Multiple Satellite Views of a Melting Snow Pack
What are the benefits and limitations of using various satellite measurements of melting snow packs?
How can proper interpretation of operational remotely sensed snow pack data lead to more accurate water supply measurements and snow pack runoff forecasts?
Need and Benefit
Future documentation of snow pack in both mountainous and plains areas primarily will rely upon satellite measurements through optical (visible and infrared) and microwave measurements. The recently ended (2002 and 2003) measurements (surface, airborne, satellite) of the Cold Land Processes Experiment (CLPX) held in the Colorado Rockies involved numerous agencies and universities to address the needs and challenges of remotely measuring the snow water equivalent (SWE) and melting characteristics of the snow pack. The outputs of such future remote sensing measurements will help water managers who need to know the changing water contents of a snow pack for forecasting runoff rates and totals.
There are important interpretation problems with the remotely sensed data, which can affect the quality of the results. These were voiced at a recent CLPX workshop (March 2003). The top four were:
* Vegetative cover
* Surface wetting during melting
* Topography (elevation, slope, aspect)
* Spatial resolution
The proposed research seeks to use a series of satellite measurements on April 3-5, 2002, as an intercomparison to quantify these effects on snow pack measurements. The data set also will show the combined effects of solar and viewing angles as trees and topography create varying shadows and blockages. Landsat7 and ASTER measurements on the 4th at 30 and 15 meter resolution will provide the detailed views for comparison with the coarse (250 to 1,000 meters) pixels of the simultaneous MODIS measurements. These three also provide detailed spectral information that may be related to snow grain size and wetting.
Coarse (1 kilometer) satellite views are the basis for present snow pack observations. On the adjacent days, MODIS has different viewing angles, as does AVHRR on all days. This allows studies of different viewing geometries with respect to sun angle, tree shadows, and terrain. The GOES-E and GOES-W satellites offer 15-minute views in real-time; image frequency in archives has yet to be learned. Such imagery over the 3 days will document in greater detail the effects of changing solar illumination and its interaction with shadows from trees and terrain.
These studies will provide valuable information to the CLPX investigations, which in turn are designed to lead to a future capability to accurately map snow pack water contents in both mountainous and plains environments. That will aid water managers to more efficiently control our water supplies.
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