A simulation model for frost penetration beneath the ground on the basis of equilibrium surface temperatures

It is often required to predict frost penetration beneath the ground using limited weather records. Previously, estimations of frost penetration beneath the ground were based mainly upon accumulated degree days of frost. These methods, however, caused non-negligible errors in estimation because of an unrealistic assumption that the air temperature is equal to the surface temperature of the ground.For the improvement of simulation models for estimating frost penetration beneath the ground, the authors used the equilibrium surface temperature which is associated with the thermal regime of the energy balance at the ground surface. In the model, time-variable surface boundary conditions were used and the quilibrium surface temperature was calculated in each time interval. Records of total solar radiation, wind velocity, air temperature, cloud cover and atmospheric pressure were used for the simulations.In addition to the equilibrium surface temperature, the present computer simulation model employs a finite difference solution to a heat conduction equation with phase changes of soil moisture. The results of simulations are compared with the results of on-the-spot investigations at various places. The model is also applied to the palaeoclimate in Hokkaido during the last glaciation. The results show good agreement with the presence of permafrost.