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M. Neil Ward and Brian J. Hoskins

Abstract

Near-surface wind over the ocean is a key variable for studies of climate variability and change. Millions of ship reports of near-surface wind are now held in computerized datasets like the Comprehensive Ocean-Atmosphere Data Set (COADS). These data are potentially well suited to many climate studies. However, the reliability of the record is controversial. In particular, the reality of an upward trend in reported wind strength since the 1940s has been questioned and attributed by some to an increasing fraction of anemometer readings on-board ship, compared to reports based on estimation of the sea state. Analyzing the COADS record for 1949–1988, this paper provides evidence on the reliability of the data, corrects the trends in the data, and finally presents some analysis of the multidecadal variability that remains in the data after correction. These multidecadal analyses are offered as a preliminary assessment of the revised wind data and the impact of the corrections.

The reliability of the data is assessed by comparing the 10°lat × 10°long seasonal mean reported wind with a wind derived from seasonal mean sea level pressure (SLP) gradients. The wind is derived using the balanced friction flow (BFF) method, which assumes a three-way balance of forces between pressure gradient, Coriolis, and friction, represented using a drag coefficient. The interannual variability of observed and BFF wind generally agree well (except close to the equator, 10°N–10°S), giving good confidence in the data and the BFF theory. However, on the trend timescale, them is considerable disagreement. Globally averaged, the results suggest that there has been an upward trend in reported circulation strength of about 14% over 1949–1988 (highly statistically significant, <1% level), whereas there is no trend of any substance in the globally averaged BFF wind. Since there is an explanation for at least some of the observed wind trend through changes in observational practice and since no serious problems with the BFF trends could be found, the BFF trends have been treated as true and the observed trends corrected to the BFF trends. For this, a smooth field of the trend correction needed at each 10°lat × 10° long box was calculated.

Although there is no globally averaged trend in circulation strength in the corrected wind data, there are regional patterns of upward trend (notably in the tropical North Atlantic and the extratropical North Pacific) and downward trend (notably in the equatorial and tropical South Atlantic and the subtropical North Pacific). The changes in the tropical Atlantic are particularly strong in the July–September season, when they accompany the strong downward trend in July–September Sahelian rainfall. The changed wind pattern corresponds to a substantial reduction in cross-equatorial flow. The impact of this on ocean currents and cross-equatorial heat flux should be assessed using ocean numerical models forced with observed wind stress changes. The wind data developed here from a smooth field of corrections are potentially well suited to development for this application.

The agreement between BFF winds and observed winds on the interannual timescale was weakest in equatorial latitudes. The utility of the wind data in the equatorial western Pacific is, however, very strongly supported by the close agreement between interannual variability of near-surface divergence, calculated from the corrected near-surface wind, and sea-surface temperature. For interannual studies, the corrections are of minimal importance, so these results, along with the close agreement between BFF and observed winds away from the equator, give a very positive endorsement of the uncorrected COADS's utility for studies of large-scale interannual climate variability.

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Sebastian Landwehr, Niall O’Sullivan, and Brian Ward

Abstract

Ship-based measurements of wind speed and direct fluxes are affected by airflow distortion that can lead to a tilt of the wind vector as well as acceleration or deceleration of the wind speed. Direct flux measurements are additionally affected by the fluctuating velocity of the platform. The classic approach is to first correct the wind speed for angular and translational platform velocities and thereafter rotate the wind vector into the mean flow. This study finds that for ships under way, this leads to an overestimation of the vector tilt and biased flux estimates. This may explain the common observation that flux estimates from ships in transit have lower quality than measurements taken on station. Here an alternative approach is presented, where the flow-distortion-induced tilt of the wind vector is estimated from the 3D wind speed measurements and applied to the apparent wind vector. The tilt correction is carried out after correction for the fluctuating part of the platform velocity but before removing the ship’s mean translational velocity. This new method significantly improved the agreement of direct momentum flux measurements made from a ship under way with the parameterization of the COARE3.5 bulk model. The sensitivity of the eddy covariance measurements of momentum and scalar fluxes to the choice of the tilt-motion correction method is analyzed, and this study proposes that a reanalysis of previous direct flux measurements with the new method discussed here can improve researchers’ understanding of air–sea interaction.

