1. Satellite Observations
Gerald van der Grijn
Meteorological Operations Section
Thanks to:
Anthony McNally (satellite section)
Lars Isaksen (data assimilation section)
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

2. Coverage of conventional observations used in NWP
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

3. Coverage of satellite observations used in NWP
NOAA AMSUA/B HIRS, AQUA AIRS
DMSP SSM/I
SCATTEROMETERS
GEOS
TERRA / AQUA MODIS
OZONE
7 June 2004
Met-OP Training Course
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4. Current data count 28R1 (10/03/04 00Z)
Screened
Assimilated
Synop: (0.27%)
Aircraft: (0.36%)
AMV’s: (3.06%)
Dribu: (0.02%)
Temp: (0.16%)
Pilot: (0.13%)
UpperSat: (95.38%)
PAOB: (0.00%)
Scat: (0.35%)
TOTAL:
Synop: (1.57%)
Aircraft: (6.35%)
AMV’s: (2.95%)
Dribu: (0.14%)
Temp: (2.67%)
Pilot: (2.00%)
UpperSat: (79.72%)
PAOB: (0.01%)
Scat: (4.59%)
TOTAL:
99.06% of screened data come from satellites
87.26% of assimilated data come from satellites
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

5. Evolution of Forecast Skill
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

6. Met-OP Training Course Use and Interpretation of ECMWF Products
Impact of withdrawing different types of observations on forecast quality
Anomaly correlation of 500hPa height for Southern Hemisphere
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

7. Met-OP Training Course Use and Interpretation of ECMWF Products
Impact of withdrawing different types of observations on forecast quality
Anomaly correlation of 500hPa height for Northern Hemisphere
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

8. Met-OP Training Course Use and Interpretation of ECMWF Products
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

9. Importance of Satellite Data
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
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

10. Two Types of Satellites
Polar Orbiters
Orbits are sun-synchronous circular orbits that almost pass over the poles.
Altitudes are typically at 850 km.
Each satellite will complete about 14 orbits in one day.
Imagery from successive orbits overlay with each other to give global daily coverage. This data is used in NWP models.
Geostationary Satellites
Orbit at a height of 35,800 km
At this height the satellite is stationary with respect to a point on the earth’s surface.
High temporal resolution
Ideal for making sequential observations of clouds
Polar orbiter
at 850 km.
Geostationary satellite
at 35,800 km.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

11. Coverage of polar orbiter
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

12. Instruments / Satellites currently used at ECMWF
Polar Orbiters
NOAA:
High Resolution IR Sounder (HIRS) on NOAA-17
Advanced Microwave Sounding Unit (AMSU-A and AMSU-B) on NOAA-14, NOAA-15, NOAA-16, NOAA-17
Solar Backscatter UltraViolet radiometer (SBUV/2)
DMSP
Special Sensor Microwave Imager (SSM/I) on DMSP-13, DMSP-14, DMSP-15
Aqua
Atmospheric InfraRed Sounder (AIRS)
MODerate resolution Imaging Spectroradiometer (MODIS)
TERRA
Quikscat
Scatterometer
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

13. Instruments (or satellites) currently used at ECMWF
Polar Orbiters cont’d
ERS-2
Radar Altimeter
Synthetic Aperture Radar (SAR)
ENVISAT
Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)
Geostationary Satellites
ATMOSPHERIC MOTION VECTORS (satellite derived winds)
MET-5, MET-7, GOES-10, GOES-12, (MET-8 under evaluation)
RADIANCES
MET-5, MET-7,GOES-9, GOES-10, GOES-12, (MET-8 under evaluation)
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

14. What do satellites actually measure ?
They DO NOT measure TEMPERATURE
They DO NOT measure HUMIDITY
They DO NOT measure WIND
Satellite observations are obtained using remote sensing techniques based on measurements of
electromagnetic radiation.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

15. Electromagnetic Radiation
Every object with a temperature larger than 0 K emits electromagnetic radiation. Electromagnetic radiation therefore extends over a wide range of energies and wavelengths. The distribution of all radiant energies can be plotted in a chart known as the electromagnetic spectrum.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

16. Electromagnetic Radiation
In the earth’s atmosphere, the radiation is partly to completely transmitted at some wavelengths; at others those photons are variably absorbed by interaction with air molecules.
Blue zones mark minimal passage of incoming and/or outgoing radiation, whereas, white areas denote atmospheric windows in which the radiation doesn’t interact much with air molecules.
Most remote sensing instruments operate in one of these windows by making their measurements tuned to specific frequencies that pass through the atmosphere. Some sensors, especially those on meteorological satellites, directly measure absorption phenomena.
7 June 2004
Met-OP Training Course
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17. What do satellites actually measure ?
In essence, satellite instruments measure the radiance L that reaches the top of the atmosphere at a certain frequency v.
Radiance transfer equation:
where is the black body emission at a given temperature at
altitude , and the change in transmittance with height.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

