ann-sofi smedman uppsala university uppsala, sweden · ann-sofi smedman uppsala university uppsala,...
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Effects of transfer processes on marine atmosphericboundary layer
orEffects of boundary layer processes on air-sea
exchange
Ann-Sofi SmedmanUppsala University
Uppsala, Sweden
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Swell
Long wavescreateform drag
Upward momentum flux
Quasi-frictional decouplingat the surface
Turbulence resembles free convection conditions
Ocean
MABL
Effect of transfer process on MABL
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Effect of MABL process on air-sea exchange
MABL
Detached eddiesimpinging on the surface
Increased small scaleheat flux
Increased CHNand CEN
Ocean
UVCN regime
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In models air-sea exchangeis described through Monin-Obukhov similarity
theorya theory which is well tested over land
but
is it valid over the ocean?
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Large heat capacity
Small diurnal variation
Roughness (waves)increasing with
increasing wind speed
What are the differences ?
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10002200
2 buoys (temp, waveheight , dir. and CO2 Footprint area
Tower 30 mTemp and wind profiles 5 levels
Turbulence 3 levelsHumidity and CO2 at 2 levels
Long term measurements in the Baltic Sea
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Turbulence, wind speed, temperature and waveparameters
Short term experiments with RV Aranda and ASIS buoy
In the mean, excellent agreement between tower and ASIS
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Data set from Agile experiment in lake Ontario 1994-95
A 15- m research vessel Agile was equipped to measure waves and turbulence with
• Sonic R2A• Thermocouple• Licor• Wave staff array
Measurements were performed at 7.8 m during two autumn periods
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Growing sea (slow waves)Unstable stratification
• Monin-Obukhov similarity theory is validbut
roughness length, zo, is a function of wave age
0.04 0.05 0.06 0.0710
−6
10−4
10−2
Growing sea
u*/c
p
z 0/σ
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Swell (long waves traveling faster than the wind)
andunstable stratification
• A quite different type of boundary layer develops
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The swell driven BL is characterized by
• Small or positive momentum flux• Low level jet• M-O scaling does not hold• The vertical extent can be global• Turbulence spectra resemble free convection conditions• Swell represents reverse atmospheric-ocean coupling
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−0.04 −0.035 −0.03 −0.025 −0.02 −0.015 −0.01 −0.005 0 0.0050
5
10
15
20
25
30
u’w’ (m2s−2)
heig
ht (
m)
C2 C
p/U
8=1.79C3
CP/U
8=1.61Shearing stress
C1C
p/U
8=4.72
F1C
p/U
8=4.66
F2C
p/U
8=1.73
Small momentum flux
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uw x 1000
positive uw
Aircraft measurements
Tower measurements
Positive momentum flux
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LES simulations Peter Sullivan
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0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 20
5
10
15
20
25
30
35
U (ms−1)
heig
ht (
m)
Mean wind profiles
Cp/U
8 = 4.72 C
p/U
8 = 4.60
Case C1 Case F1
Low level jet
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2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 70
5
10
15
20
25
30
35
U (ms−1)
heig
ht (
m)
Mean wind profiles
Cp/U
8 = 1.79 C
p/U
8 = 1.61 C
p/U
8 = 1.73
Case C2 Case C3 Case F2
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0 2 4 6 8 1010
0
101
102
103
104
U (m/s)
z (m
)
Pibal tracking 03090610−23 LST
The vertical extent is global
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−1 0 1 2 3 4 5 6 70
5
10
15
20
25
30
φm
heig
ht (
m)
φm
= (1−15z/L)−1/4for L= −30 m
