Stuart D. Smith

Sea surface wind stress and drag coefficients: The hexos results

Turbulent fluxes have been measured in the atmospheric surface layer from a boom extending upwind from the Dutch offshore research platform Meetpost Noordwijk (MPN) during HEXMAX (Humidity Exchange over the Sea Main Experiment) in October–November, 1986. We started out to study eddy flux of water vapour, but discrepancies among simultaneous measurements made with three different anemometers led us to develop methods to correct eddy correlation measurements of wind stress for flow distortion by nearby objects. We then found excellent agreement among the corrected wind stress data sets from the three anemometers on the MPN boom and with eddy correlation measurements from a mast on a tripod. Inertial-dissipation techniques gave reliable estimates of wind stress from turbulence spectra, both at MPN and at a nearby ship. The data cover a range of wave ages and the results yield new insights into the variation of sea surface wind stress with sea state; two alternative formulas are given for the nondimensional surface roughness as a function of wave age.

On the dependence of sea surface roughness on wave development

Abstract The aerodynamic roughness of the sea surface, z0, is investigated using data from Lake Ontario, from the North Sea near the Dutch coast, and from an exposed site in the Atlantic Ocean off the coast of Nova Scotia. Scaling z0 by rms wave height gives consistent results for all three datasets, except where wave heights in the Atlantic Ocean are dominated by swell. The normalized roughness depends strongly on wave age: younger waves (traveling slower than the wind) are rougher than mature waves. Alternatively, the roughness may be normalized using the friction velocity, u*, of the wind stress. Again, young waves are rougher than mature waves. This contradicts some recent deductions in the literature, but the contradiction arises from attempts to describe z0 in laboratory tanks and in the field with a single simple parameterization. Here, it is demonstrated that laboratory waves are inappropriate for direct comparison with field data, being much smoother than their field equivalents. In the open ocea...

Air‐sea exchange of water vapor and sensible heat: The Humidity Exchange Over the Sea (HEXOS) results

Surface layer fluxes of sensible heat and water vapor were measured from a fixed platform in the North Sea during the Humidity Exchange over the Sea (HEXOS) Main Experiment (HEXMAX). Eddy wind stress and other relevant atmospheric and oceanic parameters were measured simultaneously and are used to interpret the heat and water vapor flux results. One of the main goals of the HEXOS program was to find accurate empirical heat and water vapor flux parameterization formulas for high wind conditions over the sea. It had been postulated that breaking waves and sea spray, which dominate the air-sea interface at high wind speeds, would significantly affect the air-sea heat and water vapor exchange for wind speeds above 15 m/s. Water vapor flux has been measured at wind speeds up to 18 m/s, sufficient to test these predictions, and sensible heat flux was measured at wind speeds up to 23 m/s. Within experimental error, the HEXMAX data do not show significant variation of the flux exchange coefficients with wind speed, indicating that modification of the models is needed. Roughness lengths for heat and water vapor derived from these direct flux measurements are slightly lower in value but closely parallel the decreasing trend with increasing wind speed predicted by the surface renewal model of Liu et al. [1979], created for lower wind speed regimes, which does not include effects of wave breaking. This suggests that either wave breaking does not significantly affect the surface layer fluxes for the wind speed range in the HEXMAX data, or that a compensating negative feedback process is at work in the lower atmosphere. The implication of the feedback hypothesis is that the moisture gained in the lower atmosphere from evaporation of sea spray over rough seas may be largely offset by decreased vapor flux from the air-sea interface.

The spray contribution to net evaporation from the sea: A review of recent progress

The part that sea spray plays in the air-sea transfer of heat and moisture has been a controversial question for the last two decades. With general circulation models (GCMs) suggesting that perturbations in the Earths surface heat budget of only a few W m−2 can initiate major climatic variations, it is crucial that we identify and quantify all the terms in that heat budget. Thus, here we review recent work on how sea spray contributes to the sea surface heat and moisture budgets. In the presence of spray, the near-surface atmosphere is characterized by a droplet evaporation layer (DEL) with a height that scales with the significant-wave amplitude. The majority of spray transfer processes occur within this layer. As a result, the DEL is cooler and more moist than the atmospheric surface layer would be under identical conditions but without the spray. Also, because the spray in the DEL provides elevated sources and sinks for heat and moisture, the vertical heat fluxes are no longer constant with height. We use Eulerian and Lagrangian models and a simple analytical model to study the processes important in spray droplet dispersion and evaporation within this DEL. These models all point to the conclusion that, in high winds (above about 15 m/s), sea spray begins to contribute significantly to the air-sea fluxes of heat and moisture. For example, we estimate that, in a 20-m/s wind, with an air temperature of 20°C, a sea surface temperature of 22°C, and a relative humidity of 80%, the latent and sensible heat fluxes resulting from the spray alone will have magnitudes of order 150 and 15 W/m2, respectively, in the DEL. Finally, we speculate on what fraction of these fluxes rise out of the DEL and, thus, become available to the entire marine boundary layer.

