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“ Combining Ocean Velocity Observations and Altimeter Data for OGCM Verification ”. Peter Niiler Scripps Institution of Oceanography with original material from N. Maximenko, M.-H.Rio, L. Centurioni, C. Ohlmann, B. Cornuelle, V. Zlotnicki,, D.-K. Lee. - PowerPoint PPT Presentation
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“Combining Ocean Velocity Observations and Altimeter Data for
OGCM Verification”
Peter Niiler
Scripps Institution of Oceanography
with original material from
N. Maximenko, M.-H.Rio, L. Centurioni, C. Ohlmann, B. Cornuelle, V. Zlotnicki,, D.-K. Lee
Method of Calculating Ocean Surface Circulation Combines Drifter and
Satellite Observations Between 1/1/88 and 12/1/06 1988 10,561 drifters drogued to 15m depth
were released in the global ocean, with array of 1250 since 9/18/05
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Satellite Observationssea level = altimeter height - geoid height
• Altimeters: GEOS, T/P, JASIN, ERS I&II• Data from 1992 - Present (rms noise: +/- 4cm
relative to geoid)• GRACE’04: Estimated accuracy of geoid: +/-3 cm
at 400 km horizontal scale• Sea level gradient, or geostrophic velocity,
depends upon method and scale of averaging, or mapping, of sea level data
Drifter velocity observations are accurate (+/- 0.015 m/sec daily averages), but spatial distribution of data
can result in biased averages in space and time
Altimeter data is used to calculate geostrophic velocity with “smoothing” scales (and amplitude
correction) consistent with drifter data: e.g. AVISO
0 10 20 30 40 50
40
60
80
100
120
140
160
Bin Center Latitude
Geostrophic Velocity Smoothing Scale vs. Latitude
N. Hemi
S. Hem i
N. Hemi dmn
S. Hemi dmn
N. Hemi nolev
S. Hemi nolev
• • •
••
••
••
• N/S ; *E/W AVISO Correlation Scales
*
*
B. Cornuelle
Drifter observed rms velocity variance [<u’2>+<v’2>]1/2
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N.Maximenko
Log10 (Eddy Energy/ Mean Energy)1/2
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N. Maximenko
The” simple method” of obtaining a velocity map
€
rV G =
r V o + A <
r V 'G−AVISO >
r V GD =
v V D −
r V EK (W , f ,...)
Minimize E relative to A,r V o :
E = <r V GD −
drifter obs<>
∑r V o − A
r V 'G−AVISO >2
Solution depends on vector correlation : C =<
r V 'DG ⋅
r V 'G−AVISO >
[<r V 'DG ⋅
r V 'DG ><
r V 'G−AVISO ⋅
r V 'G−AVISO >]1/ 2
MAP :r V MAP =
r V o + A
r V 'G−AVISO +
r V EK
Vector Correlation between drifter and altimeter
derived AVIO geostrophic velocity anomalies
N.Maximenko
“East Sea”: 3 day average velocity from “simple method” vs drifter obs. 8/01-11/03
D.-K. Lee
Comparison drifter and ECCO 15m zonal velocity components in tropical Pacific
18
Mean U (cm/s) at 15m depth from Drifters
B. Cornuelle
Vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO in California Current
L.Centurioni
HYCOM NLOM POP ROMS
spatial domain global global global ~1000 x 2000 km (USWC)
vertical coordinates hybrid layers levels sigma (ETOPO5)
horizontal resolution 1/12° (~7 km) 1/32° (~3.5 km) 1/10° (~10 km) ~5 km
vertical layers/levels 26 6 + ML 40 20
time step 6 hour 6 hour 6 hour 15 minute
mixed layer KPP Kraus-Turner KPP KPP
wind forcing ECMWF NOGAPS/HR NOGAPS COADS (seasonal)
heat forcing ECMWF NOGAPS ECMWF COADS (seasonal)
buoyancy forcing COADS(restored to
Levitus)
Levitus(restoring)
Levitus (restoring)
COADS (seasonal); parameterization for
Columbia River outflow
integration time 1990-2001 1991-2000 1990-2000 9 years
assimilation none SST, SSH none none
other Low computational
cost
open boundaries
C. Ohlmann
L.Centurioni and C. Ohlmann
Unbiased drifter and satellite derived geostrophic 15m velocity (on left) and ROMS 5km resolution sea level (right)
Geostrophic zonal velocity from drifter and altimeter data
L. Centurioni
Decadal MEAN SEA LEVEL (cm) in models of the California Current
C. Ohlmann
OGCM (Eddy Energy)1/2: California Current
NLMHYCO
POP ROMS
Geostrophic EKE0.5 ROMS (left) corrected AVISO (right) (0-20 cm s-1)
L. Centurioni and C. Ohlmann
Ageostrohic 15m velocity and MSL in 5km resolution ROMS of California Current
C. Ohlmann
THE GLOBAL SOLUTIONS
1. Time mean surface momentum balance for surface sea level gradient:
• Observed drifter = “D”
• Computed Ekman = “E”
€
−< ∇η >= ˆ f x<r v D −
r v E > + < d
r v D / dt >≡ −
r F
€
uE + ivE = A(lat)(Wx + iWy )( NCCEP wind)
2. Compute sea level that minimized the global cost function in least square
The solution is also minimized relative to parameters of Ekman force and GRACE altimeter referenced sea level, Go, is averaged on 1000km scales.
