Developments on the interpretation and assimilation of ... · Developments on the interpretation...
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Developments on the interpretation and assimilation
of GPSRO data atEnvironment Canada
Josep M. Aparicio, Stéphane LarocheData Assimilation and Satellite Meteorology
Environment Canada
6th COSMIC Data Users WorkshopBoulder, Oct 31st, 2012
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Outline
• Considerations around unbiased observations• Realization of GPSRO as a calibrated source
– Quality of our knowledge– Air compressibility– Expression of refractivity
• Exploration of calibration’s forecast value– Direct value– Indirect value through radiance bias correction
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Information flow from data
Absolute Data(eg GPSRO)
Directassimilation
Assimilation of Dataimpacted by
bias correction loop
NumericalFields
Absolute Data(eg GPSRO) Assimilation
NumericalFields
AtmosphericFields
Relative Data(radiances)
Bias correction
∑
−=
2
2
1
σBO
JObs
Background
Double path for impact:
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The tolerance to bias (in NWP) : 1
Standard view within the GPSRO community:• “GPSRO is self-calibrating, unbiased”
But:– 1: Is it true?– 2: Is it verifiable?– 3: Does it require a careful procedure? (to realize the accuracy)
Most measurements in NWP (radiances) are more biased (10x-100x)
– But nobody is claiming that they are not– They don’t receive the responsibility to calibrate other data
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The tolerance to bias (in NWP) : 2
• From an NWP user perspective, the no-bias claim means:
“Sufficiently unbiased to avoid degrading forecast performance”
• Window of optimum forecast quality is very narrow– Verified in different ways at EC, ECMWF, NCEP.– Width of this window about 0.05% (O-B)/B
• Not so surprising:– GPSRO injects information at fractional levels around 0.5%
(O-B)/B, leaving little room to accept a bias
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Agreement of several « anchors »V
aisa
la R
S92
Sip
pica
n M
ark
IIA
AV
K-M
RZ
VIZ
-B2
Coincident (>104) GPSRO vs several RS types, at several sun elevations
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The Refractivity-Atmosphere link
• Measurement is– Or equivalent , or other
• Interpreted as field of (P,T,q)
• Required– Refractivity expression N ↔ (P,T,q)
Local relationship (thermodynamic)
– Structure of the atmosphereNonlocal (hydrostatic eqn, etc)
Note: NWP Obs operators must include both relationships
)(xN
),,( qTPx ↔
)(hN )(aα
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1: Structure of the atmosphere
• Essentially, the hydrostatic equation
• We need there the equation of state (EOS)
• Already found that the deviation of EOS from ideal is non-negligible
• Non-local
• 0.05% relevant for NWP if systematic (affects the anchor of radiances)
ρ)(xgP−=∇
),,( wxTP ρ
Ideal gas Non-ideal
Surface
Impact non-localLevels displaced(even if EOS is locally identical)
0.02%-0.1%5-20 m at T/P
EOS differs locally
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Proposed setup
Hydrostatic equation• Should consider
• EOS should include compressibility
Refractivity expression• Calibration should have included compressibility
• Expressions of the form
cannot attain stated accuracy (for any set of coefficients)
• By theory or experiment should consider– Air composition
– Molecular polarizability
– Electric dipoles (H2O)
– Magnetic (O2) dipoles
– Dielectric enhancement
– Univocal meaning
1/15.273
)886.6385605.6701(
)069.0682.222(
)6/101(
:Pr
0
600
−=⋅⋅+
+⋅⋅+=⋅+= −
T
N
NNN
oposal
w
d
τρτρτ
),,( wxTPρ),( hg λ
2321 /// TPkTPkTPkN wwd ++=
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Forecast impact of the calibration I
• Different implementations of GPSRO calibration– Our first (RU02)– Our refined (see
former viewgraphs, AL11)
– Other tests (SW53)
• Good tropospheric temperatures at stake
RS Temperatures (World AVG)
All Jan 2009
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Forecast impact of the calibration II
• Bias correction– Each RO implementation
blocked/allowed to calibrate radiances
• Blocking/allowing (DYN) bias correction feedback loop between implementations
• Impact smaller, but comparable to differences between calibrations
• Indirect impact of RO assimilation comparable to direct impact
RS Temperatures (World AVG)
All Jan 2009
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GPSRO calibration and denial tests
• Cycles– Best estimate, with its
own bias correction
– No GPSRO assimilation, but bias correction from best estimate retained
– No GPSRO assimilation, and bias correction recalculated
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Impact of the calibration and denial
• We use NO-GPSRO as reference
• Blocking/allowing (DYN) bias correction feedback loop between implementations
• A large fraction of impact depends on calibration
• Indirect impact (through bias corr) of RO assimilation comparable to direct impact
Anom CORR GZ 500
All Jan 2009, World
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Caveat:The danger of success
• GPSRO a very important addition
• Can even mask issues (testing new radiances)– Nothing special with
GPSRO– Large polar Temp bias
fluctuations without (builds with as little as ~3 days of data loss)
• Exposure danger to depend on a non-fully operational system
Antarctica
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Conclusion
• As calibrated data, GPSRO has– Direct impact (entered in the cost function)– Indirect impact (anchors radiance bias correction)
• Both impact paths have forecast value• Different calibrations lead to different fcst performance• Indirect impact smaller than direct, but not negligible
We recommend:• A careful revision of inter-calibration for optimal results• A revision to rule out critical dependencies
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Thank you!Thank you!