Future Snow Water Equivalent Satellite Mission Considerations

Today we have the remarkable capability to quantify key elements of the global water cycle in ways that were barely conceivable only two or three decades ago. Water storage elements such as soil moisture and ground water are now routine data products enabled by satellite observations, for example, from SMOS, SMAP, and GRACE. Fluxes such as precipitation are observed by the global constellation of satellites known as Global Precipitation Measurement (GPM). Models are steadily helping to tie together observations of these storage elements and fluxes, forming integrated pictures of portions of the water cycle. However, to fully realize the integrated framework of observations and models of the global water cycle envisioned by the Water Panel in the recent 2017 Earth Science Decadal Survey, certain components need to be quantified with better fidelity.

One of these is the storage element represented by seasonal terrestrial snow. Indeed, the Decadal Survey identified snow water equivalent (SWE) as a highly desirable measurement. A variety of SWE remote sensing techniques have been exploited (with varying degrees of success) over the years, and no single technique works well enough under all snow types and confounding factors to be the obvious choice for a future SWE satellite mission. And, existing global SWE estimates (Globsnow, etc) still have limitations in important regions—forests and complex terrain.

A strategy of achieving global SWE through combining multiple sensing techniques along with models appears to hold promise. This approach can leverage the many existing and planned active and passive optical and microwave satellite sensors, along with the more rigorous understanding of the strengths and limitations of the various sensing techniques that is evolving through ongoing research.

Field campaigns such as NASA's SnowEx are being designed to provide data to enable trade studies that will guide us to optimize the exact combination of sensors and models that will lead to a new global SWE product.

This paper will discuss the current understanding of the sensing techniques, how they might be combined into a new global SWE observing system, along with models, and some of the trade studies that are needed in order to move toward a new global picture of SWE, and enable the novel science and applications that will follow.