Ocean Surface Topography
Calibration and Calibration and Performance Assessment of Performance Assessment of
the JMR and TMRthe JMR and TMRShannon Brown, Shailen Desai, Wenwen Lu
NASA Jet Propulsion Laboratory
Brown et al. OSTST07-Hobart
JMR Calibration StatusJMR 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 StatusTMR 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 TOn-Earth TBB References 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 VariabilitySensitivity 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 18.0 GHz Amazon TBsObserved TMR 21.0 GHz Cold Reference
Annual harmonic fit + linear drift
Brown et al. OSTST07-Hobart
Hot Reference Model 1992-2005Hot 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 StabilityCold Reference Stability• SST, PWV and TB joint statistics computed using NCEP/NCAR Re-
analysis 4x-daily fields for 1998
18.0 GHz 21.0 GHz 37.0 GHz
PWV
SST
Brown et al. OSTST07-Hobart
Cold Reference StabilityCold 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
21.0 GHz
37.0 GHz
Brown et al. OSTST07-Hobart
Sensitivity of Cold ReferenceSensitivity 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 TTMR TBB Drift 1992-2005 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) DependencyInstrument 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
RaOb-TMR PD vs Instrument Temperature
dPD/dT = 0.36 mm/K
dPD/dT = 6e-5 mm/K
Recalibrated TMR
GDR TMR
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
• Recalibration reduces instrument temperature dependence to < 0.1 K peak-to-peak at cold end
Brown et al. OSTST07-Hobart
TMR PDs compared to SSM/ITMR 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.)
*SSM/I vapor fields acquired from Remote Sensing Systems
SSM/I – TMR PD
Recalibrated TMR
Brown et al. OSTST07-Hobart
Path Delay ValidationPath Delay Validation
Cycles 344-481-0.005-3.6ECMWF
Cycles 1-355+0.0044.4GPS
Cycles 1-481-0.0052.5RaOb
Cycles 344-3651x10-40.36JMR
NotesScale Error [mm/mm]
Bias [mm]
Cycles 344-481-0.005-3.6ECMWF
Cycles 1-355+0.0044.4GPS
Cycles 1-481-0.0052.5RaOb
Cycles 344-3651x10-40.36JMR
NotesScale Error [mm/mm]
Bias [mm]
TMR Shows Negligible Bias and Scale Error Compared to Other Sources
TMR PD vs RaOb PD 1992-2005
• 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 TBs
• 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
*
Brown et al. OSTST07-Hobart
Comparisons to JMRComparisons 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
JMR PD – TMR PD vs JMR PDJMR PD – TMR PD vs JMR PD• Additionally, scale error in JMR compared to ECMWF and GPS, which was reported at the Venice SWT, is removed with PD coefficient adjustment
Additional details in presentation by S. Desai
Brown et al. OSTST07-Hobart
JMR – TMR Regional Biases < 3 mmCycles 1-21 JMR – TMR Regional Biases on 0.4ox0.4o grid
dPD [mm]
Brown et al. OSTST07-Hobart
JMR-TMR Regional Error StatisticsJMR-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
SummarySummary
• 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 RecordTMR Climate Record
Brown et al. OSTST07-Hobart
BackupBackup
Brown et al. OSTST07-Hobart
TMR Vapor Trends 1992-2005TMR Vapor Trends 1992-2005
• Work is on-going to produce climatology from TMR data
288
182.10
SSTae
RHw
SSTSSTaw
w
064.0
• Stevens (1990) derives approximate mean relationship between integrated vapor and SST
• Differentiating givesâ = 0.05, R2=0.46: all SST
â = 0.06, R2=0.36: SST > 15oC