Distribution of the surface energy budget: preliminary analysis on the incoming solar radiation. The case study of the Forni Glacier (Italy)

Abstract

This study represents a contribution to distribution of the surface energy budget of the Forni Glacier (Ortles-Cevedale Group, Upper Valtellina, Italy). The analyses are based on data acquired at S. Caterina Valfurva (a village in the glacier valley at 1768 m ellipsoidal elevation WGS84) by an Automatic Weather Station (AWS) installed and managed by the Lombardy Agency for the Environment («ARPA Lombardia»). We focus on the two most important meteorological parameters affecting surface energy budget: air temperature (T) and incoming shortwave radiation (SWin). Data collected from the ARPA AWS are used to evaluate these parameters at the glacier surface during the meteorological summer 2009 (from 1^st June to 31^st August 2009) and then the computations are validated through comparison with data recorded by an AWS installed at the surface of Forni Glacier tongue («AWS1 Forni», 2669 m ellipsoidal elevation WGS84). The analysis of the distributed air temperature data enabled identification of the lowest value (–11.9°C), found at the Mount S. Matteo peak (3669 m) on 22^nd June at 8:00 pm, and the highest value (+16.1°C), recorded at the glacier terminus (2497 m) on 23^rd July at 2:00 pm. The seasonal temperature amplitude (Tmax-Tmin) was 28°C. The hottest week was 20^th-26^th July 2009 and the coldest was 1^st-7^th June 2009. Regarding daily SWin distribution, the maximum value (406.9 Wm^–2) was recorded on 13^th June and the minimum (28.5 Wm^–2) on 6th June. From the analysis of hourly SWin values we could distinguish between days with clear sky conditions and days with intense cloud cover. Weekly mean SWin data showed the greatest value (327.1 Wm^–2) from 20^th-26^th July 2009 and the lowest (207.8 Wm^–2) from 22^nd-28^th June 2009. Furthermore, in analysing SWin it is critical to take into account the problem of shading. Using the Hillshade tool of ArcGIS, which takes into account only the slope and the aspect of each grid cell neglecting the surrounding topography effect, we compiled 66 shadow maps. Finally this study represents a first approach in modelling the distributed incoming solar radiation. In fact the considered driving factors are the elevation, the slope and the aspect of each grid cell. The next step will consist in taking into account the surrounding topography and the actual atmosphere conditions as well.