# The Combined ASTER MODIS Emissivity over Land (CAMEL) Part 2: Uncertainty and Validation
## 4 Validation with Laboratory Spectra
### CAMEL Intercomparison with IASI Emissivity  
This section shows a comparison of CAMEL to the 8-year averaged IASI HSR emissivity dataset, which is derived from IASI radiance observations and uses a retrieval method based on Principle Component Analysis (PCA) of lab spectra [5]. To facilitate the comparison at the 13 CAMEL hinge points, the CAMEL 13 hinge point product is first resampled to a lower spatial resolution by applying a 5 \(\times\) 5 pixel running average over the CAMEL grids to match the 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) spatial resolution of the IASI dataset. CAMEL 8-year averaged monthly emissivities are then computed corresponding to the years over which the IASI dataset is averaged. For the months of January through May the years 2008-2015 are averaged, and for June through December the years of 2007-2014 are averaged. To compare the HSR CAMEL product to the HSR IASI emissivity, the individual CAMEL HSR spectra are simply computed and averaged over the proper years. Since the CAMEL HSR 8-year monthly averages actually represent a 0.05\({}^{\circ}\)\(\times\) 0.05\({}^{\circ}\) resolution, the HSR uncertainty estimate, which includes a measure of the surrounding 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) standard deviation, is used to provide a measure of the potential error from the spatial resolution mismatch (i.e., of the CAMEL 0.05\({}^{\circ}\)\(\times\) 0.05\({}^{\circ}\) to the IASI 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\)). Figure 4 shows the monthly 8-year averages of CAMEL HSR emissivities compared against (i) laboratory validation data (where it is available) and (ii) the IASI HSR dataset for various case sites. The 8-year averaged HSR emissivities are overlaid in the top panels, including individual CAMEL spectra for each of the years averaged, and the bottom panels show the 8-year average differences bounded by the CAMEL uncertainty estimate. At the top of Figure 4, the Namib and Tucson, AZ desert cases show emissivity differences between the CAMEL and lab emissivities of less than 0.05 for wavelengths greater than 5 \(\upmu\)m. Within the quartz spectral region, CAMEL agrees more closely with the lab measurements than the IASI emissivity. For the Greenland case site, differences between the lab and CAMEL emissivities are less than 0.01. The IASI emissivity reflects a subtle quartz signature which is absent in CAMEL. This case shows the importance of the selection of laboratory measurements for the PCA method. For snow or ice-covered cases the CAMEL HSR Algorithm includes a separate set of laboratory measurements of snow and ice spectra only. The Yemen case site shows the 8-year average CAMEL emissivity having a \(\sim\)0.15 maximum difference from the lab measurements around the 6.5 \(\upmu\)m carbonate spectral feature. While CAMEL does not capture the full extent of this feature as shown by the lab measurements, it is an improvement upon the UW BF and IASI emissivity, as the feature is present, but lower in magnitude. For the ARM SGP site, the CAMEL and IASI 8-year averages have close resemblance across the spectrum with the exception of the 8-9.5 \(\upmu\)m region where IASI resolves the sharper features of the quartz signature. Differences between the CAMEL and lab emissivity are generally less than 0.025 with a small exception in the SW region at 5 \(\upmu\)m. The last case site (no lab validation data is available) over Mt. Massive, located in the Colorado Rocky Mountains, is shown for the month of November. The overlaid November CAMEL HSR estimates for the years 2007-2014  
located in the top panel reveal the year to year variation which is captured by the monthly CAMEL product. In some years, HSR emissivities reflect snow/ice signatures (version number of the laboratory dataset: Vs = 12 and #PC = 2), while others reflect more quartz-like signatures of bare rocks (Vs = 8 and #PC = 7), reflecting the dynamics of annual snow cover and snow-melt across the region.