Transcript Cold Reference [K]

Ocean Surface Topography
Calibration and
Performance Assessment of
the JMR and TMR
Shannon Brown, Shailen Desai, Wenwen Lu
NASA Jet Propulsion Laboratory
JMR Calibration Status
• Most recent calibration on JMR version B GDR
• No large calibration offsets observed after Nov. 2006
safehold
• Slight change in 23.8 GHz ND 1 observed, should have <
2mm effect on PDs
• Cause of the JMR PD scale error reported at Venice SWT
was identified and corrected
– This will be implemented in the version-C GDRs
– Users can apply ad-hoc correction by dividing PDs by 1.023
Brown et al. OSTST07-Hobart
TMR Calibration Status
• End-of-mission GDR calibration effort completed
• TMR replacement product is available
• Details of the calibration methodology and results
are presented here (and in poster in C/V room)
Brown et al. OSTST07-Hobart
On-Earth TB References
• Tune radiometer to on-Earth hot and cold TB references
– Vicarious Cold Reference
(Ruf, 2000, TGARS)
• Stable, statistical lower bound on ocean surface brightness temperature
– Amazon pseudo-blackbody regions (Brown and Ruf, 2005, JTECH)
• THOT(frequency, incidence angle, Local Time, Time of year)
SSM/I 37.0 GHz V-pol – H-pol TB
Histogram
of Cold TBs
Hot Reference Regions
Brown et al. OSTST07-Hobart
Sensitivity of References to Climate Variability
• Cold reference
– Small annual signal present (~0.2-0.3 K peak to peak)
– Stable over many years?
• Hot reference
– Significant diurnal (~6K) and annual (~2K) signal present
– Minimum annual signal in early morning hours
– Affected by 1997-98 El Nino/La Nina
Observed TMR 21.0 GHz Cold Reference
Observed TMR 18.0 GHz Amazon TBs
Annual harmonic fit +
linear drift
Brown et al. OSTST07-Hobart
Hot Reference Model 1992-2005
• Use NCEP/NCAR Reanalysis-1 4x-daily fields to model Amazon regions
–
–
–
–
Surface emissivity estimated from SSM/I for each TMR frequency
NCEP provides temperature, pressure and humidity for radiative transfer
4-x daily modeled TBs are interpolated to TMR observation times
Model is able to replicate the observations during 1997/98 El Nino
Observed TMR 18.0 GHz Amazon TBs
Modeled 18.0 GHz Amazon TBs
*NCEP Reanalysis data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA,
from their Web site at http://www.cdc.noaa.gov/Brown et al. OSTST07-Hobart
Cold Reference Stability
• SST, PWV and TB joint statistics computed using NCEP/NCAR Reanalysis 4x-daily fields for 1998
18.0 GHz
21.0 GHz
PWV
SST
Brown et al. OSTST07-Hobart
37.0 GHz
Cold Reference Stability
• Cold reference TB for each channel is sensitive to different
areas of joint probability distribution of SST and water
vapor
18.0 GHz
37.0 GHz
21.0 GHz
Brown et al. OSTST07-Hobart
Sensitivity of Cold Reference
• SST/PWV joint probability distribution is artificially perturbed to assess
impact on cold reference
• No significant changes in cold reference until probability of occurrence
in optimum SST/PWV regions decreases by about 40%
• Contend that cold reference is stable over TMR lifetime and any drifts
are related to calibration errors
21.0 GHz Cold Reference Deviation
37.0 GHz Cold Reference Deviation
Brown et al. OSTST07-Hobart
TMR TB Drift 1992-2005
•
•
•
•
Known 1.5 K drift in TMR 18.0 GHz channel, attributed to drift in cold sky horn
switch isolation (Ruf, 2002 TGARS)
Small drifts also observed in 21.0 and 37.0 GHz cold TBs <0.5 K over 13
years
0.5 K drift observed in 37.0 GHz hot TB, little drift observed in 18.0 and 21.0
GHz hot TB
Gain and offset errors observed
TMR - Cold Reference [K]
TMR - Hot Reference [K]
Brown et al. OSTST07-Hobart
Instrument Temperature (Yaw State) Dependency
• Sample on-Earth references w.r.t.
