Microwave Land Emissivity Calculation

The method

• The land emissivities are calculated from satellite observations under the assumption that the surface is flat and specular. When no a priori information about the surface is available, this assumption is useful and has been adopted by many authors (Jones and Vander haar 1997 ; Prigent et al. 1997 ; Karbou et al. 2005, Weng et al. 2001 among many others) However, Matzler (2005) suggests that the use of this approximation for nadir viewing sensors is questionable in very specific cases, because the lambertian component could be of some importance. We have shown that the impact of the specular assumption on the retrieved near-nadir AMSU emissivities is well below 1% of emissivity bias in most atmospheric situations over natural snow-free areas (Karbou and Prigent 2005). Ideally, and as suggested by Matzler (2005), a specularity parameter could be used to reduce the already small effect of the specular approximation.
• For a non scattering plane-parallel atmosphere and for a given AMSU path zenith angle and frequency, the emissivity could be retrieved using :
$\epsilon \left(p,u\right) = \frac$ T(p,u) - T(u,\uparrow) - T(u,\downarrow) * \tau \tau x ( Ts - T(u,\downarrow) ) \$ ;
the emissivity is then function of the observed brightness temperature, the skin temperature, the atmospheric down-welling and upwelling Tbs and the net atmospheric transmissivity.
• Short range forecast temperature and humidity profiles (6h or 12h) have bee used as inputs to the RTTOV model to compute the atmopheric contribution to the measured radiances.

Sensors and frequencies

The land emissivities have been calculated using :

• Observations from AMSU-A and AMSU-B window channels :
• 23.8, 31.4, 50.3, 89 GHz for AMSU-A
• 89 and 150 GHz for AMSU-B
• All scan positions (this corresponds to +/- 58° of zenith angle change)
• The polarization varies with the zenith angle
• Observations from SSM/I channels :
• 19V, 19H, 22H, 37V, 37H, 85V, 85H
• For this sensor the zenith angle is close to 53° and the polarization is V (for Vertical) or H (for Horizontal)
• Observations from SSMI/S window channels :
• This work has been done at ECMWF and partly funded by the NWPSAF
• 19.35V, 19.35H, 22.2V, 37V, 37H, 91.6V, 91.6H, 150H GHz
• For this sensor the zenith angle is close to 53° and the polarization is V (for vertical) or H (for Horizontal)
• Observations from TMI channels :
• This work has been done at ECMWF and partly funded by the NWPSAF
• 10.65 (V&H), 19.35 (V&H), 21.3 V, 37 (V&H), 85.5 (V&H) GHz
• For this sensor the zenith angle is close to 53° and the polarization is V (for Vertical) or H (for Horizontal)
• Observations from AMSR-E channels :
• This work has been done at ECMWF and partly funded by the NWPSAF
• 6.9 (V&H),10.64 (V&H), 18.7 (V&H), 23.8 (V&H), 36.5 (V&H), 89 (V&H) GHz
• For this sensor the zenith angle is close to 53° and the polarization is V (for Vertical) or H (for Horizontal)

References

1. Jones, A. S., and T. H. Vonder Haar, 1997, Retrieval of microwave surface emittance over land using coincident microwave and infrared satellite measurements, J. Geophys. Res., vol. 102, NO. D12, pp 13,609-13,626, Jun 1997.
2. . Karbou, F., C. Prigent, L. Eymard and J. Pardo, 2005, Microwave land emissivity calculations using AMSU-A and AMSU-B measurements. IEEE Trans. on Geoscience and Remote Sensing, 43, 5, 948-959
3. Karbou, F. and C. Prigent,2005,Calculation of microwave land surface emissivities from satellite observations : validity of the specular approximation over snow-free surfaces ? . IEEE Trans. Geosci. Remote Sensing Letters, 2, 3, 311-314
4. Matzler, C.,2005, On the determination of surface emissivity from satellite observations, IEEE Geoscience and remote sensing letters,. IEEE Geosci. remote sensing letters,2, 2, 160-163
5. Prigent, C., W. B. Rossow, and E. Matthews, 1997, Microwave land surface emissivities estimated from SSM/I observations, J. Geophys. Res., 102, 21867-21890
6. Weng, F., B. Yan and N. Grody, 2001, A microwave land emissivity model, J. Geophys. Res., vol. 106, NO. D17, pp 20,115-20,123