18. Different instruments and channels
Depending on the frequency, the measured radiance will be sensitive to different geophysical variables. In general the channels used for NWP can be considered as one of three different types.
Atmospheric sounding channels from passive instruments
Surface sensing channels from passive instruments
Surface sensing channels from active instruments
In practice, satellite instruments have channels which are a combination of atmospheric sounding and surface sensing.
7 June 2004
Met-OP Training Course
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19. Passive atmospheric sounders
These channels are located in parts of the infrared and microwave spectrum.
Main contribution to the measured radiance is from the atmosphere and can be written as:
They avoid frequencies for which surface radiation or cloud contribution are important.
Channels are primarily used to obtain temperature and humidity profiles.
7 June 2004
Met-OP Training Course
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20. Passive atmospheric sounders0°
30°N
60°N
150°W
120°W
90°W
60°W
30°W
30°E
60°E
90°E
120°E
150°E
200
210
220
230
240
250
260
270
280
290
300
HIRS Ch. 12
(6.7 micron)
60°S
30°S 0°
30°N
60°N
150°W
120°W
90°W
60°W
30°W
30°E
60°E
90°E
120°E
150°E
190
200
210
220
230
240
250
260
270
280
290
AMSU-A Ch. 5
(53 GHz)
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21. Passive surface sensing channels
These channels are located in atmospheric window regions at frequencies where there is very little interaction with the earth’s atmosphere.
Also known as “imaging channels”
Main contribution to the measured radiance is:
where Tsurf is the surface temperature and ε the surface emissivity.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

22. Passive surface sensing channels
Primarily used to obtain information on surface characteristics such as
Surface temperature
Quantities that influence surface emissivity
Wind (roughness over the sea)
Vegetation
Also used for
Cloud top temperatures (infrared)
Rain (microwave)
Deriving wind information through sequence of geostationary satellite images.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

23. Passive surface sensing channels
HIRS Ch. 8
(6.7 micron)
60°S
30°S 0°
30°N
60°N
150°W
120°W
90°W
60°W
30°W
30°E
60°E
90°E
120°E
150°E 90
110
130
150
170
190
210
230
250
270
290
SSM/I Ch. 7
(85 GHz)
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

24. Active surface sensing channels
These instruments illuminate the earth’s surface by emitting energy in atmospheric window regions and measure the radiance that is scattered back.
Main contribution to the measured radiance is:
Provide information on ocean winds (scatterometers)
Similar class instruments such as altimeters and SARS (Synthetic Aperture Radars) provide information on wave height and spectra.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

25. Active surface sensing channels
Big gaps in data coverage of Quikscat data due to rain contamination.
Hurricane Lili
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26. Atmospheric temperature sounding
We saw that the radiation measured by passive atmospheric sounders can be written as:
If the primary absorber is a well mixed gas (e.g. O2 or CO2) than
it can be seen that the measured radiance is essentially a weighted
average of the atmospheric temperature profile,
The function K(z) that defines this weighted average is known as a
WEIGHTING FUNCTION. It specifies the layer from which the radiation emitted to space originates, and hence it determines the region of the atmosphere which can be sensed from space at this frequency.
7 June 2004
Met-OP Training Course
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27. Ideal weighting functions
If the weighting function was a delta-function, this would mean that the measured radiance is sensitive to the temperature at a single level in the atmosphere. z
K(z)
If the weighting function was a
box-car function, this would mean
that the measured radiance is
sensitive to the mean temperature
between two atmospheric levels. z
K(z)
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

28. Real atmospheric weighting functions
High in the atmosphere very little radiation is emitted, but most will reach the top of the atmosphere. z
At some level there is an optimal balance between the amount of radiation emitted and the amount reaching the top of the atmosphere.
K(z)
A lot of radiation is emitted from the dense lower atmosphere, but very little survives to the top of the atmosphere due to absorption.
K(z)
7 June 2004
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29. Real weighting functions cont’d
The altitude at which the peak of the weighting function occurs depends on the strength of absorption for a given channel.
Channels in parts of the spectrum where the absorption is strong (e.g. near the centre of CO2 or O2 lines ) peak high in the atmosphere.
Channels in parts of the spectrum where the absorption is weak (e.g. in the wings of of CO2 or O2 lines) peak low in the atmosphere.
AMSUA
By selecting a number of channels with varying absorption strengths we sample the atmospheric temperature at different altitudes
7 June 2004
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30. Real weighting functions cont’d
HIRS
AMSUA
AIRS
Ch-14
Ch-1
Ch-13
Ch-12
Ch-11
Ch-2
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31. Characteristics of weighting functions
The weighting functions are broad, i.e. several kilometres.
the instrument can sense the mean properties of broad layers very well, but the width of the weighting functions limits the capability of satellite sounders to detect atmospheric structures which have relatively small scale in the vertical.
For most instruments the weighting functions are highly overlapping.
although the instrument may make measurements at N separate frequencies, we obtain fewer than N pieces of independent information.
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

32. Extracting temperature profiles from satellite measurements
If we know the entire atmospheric temperature profile T(z) then we can compute (uniquely) the radiances a sounding instrument would measure using the radiative transfer equation. This is sometimes known as the forward problem
In order to extract or retrieve the atmospheric temperature profile from a set of measured radiances we must solve what is known as the inverse problem
Unfortunately with a finite number of channels and weighting functions that are generally broad, the inverse problem is formally ill-posed (an infinite number of different temperature profiles could give the same measured radiances)
Extracting temperature profiles from satellite measurements
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

33. for more information on how this data is actually used:
We’ve learned what satellites measure, but …
for more information on how this data is actually used:
ECMWF MET-OP lecture: ‘Data assimilation’ by Lars Isaksen
ECMWF MET-DA lecture: “Data assimilation and use of satellite data”.
ECMWF newsletter articles ( )
Spring 2003: “Assimilation of high-resolution satellite data.”
Spring 1999: “The use of raw TOVS/ATOVS radiances in the ECMWF 4D-Var assimilation system”
ECMWF Technical Memoranda ( )
TM 345: “An improved general fast radiative transfer model for the assimilation of radiance observations”
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products

34. Satellite Data Monitoring on the Web
Products publicly available on the public ECMWF server:
Data coverage maps
Data monitoring plots
7 June 2004
Met-OP Training Course
Use and Interpretation of ECMWF Products