Assimilation of satellite data at ECMWF Prospects for use of radiooccultation measurements JeanNol T

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Assimilation of satellite data at ECMWF
Prospects for use of radio-occultation
measurementsJean-Noël ThépautECMWFthanks to
Tony McNally, Graeme Kelly, Antje Dethof and
Niels Bormann
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Outline

• Importance of current satellite observations at
  ECMWF and performances of the NWP system
• Needs for and challenges with high vertical
  resolution soundings
• AIRS
• MIPAS
• Priorities for the future Scope for using GPS
  radio-occultation measurements
• CAL/VAL and monitoring capabilities
• Acquisition of limb sounding expertise
• Solve the vertical resolution requirements?

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Importance of current satellite observations in
the ECMWF system

• Satellite data have progressively become an
  essential part of the observing system used at
  ECMWF
• Satellite data represent by far the largest
  volume of data (and associated computing cost)
  used in the ECMWF data assimilation system
• Satellite data have recently caught up
  radiosondes in terms of forecast skill impact
  over NH
• 4D-Var is particularly appropriate to assimilate
  high time frequency data

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4D-Variational Analysis of Observations (4D-Var)
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Satellite data actively used in the current ECMWF
DA system
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NOAA15-16-17 and DMSP13-14-15 satellitesAMSU/HIRS
SSM/I raw radiances
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NASA-Quikscat (Seawinds)
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Geostationary platforms AMVsWV radiances
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Polar platforms AMVs from MODIS-TERRA
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GOME and SBUV-NOAA16 ozone profiles/columns
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Current data count (31/01/2003 00Z)
Data entering the screening
Data assimilated

• Synop 166207 (1.1)
• Aircraft 251024 (1.7)
• Satob/modis 618434 (4.2)
• Dribu 7480 (0.005)
• Temp 108520 (0.7)
• Pilot 78006 (0.5)
• Upper Sat 13151980 (90)
• PAOB 538 (0.00)
• Scat 225330 (1.6)
• TOTAL 14607519 (95.8)

• Synop 37065 (2.5)
• Aircraft 157817 (10.8)
• Satob 83532 (5.7)
• Dribu 3669 (0.25)
• Temp 60887 (4.2)
• Pilot 44498 (3.0)
• Upper Sat 960561 (65.6)
• PAOB 182 (0.01)
• Scat 115692 (7.9)
• TOTAL 1463903 (79.2)

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Evolution of forecast skill for the northern and
southern hemispheres
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Latest Observing System Experiments
NH
120 days 500 hPa Z scores
SH
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Partial conclusions

• Our current data assimilation system is heavily
  constrained by nearly 2 millions of observations
  (mostly satellite data) every 12 hours
• (500 radiosondes a day have ½ the impact of
  satellite data in the Northern Hemisphere)
• We have to be realistic in terms of expectation
  from new Observing Systems
• the metric for evaluation may be more related to
  fixing a number of forecast busts than improving
  scores on average
• Our current requirements concern high vertical
  resolution (in all weather including below
  clouds) for temperature/humidity/wind

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What will future satellite systems offer?

• Advanced IR sounders will improve vertical
  resolutions (but still not resolve the humidity
  of boundary layer top and the tropopause)
• further limitations due to vertical structure
  functions in the NWP model
• further limitations due to the presence of
  clouds
• Limb data (passive and active/GPS) will provide
  very high vertical resolution
• but difficult assimilation problem in the
  horizontal (ray tracing)
• problem of accuracy in the BL
• It is hoped that Global NWP assimilation system
  is perfectly placed to exploit the synergy
  between the two

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AIRS/IASI Progress towards an assimilation
strategy at ECMWF

• By measuring radiation in thousands of different
  spectral channels, the Atmospheric InfraRed
  Sounder AIRS can provide temperature and humidity
  information at a much higher vertical resolution
  than that from current sounders
• To fully exploit the potential of AIRS, several
  issues have to be tackled
• Handle the data volume from advanced sounders
  efficiently
• Technically absorb a substantial increase of
  data count in the system
• Scientifically extract the maximum information
  content from the full spectrum
• Design an efficient cloud detection scheme
• clouds can severely limit the information from
  advanced infrared sounders (probably in the most
  crucial areas)
• Design an effective monitoring system able to
  cope with multichannel information
• Learning exercise for IASI/CRIS

