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