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L. Mahrt and Tihomir Hristov

Abstract

Observations of the heat flux over the open sea and in the coastal zone are analyzed to reexamine the relation of the heat flux to the air–sea temperature difference and wind speed. The study begins by examining problems with different methods for estimating the air–sea temperature difference. The difference between the air and within-water temperature is found to be most suitable for one dataset, while the difference between the air and the radiometrically measured sea surface temperature must be used for the second dataset. On average, the heat flux is linearly proportional to the product of the air–sea temperature difference and wind speed corresponding to approximately constant transfer coefficient for heat C H. Deviations of the heat flux from this simple relationship were generally weakly related or unrelated to the surface bulk Richardson number Rb, even for the coastal zone site. Similar results are also found for the moisture flux. In contrast to the general success of constant transfer coefficient for heat, C H plotted directly as a function of Rb is systematically related to the square root of Rb. The role of the wind speed as a shared variable between C H and Rb also predicts such a square root dependence, suggesting that the relationship could be largely due to self-correlation, which is also supported by a nominal study of self-correlation. However, confident isolation of the influences of stability, self-correlation, uncertainties in the air–sea temperature difference, and other physics requires more extensive data.

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Tihomir Hristov and Jesus Ruiz-Plancarte

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The authors analyze the influence of waves on the budgets of momentum flux and kinetic energy in the atmospheric flow over sea surface waves and use the findings to reinterpret the results from the earlier empirical studies on the subject. This analysis employs the framework of wave–mean flow interaction and experimental data collected recently over the open ocean. From a minimal set of plausible assumptions, limited to small-slope waves and uncorrelated turbulent and wave-induced motions in the wind, this study demonstrates that the budgets apply separately to the turbulent and the wave-induced flows. The explicit forms of the wave-supported fluxes of momentum and kinetic energy favor wave spectra ∝ ω β, 4 ≤ β ≤ 5 for wind–wave equilibrium. These explicit forms also show that in common conditions at heights above one significant wave height from the unperturbed surface, the wave-supported fluxes are a small fraction of the total, of the order of 5%. The wave influence on the kinetic energy budget and on the shape of the wind profile is therefore also small at these heights and thus difficult to identify experimentally next to influences from nonstationarity or horizontal inhomogeneity. Consequently, the predictions of Monin–Obukhov phenomenology show little sensitivity to wave effects. This makes the phenomenology as valid over the ocean as it is over land, but a poor instrument for studying wind–wave interaction. Describing the wind–wave interaction through the dynamics and statistics of the wave-induced motion remains a viable and productive alternative.

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Lichuan Wu, Tihomir Hristov, and Anna Rutgersson

Abstract

The wave-coherent momentum flux and velocity variances are investigated using a theoretical model and open-ocean measurements. The spectrum-integrated wave-coherent (SIWC) momentum flux and velocity variances decay roughly exponentially with height. The exponential decay coefficients of the SIWC momentum flux and velocity variances decrease with increasing peak wavenumber. The phases of the wave-coherent horizontal (vertical) velocity fluctuations are approximately 180° (90°) under waves with wind-wave angle |α 1| < 90°. In general, the ratio of the SIWC momentum flux to the total momentum flux under swell conditions is higher than that under wind-wave conditions at the same height. At a height of 9.9 m, the SIWC vertical (horizontal) velocity variances can exceed 30% (10%) of the total vertical (horizontal) velocity variances at high wave ages. The impact of SIWC momentum flux on wind profiles is determined mainly by the surface SIWC momentum flux ratio, the decay coefficient of the SIWC momentum flux, and the sea surface roughness length, with the first two factors being dominant. The results of this study suggest a methodology for parameterizing the SIWC momentum flux and the total momentum flux over the ocean. These results are important for simulating the marine atmospheric boundary layer and should be used in model development.

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Larry Mahrt, Scott Miller, Tihomir Hristov, and James Edson

Abstract

Our study analyzes measurements primarily from two Floating Instrument Platform (FLIP) field programs and from the Air–Sea Interaction Tower (ASIT) site to examine the relationship between the wind and sea surface stress for contrasting conditions. The direct relationship of the surface momentum flux to U 2 is found to be better posed than the relationship between and U, where U is the wind speed and is the friction velocity. Our datasets indicate that the stress magnitude often decreases significantly with height near the surface due to thin marine boundary layers and/or enhanced stress divergence close to the sea surface. Our study attempts to correct the surface stress estimated from traditional observational levels by using multiple observational levels near the surface and extrapolating to the surface. The effect of stability on the surface stress appears to be generally smaller than errors due to the stress divergence. Definite conclusions require more extensive measurements close to the sea surface.

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Peter P. Sullivan, James B. Edson, Tihomir Hristov, and James C. McWilliams

Abstract

Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases by a factor of 4 compared to a flat surface. The mean wind profile, turbulence variances, and vertical momentum flux are then dependent on the state of the wave field. The LES results are compared with measurements from the Coupled Boundary Layers Air–Sea Transfer (CBLAST) field campaign. A quadrant analysis of the momentum flux from CBLAST verifies a wave age dependence predicted by the LES solutions. The measured bulk drag coefficient CD then depends on wind speed and wave state. In situations with light wind following swell, CD is approximately 50% lower than values obtained from standard bulk parameterizations that have no sea state dependence. In extreme cases with light wind and persistent swell, CD < 0.

