Vol. 4, No. 2, 215-225, 2005

Abstract
A significant response of global warming is expected to occur over the high latitudes. Due to the presence of permafrost and snow cover these regions are very sensitive to an increase in air temperature. Our objective is to derive a daily Snow Cover Extent variation from satellite microwave sensors over Canada-Alaska and Northern Eurasia (latitudes >50°N), as in situ measurements do not allow a study over northern high latitudes. Passive microwave data from SSMI (Special Sensor Microwave Imager) sensors are adequate for this study as they can measure the energy emitted by the earth surface, independent of solar illumination and cloud cover. Here, we are presenting a simple method based on vertically polarized brightness temperatures at 19 and 37 GHz and the determination of an adaptative threshold.

The study of the 1988-2002 mean seasonal snow extent shows a maximum snow cover for mid-February of (9.69+/-0.06)·10⁶ km², for the Canada/Alaska area, and of (20.75+/-0.32)·10⁶ km² over Northern Eurasia. We compare the seasonal variation with two existing satellite datasets: one from the NSIDC (National Snow and Ice Data Center, Boulder, USA) derived from optical sensors, and one from IMS (Interactive Multisensor Snow and Ice Mapping System) derived from different sensors, available since 1997. Differences occur over high mountains as topographic effects hamper our method to clearly distinguish the snow signal. In some other areas we are more confident about our method. Also short snow-free episodes during winter cannot be detected, but our interest is to clearly define a long snow- / ice-free period characterizing the summer. Good concordances during key climatic months (March-April-November-December) are found between the series which allow us to study climate variability in further work.

ISSN 1729-3782