Effects of unfrozen water on heat and mass transport processes in the active layer and permafrost

Precise temperature data from four Alaskan permafrost sites (Prudhoe Bay, B arrow and two sites near Fairbanks) combined with computer modelling provid e quantitative measures of the existence and dynamics of unfrozen water in the active layer and permafrost. Unfrozen water contents are negligible for living and dead moss layers, small in the peat layers and larger in the si lts, and show significant site-to-site variation. The effect of unfrozen wa ter on the ground thermal regime is largest immediately after freeze-up and during cooling of the active layer. It is less important during warming an d thawing of the active layer and during freezing and thawing of seasonally frozen ground. The effects last less than a month in cold permafrost and t hroughout most of the freeze-up period in warm permafrost. Physically, unfr ozen water introduces a spatially distributed latent heat and changes therm al properties which retards the thermal response of an active layer or perm afrost. Unfrozen water in the freezing and frozen active layer and nearsurf ace permafrost also protects the ground from rapid cooling and creates a st rong thermal gradient at the ground surface that increases the heat flux ou t of the ground. This enlarged heat flux also enhances the insulating effec t of the snow cover. There do not appear to be any inherent difficulties in using conductive heat modelling for the active layer during the period whe n the zero curtain exists. Copyright (C) 2000 John Wiley & Sons, Ltd.