Snow

Several approaches to use satellite remote sensing to monitor the snow pack globally have been proposed, including:

• Reflectometry from Global Navigation signals
• Lidar
• Multi-frequency passive microwave radiometry
• Multi-frequency radar backscatter
• Interferometric Synthetic Aperture Radar

In the radar Science group, X. Xu has been developing algorithms to extract, from reflectometry measurements, estimates of the underlying snow water equivalent (SWE). Reflectometry utilizes existing communication and navigation satellite signals, receiving the reflected signals that carry land surface information. The P-band frequency range is particularly useful in this context due to the geosynchronous Mobile User Objective System (MUOS), which transmits signals globally. Changes in Snow Water Equivalent (SWE) create measurable shifts in signal phase between two acquisitions. When the snow is dry to moist with a liquid water content (LWC) less than or equal to 3%, the P-band signal traverses the snowpack twice—from the MUOS transmitter to the ground and then reflected back through the snow to the receiver. The phase difference between two overpasses depends on changes in the amount of ice the wave encounters on its two-way journey through the pack. Thus, for dry to moist snow (primarily during the accumulation season), SWE can be directly retrieved from phase measurements. When the snow is wet (LWC greater than 3%), the P-band signal reflects off the air-snow interface rather than penetrating the snow. Under these conditions (primarily during the melt season), phase retrievals of SWE just before the snow starts to melt, and depth retrievals of the melting snow can be estimated at the time the snow starts to melt.