CLIMATE DATA BASED ON GNSS RADIO OCCULTATION MEASUREMENTS

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CLIMATE DATA BASED ONGNSS RADIO OCCULTATION
MEASUREMENTS
Hans Gleisner Danish Meteorological Institute
(DMI), Denmark GRAS SAF
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Outline

• What is measured with the RO technique sensor
  data geophysical data
• Coverage no of measurements, geographical
  coverage, altitude range, etc.
• Applications use of RO data (NWP, climate,
  re-analysis)
• RO missions
• Role of EUMETSAT and the GRAS Satellite
  Application Facility

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Measurement principle
t2
MetOp
Measurement of phase and amplitude gives a
vertical atmospheric profile at the raystangent
point.
t1
t3
GPS
GPS signals L1 19 cm L2 24 cm
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Atmospheric sounding with GPS RO what is
measured
1. From phase to bending angle
2. From bending angle to refractive index
Bending angle is related to the vertical gradient
of the refractive index.
Under the assumption of spherical symmetry, the
bending-angle integral can be inverted with an
Abel transform.
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Atmospheric sounding with GPS RO what is
measured
3. Refractivity gt Pressure, Temperature,
Humidity
refractive index
refractivity
wet term
dry term
When the wet term is negligable, the assumption
of ideal gas and hydrostatic equilibrium gives
p,T profiles
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Atmospheric sounding with GPS RO what is
measured
3. Refractivity gt Pressure, Temperature,
Humidity
Temperature-humidity ambiguity in the
troposphere. A statistically optimal p,T,q
profile is found through a 1D variational procedur
e, using a priori information from a model, e.g.
ECMWF.
Observational error covariance
Background error covariance
Refractivity observed and background
Background
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Atmospheric sounding with GPS RO atmospheric
profiles
L 300 kilometers Z 200 meters 1.5
kilometer


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GRAS SAF climate data bending angle,
refractivity 1D-Var products
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GRAS SAF climate data 1DVar relative to ECMWF

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Sampling the atmosphere with GPS RO
distribution of profiles
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Sampling the atmosphere with GPS RO
distribution of profiles
Longitude vs. local time
Latitude vs. local time
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RO characteristics summary

• microwave frequencies, active sounding
  independent of clouds, surface, day/night
• based on time measurements, not radiances
  intrinsically stable and calibration free
• limb sounding high vertical resolution
  (0.1-1.0 km from troposphere to stratosphere)
• low horizontal resolution (300 km along
  the ray)
• global coverage sampling all lats lons,
  troposphere and stratosphere
• measures refractivity temperature/humidity
  ambiguity in the troposphere

Important for climate global coverage, high
vertical resolution, long-term stability. An
observation made today can be directly compared
to an observation made 50 years from now without
intermediate inter-calibrations.

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Applications

• Numerical Weather Prediction assimilated
  in real-time at ECMWF, Met Office, Meteo France,
  NCEP, NRL Monterey, HMC (Moscow), etc.
  complementary to radiance measurements gt RO
  provides bias corrections for other type of
  observations, which are then used more
  efficiently.
• Re-analysis assimilated into current
  re-analysis systems ERA-Interim, NCEP-NCAR
  reanalysis, and into future re-analyses
  ERA-75 (the ERA-Clim project)
• Climate climate research applications
  climate change studies, climate model testing,
  stratospheric dynamics, tropopause,
  low-latitude planetary boundary layer,
  climate monitoring alternative to MSU/AMSU
  temperatures

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RO missions past, current, and future
Satellite Instrument Time span Profiles/month C
HAMP BlackJack 2001-09 2009-08 5,000 COSMIC
(FM1-6) IGOR 2006-07 9,000 / satellite METOP
(A-C) GRAS 2007-01 20,000
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EUMETSAT GRAS SAF

• Metop / GRAS mission
• More than 15 years lifetime (3 satellites,
  Metop-A launch October 2006)
• Follow-on RO mission extends operations well into
  the 2030s
• Provision of data in NRT (3 hours) and Offline
  (30 days)
• Re-processing into climate-quality, homogeneous
  data sets
• EUMETSAT supports a Satellite Application
  Facility (SAF) dedicated to RO
• EUMETSAT, centrally
• Raw processing up to Level 1a (phase
  observations)
• Processing from Level 1a to Level 1b (bending
  angles) in NRT and Offline
• Re-processing of data from all major RO missions
  for ERA CLIM (FP7 project)
• Continued development of RO processing methods

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EUMETSAT GRAS SAF

• GRAS SAF
• Processing from Level 1a to Level 1b, 2, and 3.
• Reprocessing of RO data from all available RO
  missions.
• Current phase (2007-2012) has been focused on NRT
  data to NWP applications.
• Next phase (2012-2017) will have a stronger focus
  on climate data.
• Deliver data in NRT and Offline
• Deliver climate data from re-processing
  activities (planned for 2013 and 2016)
• Deliver relevant software (forward modelling,
  assimilation, 1D-Var retrivals, etc.)
• Helpdesk functions to users

http//www.grassaf.org
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STOP
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GRAS SAF climate data sampling
MetOp is in a Sun-synchronous orbit, whereas
CHAMP and COSMIC slowly drift in local time.
i99 deg Sun-sync orbit (MetOp)
i72 deg precessing orbit (COSMIC)
i87 deg precessing orbit (CHAMP)
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Atmospheric sounding with GPS RO measurement
principles

• Limb sounding
• High vertical resolution
• Horizontally integrating (and moving) over 200
  400 km
• measurement of opportunity only
• GPS frequencies (19 / 24 cm, 1.575 / 1.227 GHz)
• Weather independent
• No information gained from absorption
  measurements
• Measurement principle
• Measuring time differences traceable to SI
  standards
• No instrument degradation or drift
• Calibration free
• Highly accurate temperature soundings (lt 1 K) up
  to the mid stratosphere
• Complementary to radiance measurements