# The Combined ASTER MODIS Emissivity over Land (CAMEL) Part 2: Uncertainty and Validation
## 4 Validation with Laboratory Spectra
### CAMEL Intercomparison with IASI Emissivity
Figure 4: High spectral resolution (HSR) emissivity for (**a**) January Namib (\(-\)24.25N, 15.25E); (**b**) May Tucson, AZ (32.01N, \(-\)110.77E); (**c**) January Greenland (72.57N, \(-\)38.45E); (**d**) January Yemen (19.15N, 55.57E); (**e**) January Atmospheric Radiation Measurement, Southern Great Plains (ARM SGP) (36.60N, \(-\)97.48E); and (**f**) Nov Mt. Massive (39.17N, \(-\)106.47E) case sites. Top panels show CAMEL 8-year averaged (solid black), lab measurement (dotted black), and Infrared Atmospheric Sounding Interferometer (IASI) 8-year averaged (dashed black) HSR emissivity spectra overlaid on top of the individual CAMEL spectra for the years over which the aforementioned average was computed (colored). Bottom panel shows differences of the IASI minus CAMEL averages (blue solid) and lab minus CAMEL emissivity (black solid) which are both bounded by the CAMEL HSR uncertainty (dotted lines). Figure 5 shows global maps of the differences between the CAMEL and IASI 8-year averages for the months of January and July at 4 different wavelengths. The global maximum magnitude of differences is larger for the shortwave and 8-9 \(\upmu\)m channels (e.g., the 3.6 and 8.6 \(\upmu\)m channels), where differences exceed 0.1 in select places (e.g., Sahara Desert, Australia, Tibetan Plateau), typically in regions whose surfaces are barren. Comparison of the January and July results reveals a seasonal dependence in the differences for various regions and channels, seen most notably at high latitudes where snow cover varies with the seasons. In general, a stable emissivity over deserts and polar ice (Greenland and Antarctica) is expected, so the large IASI variations over deserts and polar ice is unphysical and synthetic. The IASI emissivity over permanent snow cover contains quartz signature artifacts as discussed previously. The 10.8 \(\upmu\)m channel shows slight changes from January to July and does show select areas where CAMEL emissivity is higher than IASI such as over the Tibetan Plateau, but in general CAMEL emissivities are lower than IASI at 10.8 \(\upmu\)m. The 12.1 \(\upmu\)m channel shows that for latitudes south of \(\sim\)40\({}^{\circ}\)N where there is no snow cover, that the IASI minus CAMEL 8-year average difference is qualitatively similar between January and July and is largely positive, though it is quite small, being confined to under 0.03 magnitude. Figure 5: _Cont_. Statistics of the 8-year averaged IASI minus CAMEL emissivity over IGBP land classification categories are shown in Figure 6 for the months of January and July. Top panels show the average IASI and CAMEL emissivities and the bottom panel shows the average IASI minus CAMEL difference. Differences are generally less than a magnitude of 0.02 in emissivity and could be due to multiple issues including differences of spatial resolution between the IASI and MODIS/ASTER instruments and differences between the IASI emissivity and CAMEL algorithms. While differences could also be due the assumption of equal emissivity over day and night, further investigation is needed on this topic. Largest differences between the CAMEL and IASI products are seen in the barren/sparse vegetation category, where they exceed 0.03 in magnitude. The IASI product has larger differences between January and July than CAMEL does for the barren/sparse 8-11 \(\upmu\)m region. But the CAMEL product has larger differences between January and July in the grassland 8-11 \(\upmu\)m region. The CAMEL temporal variations over temperate grasslands and stable emissivity over deserts is as expected due to changes in phenology that affect vegetation cover.