Slope scale modeling of snow surface temperature in topographically complex terrain

Abstract

In mountainous terrain, landscape can influences the thermal state of snow. Snow temperature and mass flux have been calculated using an energy balance model, Radtherm/RT, to account for the effects of topography and meteorological conditions. For a specific location, a terrain model is defined and contains a connected assemblage of elements or facets. Each element has a specified terrain type with assigned thermal properties. Meteorological data are applied, and a one dimensional energy balance is calculated for each element. This energy balance includes conduction, convection, radiation, and latent heat; however, the calculation of radiation is unique. Taking into account topography, global position, and time, the model is used to calculate incoming solar radiation for each element as well as reflected short wave radiation and the exchange of long wave radiation between terrain surfaces. Light detection and ranging topographic data with a one meter resolution were used to create separate models (on the order of 10⁴ m²) for two slopes in southwest Montana. Meteorological data were collected at these two slopes as well as a third location having a relatively unobstructed view of the sky. The results for elements in different locations and under different meteorological conditions were compared. Readily available USGS topographic data with a 30 meter resolution were used to create a model (on the order of 10⁶ m²) containing both slopes. For this model of a much larger scale, surface temperatures and mass flux were again calculated and compared with results for the slope scale models. Incoming long wave radiation from the atmosphere only was found to be critical input data for accurate temperature calculations. The set value for albedo also had a major effect. When suitable long wave data and good estimations of albedo were used, snow surface temperature was calculated with accuracies on the order of several degrees. Additionally, when surface hoar deposition and growth was observed and reasonable temperature results were achieved, calculated values of mass flux were consistently positive. In one instance, observed variations in surface hoar growth across a slope matched calculated variations in mass flux across the same slope.