intercomparison of globvapour total column water vapour (tcwv) and envisat ra2-mwr measurements...

Intercomparison of GlobVapour total column water

vapour (TCWV) and Envisat RA2-MWR measurements

(preliminary results)Bruno Picard (CLS), Jean-Rémi Deboer (CLS)

Martin Stengle (DWD), Marc Schröder (DWD)Estelle Obligis (CLS), Marie-Laure Frery (CLS)

[email protected](+33)5 6139 3737

mailto:[email protected]

Gewax /GlobVapour WS ESRIN, 08/03/2010

Objectives

• A comparison of GlobVapour TWVC against RA2-MWR product (microwave radiometer on Envisat)

• The present study is focusing on 4 monthly SSMI/Meris combined products

• The results are preliminary and show what metrics could be used when more data will be available


Context

• Altimetry missions usually embed a microwave radiometer in charge of providing the wet tropospheric correction ([cm]) to the radar signal

• MWR are 2-bands (23.8/36.5 GHz, ERS-1/2, RA2-Envisat [ESA]) or 3-bands (+18.7GHz, Topex, Jasons [NASA/CNES])


Context

• Altimetry missions usually embed a microwave radiometer in charge of providing the wet tropospheric correction ([cm]) to the radar signal

• MWR are nadir 2-bands (23.8/36.5 GHz, ERS-1/2, RA2-Envisat [ESA]) or 3-bands (+18.7GHz, Topex, Jasons [NASA/CNES])


Wv for altimetry

• The wet tropospheric correction 6 cm of wet tropo. corr = 1 g/cm2 TWVC

• « dry » term (2m) provided by meteorological models with a good accuracy

• « wet » term, almost proportional to the integrated water vapor (1cm = 1.66 kg/m2), highly variable in space and time, ranging from a few cm to about 50 cm, not provided with enough accuracy by meteorological models (actual resolution of 50km/6hours)

H

0

H

0

H

0

v62v5 dz

T

e106dz

T

e37440dz

T

p106777dh .,.,


Altimetry requirements

• To provide the wet tropospheric corrrection :

• with a good accuracy (≈ 1 cm rms)

• with a good stability (≈ 1 mm/year)

• with no geographically correlated errors

• GCOS recommandation: < 0.05mm/yr !


RA2-MWR

• RA2-MWR is a 2-bands (23.8/36.5 GHz) nadir Dicke radiometer (resolution of about 20 km)

• Wet tropo. corr and TWVC are retrieved by a neural network algorithm developed by CLS

E. Obligis, L. Eymard, et al, “First three years of the microwave radiometer aboard ENVISAT: In-flight

calibration, processing, and validation of the geophysical products,” J. Atmos. Ocean. Technol., vol. 23, no. 6, pp. 802–814, Jun. 2006.

• Data are available since 2002


Reminder

• BUT, when using RA2-MWR products, precaution should be taken on the coherency of the dataset

• Algorithms have been constantly improved leading to discontinuities

some RA2-MWR events since BOM• Oct.24, 2005, v5.02, 1st day of cycle 42, L1b upgrade: MWR Side Lobe correction upgrade

• Jan.12, 2010, v6.02, 1st day of cycle 86, L1b/L2 upgrade: new in-flight calibration / new NN algorithm


Reprocessing

• Envisat RA2 whole mission reprocessing in on-going

• The present RA2-MWR data have been reprocessed using the CLS ESL MWR L1B reference prototype applying a consistent set of algorithms


SSMI/Meris combined product

• 4 monthly mean 0.5°x0.5° SSMI/Meris combined TWVC product (prototype):

July 2007, August 2007, January 2008, August 2008

January 2008

(missing data ?)


RA2-MWR (reproc) product

• Along-track nadir TWVC over a month has been averaged on a 0.5°x0.5° map

• Altimetry validity flag has been used to edit

data of poor quality

January 2008


RA2-MWR (reproc) - GlobVapour

• mean bias = -0.74 kg/m2

• mean rms = 7.11 kg/m2

• no geographical pattern

January 2008



• mean bias = -0.74 kg/m2

• mean rms = 7.11 kg/m2

• larger dispersion around 15 kg/m2



• Impact of the reprocessing– official product

– reprocessed product

• larger bias with thereprocessed product

• BUT, more stable in time

mean of diff [kg/m2]

std of diff [kg/m2]months since 07/2008


Conclusion

• Comparison of SSMI/Meris with RA2-MWR shows good agreement

• a bias of about 0.8 kg/m2, constant in time (?)no geographical pattern


Further study

• Expected improvement for this study– > 15 kg/m2 dispersion ?

– more data from GlobVapour ! • SSMI/Meris but also MWR 1D-Var• daily product ?

– validation against:• model: ECMWF TWVC• in-situ measurements: radiosonde


THANK YOU!

[email protected](+33)5 6139 3737

intercomparison of globvapour total column water vapour (tcwv) and envisat ra2-mwr measurements...

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