Field observations and numerical model experiments for the snowmelt process at a field site

There are several levels of models for the snowmelt process in terms of the snow thermal structure: isothermal, bi-layered and multi-layered models. However, it is difficult to choose the appropriate level of complexity for application because the number of unknown variables is crucial in model handling. One of the major issues in energy balance snow models is the shape of the snow temperature vertical profile. This profile, if taken as a specified function, would simplify a snowmelt model calibration and computation significantly. In this study, in order to determine the appropriate representative snow vertical thermal profile, snow temperature measurements have been performed using five snow thermocouples placed vertically along an observation tower with insulating arms. Also, as a field experimental study of an energy balance snow model, the net radiation, air temperature, relative humidity and wind speed along with the vertical one dimensional snow temperature profile have been observed at a field site in Lake Tahoe Basin. The computational results correspond with the measured snow temperature profile and snow water equivalent reasonably well. It is illustrated that the temperature in the snow near surface (called the “active layer”) varies daily, and the lower snow layer (called the “inactive layer”) is barely affected by the atmosphere. The results of field observations and the numerical experiments show that the vertical temperature distributions in the active layer, which is the upper layer affected by energy exchange with the atmosphere, generally have an exponential shape during night time under cold weather, while snow pack stays around 0 °C during daytime. Both of the results indicate that not only the snow temperature in the top active layer, but also the thickness of snow active layer fluctuates during the snowmelt process. The observation results show that the thickness of the active layer may reach about 60 cm in Sierra Nevada, California. These results provide significant information for the development of appropriate approximations in physically based snowmelt modeling.