Direct measurements of air-sea interactionJohn Prytherch, Margaret J. Yelland, Robin W. Pascal, Ben I. Moat

With contributions from:

Meric Srokosz, Christine Gommenginger

Ian Brooks & Sarah Norris - Leeds University

Ingunn Skjelvan - University of Bergen

Tamzin Palmer - Met Office

Outline

• Motivation: uncertainty in flux parameterisations

• Recent results (HiWASE)

• Current and future work (WAGES)

Overview

• Aim. Improved parameterisation and understanding of the fluxes of momentum, sensible heat, latent heat, CO2 and sea-spray aerosol (Ian Brooks, University of Leeds).

• Improved parameterisations are needed for use in climatologies (in-situ and satellite) and coupled models.

• We are making long term direct measurements of the air-sea fluxes in the open ocean (plus mean forcing variables, directional sea-state and whitecap -fraction). Measurements made continuously for years rather than weeks.

Transfer velocity k (or transfer coefficients for heat/momentum) derived from direct measurements of other terms.

Dependence of k on seastate, whitecapping (and solubility) may explain much of the difference between studies.

e.g.: CO2 flux = k Solubility (pCO2air – pCO2sea)

Motivation: uncertainty in flux parameterisations

Woolf, D. K. (2005), Parameterization of gas transfer velocities and sea-state-dependent wave breaking, Tellus B, 57, 87-94.

Transfer velocity k (or transfer coefficients for heat/momentum) derived from direct measurements of other terms.

Dependence of k on seastate, whitecapping (and solubility) may explain much of the difference between studies.

Woolf (2005) sea-state dependent model tuned to match the spread of previous studies.

Previous studies lack data on sea-state or whitecapping.

e.g.: CO2 flux = k Solubility (pCO2air – pCO2sea)

Motivation: uncertainty in flux parameterisations

Woolf, D. K. (2005), Parameterization of gas transfer velocities and sea-state-dependent wave breaking, Tellus B, 57, 87-94.

Recent results: HiWASE(High Wind Air-Sea Exchanges)

• Long term (Sep’ 2006 to Dec’ 2009), automated continuous air-sea flux measurements.

• Direct measurements of momentum, sensible heat, latent heat and CO2 flux.

• Comprehensive sea-state measurements: 1) ship-borne wave recorder, 2) wave radar.

• Whitecap fraction from camera images.

• Max. 10 minute U10N 26 m/s, max Hs 11 m

Brooks, Yelland, et al… (2009) Physical exchanges at the air-sea interface: UK-SOLAS Field Measurements. BAMS.

http://www.noc.soton.ac.uk/ooc/CRUISES/HiWASE/OBS/data_intro.php

Autonomous flux system “AutoFlux”

Fluxes measured using “direct” Eddy Covariance (EC) and Inertial Dissipation (ID) techniques.

R3 Sonic anemometerMomentum and sensible heat fluxes

Motion pack Rate gyros & accelerometers. 6 components of ship motion.

Licor 7500 infra-red gas analyser: C2/Latent heat flux and CO2 flux

Sync signal: logged by each sensor

Yelland, M. J., R. W. Pascal, P. K. Taylor, and B. I. Moat, B. I., 2009: AutoFlux: an autonomous system for the direct measurement of the air-sea fluxes of CO2, heat and momentum. Journal of Operational Oceanography, 2, (1), 15-23.

HiWASE: gas transfer velocity

CO2 flux sensors suitable for long term autonomous deployment exhibit a large humidity cross-sensitivity error when used at sea.

Prytherch J., M. J. Yelland, R. W. Pascal, B. I. Moat, I. Skjelvan and C. C. Neill, 2010a: Direct measurements of the CO2 flux over the ocean: development of a novel method. Geophys. Res. Lett., 37, 3, doi:10.1029/2009GL041482

HiWASE: gas transfer velocity

CO2 flux sensors suitable for long term autonomous deployment exhibit a large humidity cross-sensitivity error when used at sea.

Error is due to the build up hygroscopic particles (salt) on the sensor lens.

Prytherch J., M. J. Yelland, R. W. Pascal, B. I. Moat, I. Skjelvan and C. C. Neill, 2010a: Direct measurements of the CO2 flux over the ocean: development of a novel method. Geophys. Res. Lett., 37, 3, doi:10.1029/2009GL041482

HiWASE: gas transfer velocity

CO2 flux sensors suitable for long term autonomous deployment exhibit a large humidity cross-sensitivity error when used at sea.

