ANALYSIS OF ERS-1 SYNTHETIC-APERTURE-RADAR DATA OF FROZEN LAKES IN NORTHERN MONTANA AND IMPLICATIONS FOR CLIMATE STUDIES

Lakes that freeze each winter are good indicators of regional climate change if key parameters, such as freeze-up and breakup date and maxim um ice thickness, are measured over a decade-scale time frame. Synthet ic aperture radar (SAR) satellite data have proven to be especially us eful for measurement of climatologically significant parameters charac teristic of frozen lakes. In this paper, five lakes in Glacier Nationa l Park, Montana, have been studied both in the field and using ERS 1 S AR data during the 1992-1993 winter. The lakes are characterized by cl ear ice, sometimes with tubular or rounded bubbles, and often with a l ayer of snow ice on top of the clear ice. They are also often snow cov ered. Freeze-up is detected quite easily using ERS 1 SAR data as soon as a thin layer of ice forms. The effect of snow ice on the backscatte r is thought to be significant but is, as yet, undetermined. On the fi ve lakes studied, relative backscatter was found to increase with ice thickness until a maximum was reached in February. Breakup, an often i ll-defined occurrence, is difficult to detect because surface water ca uses the SAR signal to be absorbed, thus masking the ice below. Compar ison of the bubble structure of thaw lakes in northern Alaska with lak es in northern Montana has shown that the ice structure is quite diffe rent, and this difference may contribute to differential SAR signature evolution in the lakes of the two areas.