Extended Abstract (3.0M)P4.7
High Wind Air-Sea Exchanges (HiWASE) - initial turbulent flux results from Station Mike
John Prytherch, National Oceanography Centre, Southampton, United Kingdom; and M. J. Yelland, R. W. Pascal, P. Taylor, B. I. Moat, I. Skjelvan, and C. Neill
Over the ocean the turbulent air-sea CO2 flux is poorly understood, with parameterizations of the gas transfer velocity differing significantly for winds of 7 m/s or more. In contrast the momentum flux is reasonably well understood, though there is still debate over the influence of sea-state. The heat fluxes are less well known than the momentum flux, with debate about any wind-speed dependence of their transfer coefficients. HiWASE is a UK-SOLAS project which aims to improve the parameterisation of the turbulent air-sea fluxes, particularly under high wind speed conditions.
The Norwegian Ocean Weather Ship Polarfront has been outfitted with a range of meterological and flux measurement instruments, including the air-sea flux system “Autoflux”, an underway delta pCO2 system, a directional wave radar, ship borne wave recorder and two digital cameras. The various instruments are operated by NOC, BCCR and the Norwegian Meteorological Institute (DNMI).
The AutoFlux system includes a Solent R3 sonic anemometer, a MotionPak motion sensor and two Licor-7500 open-path fast-response infrared CO2 and H2O sensors. Each Licor is alternately shrouded each month in order to obtain corrections for distortion of the sensor head. The AutoFlux system produces momentum and latent heat fluxes automatically using the inertial dissipation method. The sensible heat and CO2 fluxes calculated using the eddy correlation method are currently produced post-cruise. Near real-time (24 hours) summary data are transmitted from the ship via iridium and displayed on a project web page under http://www.noc.soton.ac.uk/ooc/CRUISES/HiWASE/index.php .
The Polarfront occupies Station Mike (66 N 2 E) year round. The instrumentation all operates continually, and will do so at least until the end of the project in late 2009. Continuous operation allows data to be obtained under a wide range of wind speeds and sea states: to date the maximum 15 minute mean wind speed is 28 m/s. This dataset will enable the fluxes to be related to the underlying forcing factors and thus determine parameterisations of the fluxes in terms of sea-state, wavebreaking and whitecap fraction as well as wind speed.
In marine environments CO2 flux measurements obtained with open-path sensors have typically been an order of magnitude larger than those estimated by other techniques and this has resulted in a long-standing controversy as to the magnitude of CO2 fluxes on small time and space scales. Initial results from the HiWASE project support the theory that this discrepancy is due to sensor cross sensitivity to water vapor fluctuations (e.g. Koshiek, 2000). An iterative corrective procedure has been developed. After correction the observed gas transfer velocities are in reasonable agreement with published values obtained using closed-path sensors or by tracer techniques, potentially resolving the controversy.
Other potential sources of flux measurement error have been tested and additional results will be presented.
Supplementary URL: http://www.noc.soton.ac.uk/ooc/CRUISES/HiWASE/index.php
Poster Session 4, POSTERS: Small-Scale Studies
Wednesday, 14 January 2009, 2:30 PM-4:00 PM, Hall 5
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