, the short gravity wave roughness being measurably greater for smoother seas.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In this paper, we study the retrieval of wind information from nautical X-band radar data. In contrast to previous studies, where data from stationary research platforms were used, this study focuses ...on data from a moving platform, encountering a larger variety of conditions than a platform at a fixed location. Compared to traditional in situ sensors, wind data derived from nautical radar images are much less susceptible to air flow distortion by the platform, since the images cover a large area around the ship. Images collected with a standard nautical HH-polarized X-band radar operating at grazing incidence exhibit a single intensity peak in upwind direction. The wind retrieval method developed here uses a harmonic function that is least-squares fitted to the radar backscatter intensity as a function of antenna look direction. The upwind direction is given by the direction that corresponds to the peak of the fitted function. An empirical model function is derived to retrieve the wind speed from the average radar backscatter intensity. Contrary to wind retrieval methods that have been proposed before, this approach is well suited for data acquired from a moving platform, as it functions well even if the radar field of view is partially shadowed and does not require ship motion correction. Here, we focus on data that were collected during two storms, using the first storm to derive and the second to test the empirical model functions. The method is validated using measurements from two ship-based anemometers.
This paper describes the new Extreme AirSea Interaction (EASI) buoy designed to measure direct airsea fluxes, as well as mean properties of the lower atmosphere, upper ocean, and surface waves in ...high wind and wave conditions. The design of the buoy and its associated deep-water mooring are discussed. The performance of EASI during its 2010 deployment off Taiwan, where three typhoons were encountered, is summarized.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A new method to determine near‐surface vertical current shear from noncoherent marine X‐band radar (MR) data is introduced. A three‐dimensional fast Fourier transform is employed to obtain the wave ...number‐frequency spectrum of a MR image sequence. Near‐surface currents are estimated from the Doppler‐shifted surface gravity wave signal within the spectrum. They represent a weighted mean of the upper ocean flow. The longer the ocean waves on which the current estimates are based, the greater their effective depth. The novelty lies in the wave number‐dependent retrieval method, yielding ∼100 independent current estimates at effective depths from ∼2 to 8 m per ∼12 min measurement period. First, MR near‐surface vertical current shear measurements are presented using data collected from R/V Roger Revelle during the 2010 Impact of Typhoons on the Ocean in the Pacific experiment in the Philippine Sea. Shipboard acoustic Doppler current profiler (ADCP) and anemometer measurements as well as WAVEWATCH III (WW3) model results are used to demonstrate that results are in accord with physical expectations. The wind and wave‐driven Ekman flow is obtained by subtracting ADCP‐based background currents from the radar measurements. At ∼2 m, it is on average ∼1.6% of the wind speed and ∼38.9° to the right of the wind. With increasing effective depth, the speed factor decreases and the deflection angle increases. Based on WW3 results, the MR‐sensed Stokes drift speed is ∼50% of the Ekman flow at ∼2 m and ∼25% at ∼8 m. These findings are consistent with previous observations and Ekman theory.
Key Points:
A new marine radar near‐surface vertical current shear retrieval method is introduced
The mean ∼2 m Ekman flow has a ∼1.6% speed factor and ∼38.9° deflection angle
The filtered Stokes drift speed at ∼2 m is ∼50% of the Ekman flow and at ∼8 m it is ∼25%
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