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Bernard A. Silverman, Brian J. Thompson, and John H. Ward

Abstract

An instrument is described for determining the size distribution of fog droplets 4 microns in diameter and larger. A film record of the fog droplets is obtained as they pass relatively undisturbed through the collimated beam of light from a Q-switched pulsed ruby laser. The recording plane is located in the far field of the individual droplets but not in the far field of the diameter of the whole sample volume. Diffraction patterns associated with the individual droplets are then observed and recorded. Measurement of the characteristic dimensions in the diffraction patterns allows the droplet diameters to be accurately calculated from well-established diffraction relationships.

The laser disdrometer is capable of sampling volumes up to five cubic centimeters on each frame of 35 mm film at a rate of 10 frames per minute. The short pulse length of the laser, as short as 1.0 microsecond, enables measurements to be made in moderately high winds without loss of accuracy. The measured distribution is relatively unaffected by the measuring technique, since no sample collection or dilution is involved.

The method is applicable to sizing both opaque and transparent particles of all geometries and has, therefore, a wider application than discussed in this paper. In addition, availability of higher repetition rate lasers makes measurements from aircraft feasible.

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Brian Ward, Rik Wanninkhof, Peter J. Minnett, and Michael J. Head

Abstract

The Skin Depth Experimental Profiler (SkinDeEP) is an autonomous, self-contained, hydrodynamic instrument capable of making repeated, high-resolution profiles of temperature and conductivity within the ocean's upper decameter. Autonomous profiling operation is accomplished through SkinDeEP's ability to change its density: positive buoyancy is achieved by pumping air from inside the body of the profiler into an external, neoprene, inflatable sleeve; the instrument sinks when the sleeve is deflated by returning the air to the interior. The sensors are mounted some distance from the top endcap and data are recorded only during the ascending phase of the profile so as to minimize disruption of a naturally occurring scalar structure by the presence of the instrument. Temperature and conductivity are measured with resolutions in the submillimeter and millimeter ranges, respectively. Highly accurate and slower sensors are installed for calibration purposes. These data are used to study exchange processes at the air–sea interface and the structure of the ocean just below.

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Brian Scanlon, Øyvind Breivik, Jean-Raymond Bidlot, Peter A. E. M. Janssen, Adrian H. Callaghan, and Brian Ward

Abstract

High-resolution measurements of actively breaking whitecap fraction (W FA) and total whitecap fraction (W FT) from the Knorr11 field experiment in the Atlantic Ocean are compared with estimates of whitecap fraction modeled from the dissipation source term of the ECMWF wave model. The results reveal a strong linear relationship between model results and observed measurements. This indicates that the wave model dissipation is an accurate estimate of total whitecap fraction. The study also reveals that the dissipation source term is more closely related to W FA than W FT, which includes the additional contribution from maturing (stage B) whitecaps.

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Brian J. Thompson, Georgy B. Parrent, John H. Ward, and Bruce Justii

Abstract

Recently a new instrument termed the laser fog disdrometer was introduced by Silverman, Thompson and Ward. As implied by the name, the function of the instrument is the determination of the size distribution of fog droplets. In design, operation and analysis this instrument represents a significant departure from the customary approaches to the problem. The basic principle of the instrument may be summarized as follows: by suitably storing the diffraction pattern associated with a droplet, both the precise size and location of the droplet may be determined. This principle can be utilized to obtain size distributions without disturbing the statistics of the sample, i.e., finite volumes may be sampled without dilution.

Originally, the data were read directly from the diffraction pattern. This type of readout is subject to two fundamental difficulties: 1) the geometry of droplets is difficult to ascertain except for simple structures; 2) if several droplets are relatively near each other in the sample volume, the resultant diffraction pattern is difficult to interpret. This first consideration does not represent a severe limitation for this application; however, it would be a serious limitation in other applications where non-spherical droplets exist. Both of these restrictions, however, are removed by the present readout technique. Physically, the new readout is based on the realization that the diffraction patterns stored by the instrument are, in fact, a new kind of hologram. Hence, the stored diffraction pattern can be used to create a real three-dimensional image of the sample volume. Since the image is fixed in time, the volume may be explored at will and the size and shape of each particle as well as its position relative to the other particles in the sample may be determined. In the present paper the concept and design of the disdrometer is reviewed and the new readout technique is discussed from both a theoretical and experimental point of view. Typical experimental results are also illustrated.