C1Eq.C2F1F2C3
Φm calculated for five swell cases
Cp/U8=4.7
Cp/U8=1.6
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Turbulent kinetic energy budget
ερ
θ +∂′∂
+∂
′∂⋅+′−
∂∂⋅′′=
zew
zwpw
Tg
zUwu
oo
10
P B Tp Tt D
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−7 −6 −5 −4 −3 −2 −1 0 1 2 3 4 5
x 10−3
0
5
10
15
20
25
30
TKE (m2s−3)
heig
ht (
m)
P B Tp Tt D
Growing sea
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
Frequency − n (Hz)
Ene
rgy
− S
(n)
(m2 s−
1 )
Growing sea case
npeak
= 0.42 Hz
Turbulent energy budgetGrowing sea
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−3 −2.5 −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 2.5 3
x 10−4
0
5
10
15
20
25
30
TKE (m2s−3)
heig
ht (
m)
Case F1
BTp P Tt ε
Turbulent energy budgetcp/U=4.7
0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 20
5
10
15
20
25
30
35
U (ms−1)
heig
ht (
m)
Mean wind profiles
Cp/U
8 = 4.72 C
p/U
8 = 4.60
Case C1 Case F1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Frequency − n (Hz)
Energ
y −
S(n
) (m
2s
−1)
Case F1
nswell
= 0.22 Hz
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−5 −4 −3 −2 −1 0 1 2 3 4
x 10−3
0
5
10
15
20
25
30
TKE (m2s−3)
heig
ht (
m)
Case C3
P B Tt Tpε
2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 70
5
10
15
20
25
30
35
U (ms−1)
he
igh
t (m
)Mean wind profiles
Cp/U
8 = 1.79 C
p/U
8 = 1.61 C
p/U
8 = 1.73
Case C2 Case C3 Case F2
Turbulent energy budgetcp/U=1.6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.005
0.01
0.015
0.02
0.025
0.03
Frequency − n (Hz)
Energ
y −
S(n
)(m
2s−
1)
Case C3
nswell
= 0.22 Hz
nmin
=0.35 Hz
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)2
''''(''
1 2
εθ ++∂∂
+−−=∂∂
pv Tqwz
wTg
wuzU
'')(
wuT
zU p
Tp −=∂∂
dzuT
mUzUz
m
p∫=−5.2
2*
)5.2()(
From the turbulence energy eq,
The effect of Tp can be isolated
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4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9 60
5
10
15
20
25
30
35
U (ms−1)
heig
ht
F2 (based on numerical expressions for Tp)
blue: measured U−curve
red: measured U−curve minus Tp/u*2
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Heat fluxes
All turbulent fluxes in models must be expressed as mean variables
)( soH TTU
wC−′′
=θ
Wind speed Air-sea temperaturedifference
Stanton numberHeat flux
In the same way for latent heat, CE
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)}/)}{ln(/{ln( 00
2
THN zzzz
C κ=
Reduced to neutral stability
Von Karman constant=0.4
Roughness length Roughness lengthfor temperature
In the same way for latent heat, CEN
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Heat fluxes, unstable stratificationΔT>3o
• Traditional parametrization (COARE algorithm) is valid
• No variation with wave age
Heat fluxes, stable stratification
• Low-level jets cause shear suppression leading to decreased fluxes
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Heat fluxes, unstable stratification, very close to neutral
Wind speed > 10 m/s and ΔT< 3o
Fluxes are underestimated with COARE algorithm
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10−4
10−3
10−2
10−1
100
101
102
10−4
10−3
10−2
10−1
100
101
nSu,
w(n
)/u*2 Φ ε2/
3 nS T(n
)/T*2 Φ ε−1
/3Φ N
T
f=nz/U
Tuw
Unstable stratification -- horizontal rolls
U T
w
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10−4
10−3
10−2
10−1
100
101
10−3
10−2
10−1
100
101
f=nz/U
nSu,
w(n
)/u *2 Φ
ε2/3 n
ST(n
)/T
*2 Φε−1
/3Φ
NT
Tuw
Unstable near neutral -- detached eddies
UT
w
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10−4
10−3
10−2
10−1
100
101
102
0
0.05
0.1
0.15
0.2
0.25
f=nz/U
nCO
wt(n
)/w
t
Norm. heat flux Östergarnsholm
L=−10 m
L=−42 mL=−140 m
L=−250 m
Normalized heat fluxes
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2 4 6 8 10 12 140
0.5
1
1.5
2
2.5
3x 10
−3
U10
CH
NMIUU, L > −150m
data2
R2, 1 m/s bins
MIUU, L <−150m; 0.5<(Tw − T10) <1.0K
MIUU, L < −150m; 1.0 < (Tw − T10) < 1.5K
MIUU, L < −150m; 1.5 < (Tw − T10) < 2.0 K
ΔT<3o
Sonic temp.
