Transcript WELCOME TO THE 7TH NON COOPERATIVE MOBILE …

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
Objectives
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• 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
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Context
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• 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])
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Context
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• 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, ERS1/2, RA2-Envisat [ESA]) or 3-bands
(+18.7GHz, Topex, Jasons [NASA/CNES])
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Wv for altimetry
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• The wet tropospheric correction
 6 cm of wet tropo. corr = 1 g/cm2 TWVC
dh  77,67.10
5
H
H
ev
p
6
 T dz  0,3744  T 2 dz  6.10
0
0
H
ev
 T dz
0
• « 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)
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Altimetry requirements
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•
•
•
•
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 !
Gewax /GlobVapour WS
ESRIN, 08/03/2010
RA2-MWR
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• 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
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Reminder
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• 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
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Reprocessing
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• 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
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• 4 monthly mean 0.5°x0.5° SSMI/Meris
combined TWVC product (prototype):
July 2007, August 2007, January 2008, August 2008
(missing data ?)
January 2008
Gewax /GlobVapour WS
ESRIN, 08/03/2010
RA2-MWR (reproc) product
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• 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
Gewax /GlobVapour WS
ESRIN, 08/03/2010
RA2-MWR (reproc) - GlobVapour
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• mean bias = -0.74 kg/m2
• mean rms = 7.11 kg/m2
• no geographical pattern
January 2008
Gewax /GlobVapour WS
ESRIN, 08/03/2010
RA2-MWR (reproc) - GlobVapour
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• mean bias = -0.74 kg/m2
• mean rms = 7.11 kg/m2
• larger dispersion
around 15 kg/m2
Gewax /GlobVapour WS
ESRIN, 08/03/2010
RA2-MWR (reproc) - GlobVapour
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• Impact of the reprocessing
– official product
– reprocessed product
• larger bias with the
reprocessed product
mean of diff [kg/m2]
• BUT, more stable
in time
Gewax /GlobVapour WS
ESRIN, 08/03/2010
std of diff [kg/m2]
months since 07/2008
Conclusion
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• Comparison of SSMI/Meris with RA2-MWR
shows good agreement
• a bias of about 0.8 kg/m2, constant in time (?)
no geographical pattern
Gewax /GlobVapour WS
ESRIN, 08/03/2010
Further study
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• 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
Gewax /GlobVapour WS
ESRIN, 08/03/2010
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THANK YOU!
[email protected]
(+33)5 6139 3737
Gewax /GlobVapour WS
ESRIN, 08/03/2010