7 June 2004Met-OP Training Course Use and Interpretation of ECMWF Products 1 Satellite Observations Gerald van der Grijn Meteorological Operations Section

7 June 2004 Met-OP Training CourseUse and Interpretation of ECMWF Products

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Satellite Observations

Gerald van der GrijnMeteorological Operations Section

Thanks to:

Anthony McNally (satellite section)

Lars Isaksen (data assimilation section)


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Coverage of conventional observations used in NWP


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NOAA AMSUA/B HIRS, AQUA AIRS DMSP SSM/I

SCATTEROMETERS GEOS

TERRA / AQUA MODISOZONE

Coverage of satellite observations used in NWP


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Current data count28R1 (10/03/04 00Z)

• Synop: 193616 (0.27%)

• Aircraft: 254294 (0.36%)

• AMV’s: 2186744 (3.06%)

• Dribu: 10803 (0.02%)

• Temp: 116442 (0.16%)

• Pilot: 94889 (0.13%)

• UpperSat: 68105926 (95.38%)

• PAOB: 814 (0.00%)

• Scat: 247320 (0.35%)

TOTAL: 71.210.848

• Synop: 39142 (1.57%)

• Aircraft: 158219 (6.35%)

• AMV’s: 73574 (2.95%)

• Dribu: 3547 (0.14%)

• Temp: 66405 (2.67%)

• Pilot: 49818 (2.00%)

• UpperSat: 1985939 (79.72%)

• PAOB: 290 (0.01%)

• Scat: 114290 (4.59%)

TOTAL: 2.491.224

Screened Assimilated

99.06% of screened data come from satellites

87.26% of assimilated data come from satellites


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Evolution of Forecast Skill


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40%

50%

60%

70%

80%

90%

100%

Day 3 Day 5 Day 7

Control

No sondes

No satellite

Anomaly correlation of 500hPa height for Southern Hemisphere

Impact of withdrawing different types of observations on forecast quality


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40%

50%

60%

70%

80%

90%

100%

Day 3 Day 5 Day 7

Control

No sondes

No satellite

Anomaly correlation of 500hPa height for Northern Hemisphere

Impact of withdrawing different types of observations on forecast quality


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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


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Two Types of SatellitesPolar 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

Geostationary satelliteat 35,800 km.

Polar orbiterat 850 km.


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Coverage of polar orbiter


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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• MODerate resolution Imaging Spectroradiometer (MODIS)

– Quikscat• Scatterometer


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Polar Orbiters cont’d– ERS-2

• Radar Altimeter

• Synthetic Aperture Radar (SAR)

– ENVISAT• Radar Altimeter

• 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)

Instruments (or satellites) currently used at ECMWF


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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.


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Electromagnetic RadiationEvery 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.


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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.


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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.

on)contributin (cloud/rai

)scattering (surface )reflection (surface emission) (surface

)())(,()(

0

dzdz

vdzTvBvL

))(,( zTvB T

Zdz

vd )(


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Different instruments and channels

• Atmospheric sounding channels from passive instruments

• Surface sensing channels from passive instruments

• Surface sensing channels from active instruments

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.

In practice, satellite instruments have channels which are a combination of atmospheric sounding and surface sensing.


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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.

dzdz

vdzTvBvL

0

)())(,()(


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Passive atmospheric sounders

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

150°W

150°W120°W

120°W90°W

90°W 60°W

60°W 30°W

30°W 0°

0° 30°E

30°E 60°E

60°E 90°E

90°E 120°E

120°E 150°E

150°E200210220230240250260270280290300

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

150°W120°W90°W 60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E190200210220230240250260270280290

HIRS Ch. 12(6.7 micron)

AMSU-A Ch. 5(53 GHz)


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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:

,...)(,emission Surface )( windTvL surf

where Tsurf is the surface temperature and ε the surface emissivity.


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• 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.


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HIRS Ch. 8(6.7 micron)

SSM/I Ch. 7(85 GHz)

60°S60°S

30°S 30°S

0°0°

30°N 30°N

60°N60°N

150°W120°W90°W 60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E90110130150170190210230250270290


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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.

)(scattering Surface )( windvL


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Active surface sensing channels

Big gaps in data coverage of Quikscat data due to rain contamination.

Hurricane Lili


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dzdz

dzTBL

0

)())(,()(

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) thanit can be seen that the measured radiance is essentially a weighted average of the atmospheric temperature profile,

dzzKzTBL

0

)())(,()(

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.

Atmospheric temperature sounding


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K(z)

z

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.

K(z)

z

If the weighting function was a box-car function, this would meanthat the measured radiance is sensitive to the mean temperaturebetween two atmospheric levels.

Ideal weighting functions


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A lot of radiation is emitted from the dense lower atmosphere, but very little survives to the top of the atmosphere due to absorption.

At some level there is an optimal balance between the amount of radiation emitted and the amount reaching the top of the atmosphere.

High in the atmosphere very little radiation is emitted, but most will reach the top of the atmosphere.

K(z)K(z)

z

Real atmospheric weighting functions


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• 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

Real weighting functions cont’d

By selecting a number of channels with varying absorption strengths we sample the atmospheric temperature at different altitudes


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HIRS AMSUA AIRS

Ch-14

Ch-13

Ch-12

Ch-1

Ch-2Ch-11

Real weighting functions cont’d


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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.


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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


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We’ve learned what satellites measure, but …

ECMWF MET-OP lecture: ‘Data assimilation’ by Lars Isaksen

ECMWF MET-DA lecture: “Data assimilation and use of satellite data”.

ECMWF newsletter articles ( http://www.ecmwf.int/publications/newsletters/ )

• 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 ( http://www.ecmwf.int/publications/library/do/references/list/14 )

• TM 345: “An improved general fast radiative transfer model for the assimilation of radiance observations”

for more information on how this data is actually used:


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Satellite Data Monitoring on the Web

Data monitoring plots

http://www.ecmwf.int/products/forecasts/d/charts/monitoring/satellite

Products publicly available on the public ECMWF server:

Data coverage maps

http://www.ecmwf.int/products/forecasts/d/charts/monitoring/coverage/dcover

http://www.ecmwf.int/products/forecasts/d/charts/monitoring/satellite

http://www.ecmwf.int/products/forecasts/d/charts/monitoring/coverage/dcover

Documents

7 June 2004Met-OP Training Course Use and Interpretation of ECMWF Products 1 Satellite Observations Gerald van der Grijn Meteorological Operations Section