Transcript Analyzing and Modelling CO2 fluxes across the air

Analysing and Modelling CO2
fluxes across the air-sea
boundary
Anthony Bloom
Project Supervisors: Ian Brooks, Conny Schwierz
Analyzing and Modelling CO2 fluxes across the air-sea boundary
CO2 fluxes:
Air-Sea CO2 Flux driven by a CO2 partial pressure difference, ΔpCO2:
pCO2 air
ΔpCO2 > 0
pCO2 sea
ΔpCO2 < 0
CO2 gas transfer velocity:
• dependent on friction velocity, white-capping and bubble mediated gas transfer.
Bulk Flux Relationship: FCO2 = kCO2 s ΔpCO2
Analyzing and Modelling CO2 fluxes across the air-sea boundary
CO2 transfer velocity parameterisations:
• Non-linear relationship with wind speed
• No direct physical relationship with wind speed
• Considerable variation between parameterisations
• Significant divergence of parameterisations at wind speeds > 10ms-1
• Wind speed CO2 gas transfer parameterisations used as sole
constraints in GCMs.
•Few open ocean measurements
Analyzing and Modelling CO2 fluxes across the air-sea boundary
Cruise D317
North Atlantic:
• A large global CO2 sink.
• High wind speeds are expected in
springtime mid-latitude weather systems.
Analyzing and Modelling CO2 fluxes across the air-sea boundary
Eddy Covariance Method
Eddy Covariance Fluxes: F = ρa· (w’ · c’)
Required Time Series include:
• Vertical Wind Speed (w)
• Atmospheric CO2 concentration (c)
• Horizontal Wind Speed
• Air and Sea Surface Temperatures
• Atmospheric H2O concentration
• Air and Sea pCO2
• Platform motion
Analyzing and Modelling CO2 fluxes across the air-sea boundary
RRS Discovery
Suitable ‘moving platform’ for eddy covariance flux measurements with
a modest wind flow distortion.
LICOR &
MOTIONPACK
SURFMET
Instrument Package
Measurements
Sampling Interval (Freq)
Group
LiCOR 7500
CO2/H2O concentrations,
0.05s – (20Hz)
Leeds
Motion Pack
Acceleration, pitch, roll, heading.
0.05s – (20Hz)
Leeds/NOC
SURFMET
Wind Speed and Direction, Rel Hum, Air
Temperature, Sea Surface Temperature, Sea
Water Salinity, Atmospheric Pressure .
30 s – (3 x 10-2 Hz)
Leeds
PML air-sea pCO2
instrumentation
Air/Sea pCO2
58 min – (3x10-4 Hz)
CAXIS/PML
Analyzing and Modelling CO2 fluxes across the air-sea boundary
Motion Correction
TRUE VERTICAL
APPARENT VERTICAL
Analyzing and Modelling CO2 fluxes across the air-sea boundary
Ogive Functions
B
A
Ogive Function = running integral from the highest to lowest frequencies
of the co-spectral density.
OR: Flux contributions at different frequencies.
Well-behaved ogive functions chosen in order to eliminate:
• Erroneous CO2 concentration effects
• Wave motion contamination effects
Main Filtering Criteria:
• Wind speed direction constrained
• Confined maximum correlation
between w and c
• Well-behaved ogive functions
• Stability - unstable profiles only
• CO2 concentration time series
(<50mmol m-3)
CO2 – H2O concentration relationship: Taylor et al. (2007) correction
1. Removal of third order polynomial
2. Derivation of CO2 flux from corrected data
dc = c*
dq
q*
(Iterative method converges in 3 steps)
3. Addition of ‘derived gradient’
CO2 Transfer Velocities (1)
CO2 Transfer Velocities (2)
CO2 Transfer Velocities (3)
Air-Sea CO2 Flux Model (1)
Mock spatial fields constructed for:
• Wind Speed
• Sea Surface Temperature
• ΔpCO2
(Easily replaceable by real spatial fields)
Air-Sea CO2 Flux Model (2)
Model Runs:
• McGillis et al., 2001 (M01) and
Wanninkhof, 1992 (W92)
parameterisations employed
• 36h model run at 6h time steps
•Daily/Yearly CO2 Fluxes Derived
Resulting CO2 fluxes:
• W92 yearly fluxes are 25% greater than
M01 fluxes
Conclusions
• Successful employment of the Eddy Covariance Method in the open
Ocean.
• Extensive Measurements at high wind speeds are required to better
constrain k at high windspeeds.
• Further research into the relationship between k and other sea state
parameters is essential.
THE END