instrument temperature
• Temperature dependency as high
as 0.7 K peak-to-peak at cold end
Channel
dTCOLD/dTInst
dTHOT/dTInstf
18.0 GHz
0.049 K/K
0.036 K/K
21.0 GHz
-0.013 K/K
0.059 K/K
37.0 GHz
0.022 K/K
0.084 K/K
• Recalibration reduces instrument temperature
dependence to < 0.1 K peak-to-peak at cold end
Channel
dTCOLD/dTInst
dTHOT/dTInst
18.0 GHz
-0.0032 K/K
-5e-4 K/K
21.0 GHz
0.0079 K/K
0.0030 K/K
37.0 GHz
-0.0033 K/K
0.017 K/K
Brown et al. OSTST07-Hobart
RaOb-TMR PD vs
Instrument Temperature
GDR TMR
dPD/dT = 0.36 mm/K
Recalibrated TMR
dPD/dT = 6e-5 mm/K
TMR PDs compared to SSM/I
• Recalibrated PD drift compared to SSM/I derived PDs is
reduced to 0.006 mm/yr over 13 years
• Validates that cold reference is stable over this time period
• Minimization of yaw state errors evident in reduced noise (std.
dev. of difference is 1.1 mm for TMR recal.)
Recalibrated
TMR
SSM/I – TMR PD
*SSM/I vapor fields acquired from
Remote Sensing Systems
Brown et al. OSTST07-Hobart
Path Delay Validation
•
During the initial TMR post-launch cal/val, the PD coefficients were increased
by ~5% to remove a PD scale error
– This was attributed to the model for the water vapor absorption line strength being
too low
•
•
•
The PD coefficient scaling was compensating for the large gain errors in the
T Bs
After correcting the gain errors in the TBs, a 5% scale error became evident in
the PDs
This was removed by reverting to the pre-launch TMR PD coefficients
TMR PD vs RaOb PD 1992-2005
TMR Shows Negligible Bias and Scale
Error Compared to Other Sources
Bias
[mm]
Scale Error
[mm/mm]
Notes
JMR*
0.36
1x10-4
Cycles 344-365
RaOb
2.5
-0.005
Cycles 1-481
GPS
4.4
+0.004
Cycles 1-355
ECMWF
-3.6
-0.005
Cycles 344-481
Brown et al. OSTST07-Hobart
Comparisons to JMR
• JMR PD coefficients also require adjustment to account for
spurious increase in water vapor absorption model line
strength
• After JMR PD coefficient adjustment, scale error is
negligible between JMR and TMR
• Additionally, scale error
in JMR compared to
ECMWF and GPS, which
was reported at the Venice
SWT, is removed with PD
coefficient adjustment
JMR PD – TMR PD vs JMR PD
Additional details in presentation
by S. Desai
Brown et al. OSTST07-Hobart
ox0.4o grid
JMR––TMR
TMRRegional
RegionalBiases
Biaseson
< 30.4
mm
Cycles 1-21 JMR
dPD [mm]
Brown et al. OSTST07-Hobart
JMR-TMR Regional Error Statistics
• Averaged JMR/TMR differences on a 0.4o lat/lon grid for
cycles 1-21
• Nearly half of the averages have differences of < 1 mm
• 90 % have differences of < 3 mm
Brown et al. OSTST07-Hobart
Summary
• TMR recalibration is complete
• TB drifts, gain and offset errors, and instrument
temperature dependent errors were removed
• PD coefficients were reverted to pre-launch values
• TMR PDs are in good agreement with several validation
sources
– No drift compared to SSM/I
– Low bias and negligible scale error compared to RaOb, GPS, and
ECMWF
• After JMR PD coefficient adjustment, TMR and JMR are in
excellent agreement
– Although, there is still room for regional improvement
– JMR calibration will be updated on version-C GDRs
Brown et al. OSTST07-Hobart
TMR Climate Record
Brown et al. OSTST07-Hobart
Backup
Brown et al. OSTST07-Hobart
TMR Vapor Trends 1992-2005
• Work is on-going to produce climatology from TMR data
• Stevens (1990) derives approximate mean
relationship between integrated vapor and SST
 RH  a  SST  288
w  10.82
e
1




• Differentiating gives
â = 0.05, R2=0.46: all SST
w
 aSST  0.064SST
w
â = 0.06, R2=0.36: SST > 15oC
Brown et al. OSTST07-Hobart