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Brian Ward, Tim Fristedt, Adrian H. Callaghan, Graig Sutherland, Xavier Sanchez, Jérôme Vialard, and Anneke ten Doeschate

Abstract

The upper few meters of the ocean form a critical layer for air–sea interaction, but because of observational challenges this region is undersampled. However, the physical processes controlling momentum transfer, gas exchange, and heat transfer are all concentrated in the uppermost region of the ocean. To study this region, the Air–Sea Interaction Profiler (ASIP) was developed. This is an autonomous microstructure vertical profiling instrument that provides data from a maximum depth of 100 m to the ocean surface and allows measurements to be performed in an undisturbed environment. The core sensor package on ASIP includes shear probes, microstructure and CTD-quality temperature and conductivity sensors, a photosynthetically active radiation (PAR) sensor, and an oxygen optode providing a repeated high-resolution dataset immediately below the air–sea interface. Autonomous profiling is accomplished with thrusters that submerge the positively buoyant instrument. Once the desired depth is reached, ASIP ascends through the water column acquiring data. At the surface, ASIP acquires its position and transmits this over the Iridium satellite network. ASIP is then placed in a low-power mode for a specified period, whereupon it repeats the profile cycle. Two-way communication over the Iridium network allows mission parameters to be changed in real time. ASIP has been used to study several scientific questions, such as the impact of diurnal warming on atmospheric processes, turbulence scaling in the upper ocean, parameterizing air–sea gas exchange, salinity gradients in the ocean surface boundary layer (OSBL), and consequences for remote sensing.

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Graig Sutherland, Louis Marié, Gilles Reverdin, Kai H. Christensen, Göran Broström, and Brian Ward

Abstract

Detailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate ε below the depth of the diurnal jet, there was an observed increase in ε within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of ε. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diurnal heating limits the depth of the momentum flux due to the wind, shear instability provides an additional source of turbulence that interacts with the enhanced shear of the diurnal jet to increase ε within this shallow layer.

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Erik Nilsson, Hans Bergström, Anna Rutgersson, Eva Podgrajsek, Marcus B. Wallin, Gunnar Bergström, Ebba Dellwik, Sebastian Landwehr, and Brian Ward

Abstract

Global oceans are an important sink of atmospheric carbon dioxide (CO2). Therefore, understanding the air–sea flux of CO2 is a vital part in describing the global carbon balance. Eddy covariance (EC) measurements are often used to study CO2 fluxes from both land and ocean. Values of CO2 are usually measured with infrared absorption sensors, which at the same time measure water vapor. Studies have shown that the presence of water vapor fluctuations in the sampling air potentially results in erroneous CO2 flux measurements resulting from the cross sensitivity of the sensor. Here measured CO2 fluxes from both enclosed-path Li-Cor 7200 sensors and open-path Li-Cor 7500 instruments from an inland measurement site are compared with a marine site. Also, new quality control criteria based on a relative signal strength indicator (RSSI) are introduced. The sampling gas in one of the Li-Cor 7200 instruments was dried by means of a multitube diffusion dryer so that the water vapor fluxes were close to zero. With this setup the effect that cross sensitivity of the CO2 signal to water vapor can have on the CO2 fluxes was investigated. The dryer had no significant effect on the CO2 fluxes. The study tested the hypothesis that the cross-sensitivity effect is caused by hygroscopic particles such as sea salt by spraying a saline solution on the windows of the Li-Cor 7200 instruments during the inland field test. The results confirm earlier findings that sea salt contamination can affect CO2 fluxes significantly and that drying the sampling air for the gas analyzer is an effective method for reducing this signal contamination.

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Ian M. Brooks, Margaret J. Yelland, Robert C. Upstill-Goddard, Philip D. Nightingale, Steve Archer, Eric d'Asaro, Rachael Beale, Cory Beatty, Byron Blomquist, A. Anthony Bloom, Barbara J. Brooks, John Cluderay, David Coles, John Dacey, Michael DeGrandpre, Jo Dixon, William M. Drennan, Joseph Gabriele, Laura Goldson, Nick Hardman-Mountford, Martin K. Hill, Matt Horn, Ping-Chang Hsueh, Barry Huebert, Gerrit de Leeuw, Timothy G. Leighton, Malcolm Liddicoat, Justin J. N. Lingard, Craig McNeil, James B. McQuaid, Ben I. Moat, Gerald Moore, Craig Neill, Sarah J. Norris, Simon O'Doherty, Robin W. Pascal, John Prytherch, Mike Rebozo, Erik Sahlee, Matt Salter, Ute Schuster, Ingunn Skjelvan, Hans Slagter, Michael H. Smith, Paul D. Smith, Meric Srokosz, John A. Stephens, Peter K. Taylor, Maciej Telszewski, Roisin Walsh, Brian Ward, David K. Woolf, Dickon Young, and Henk Zemmelink

As part of the U.K. contribution to the international Surface Ocean-Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosolsize spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.

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