Investigation of Near-Surface Soil Freezing and Thawing in North America

Freezing and thawing processes of soils have a significant impact on energy and water transfer between the atmosphere and the land surface, surface hydrology, plant growth, agriculture, and ecosystem as a whole. The timing, duration, areal extent, and depth of surface soil freezing and thawing are important parameters for regional climatic and hydrologic studies, and changes in these parameters are important climatic indicator and integrator. The conventional point measurement of surface soil freezing and thawing provide information for process studies in local area. It is impractical to conduct large- or regional-scale investigation of surface soil freezing and thawing by using the point measurement method. In this study, the timing, duration, areal extent, and depth of near-surface soil seasonal freezing and thawing will be investigated using available in situ measurements, numerical modeling, and satellite remote sensing data. Charateristics of near-surface soil freezing and thawing under different climate conditions over the North American are studied using in situ measurements from various sources. These in situ measurements are also used to validate algorithms to detect surface soil freezing/thawing status over regional-scale. The preliminary results obtained from satellite remote sensing data indicate that frozen soil extent is generally in good agreement with zero deg. Celsius isotherm of air temperature. The boundary of frozen soil extent moved southward as the cold air was invading from north in northern Plains during the early winter. Duration of surface soil freezing varies from a few days in south to several months in north and mountains regions. Investigation from in situ measurements indicates that depth of the near-surface soil freezing varies from a few centimeters in the south to greater than a meter in the north.