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
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
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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])
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Gewax /GlobVapour WS ESRIN, 08/03/2010
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])
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Gewax /GlobVapour WS ESRIN, 08/03/2010
Wv for altimetry
• The wet tropospheric correction 6 cm of wet tropo. corr = 1 g/cm2 TWVC
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• « 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)
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Gewax /GlobVapour WS ESRIN, 08/03/2010
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 !
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Gewax /GlobVapour WS ESRIN, 08/03/2010
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
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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
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Reprocessing Page 9
• 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
Gewax /GlobVapour WS ESRIN, 08/03/2010
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
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January 2008
(missing data ?)
Gewax /GlobVapour WS ESRIN, 08/03/2010
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
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January 2008
Gewax /GlobVapour WS ESRIN, 08/03/2010
RA2-MWR (reproc) - GlobVapour
• mean bias = -0.74 kg/m2
• mean rms = 7.11 kg/m2
• no geographical pattern
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January 2008
Gewax /GlobVapour WS ESRIN, 08/03/2010
RA2-MWR (reproc) - GlobVapour
• mean bias = -0.74 kg/m2
• mean rms = 7.11 kg/m2
• larger dispersion around 15 kg/m2
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RA2-MWR (reproc) - GlobVapour
• Impact of the reprocessing– official product
– reprocessed product
• larger bias with thereprocessed product
• BUT, more stable in time
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mean of diff [kg/m2]
std of diff [kg/m2]months since 07/2008
Gewax /GlobVapour WS ESRIN, 08/03/2010
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
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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
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