Precipitation Validation Network in West Africa set up for AMMA Perspectives for MeghaTropiques and

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Precipitation Validation Network in West Africa
set up for AMMA Perspectives for
Megha-Tropiques and GPMMichel Desbois et al.
(LMD), Thierry Lebel et al. (IRD/LTHE)Abdou Ali
et Abou Amani (AGRHYMET)
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Quick reminder what is AMMA ?
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What is available as pluviometric data over Sahel
?
Cilss network 650 stations available several
weeks after the rainy season CRA network
average of 280 stations available every 10
days SYN average of 85 stations available each
day
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Summary on the kriging method used to create the
areal rainfall for validation of satellite
rainfall estimates

•  Regression kriging method is used to estimate
  the mean areal rainfall for the three grids
  (0.5x0.5, 1x1 and 2.5x2.5) from AGRHYMET
  rainfall data base (around 650 gauges on average)
•  
• 2.      A drift model with quadratic component
  for the latitude and linear one for the longitude
  is used
•  
• the values of the coefficients a, b, c and d
  depend on the period (on the 10-day under
  consideration). There are estimated using the
  generalized least squares (GLS) in order to
  account for the spatial correlation of residuals.
•  
• 3.      Also in order to account for the
  non-homogeneity of the drift coefficients in the
  Sahelian region, the studied area is divided in
  five regions. The central region between
  longitude 13W and 10E and latitude 12N and
  15N. The different sides of this central region
  are considered separately.
•  
• 4.      A nested variogram using two anisotropic
  exponential functions with an anisotropy
  coefficient of for the first structure
  (decorrelation distance 0.2) and 2/3 for the
  second structure nested (decorrelation distance
  3) is used.
•  
• 5.      For details on the kriging method and
  raingauge data description used to compute the
  validation areal rainfall, see Ali et al., J.
  Appl. Meteo, 2005 (accepted).
•  

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Regional  supersites  are available for higher
resolution studies
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Supersites of Niamey and Djougou
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Raingauge network Mesoscale sites Available
data sets for 2004

• Niamey square degree
• 33 recording rain gauges
• Raw data 5 min, from May to September (depends
  on the station)
• Validated data
• 10-day periods - station and grid (krigging 5
  km x5 km)
• rainfall events (i.e. gt30 rainy stations) -
  station and grid (krigging 5 km x5 km)

North Benin 35 recording rain gauges Raw data 5
min, all over the year (with some missing
data) Validated data daily rainfall
(stations) Gridded estimates are not available
(yet) by lack of climatological knowledge
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Example of large scale validation of a particular
algorithm SRA SRA means Sliding Rescaling
Algorithm (Franck Chopin and Jean-Claude
Bergès) It is based on the estimation of
rainfall probability on MSG images (15minutes
intervals, 3 km pixels), calibrated on
coincidences with TRMM radar. A second step is
to establish maps of  precipitation
efficiencies  based on comparisons of monthly
rainfall accumulation with rainfall
probabilities Then, precipitation estimates can
be provided at all time scales below one month
(but errors need to be estimated)
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Data and Experimentation area
Input dataset ? Full flow of MSG/HRI ? TRMM/PR
from 3G68Land ? GPCP 1 daily rainfall Validation
dataset ? Dense raingauges network (IRD
AGRHYMET supplied) Experimentation Area ? A
window on W-Africa from June to Sep. 2004 (25W
to 25E and 5S to 20N) This is the AMMA West
Africa Region
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Example of Rainfall Probability Image
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Collocation between Pr image and 2A25 TRMM data
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Examples of Potential Intensity Image
Mm/h
Mm/h
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Example of rainfall estimation image
mm
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Results for 3rd decade of August 2004
Krigged data set
Number of stations
SRA on area and space resolution of krigged data
set
SRA result
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Third decade of August, 2004
GPCP 1DD
raingauges
Duration of rain (hours)
SRA
raingauges
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Third decade of August 2004
Number of raingauges
Absolute value difference between raingauges and
SRA
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Results for all the decades of 2004 are better
for most of the indicators for SRA than for GPCP.
But better validation fields are needed at
smaller scales.
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Present work for Amma is concentrated on giving
results at smaller scales, for needs of soil
moisture modelling and hydrology. Further
validation will use the supersites of AMMA, which
were already operated in 2004. Year 2005 will
also be processed, as well as 2006 (the special
observing period of AMMA, and 2007). After 2007,
it is foreseen to maintain these supersites, in
particular for the needs of Megha-Tropiques and
GPM ground validation. It seems very difficult
in Africa to establish and maintain validation
sites in the definition of GPM, in particular
with well calibrated rainfall radars. However,
considering the crucial and specific problems of
rainfall estimation in these regions of the
world, the Megha-Tropiques science team is
considering AMMA-type supersites as necessary for
ground validation over these regions. (does GPM
support this statement ?)
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Perspectives of Megha-Tropiques (launch
2008-2009) nominal life time 3 years
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• ScaRaB a wide band radiometer for radiative
  fluxes measurement (transversal scan, 40 km)
• Saphir a microwave water vapour sounder. 6
  channels around 183.31 GHz. (transversal scan, 10
  km)
• Madras a microwave imager
• 5 channels 18, 23, 37, 89 and 157 GHz
• (conical scan, lt10 km to 40 km)
• Marfeq is the radiofrequency part of this
  instrument

