Radiative transfer in the thermal infrared and the surface source term Session 2 - Impact of thermal infrared surface emissivity uncertainty on trace gas retrieval Introduction Radiative transfer in the thermal infrared and the surface source term How is emissivity taken into account in the trace gas retrieval algorithms Example of IASI retrieval algorithm (FORLI) Spectral emissivity in the CO retrieval spectral range Example of Zhou et al. climatology (from IASI on IASI sampling) Impact of emissivity on the CO retrievals –preliminary results-

Preliminary remark Thermal infrared Shortwave infrared IASI - FORLI MOPITT V5 MOPITT V6 uses TIR and SWIR channels Thermal infrared Shortwave infrared

1. Radiative transfer in the thermal infrared and the surface source term z q The general equation Radiance at the beginning of the light path Total transmittance over the light path source term from the medium (thermal emission, scattering…) Weighting function 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

(255 K on average for the troposphere) 1. Radiative transfer in the thermal infrared and the surface source term Nadir In the nadir THERMAL infrared, both term are equally important and cannot be neglected  Air temperature (255 K on average for the troposphere) surface temperature (288 K on global average)

Grey-body surface emission 1. Radiative transfer in the thermal infrared and the surface source term Nadir thermal infrared – more details – Looking at 180° (no angle) TOA surface source term? zTOA = surface spectral emissivity = effective reflectivity Grey-body surface emission Reflected solar radiation (negligible below ~2200 cm-1) Reflected downward atmospheric radiation

IASI radiances (W / cm2 sr cm-1) 1. Radiative transfer in the thermal infrared and the surface source term Concretely IASI radiances (W / cm2 sr cm-1) Total atmospheric transmittance Atmospheric source term (emission + scattering) Transmittance from z’ to z Surface source term Dominant term = Emissivity × Blackbody Surface thermal emission Reflected solar radiance Reflected downward radiance from atmosphere Becomes significant when e < 0.95 Becomes significant above 2200 cm-1 (daytime)

IASI radiances (W / cm2 sr cm-1) 1. Radiative transfer in the thermal infrared and the surface source term Concretely IASI radiances (W / cm2 sr cm-1) Total atmospheric transmittance Atmospheric source term (emission + scattering) Transmittance from z’ to z Surface source term >> Brightness -equivalent blackbody- temperature (K) Tskin if e =1

!!! Emissivity !!! IASI radiances (W / cm2 sr cm-1) 1. Radiative transfer in the thermal infrared and the surface source term Concretely IASI radiances (W / cm2 sr cm-1) Total atmospheric transmittance Atmospheric source term (emission + scattering) Transmittance from z’ to z Surface source term >> Equivalent blackbody (brightness) temperature !!! Emissivity !!! Sand emissivity from Zhou et al. climatology

2. How is emissivity taken into account in the retrieval algorithms ? 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 Wavenumber-dependence of TIR land emissivity

Grey-body surface emission 2. How is emissivity taken into account in the retrieval algorithms ? Example for IASI (FORLIv20100815) = surface spectral emissivity = effective reflectivity Grey-body surface emission Reflected solar radiation (negligible below ~2200 cm-1) Reflected downward atmospheric radiation 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 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. 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.

Monthly global variability of emissivity at 2150cm-1 3. Spectral emissivity in the CO retrieval spectral range spatial and temporal variability of TIR land emissivity Zhou et al. climatology Monthly global variability of emissivity at 2150cm-1 Jan Feb March Apr May June July Aug Sept Oct Nov Dec

3. Spectral emissivity in the CO retrieval spectral range Zhou et al. climatology variability of emissivity between 1800 and 2760 cm-1 above given surfaces Sahara Western US CO retrieval spectral range for IASI FORLI 2143-2181.25 cm-1 Greenland Europe Figures M. Van Damme Dashed lines: min and max Plain line: mean Shadow: standard deviation spatial and temporal variability of TIR land emissivity Wavenumber-dependence of TIR land emissivity

3. Spectral emissivity in the CO retrieval spectral range Zhou et al. climatology variability of emissivity between 1800 and 2760 cm-1 above given surfaces Sahara Western US CO retrieval spectral range: 2143-2181.25 cm-1 Greenland Europe Figures M. Van Damme

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

Residual bias (W/(cm2.sr.cm-1)) 4. What is the impact of emissivity on the IASI CO retrievals? –preliminary results– Larger RMS and biases above hot surfaces, including deserts Impact on total retrieval error is, however, limited JUNE 2008 IASI morning overpass Retrieval error (%) CO total column (molec/cm²) Residual bias (W/(cm2.sr.cm-1)) RMS (W/(cm2.sr.cm-1)) Figures M. Van Damme

Residual bias (W/(cm2.sr.cm-1)) 4. What is the impact of emissivity on the IASI CO retrievals? –preliminary results– Larger RMS and biases above hot surfaces, including deserts Impact on total retrieval error is, however, limited Diurnal variability of e? JUNE 2008 IASI evening overpass Retrieval error (%) CO total column (molec/cm²) RMS (W/(cm2.sr.cm-1)) Residual bias (W/(cm2.sr.cm-1)) Figures M. Van Damme

Total retrieval errors 4. What is the impact of emissivity on the IASI CO retrievals? –preliminary results– emissivity emissivity emissivity IASI morning overpass JUNE 2008 Total retrieval errors RMS (W / cm2 sr cm-1) Bias (W / cm2 sr cm-1) emissivity emissivity emissivity IASI evening overpass JUNE 2008 Figures M. Van Damme Some correlations between decreasing emissivity and larger errors/biases/RMS Some (but weak) differences in correlation patterns day and night Other impacts still to be verified (temperature vs. emissivty?)