ORBITAL REMOTE SENSING OBSERVATION OF RIVER ICE, SIBERIA AND ALASKA

The Advanced Microwave Scanning Radiometer (AMSR-E on AQUA) provides global coverage of the Earth's land surface on a near-daily basis and without severe interference from clouds. Using a strategy first developed for wide-area optical sensors, AMSR-E can be used to measure river discharge changes and river ice status. We use the band at 36.5 GHz, descending orbit only, horizontal polarization, and the resampled Level-3 daily global data product. The discharge estimator (“HR”) is a ratio of: calibration target emission (C, for a local land parcel unaffected by the river), to measurement target emission (M, for a pixel centered over the river reach). Pixel dimensions are approximately 25 km. Due to low emission from water surfaces, HR values increase as discharge and in-pixel water area increases: discharge can thereby be monitored through this approach. As high latitude rivers acquire an ice cover, HR values decrease, until surface water is entirely lacking. In contrast, at the time of ice breakup and removal in the spring, as surface water appears, a sudden decrease in microwave emission from the river pixel occurs and HR values sharply increase. For example, along the Lena River during the summer, measurement reach brightness temperatures are much lower than the nearby calibration pixel. Once the river freezes in late October, the relative brightness temperatures are reversed. Spring ice thaw and breakup are preceded by rises in both measurement and calibration pixel brightness temperatures, but then actual ice out causes a sharp drop for the measurement pixel. A similar pattern occurs for the Yukon River in Alaska. The timing of freezing and thawing of rivers across the Arctic can be accurately and consistently monitored through such freely-available remote sensing information and using this methodology. Along the Lena (two stations), ice break up in 2003 was June 5 and May 24, in 2004, May 29 and May 21, in 2005, May 21 and May 13, and in 2006 May 24 and May 18. In this case, a trend towards earlier ice removal is apparent.