High Wind Gas Exchange Study (HiWinGS)
Collaborators: Byron Blomquist (University of Hawaii)
Ian M. Brooks (University of Leeds)
Christopher W. Fairall (NOAA)
Barry Huebert (University of Hawaii)
Ming-Xi Yang (Plymouth Marine Laboratory)
In the Fall of 2013 we participated in the High Wind Gas Exchange Study (HiWinGS) cruise aboard the R/V Knorr at the invitation of B. Blomquist, B. Huebert, and C. Fairall. The HiWinGS data set presents the unique opportunity to gain new insights on the poorly understood aspects of air-sea interaction under high winds.
Poor understanding of the complex physical controls of air-sea exchanges under high winds, in particular with respect to uptake and release of greenhouse gases and pollutants/particulate material, remains one of the major uncertainties in biogeochemical models and climate predictions. Adequate characterization of gas transfer across the air-sea interface is not only essential to quantify local and global sinks and sources of CO2 but also to budget many other trace gases that influence Earth's radiation. These include, amongst others, marine aerosol producers (such as DMS) and volatile organic compounds that contribute to tropospheric ozone formation and destruction and/or that alter the oxidative capacity of the atmosphere (such as acetone and methanol) and thereby change the life time of gases like methane. Transfer coefficients up to wind speeds of ~15 m s-1 have been obtained by many research programs over the past two decades. However, few gas transfer observations at wind speeds of 15 to 30 m -1 exist, and almost none with coincident wave physics observations. In physical models the effects of whitecaps, sea spray, and sea state on gas transfer remain crudely parameterized. Further progress requires guidance from field studies under high wind speed conditions.
Global air-sea gas flux estimates are based on parameterizations of the gas transfer velocity k. To first order, k is dictated by wind speed (U) and is typically parameterized as a non-linear function of U. There is however a large spread in k predicted by the traditional parameterizations, especially at high wind speed. This is because a large variety of environmental forcings and processes actually influence k and wind speed alone cannot capture the variability of air-water gas exchange. At high wind speed, breaking waves become a key factor to take into account when estimating gas fluxes. Wave breaking results in additional upper ocean turbulence and generation of bubble clouds.
The HiWinGS cruise took place on board the R/V Knorr, in the North Atlantic, (Figure 1) departing Nuuk, Greenland, on October 9th 2013 and ending at Woods Hole, USA on November 14th 2013. The ship's track was chosen based on daily analysis of weather maps and forecasts from the European Centre for Medium-Range Weather Forecast model provided by the Icelandic Met Office as well as from PassageWeather.com with the aim of maximizing the amount of time spent in the strongest winds. Along the track, the ship stopped at several stations for buoy deployments. While on station the ship was positioned bow pointing into the wind.
The ship remained in the Labrador Sea, south of Greenland for the first ~20 days of the cruise. Sea surface temperature and salinity were around 6-8 oC and 34-34.5 respectively at the first 6 stations (Figure 2). The ship then transitioned through the Gulf of St Lawrence from November 4th to 6th, and the last station was south of Nova Scotia where warm and salty Gulf Stream waters were encountered with SST of ~20oC and salinity of 36. Wind speeds exceeded 15 m s-1 25% of the time amounting to a total of 189 hours of wind speeds above 15 m s-1 of which 48 hours wind speeds greater than 20 m s-1 (Figure 2). On October 25th (station 4), wind speeds exceeded 25 m s-1 with gusts of 35 m s-1 during the St Jude storm.
Figure 3 depicts the experimental setup at the bow and flying bridge of the R/V Knorr. One of focal point of the deployment is the mast system at the bow of the ship. It included a complete meteorological package deployed by C. Fairall (NOAA), turbulent flux heat, momentum and gas measurements, and a LIDAR system at the top of the mast. Furthermore, the wave breaking measurements were made from the flying bridge. All measurements on the mast system were continuous and viewed the surface at an angle such that it minimizes the view of the wave distortion caused by the bow of the ship.
Fluxes of DMS and CO2 were measured by B. Blomquist [Blomquist et al., 2010] of the University of Hawaii, while fluxes of methanol and acetone where measured by M. Yang [Yang et al., 2014] from Plymouth Marine Laboratory (UK). Wave measurements where obtained from a Riegl laser altimeter as well as from a Datawell DWR-4G waverider deployed by I. Brooks from the University of Leeds (UK) and wave wires equipped buoys deployed by R. Pascal from the University of Southampton (UK). Breaking wave statistics will be derived from high resolution visible imaging based on the spectral framework by Phillips .
We recorded ~500 20-minute runs during the HiWinGS cruise, of which 50 were taken during the St Jude storm. The imaging system consisted of two obliquely-looking Imperx model Lynx 1M48 digital video cameras with a sensing array of 1000 x 1000 elements. Both visible cameras have a wide field-of-view (FOV; 6mm focal length) with one directed starboard and one directed port to accommodate all lighting conditions depending on the direction of the ship into the wind. The systems were mounted on the flying bridge, roughly 15 m above the water line. For an instrument elevation of 15 m above the water surface at an incidence angle of 65o, the resolution for the visible camera systems in the usable portion of the imagery will be 0.1 m with usable image FOV of roughly 50 m by 50 m. The visible cameras ran at a frame rate of 20 Hz. In addition to the camera, each system had an Xsens model MTi IMU that recorded the pitch, roll and yaw of the camera and will be used to correct for image distortion employing Holland et al. . These IMUs were referenced to the GPS heading on the mast. Motion corrections will be made using IMU as according to Edson et al., .
We are currently analyzing the ship-based wave and breaking measurements, to relate the breaking statistics to gas transfer measurements, and to apply insights gained to improve physical gas transfer parameterizations such as COAREG.
Blomquist, B. W., B. J. Huebert, C. W. Fairall, and I. C. Faloona (2010), Determining the sea-air flux of dimethylsulfide by eddy correlation using mass spectrometry, Atmospheric Measurement Techniques, 3(1), 1-20.
Holland, K. T., R. A. Holman, T. C. Lippmann, J. Stanley, and N. Plant (1997), Practical use of video imagery in nearshore oceanographic field studies, IEEE Journal of Oceanic Engineering, 22(1), 81-92.
Phillips, O. M. (1985), Spectral and statistical properties of the equilibrium range in wind-generated gravity waves, Journal of Fluid Mechanics, 156, 505-531.
Yang, M., B. W. Blomquist, and P. D. Nightingale (2014), Air-sea exchange of methanol and acetone during HiWinGS: Estimation of air phase, water phase gas transfer velocities, Journal of Geophysical Research: Oceans, 119(10), 7308--7323.