# The Combined ASTER MODIS Emissivity over Land (CAMEL) Part 2: Uncertainty and Validation
## 4 Validation with Laboratory Spectra
### RTTOV IASI Brightness Temperature Calculations
#### 4.3.2 Results  
Differences between the calculated and observed IASI BTs are computed over the 616 channel set and separated by land classification type using the IGBP categories. The improper skin temperature determination can cause a bias in the BT calculation. This bias between ECMWF and IASI has a magnitude typically around 1-2 K in the Northern Hemisphere and 2-3 K in the Southern Hemisphere. To avoid it the de-biased variances of the calculated minus observed BTs differences for the 3.6-5, 8-9, and 10-13 \(\upmu\)m spectral regions are used as indicators for a better emissivity estimate. To compute the de-biased variances for each spectrum, the IASI BT calculation minus observation difference is found and the result is then fitted to the 10.5-13 \(\upmu\)m window region by subtracting the calculation minus observation 10.5-13 \(\upmu\)m spectral domain average from the calculated spectrum. This de-biasing using the window region is assumed to only account for differences in skin temperature, though it's possible that the de-biasing could account for other errors of lesser magnitudes such as inaccurate surface emissivity characterization and errors induced from the inclusion of cloudy scenes in the analysis. Three separate spectral variances over the 3.6-5, 8-9, and 10-13 \(\upmu\)m regions are then computed for the window-fitted BT difference. Using this method, there are 3 separate variances computed for each IASI calculation. The de-biased variance statistics corresponding to the CAMEL and 0.98 emissivity are compared for different time and space domains by taking averages of the variances over the specific regions. Figure 7 shows mean CAMEL and 0.98 constant emissivity spectra and calculated minus observed IASI BT biases for a nighttime granule over Egypt. The 8.5 \(\upmu\)m map of the calculated CAMEL minus observed BTs illustrates the spatial distribution and magnitude of the differences. This example shows  
Figure 6: Top panels show IASI (black) and CAMEL (red) 8-year average emissivity averaged over the (**a**) evergreen broadleaf forest; (**b**) barren/sparse vegetation; (**c**) grassland; and (**d**) cropland IGBP land classifications for the months of January (solid line, circle markers) and July (red line, ’X’ markers). Bottom panels show the associated averaged IASI minus CAMEL again for January (black dashed ’X’ markers) and July (black solid circle markers). Number of samples, N, noted in bottom panel’s legend. the very large improvement of over 5 K when using CAMEL emissivity as opposed to the constant 0.98 emissivity value within the 8-10 \(\upmu\)m region Reststrahlen band region. De-biased variance values over the three spectral regions can be found in the upper panel and show lower, improved values for the CAMEL dataset. The CAMEL calculated minus observed BT bias has increased magnitudes in spectral regions where atmospheric gases strongly absorb, in particular the 4.5-5 \(\upmu\)m carbon monoxide and carbon dioxide region, the 6-8 \(\upmu\)m water vapor region, and 9.6-10 \(\upmu\)m ozone absorption region. Figure 8 shows a second nighttime example of statistics from a granule over the coast of Antarctica. Though not many samples are included in this granule, results are representative of snow and ice scenes. The BT bias corresponding to the CAMEL emissivity calculations is greater than the 0.98 emissivity bias from approximately 10-10.8 \(\upmu\)m, however, it is less within the \(\sim\)11-13 \(\upmu\)m region where most of the spectral variation in snow/ice emissivity occurs. The magnitude of the BT bias difference is just under 1 K and represents how the CAMEL emissivity changes with respect to the 0.98 constant emissivity in the bottom panel. While the 8-9 \(\upmu\)m de-biased variance is greater for CAMEL than the 0.98 constant, the 10-13 \(\upmu\)m value is overall decreased.