A Microwave Land Surface Emissivity Model for the Hyperspectral Future

This project proposes the development of a surface emissivity model to be used in PBL investigations using new hyperspectral microwave technology, with further applications to precipitation via the GPM constellation and general atmospheric remote sensing in the microwave. An upcoming field campaign being led by GSFC will test such technology in anticipation of a pathfinder spaceborne mission to fly in three years with a goal of demonstrating atmospheric sounding in the planetary boundary layer (PBL) as part of a Decadal Survey Incubator project. Such remote sensing near the surface requires observations close to atmospheric window channels and at the far end of absorption bands, which involve a contribution from poorly understood surface emission. This project will develop a microwave land surface emissivity model for hyperspectral applications with additional applications toward improving currently operating microwave missions. Such a model will address a crucial shortfall in passive microwave remote sensing over land – one that has been well known for current applications but becomes even more crucial looking forward to a future PBL mission. Successful implementation will both enable future hyperspectral science and have the potential to significantly improve current atmospheric retrievals over land surfaces. The model development involves two broad components, including development of a physical emissivity model at 10 or 19 GHz followed by an empirical component derived from channel covariances using a combination of field campaign data and program of record radiometer data.

In the microwave region, emissivity is a large and dynamic contribution to the upwelling radiation. Often desired retrieval variables have a dynamic signal much smaller than the background emission. For this reason, the surface contribution must either be eliminated, as is the case for empirical, scattering-based precipitation algorithms for example, or modeled to a sufficiently high degree of accuracy so as to distinguish the contribution from the variable of interest. For any application involving forward modeling – physical retrievals, OSSE studies, and anything leveraging radiative transfer, the emissivity is a necessary input. As we move into the hyperspectral future, the need multiplies to understanding emissivity at frequencies not previously modeled or even observed. The emissivity model developed here is designed to be part of OSSE experiments and algorithm development for future PBL-focused missions. Continual improvements and hyperspectral expansion can be made as more observational data is collected and this could be funded through algorithm development for these missions following successful demonstration. In addition, the model produced here has a real chance of being implemented for other operational microwave sensors, such as the passive constellation used for retrievals as part of the GPM mission.

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