ra-2 & mwr ccvt – esa/esrin – 25 to 27 march, 2003 mwr calibration & validation status...

RA-2 & MWR CCVT – ESA/ESRIN – 25 to 27 March, 2003

MWR Calibration & Validation status

MWR Cal/Val team: Ngan Tran, Estelle Obligis, Laurence Eymard,

Michel Dedieu,

Claire Marimont, Joël Dorandeu, Yannice Faugère


Instrumental parameters monitoring

-2.8%



-3.0%



-3.8%



=> Impact direct sur Ta(ERS2 < 0.5K)

-2.2K

1.3K


Impact of drift on the Brightness Temperatures

Drift of -1.06K/year on ENVISAT TBs

Drift of -1.90K/year on ENVISAT TBs

Monitoring over continental areas


Impact of drift on the Brightness Temperatures

Monitoring over coldest ocean points

Selecting method of the coldest points over ocean: -1.2K /year


MWR 36 GHZ Drift Analysis

• The sky horn counts, hot load counts and gain have decreased by about 4% since launch

• Different methods provide a similar estimation for the drift of the brightness temperatures

• Differences between ERS and ENVISAT TBs present a drift, which is between +1 and +2 K/year

=> A drift of 1 to 2K/year corresponds to a drift between 0.7mm and 3 mm per year depending on the brightness temperature.

• Action on ALS (MWR manufacturer) to perform the investigations on the too high Te and counts and gain drifts cause and provide recommendations

• If no MWR stabilization, a Level 1 B algorithm upgrade to correct for the drift could be envisaged in a later stage


Methodology for the MWR Calibration

• Retrieval algorithms formulated using the 2000/2001 analyses of the

ECMWF model / simulations by a radiative transfer model / regression

• Important change in the model in January 2002

Mean bias(K) 2000/2001 2002 difference

SIMU/ERS2 23.8 GHz +4.91 +2.75 +2.16

36.5 GHz +2.16 +0.30 +1.86

• Algorithms and calibration have to be consistent

• Not suitable to calibrate ENVISAT/MWR over 2002 analyses

> Calibration performed on ERS2 TBs adjusted on 2000/2001 database


+4.91 K +2.16 K

Comparison measurements/simulations ERS2 - 2000/2001 database


ERS2 vs ENV dh computed with

adjusted sigma0_Ku

High |dh| higher

Low |dh| lowerBias=4.5 mmStdev=10mm


ENV dh computed with adjusted sigma0_Ku vs

ECMWF dh

Bias=3.7 mm


ERS2 vs radiosounding wet

tropo

With Envisat we will get a better agreement with radiosounding measurements


MWR Algorithm Upgrades• Level 2 Neural Network Algorithm

Already implemented and validated in Reference Processor Expected operational date in IPF and CMA by mid-2003

Bias +1.46 cm Bias +4.5 mmNeural productsBetter behavior for dry atmosphere (no more necessary to add a particular correction)

Current products


MWR Algorithm Upgrades

• Level 2 MWR Side Lobe Algorithm

– Standard algorithm :

• Tsl constant : Tsl_24=8.21K and Tsl_36=0.413K

– Improved Side Lobe algorithm

• Tsl=eta_sun x Tsun + eta_sky x Tsky +eta_earth x Tearth + eta_sat x Tsat

• The satellite is supposed to be at the earth temperature

• Tsl=eta_sun x Tsun + eta_sky x Tsky +(eta_earth + eta_sat) x Tearth

• Tearth estimated from 1 year of ERS2/MWR measurements

• function of the position(1°x1° meshes)/season(4)/frequency(2)


Mean Side Lobe Contribution for 23.8 GHz channel - Spring

Actual constant value


Difference between TB_23.8 computed with improved and operational algo for cycle 10


Difference between TB_36.5 computed with improved and operational algo for cycle 10


Difference between Dh computed with improved and operational algo for cycle 10


Validation of the new Side Lobe algorithm

Validation method– Comparison between BTs Envisat and ERS-2 over strongly

contaminated zones– With the new algorithm, the difference in BT between ERS and

Envisat shall be closer than the one observed in open ocean

But– Can we consider the ERS-2 BT as a reference ?– Are all the Envisat efficiencies values valid ?– Can the Envisat Platform be considered as a perfect reflector ?


Conclusions / Recommendations

• The Envisat MWR has been calibrated to get consistency with the 2000/2001 database used for the computation of the retrieval algorithms

• Mean bias around 5 mm between ERS2 and ENVISAT dh => we will get a better agreement with radiosounding measurements

• Neural algorithm ready for operational application– Linear correction factors shall be added on BTs and Sigma0– Sigma0_Ku 1 dB higher (in MWR NN level 2 processing) to provide

reliable radiometer products

• The impact of the new side lobe algorithm has been estimated (up to 1 cm near the costs). The validation is on the way

• There is a drift of the ENVISAT 36.5 GHz gain. Investigation is on-going. Not taken into account because the calibration has been performed at the end of September and is valuable for October data


Perspectives • Validation of the Side Lobe algorithm near the costs using ERS2 TBs (-

> mid 03)

• Comparison with other radiometers (Jason, AMSU-A - 1 common channel)

• Comparison with radiosounding measurements (when enough coincidences : mid 2003 ?)

• Check/change the calibration depending on the drift explanation (-> April/May 03)

• After adjustment of the ERS2 TBs as well as correction of the 23.8 GHz drift, use of the same algorithms than for Envisat => better agreement and continuity between the 2 missions

ra-2 & mwr ccvt – esa/esrin – 25 to 27 march, 2003 mwr calibration & validation status...

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