Error is due to the build up hygroscopic particles (salt) on the sensor lens.

A novel method for correcting this error (the ‘PKT’ correction) was developed.

Prytherch J., M. J. Yelland, R. W. Pascal, B. I. Moat, I. Skjelvan and C. C. Neill, 2010a: Direct measurements of the CO2 flux over the ocean: development of a novel method. Geophys. Res. Lett., 37, 3, doi:10.1029/2009GL041482

Gas transfer velocity results

Gas transfer velocity data to nearly 20 m/s.

New parametersiation: k = 0.51+0.034 U3

Lower k relationship found for DMS: may be due to a sea-state dependence of the gas transfer.

HiWASE data too noisy to see a sea-state signal directly.

Huebert, B., B. Blomquist, M.X. Yang, S. Archer, P. Nightingale, M. J. Yelland, R. W. Pascal, B. I. Moat, 2010: Linearity of DMS Transfer Coefficient with Both Friction Velocity and Wind Speed in the Moderate Wind Speed Range. Geophys. Res. Lett., 37, L01605, doi:10.1029/2009GL041203

Prytherch J., M. J. Yelland, R. W. Pascal, B. I. Moat, I. Skjelvan and M. A. Srokosz, 2010b: A new parameterisation of the open ocean CO2 gas transfer velocity. Geophys. Res. Lett., 37, L23607, doi:10.1029/2010GL045597

Gas transfer - requires improved understanding of whitecap and wave breaking

40 years of camera studies produced 800 data points and 30 different relationships!

2 to 3 orders of magnitude difference between one relationship and another.

Influences affecting breaking and whitecapping include:- sea-state (fetch/development/swell), SST, wind stress and atmospheric stability.

Previous studies are limited by:-• Few data• Limited wind speed range• A lack of sea-state, SST etc information

Anguelova, M. D. & Webster, F. (2006) Whitecap coverage from satellite measurements: A first step toward modeling the variability of oceanic whitecaps. Journal of Geophysical Research-Oceans, 111, C03017.

Whitecap fraction from digital cameras

• Images every 5 minutes.

• Have developed a semi-automated analysis

technique based on the method of Callaghan

and White (2009)

• Labour-intensive but have about 10,000 data

• Beware sea gulls!

Initial results – open ocean whitecap coverage (W)

Whitecap measurements obtained in winds up to 23 m/s – vary with (U10n)3

Reasonable agreement of HiWASE / SEASAW results and Callaghan & White, (2008).

Ongoing challenges include sampling, viewing angle etc

Waves, Aerosols and Gas Exchange Study “WAGES”:

Measurement campaign transferred to RRS James Clark Ross, summer 2010 - September 2012

• Added CLASP to the AutoFlux system to obtain sea-spray aerosol fluxes• Manned process cruises, with deployments of a spar buoy with capacitance wave-wires and an air-borne camera (balloon) to obtain whitecap fraction over a larger surface area

JCR ship track since 2010

Initial results - direct measurement of breaking waves.

True wind speed (m/s)

Pascal R. W., M. J. Yelland, M. A. Srokosz, B. I. Moat, E. Waugh, D. Comben, A. Cansdale, M. Hartman, D. Coles, P. C. Huseh and T. G. Leighton. 2010: A spar buoy for high frequency wave measurements and detection of wave breaking in the open ocean. Journal of Atmospheric and Ocean Technology.Yelland, M. J., R. W. Pascal, M. A. Srokosz, B. I. Moat 2011: Ocean wave breaking and whitecap events as detected by capacitance wave wires. Geophys. Res. Letts IN PREP

Developed a novel method to distinguish between actively breaking waves and advected whitecap.

Recent results (January, 2011) show fetch dependence of wave breaking.

Recent JCR WAGES/Drake Passage cruise:

2 spar buoy deployments (1 coastal, 1 open ocean) in >30 knot winds.

Recovery proved challenging….

Recent JCR WAGES/Drake Passage cruise:

2 spar buoy deployments (1 coastal, 1 open ocean) in >30 knot winds.

Recovery proved challenging….

But all data and most instruments recovered.

Summary

HiWASE and WAGES: Unique in-situ flux and wave datasets.

Large number of direct flux measurements in a wide range of conditions.

Cubic dependence of gas transfer on wind speed.

Possible fetch dependence of wave breaking.