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HIRS channel weighting functions
AIRS channel weighting functions (1/10th of
channels)
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Future instruments ADVANCED IR NADIR SOUNDERS
AIRS averaging
HIRS averaging
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Cloud cover and sensitivity
Average model low cloud cover in June 2000
(contour 50/75))
Monthly mean keyanalysis errors in June 2000
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The robustness of the current system makes it
hard to demonstrate quickly a spectacular impact
of a new satellite instrument
Day-3
RMS of 500hPa geopotential forecast error
averaged over 40 days (Dec 02/ Jan 03) AIRS
error minus CTRL error
Day-5
The assimilation of AIRS radiances shows a small
but consistent positive impact on forecast
quality in all areas
Day-7
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ENVISAT (ESA)

• ECMWF supports the ENVISAT CAL/VAL.
• ENVISAT will enhance our capabilities to monitor
  and assimilate ozone
• ECMWF will study the feasibility of assimilation
  of MIPAS limb sounding radiances

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ASSET AT ECMWF Assimilation of MIPAS IR limb
radiances

• Idea
• Use radiances as observations, rather than
  retrieved profiles of temperature, humidity,
  ozone,
• Why?
• Very successful at ECMWF for nadir sounders
  flexibility
• Estimation of observation error and bias
  characteristics easier for radiances than for
  retrievals
• Avoids having to account for the use of a priori
  information in the retrievals
• Some challenges
• Limb geometry, ray-tracing
• Channel/data selection out of about 60,000
  points per sweep
• Fast radiative transfer model
• Error characteristics (background observations)

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Priorities for the future Scope for using GPS
radio-occultation measurements

• Impact of the background error covariance to
  propagate information from observations in the
  vertical (how to best formulate B and how to make
  it evolve while high vertical resolution data
  become available)?
• Is there a standard accurate enough to allow the
  detection and correction of radiance biases?
• Can radio-occultation measurements help for the
  calibration?
• do we need active sensors?
• How best develop expertise in limb sounding?
• Progressive approach

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ECMWF could offer monitoring and cal/val
capabilities for this new type of observations

• Passive assimilation Data are not assimilated
  actively, but go through the assimilation system
  and statistics (e.g. first-guess departures) are
  calculated (offers Near Real Time quality
  control). Not too complicated observation
  operator required!
• Use assimilation system to evaluate data
  quality, biases, instrument and algorithm
  stability (can also show model problems)
• Output statistics
• - Number of data
• - Mean
• - Stdev of
• - time series

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Example of ECMWF / SAF external web
operational (NRT) satellite data monitoring
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New ECMWF / SAF external web (time series
for limited area statistics)
Disturbance to global statistics caused by
sudden warming over North Pole
Large disturbance in global statistics for
stratospheric AMSUA channels
No disturbance seen in statistics for Tropics
or Southern Hemisphere
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ECMWF plans for the assimilation of GPS
radio-occultation measurements (1)

• New field in data assimilation at ECMWF
• all weather, high vertical resolution and
  self calibration capabilities are attractive
  features for global NWP
• We would love to verify this !!
• But importance of the characterisation of the
  errors to fully exploit these data
• 4DVAR offers the best dynamically consistent
  ancillary data to perform the RO inversion

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ECMWF plans for the assimilation of GPS
radio-occultation measurements (2)

• Investment worth the effort because
• GPSGALILEO will increase the number of possible
  occultations
• Research and Operational missions are up or
  underway
• CHAMP (Germany, NASA,),COSMIC (NASA,NOAA,UCAR,NSF
  ,JPL,Taiwan,), ACE (ESA GPS/GALILEO-LEO
  LEO-LEO), GRAS (EUMETSAT)
• A stepwise approach is possible
• Assimilation of temperature/humidity profiles
  (unlikely!)
• Assimilation of refractivity profiles
• Assimilation of bending angles (is it necessary?)

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ECMWF/EUMETSAT fellowship on RO assimilation

• 3-year (12) EUMETSAT funded fellowship
• Will start 1st September 2003
• Several-fold objective
• Develop in-house expertise with limb geometry
  (observation operator)
• Exploit the RO data for model (bias) validation
• CAL/VAL the METOP-GRAS instrument
• Get involved in the GRAS-SAF products validation
• Investigate the feasibility of level-1ish RO
  assimilation
• Prepare for future missions (COSMIC, ACE,)