Maximenko-Niiler
€
∂∂ηo
{4π 2
L2| Go
globe
∫∫ − ηo |2 dS + | ∇η o −r F |2
globe
∫∫ dS + 4δ 2 | ∇2η o |2
globe
∫∫ dS} = 0
with (ηoglobe
∫∫ )dS = 0; L = 1000km; δ =1
2
o
(km)€
ηo
€
ηo
3. Perform an objective mapping of sea level, with mesoscale based, geostrophic, correlation
functions, as a linear combination of:
• Levitus 1500m relative steric level, • GRACE referenced altimeter derived sea level
• Drifter geostrophic velocity.
RIO(05), Knudsen-Andersen
1992-2002 Mean Sea Level: Maximenko (05)
Zonal, unbiased geostrophic velocity (-10,+10 cm/sec)
1993-1999 Mean Sea Level: RIO (05)
M.-H. Rio
Difference between Maximenko(‘05)-Rio(‘05) MSL with both data adjusted to 1993-1999 period
M.-H. Rio : RMS difference of 5cm
Comparison of 15m velocity from SURCOLF and MERCATOR near real time maps of Gulf Stream
region with drifter data
M.-H. Rio
Mean Sea Level: Knudsen-Anderson
V. Zlotnicki
ECCO-2 CUBE49 (18km horizontal, global assimilation with flux and diff.par.optim.)
V. Zlotnicki
ECCO-2 cube 37 and 49 east velocity difference from Maximenko (05) and Knudsen-Anderson
V. Zlotnicki
CONCLUSIONS
• Combined drifter and altimeter derived velocity anomalies can be used to make regional, realistic, near real time maps of 15m ocean circulation.
• Global, absolute sea level on 50km scale from combined data displays new circulation features.
• OGCM solutions are most stringently tested with velocity fields derived from combined drifter and altimeter observations.
“Why does our view of ocean circulation always have such a
dreamlike quality…”...Henry Stommel
THE DREAM HAS COME TRUE…
we are observing the circulation
peter niiler
East-west average vorticity balance at 15m depth (black line is from Ekman’s 1906 model, shaded is drifter data; 100 km coastal and western
boundary currents excluded)
The eddy transport of vorticity
The eddy transport vector of vorticity is computed around the Gulf Stream eddy energy maximum.
€
< dr v /dt > −d <
r v > /dt =
1
2∇(<
r v '⋅
r v '>) + ˆ k × < ς '
r v '>
The eddy vorticity transport vector is < ς 'r v '>, where
relative vorticity is ς = ∂v /∂x − ∂u /∂y. In quasi − geostrophic
theory, ς '= (g∇ 2η ') / f0;η ' is obtained from altimeters.
North Atlantic:
0.25º resolution sea level (upper) and “simple” geostrophic velocity (lower).
The 1992-2000 time average quasi-geostrophic eddy vorticity flux vector in the
Gulf Stream region.
The mean kinetic energy at 15m depth from drifters. This quantity graphed is (<v’•v’>/2g) and represents the sea level change caused by Bernoulli effect
of ocean time variable eddies.
SST convergence (x10-7Cºsec-1) at 15m depth:
€
< r
v > ⋅∇(< SST >)
€
1978-2003 Average drifter velocity with QSCT/NCEP blended wind-stress divergence
Conservation of vorticity in the Agulhas Extension Current
Drifter geostrophic velocity compared with ocean circulation model sea-level in California Current in POP (left) and UCLA/ROMS (right)
POP SSH (1990-2000) and geostrophic drifter velocity
132oW 128oW 124oW 120oW 116oW
28oN
32oN
36oN
40oN
44oN
20 cm s 1
10
15
20
25
Global streamlines of 1992-2002 average 15m depth velocity
Zonal, unbiased geostrophic velocity (-40,+40cm/sec)