Uncertainties in wind speed dependent CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; transfer velocities due to airflow distortion at anemometer sites on ships

F Griessbaum,O Klemm,M J Yelland,Y Narita,M Uematsu,B I Moat

doi:10.5194/acp-10-5123-2010

Abstract

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. 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 methods.

Highlights

IntroductionThe gas transfer velocity k can be estimated from measurements of the air-sea CO2 flux F and differences between the partial pressures of CO2 in the ocean pCO2 sw and atmosphere pCO2 atm ( pCO2);
Biases in wind speeds caused by flow distortion were obtained at various anemometer locations using a threedimensional Large Eddy Simulation (LES) approach, implemented in the open source code GERRIS
Detailed geometries of the research vessels Hakuho Maru and Mirai were modelled in order to obtain the mean biases in wind speed at anemometer locations on the foremast, the funnel mast, a hypothetical foremast anemometer location close to the bow, and at a hypothetical boom in front of the bow

Summary

Introduction

The gas transfer velocity k can be estimated from measurements of the air-sea CO2 flux F and differences between the partial pressures of CO2 in the ocean pCO2 sw and atmosphere pCO2 atm ( pCO2);. The parameterisation of k is subject to numerous uncertainties caused by e.g. measurement errors, surfactants and sea-state. These wind speed dependent parameterisations of k are used to obtain the air-sea flux of CO2 when measurements of the flux themselves are not available. F. Griessbaum et al.: Uncertainties in wind speed dependent CO2 transfer velocities global ocean. The delta pCO2 climatologies are derived from in-situ measurements made from e.g. platforms, research ships and voluntary observing ships A global map of all observation obtained between 1968 and 2008 is available from the Carbon Dioxide Information Analysis Center (CDIAC, 2010)

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