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30 N
15 N
15 S
30 S
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2005 new start of the scientific studies

• 2004 French mission team constituted Michel
  Desbois (P.I.) Laurence Eymard (SAPHIR) Rémy
  Roca (mission) Nicolas Viltard (MADRAS) Michel
  Viollier (ScaRaB) Sophie Cloché (data bases)
• January 2005 1st meeting of a preliminary
  French  science team 
• April 2005 Indo-French meeting of the mission
  teams in Bangalore (Indian team chaired by R.
  Narasimha, PIs J. Srinivasan, M.S. Narayanan,
  participants P.C. Joshi, V.K. Agarwal, Kusuma
  Rao, members of SAC Ahmedabad, of ISRO HQ)
• October 2005 enlarged Indo-French meeting in
  Ahmedabad aimed to start common projects around
  missions Megha-Tropiques and Oceansat III (French
  altimeter Altika)

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Preliminary list of actions

• Actions devoted to algorithm developments
• Satellite data bases for testing algorithms and
  processing procedures of the Megha-Tropiques
  instrumental package.
• Microwave rain algorithm developments
  adaptation and evaluation of TRMM-based
  algorithms specific effort on the ice phase.
• Combined geostationary - microwave algorithms.
• Radiative fluxes retrievals at different
  space-time scales.
• -Humidity retrievals in the perspective of
  SAPHIR.

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• Actions devoted to validation of satellite
  products
• Precipitation retrieval validation (raingauges
  and radars) sampling and scaling effects.
• Validation for other quantities (ice
  microphysics, others)
• Intercomparisons of methods for water vapour
  determination (IR and MW sounders, GPS, )
• General use of the AMMA campaigns and data bases
  for satellite validation studies
• - Promoting specific campaigns for water vapour,
  clouds and radiative budget .

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• Actions devoted to MT sampling effects
• Use of geostationary observations to simulate the
  MT sampling of convective systems and their life
  cycle. To be performed over Africa, India and
  Brazil.
• Development of a simulator for MT from analyses /
  satellite observations to analyze the impact of
  MT orbit on the various fields retrieved
  (radiative budget, water vapour )

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• Actions devoted to MT data scientific
  exploitation
• Assimilation of microwave data including MT in
  forecast models.
• Impact of dry subsidence areas in the subtropics
  on the global climate .
• Upper tropospheric drying influences on monsoon
  cloud systems and hence on monsoon variability.
• Construction of cloud climatology over the Indian
  ocean area.
• Structure of mesoscale convective
  complexes/systems over Indian Monsoon Region.
• Monsoon systems in India and Africa
• Use of microwave emissivity in complement of
  VIS-IR data for land surface properties.
• Cloud resolving models for Indian region using
  Microwave and radar data.

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Conclusion

• Numerous research subjects related to the
  satellite mission, but also to general tropical
  climate issues
• Presently, not enough scientists between France
  and India to perform all this research
• Mission teams in France and India look for a
  widening of the science community working on
  Megha-Tropiques related themes
• Concerning ground validation, same tropical sites
  should be used for GPM and MEGHA-TROPIQUES