ICOS Science Conference, Session 4

1. Evaluation of a turbulence-based description of
the air-water gas transfer velocity
Leonie Esters, Erik Sahl´ee, Erik Nilsson, Lucia Guti´errez-Loza,
Anna Rutgersson
ICOS Science Conference 2020
15.09.2020
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 1

2. Turbulence and Air-Sea Gas Transfer
• The gas transfer velocity k
is usually parametrised as
function of wind speed
• There are attempts to
relate k directly to
near-surface turbulence
⇒ k = A · ( ν)1/4 Sc−n
(Lamont and Scott, 1970)
: dissipation rate of TKE
ν: viscosity of water
Sc: Schmidt number
n: Sc exponent
1/2 ≤ n ≤ 2/3
A: proportionality coefficient
0 2 4 6 8 10 12 14 16
0
20
40
60
80
100
120
Wanninkhof (1992)
Wanninkhof + Mc Gillis (1999)
Liss + Merlivat (1986)
k660
[cmhr
−1
]
u
10
[m s−1
]
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 2

3. 5-beam Acoustic Doppler Current Profiler (ADCP)
Simpson et al. 2001
• ADCPs measure the 3D current
field
• They send sound waves, which are
reflected by particles in the water
• The particles velocity is
determined using the Doppler
effect
• Measurements of:
• water current speed
• water turbulence
• wave characteristics
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 3

4. Deployment in lake Erken
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 4

5. Observations 2018
• 21 days of data
• No reliable
observations of k
• Waves follow the
wind
• Deepening of the
mixing layer depth
(XLD) coincide with
increased winds and
waves
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 5

6. Observations
• 73 days of data
• No observations of k
• Waves follow the
wind
• Deepening of the
XLD coincide with
increased winds,
waves, and positive
buoyancy flux
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 6

7. Surface turbulence versus wind speed
• Total of 94 days
of data
• Filled circles:
2018,
open circles:
2019
• Linear relation
between surface
turbulence and
wind speed
⇒ Turbulence is
wind-driven
-9.5 -9 -8.5 -8 -7.5 -7 -6.5 -6
log10( 05
) [m2
s-3
]
0
2
4
6
8
10
12
14
u10
[ms-1
]
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 7

8. Relating surface turbulence to k
• Wind-speed based
parametrisation for
k by Cole and
Caraco (1998):
k600 = 2.07+0.215u1.7
10
⇒ turbulence-based
model explains the
gas transfer velocity
calculated following
Cole and Caraco
(1998).
100
101
A Sc-n
( 05
)1/4
[cm hr-1
]
100
101
k600
CC
[cmhr-1
]
A = 1.3
R2
= 0.797
2
4
6
8
10
12
14
u10[ms-1
]
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 8

9. Outlook: ¨Ostergarnsholm
• At the moment, the ADCP is deployed in
the footprint of the flux tower on
¨Ostergarnsholm Island in the Baltic Sea.
• There, we have observations of k.
⇒ Surface turbulence can be directly related
to the measured k.
Leonie Esters Turbulence-based description of the air-water gas transfer velocity 9