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Scott D. Miller, Tihomir S. Hristov, James B. Edson, and Carl A. Friehe

Abstract

Platform motion contaminates turbulence statistics measured in the surface layer over the ocean and therefore adds uncertainty to the understanding and parameterization of air–sea exchange. A modification to the platform motion–correction procedure of Edson et al. is presented that explicitly accounts for misalignment between anemometers and motion sensors. The method is applied to a high-resolution dataset, including four levels of turbulence within 20 m of the ocean surface, measured over deep ocean waves using the stable research platform R/P FLIP. The average error magnitude of the air–sea momentum flux (wind stress) from the four sensors during a 6-day period (10-m wind speed 2–14 m s−1) was 15% ± 1%, and varied systematically with measurement height. Motion and sensor-mounting offsets caused wind stress to be underestimated by 15% at 18.1 m, 13% at 13.8 m, and 11% at 8.7 m, and to be overestimated by 3% at 3.5 m. Sensor misalignment contributed to one-third of the correction to the wind stress. The motion correction reduced some measured artifacts in the wind that could otherwise be interpreted in terms of air–sea interaction, such as the angle between wind and wind stress vectors, while other features remained in the corrected wind, such as apparent upward momentum transfer from ocean to the atmosphere during low wind. These results demonstrate the complex interaction between motion and wind turbulence, and reinforce the necessity to measure and correct for platform motion. Finally, it is shown that the effects of motion on wind stress measured using R/P FLIP are much smaller than in situ measurements made using a conventional research ship.

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Kenneth Anderson, Barbara Brooks, Peter Caffrey, Antony Clarke, Leo Cohen, Katie Crahan, Kenneth Davidson, Arie De Jong, Gerrit De Leeuw, Denis Dion, Stephen Doss-Hammel, Paul Frederickson, Carl Friehe, Tihomir Hristov, Djamal Khelif, Marcel Moerman, Jeffery S. Reid, Steven Reising, Michael Smith, Eric Terrill, and Dimitris Tsintikidis

In the surface layer over the ocean the Monin–Obukhov similarity theory is often applied to construct vertical profiles of pressure, temperature, humidity, and wind speed. In this context, the rough boundary layer is derived from empirical relations where ocean wave characteristics are neglected. For seas where wind speed is less than ~ 10 m s−1 there is excellent agreement for both meteorological and microwave propagation theory and measurements. However, recent evidence indicates that even small waves perturb these profiles. It is, therefore, hypothesized that mechanical forcing by sea waves is responsible for modifying scalar profiles in the lowest portion of the surface layer, thereby reducing the effects of evaporation ducting on microwave signal propagation. This hypothesis, that a rough sea surface modifies the evaporation duct, was the primary motivation for the Rough Evaporation Duct (RED) experiment.

RED was conducted off of the Hawaiian Island of Oahu from late August to mid-September 2001. The Scripps Institution of Oceanography Research Platform Floating Instrument Platform, moored about 10 km off the northeast coast of Oahu, hosted the primary meteorological sensor suites and the transmitters for both the microwave and the infrared propagation links. Two land sites were instrumented—one with microwave receivers and the other with an infrared receiver—two buoys were deployed, a small boat was instrumented, and two aircraft flew various tracks to sense both sea and atmospheric conditions.

Through meteorological and propagation measurements, RED achieved a number of its objectives. First, although we did not experience the desired conditions of simultaneous high seas, high winds, and large surface gradients of temperature and humidity necessary to significantly affect the evaporation duct, observations verify that waves do modify the scalars within the air–sea surface layer. Second, an intriguing and controversial result is the lack of agreement of the scalar profile constants with those typically observed over land. Finally, as expected for the conditions encountered during RED (trade wind, moderate seas, unstable), we show that the Monin–Obukhov similarity theory, combined with high-quality meteorological measurements, can be used by propagation models to accurately predict microwave signal levels.

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James Edson, Timothy Crawford, Jerry Crescenti, Tom Farrar, Nelson Frew, Greg Gerbi, Costas Helmis, Tihomir Hristov, Djamal Khelif, Andrew Jessup, Haf Jonsson, Ming Li, Larry Mahrt, Wade McGillis, Albert Plueddemann, Lian Shen, Eric Skyllingstad, Tim Stanton, Peter Sullivan, Jielun Sun, John Trowbridge, Dean Vickers, Shouping Wang, Qing Wang, Robert Weller, John Wilkin, Albert J. Williams III, D. K. P. Yue, and Chris Zappa

The Office of Naval Research's Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Martha's Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over scales of O(1–104 mm) using a mesoscale mooring array, aircraft-borne remote sensors, drifters, and ship surveys. To our knowledge, the former represents the first successful attempt to directly and simultaneously measure the heat and momentum exchange on both sides of the air–sea interface. The latter provided a 3D picture of the oceanic boundary layer during the month-long main experiment. These observations have been combined with numerical models and direct numerical and large-eddy simulations to investigate the processes that couple the atmosphere and ocean under these conditions. For example, the oceanic measurements have been used in the Regional Ocean Modeling System (ROMS) to investigate the 3D evolution of regional ocean thermal stratification. The ultimate goal of these investigations is to incorporate improved parameterizations of these processes in coupled models such as the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to improve marine forecasts of wind, waves, and currents.

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