Platinum sensor ( o ***)
Heat flux from Östergarnsholm
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Latent heat flux, Östergarnsholm
COARE
ΔΤ<2ο
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2 4 6 8 10 12 14 16 180.5
1
1.5
2
2.5
3
3.5
4x 10
−3
CH
N
U10
SonicTC
ΔT>3
ΔT>3
0.8
0.9
0.8
0.6 0.7
2.18
0.752.2
2.7
2.0 3.0
2.3 2.0
..
Agile heat flux
thermocouple
sonic
ΔT<3o
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2 4 6 8 10 12 140
0.5
1
1.5
2
2.5
3x 10
−3
U (ms−1)
CE
N
2.98 3.04
3.1
3.13.17
3.13
3.2
2.06
2.05
2.32
6.19
6.14
6.22
5.562.26
Agile Licor, ΔT<3
Agile latent heat flux
Solid curves data from Östergarnsholm
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2 4 6 8 10 12 14 16 18−0.01
−0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04Comparison of heat fluxes, differences, Agile−94,95
w´t
´ TC −
w´t
´ S (
m s
−1 K
)
U10
(m s−1)
Agile heat flux: (wt)TC- ( wt) sonic
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The unstable very close to neutral regime
• The UVCN regime is defined for ΔT<3o and U>10m/s
• It is found both over land and sea in the lowest region (z<15 m) of the surface layer
• Small scale heat flux is a result of detached eddies originating from the upper part of the surface. This is in contrast to a convective surface layer ruled by horizontal rolls.
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Frequency (%) of occurrence of UVCN conditions
ERA-40 reanalysis
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Increase (W/m2) in latent heat flux with UVCN regime included
ERA-40 reanalysis
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Global mean values of the increase of heat fluxes with UVCN regime included
• Sensible heat flux= 0.8 W/m2
• Latent heat flux (evaporation)= 2.4 W/m 2
• The effect of increased anthropogenic greenhouse gases (2006) on net radiation = 2.63±0.26 W/m 2
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Conclusions
Unstable stratification
• M-O similarity theory is only valid for growing waves
During swell the MABL is characterized by
• Low-level jet (LLJ) at 5-10 m height• Constant wind speed above LLJ• Small or positive momentum flux varying with height• Energy is transported from waves to atmosphere by
pressure transport term Tp
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Mixed seas
As soon as a small portion of the waves are travelling faster than the wind (mixed seas) MABL is slowly changing from an ordinary boundary layer to a ‘swell dominated ‘ boundary layer and
Monin-Obukhov similarity theory is not valid
During stable stratification wave effects are small
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Heat fluxes
• The dynamics of the MABL influence the air-sea exchange of sensible and latent heat.
• When the convective boundary layer is ruled by horizontal rolls the well known COARE algorithm is valid.
• When detached eddies originating from the upper part of the surface layer is present (UVCN regime) the heat fluxes are increased and the Φh-function goes to zero
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Why has not the UVCN regime been observed earlier?
• sonic temperature fails to measure temperature fluctuations correctly when σT is small (high wind speed, small ΔT)
• data with small ΔT are often removed from the data set
• measurements are taken at heights above the UVCN layer.
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Thanks to• Ulf Högström, Erik Sahlee, Anna Rutgersson,
Hans Bergström, Cecilia Johansson, MIUU
• Will Drennan, RSMAS, Miami, Kimmo Kahmaand Heidi Pettersson, FMRI, Helsinki
• Phd-students: Andreas Karelid , Björn Carlsson, Maria Norman, Alvaro Semedo
• Former Phd-students: Cecilia Johansson Birgitta Källstrand, Anna Rutgersson, Anna Sjöblom, Xiaoli Guo Larsen