Basics about observation techniques - Earth OnlineDay 2 Lecture 3 Basics about observation...

Day 2 Lecture 3 Basics about observation techniques - Bruno Carli 1

DRAGON ADVANCED TRAINING COURSE IN ATMOSPHERE REMOTE SENSING

Basics about observation techniques

Bruno Carli



Table of Contents

• Geometry of observation: zenith, nadir and limb sounding.

• The sources• Spectroscopy• Spatial resolution• Time resolution



Geometry of observation

• Zenith sounding• Nadir sounding• Limb sounding



Geometry of observationLimb sounding



The sources

( ) σσσσσσσσ αα JsTBIs

dxdI

⋅+⋅+⋅+−= )(

Sun/StarMoon

Earth/planet Atmosphere

SunEarth/atmosphere



Thermal IR

Thermal infraredSources

• Sun: solar occultation

• Atmosphere: emission sounding



Near IR and Visible UV

Near IR and Visible/UVSources

• Sun: solar occultation and scattering

• Moon: moon occultation

• Stars : star occultation



Spectroscopy

Rotational spectra

Vibrational spectraElectronic spectra



Spectroscopy

Main spectroscopic constituents of the Earth’s atmosphere:

• water vapor (••)

• ozone (•••)

• carbon dioxide (•)

• methane (•)

• nitrous oxide (••)

• nitric acid (••)

Rotational spectra

Vibrational spectra

Electronic spectra



Spectroscopy

Wavenumber cm-1

Alti

tude

km

40

10

30

80

100

90

70

60

50

20

40

10

30

80

100

90

70

60

50

20

110100100010000

LINE WIDTH

Temperature broadening:Gaussian line shape

Pressure broadening:Lorentzian line shape

T⋅∝∆ σσ

P∝∆σ

Voight line shape equal convolution between Gaussian and Lorentzian distributions.



The vertical and horizontal resolution depend on the geometry of observation.

Typical resolution of nadir measurements is 10 km horizontal and 10 km vertical. Typical resolution of limb measurement is 2 km vertical and 700 km horizontal.

Spatial resolution



Spatial resolutionA large number of observations (both in space and spectral domain) improves the “conditioning” of the inversion problem.

In nadir sounding the good conditioning can be exploited for attaining 1 km vertical resolution.

In limb sounding the good conditioning can be exploited for attaining 50 km horizontal resolution.



Time resolutionspacecraft orbitspacecraft orbit

Polar platforms are usually the preferred choice because of global coverage requirements.

Meteorological, and now-casting applications in general, require frequent observations that can be obtained with geostationary platforms.



RadiativeRadiative TransferTransfer

( ) ( ) ( ) ( )

( ) ( ) ( )( ) ( ) ( )∑ ∏

∏

= +=

∆−∆−

=

∆−

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅−⋅+

+⎟⎟⎠

⎞⎜⎜⎝

⎛⋅=

N

l

N

lk

kkll

N

l

ll

eeTlB

eILI

1 1

1,,

1,

0

σσ

σ

ασσ

αθσθσ

The radiative transfer calculates the atmospheric spectral intensity at a point in space, as a function of the spectral frequency and of the direction of propagation (determined by the geometry of observation).

Discrete integration of the radiative transfer:



The forward model calculates the spectrum measured by the instrument.

This is equal to the atmospheric spectral intensity Iσ,θ (L), obtained with the

radiative transfer model, convoluted with instrument effects.

( ) ( )LIIL ϑσϑσ ,, =

Forward Model of the measurementsForward Model of the measurements

Where ILS is the “instrument line shape” and IFOV is the “instantaneous field of view “ of the instrument.

( ) ( ) ( )( ) ( ) '''',, ϑθϑσσσϑσθσ dIFOVdILSIS LL ∫∫ −⋅−⋅=



The inversion problem

Forward problem

Inverse problem

Basics about observation techniques - Earth OnlineDay 2 Lecture 3 Basics about observation...

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