The heat budget at ocean weather station Bravo

Abstract The monthly surface heat budget and wind stress are calculated from three‐hourly meteorological data obtained at OWS B from 1946 to 1974 using formulae based on the best available measurements, and are then compared with earlier estimates. The surface heating is compared with the monthly heat storage in the water column calculated from hydrographic casts from 1964 to 1973. In spite of the scatter in the individual monthly heat storage values, the mean seasonal cycle agrees well with that of the surface flux. The residual, attributed to advection, is highly variable and does not have a strong seasonal cycle at this location. The long‐term heat loss from the ocean to the atmosphere at OWS B is found to be 28 W m−2, much smaller than the 98 W m−2 loss computed from the same data using Bunkers widely accepted formulae. The difference is accounted for by Bunkers 28% underestimate of the incoming short‐wave radiation and his 74 and 43% overestimates of the outgoing sensible and latent heat fluxes, re...

Air-Sea Fluxes: 25 Years of Progress

During the past quarter century the study of air-sea interaction has evolved from a small branch of marine climatology to play a key role in the modelling of the coupled system of ocean and atmosphere. Knowledge of air- sea fluxes has grown, based on Monin-Obukhov similarity theory for surface boundary layers and on direct and indirect techniques of measuring the fluxes. This has been the basis for providing boundary conditions needed to couple atmospheric and oceanic circulation models that are used to forecast weather and climate. An overview of current understanding is followed by a discussion of parameterisation schemes and a chronicle of some of the experimental work that has tested theories and quantified their conclusions.

HEXOS—Humidity Exchange Over the Sea A Program for Research on Water-Vapor and Droplet Fluxes from Sea to Air at Moderate to High Wind Speeds

HEXOS is an international program for the study of evaporation and spray-droplet flux from sea to air. The program includes measurements in the field at moderate-to-high wind speeds, wind-tunnel studies, instrument development, boundary-layer modeling, and subsequent development of parameterization for use in synoptic and climatic models of the atmosphere and the ocean. Present accomplishments of the program are 1) a wind-tunnel study of the flow distortion around the Dutch research platform, Meetpost Noordwijk, 2) a pilot experiment at this platform in November 1984, and 3) an investigation of processes near the air-sea interface in a wind-wave simulation tunnel. The main field experiment, taking place in the autumn of 1986 at and around the Noordwijk platform, includes measurements of the fluxes of water vapor, spray droplets, sensible heat, and momentum, as well as the structure of the planetary boundary layer and the state of the sea. This multidisciplinary effort involves direct measurements from the...

Measuring the relationship between wind stress and sea state in the open ocean in the presence of swell

Abstract Wind stress and directional wave spectra were measured during the Grand Banks ERS‐1 SAR wave spectra validation experiment. Swell dominated the wave spectral energy, so that existing relations between wind stress and sea state at sheltered sites were not directly applicable. To test the idea that simple deletion of the swell from the wave spectra would produce a sea state/wind stress relation suitable for general use, a spectral method for separating sea from swell was developed. The resulting open‐ocean wind stress/sea‐state relation does not differ from earlier results without swell, but, as for the swell‐dominated part of the HEXOS (Humidity Exchange Over the Sea) dataset, the statistical significance is low.

Flow Distortion Calculations and Their Application in HEXMAX

Abstract Several methods are examined for correction of turbulence and eddy fluxes in the atmospheric boundary layer, two of them based on a potential-flow approach initiated by Wyngaard. If the distorting object is cylindrical or if the distance to the sensor is much greater than the size of the body, the undisturbed wind stress can be calculated solely from measurements made by the sensor itself; no auxiliary measurements or lengthy model calculations are needed. A more general potential-flow correction has been developed in which distorting objects of complex shape are represented as a number of ellipsoidal elements. These models are applied to data from three turbulence anemometers with differing amounts of flow distortion, operated simultaneously in the Humidity Exchange over the Sea (HEXOS) Main Experiment. The results are compared with wind-stress estimates by the inertial-dissipation technique; these are much less sensitive to local flow distortion and are consistent with the corrected eddy correl...

Eddy flux measurements over Lake Ontario

Wind, temperature and humidity fluctuations have been recorded using a sonic anemometer-thermometer, a thrust anemometer, and a La humidiometer. The two anemometers agree in wind speed and stress. Exchange coefficients for momentum, heat and moisture are found to agree with values measured over other bodies of water.