An energy balance snowmelt model in a Mediterranean site

Abstract

This article describes the development and calibration of a mass and energy balance snowmelt point model for the south-facing slope of the Sierra Nevada Mountains (Spain) and brings attention to snow processes taking place in a Mediterranean site, especially to high evaporation rates. This mountain range has peaks rising to 3500 m, and is located at latitude 37∘N, surrounded by a mild Mediterranean climate. Energy balance over the snowpack is subjected to very changeable weather conditions such as episodic strong low humidity winds, clear skies with very low nocturnal temperatures, intense solar radiation and sudden increases in temperatures. We found the behavior of the snow to be extremely variable throughout the year, especially in regards to melting cycles and evaporation. Simulations, based on snow water equivalent measurements, show that during 2004–2005, 42% of the total snow precipitation evaporated. Only 124 mm of the 300 mm of snowfall collected at the monitoring point actually melted. However, evaporation can range from 80% in December to 20% during the spring months and from 20% to 40% between years. The variability of the balance of energy fluxes acting on the snow cover in this basin means that all fluxes will be important at some point during the accumulation process. This is an excellent context for testing an energy balance model because of the widely diverse situations occurring at this location during the same time period. In order to calibrate the model, it was necessary to remove the stability-correction factors for non-adiabatic temperature gradients of the turbulent transfer, which are important for evaporation. The sensible-heat transfer coefficient in windless conditions is raised to 6W/m2K. An empirical relationship for atmospheric emissivity calculation is derived from the observed relative humidity, as the well-known Brutsaert’s formula clearly underestimated this value under cloudy skies.