Uncertainties in wind speed dependent CO2 transfer velocities due to airflow distortion at anemometer sites on ships 1Ocean Research Institute, The University of Tokyo, Tokyo, Japan
2Institute for Landscape Ecology – Climatology, University of Münster, Münster, Germany
3National Oceanography Centre, Southampton, UK
*Invited contribution by F. Griessbaum, recipient of the EGU Young Scientists Outstanding Poster Paper Award 2008.
Received: 22 Aug 2009 – Published in Atmos. Chem. Phys. Discuss.: 10 Sep 2009 Abstract. Data from platforms, research vessels and merchant ships are used to
estimate ocean CO2 uptake via parameterisations of the gas transfer
velocity (k) and measurements of the difference between the partial pressures
of CO2 in the ocean (pCO2 sw) and atmosphere (pCO2 atm) and of
wind speed. Gas transfer velocities estimated using wind speed dependent
parameterisations may be in error due to air flow distortion by the ship's
hull and superstructure introducing biases into the measured wind speed. The
effect of airflow distortion on estimates of the transfer velocity was
examined by modelling the airflow around the three-dimensional geometries of
the research vessels Hakuho Maru and Mirai, using the Large Eddy Simulation code GERRIS. For
airflows within ±45° of the bow the maximum bias was +16%.
For wind speed of 10 m s−1 to 15 m s−1, a +16% bias in wind
speed would cause an overestimate in the calculated value of k of 30% to
50%, depending on which k parameterisation is used. This is due to the
propagation of errors when using quadratic or cubic parameterisations.
Recommendations for suitable anemometer locations on research vessels are
given. The errors in transfer velocity may be much larger for typical
merchant ships, as the anemometers are generally not as well-exposed as
those on research vessels.
Revised: 17 May 2010 – Accepted: 26 May 2010 – Published: 09 Jun 2010
Flow distortion may also introduce biases in the wind speed dependent k
parameterisations themselves, since these are obtained by relating
measurements of the CO2 flux to measurements of the wind speed and the
CO2 concentration difference. To investigate this, flow distortion
effects were estimated for three different platforms from which wind speed
dependent parameterisations are published. The estimates ranged from −4%
to +14% and showed that flow distortion may have a significant impact on
wind speed dependent parameterisations. However, the wind biases are not
large enough to explain the differences at high wind speeds in
parameterisations which are based on eddy covariance and deliberate tracer
Citation: Griessbaum, F., Moat, B. I., Narita, Y., Yelland, M. J., Klemm, O., and Uematsu, M.: Uncertainties in wind speed dependent CO2 transfer velocities due to airflow distortion at anemometer sites on ships, Atmos. Chem. Phys., 10, 5123-5133, doi:10.5194/acp-10-5123-2010, 2010.