Radiative transfer in the thermal infrared and the surface source term

1. Radiative transfer in the thermal infrared and the surface source term

2. How is emissivity taken into account in the trace gas retrieval algorithms Example of IASI retrieval algorithm (FORLI)

3. Spectral emissivity in the CO retrieval spectral rangeExample of Zhou et al. climatology (from IASI on IASI sampling)

4. Impact of emissivity on the CO retrievals –preliminary results-

Session 2 - Impact of thermal infrared surface emissivity uncertainty on trace gas retrieval

Introduction

Preliminary remark

Thermal infrared

IASI - FORLIMOPITT V5

Shortwave infrared

MOPITT V6 uses TIR and SWIR channels

0

( ; , , )( ; , ) ( ; ,0) ( ; , 0, ) ( , , )z t z zL z L t z J z dz

z

Ω

The general equation

Radiance at the end of the light path

radiance from the medium weighted by absorption through upper layers

Initial radiance transmitted through the entire path

Radiance at the beginning of the light path

Total transmittance over the light path

source termfrom the medium (thermal emission, scattering…)

Weighting function

( ; ,0)L

( ; , )L z

0 z


Nadir

( )sB T ( )zB T

surface temperature(288 K on global average)

Air temperature(255 K on average for

the troposphere)

In the nadir THERMAL infrared, both term are equally important and cannot be neglected

0

( ; , , )( ; , ) ( ; ,0) ( ; , 0, ) ( , , )z t z zL z L t z J z dz

z

Ω


zTOA

TOA

0

TOAL

0

( , )( )

z t z zJ z dz

z

0L

0

0 ( , )(0, ) ( )

zTOATOA

t z zL L t z J z dz

z

Nadir thermal infrared – more details –

0 0( ) (1 ) SunsB TL L L

Reflected solar radiation (negligible below ~2200 cm-1)

Reflected downward atmospheric radiation

Grey-body surface emission

= surface spectral emissivity = effective reflectivity

0L

0L

SunL

Looking at 180° (no angle)

surface source term?


IASI radiances (W / cm2 sr cm-1)

Surface thermal emission

Reflected downward radiance from atmosphere

Reflected solar radiance

Becomes significant above 2200 cm-1 (daytime)

Becomes significant when < 0.95

Dominant term =Emissivity × Blackbody

0

0 ( , )(0, ) ( )

zTOATOA

t z zL L t z J z dz

z

Surface source term

Total atmospheric transmittance Atmospheric source term

(emission + scattering)Transmittance from z’ to z

0 0( ) (1 ) SunsB TL L L

1. Radiative transfer in the thermal infrared and the surface source termConcretely


0

0 ( , )(0, ) ( )

zTOATOA

t z zL L t z J z dz

z

Surface source term



Tskin if =1

0 0( ) (1 ) SunsB TL L L


>> Brightness -equivalent blackbody- temperature (K)

Concretely


0

0 ( , )(0, ) ( )

zTOATOA

t z zL L t z J z dz

z

Surface source term



!!! Emissivity !!!

Sand emissivity from Zhou et al. climatology


>> Equivalent blackbody (brightness) temperature

Concretely

Spectral emissivity modifies the surface source term in two ways: • it decreases the surface thermal radiance which would otherwise be

described by pure a blackbody function• It allows for the reflection of the downwelling atmospheric radiation

While the emissivity is pretty close to unity and relatively constant over the entire infrared spectral range for some surfaces (oceans), see, it can be characterized by sharp spectral variations for several land surfaces, especially between 1000 and 1200 cm-1 and above 2100 cm-1

2. How is emissivity taken into account in the retrieval algorithms ?Concretely

Wavenumber-dependence of TIR land emissivity

• Planck blackbody function at the temperature Ts with a spectral emissivity .

• The skin temperature is retrieved together with the CO profile, using the same spectral fitting window.

• For continental surfaces the spectral emissivity relies on the climatology of [Zhou et al. 2011]. In cases of missing values in the Zhou et al. climatology, the MODIS climatology of Wan [2008] is used.

• A constant sea surface emissivity (possibly varying with wind speed) is used

The third term, accounts for the reflected solar radiance in the direction of the sounding beam. It is calculated using a • Planck blackbody function at 5700 K,

without including spectral lines,• a reflective surface combining

Lambertian and specular reflections.

0 0( ) (1 ) SunsB TL L L

Reflected solar radiation (negligible below ~2200 cm-1)

Reflected downward atmospheric radiation

Grey-body surface emission

= surface spectral emissivity = effective reflectivity

calculation of the mean radiance associated to the total downward flux reaching the surface, integrated upon all the geometries. This is done • considering a

Lambertian surface.

Example for IASI (FORLIv20100815)

2. How is emissivity taken into account in the retrieval algorithms ?

Monthly global variability of emissivity at 2150cm-1

Zhou et al. climatology

3. Spectral emissivity in the CO retrieval spectral range

Jan Feb March

Apr May June

July Aug Sept

Oct Nov Dec

spatial and temporal variability of TIR land emissivity

variability of emissivity between 1800 and 2760 cm-1 above given surfaces

CO retrieval spectral range for IASI FORLI2143-2181.25 cm-1

Sahara

Greenland

Western US

Europe

Figures M. Van Damme



Dashed lines: min and max Plain line: mean Shadow: standard deviationspatial and temporal

variability of TIR land emissivity

Wavenumber-dependence of TIR land emissivity

variability of emissivity between 1800 and 2760 cm-1 above given surfaces

CO retrieval spectral range: 2143-2181.25 cm-1

Sahara

Greenland

Western US

Europe




First FORLI version with MODIS emissivity database (12 channels only in the thermal IR)

FORLI version v20100815 with first Zhou et al. emissivity database (all IASI channels but monthly averages.

Figure by Maya George

4. What is the impact of emissivity on the IASI CO retrievals? –preliminary results-

JUNE 2008IASI morning overpass

CO total column (molec/cm²) Retrieval error (%)

RMS (W/(cm2.sr.cm-1)) Residual bias (W/(cm2.sr.cm-1))

o Larger RMS and biases above hot surfaces, including deserts

o Impact on total retrieval error is, however, limited

4. What is the impact of emissivity on the IASI CO retrievals? –preliminary results–


JUNE 2008IASI evening overpass

CO total column (molec/cm²) Retrieval error (%)

RMS (W/(cm2.sr.cm-1)) Residual bias (W/(cm2.sr.cm-1))

o Larger RMS and biases above hot surfaces, including deserts

o Impact on total retrieval error is, however, limited

o Diurnal variability of ?



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(W /

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emissivity emissivity emissivity

emissivity emissivity emissivity

o Some correlations between decreasing emissivity and larger errors/biases/RMSo Some (but weak) differences in correlation patterns day and nighto Other impacts still to be verified (temperature vs. emissivty?)



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Radiative transfer in the thermal infrared and the surface source term