G4.10 Incomplete estimates of uncertainties in land surface infrared emissivity atlases

Passive-infrared observations from satellite radiometers operating in the spectral range from 17-3.3 µm are widely used to make remote-sensing measurements of the Earth’s atmosphere and surface characteristics. Vertical profiles of humidity and temperature and surface properties such as skin temperature are derived from measurements in this spectral region.  The top-of-atmosphere (TOA) spectral signals in this range can, depending on the state of the atmosphere, comprise a significant component due to emission and reflection from the land or ocean surface. It is therefore critical that validated models of (ocean and land) surface emissivity are available for the analysis of these infrared observations. This requirement, for validated models of emissivity, spans applications ranging from the assimilation of Level-1 products (for example) in reanalysis efforts, to the generation of Level-2 (and higher) products at all levels of maturity ranging from near-real-time operational products to climate data records.

There are particular challenges to representing the emissivity of land surfaces. In contrast to the ocean, where the physical mechanisms governing the surface emission can be parameterised, the infrared land surface emission is highly dependent on properties such as land-surface coverage (vegetation, bare soil, snow and so on), roughness and moisture content. These properties may change slowly (seasonally) or rapidly (daily). As a result, it has become necessary to rely on infrared land surface emissivity atlases, which characterize in a gridded fashion the global variations in emissivity at different frequencies.

There are several notable examples of publicly available atlases. The ASTER Global Emissivity Dataset has been compiled using cloud free scenes from the Advanced Spaceborne Thermal Emission and Reflection Radiometer on the Terra satellite. Monthly emissivity maps at 5 km spatial resolution are available for the years 2000-2015 (Hulley et al., 2015). Validation with laboratory spectra from four desert sites resulted in an absolute error of approximately 1%.

Capelle et al. (2012) applied a multispectral method for the retrieval of emissivity and surface temperature from IASI clear sky fields of view. They obtained a high spectral resolution product over the tropics for the period 2007-2011. The product was validated against emissivity spectra retrieved with an airborne interferometer (Thelen et al., 2009) to within an absolute accuracy of 2%.

Borbas et al. (2007) developed the UWIREMIS global land surface emissivity atlas for the 3.7 to 14.3 µm range. The atlas was derived by regressing the MODIS operational land surface emissivity product against laboratory emissivity spectra. At the Met Office, the UWIREMIS atlas is used as a first guess in the 1-D variational retrieval of surface emissivity for IASI observations over land.

The use of infrared emissivity atlases in NWP models is evolving. At the Met Office, work is underway to incorporate emissivity estimates derived from sounders such as IASI into a dynamically updated atlas (Gray, 2016). By using a Kalman filter approach, it is intended that the atlas can be updated in near-real-time as new observations become available. Thus, it would be able to capture short term emissivity variations in a way that static atlases cannot. This methodology is promising; however, such atlases need to be validated to make sure the retrieved values have robust uncertainties associated with them.