Air Temperature Thresholds to Assess Snow Melt at the Forni Glacier Surface (Italian Alps) in the April-June Period: A Contribution to the Application of Temperature Index Models

Glacier melt occurs whenever the surface temperature is null (273.15 K) and the net energy budget is positive. These conditions can be assessed by analysing meteorological and energy data acquired by a supraglacial Automatic Weather Station (AWS). In the case this latter is not present at the glacier surface the assessment of actual melting conditions and the evaluation of the melt amount is difficult and simple methods based on Temperature index (T-index) or degree days models are generally applied. These models require the choice of a correct temperature threshold. In fact, melt does not necessarily occur at air temperatures higher than 273.15 K, since it is determined by the surface energy budget which in turn is only indirectly affected by air temperature. This is particularly the case of the late spring period when ablation processes start at the glacier surface thus progressively reducing snow accumulation. In this paper, to detect the most indicative air temperature threshold witnessing snow or ice melting in the April–June period, we analysed air temperature data recorded from 2006 to 2012 by a supraglacial AWS set up on the ablation tongue of Forni Glacier (Italian Alps). Moreover we also evaluated the glacier energy budget and the Snow Water Equivalent (SWE) value during this time-frame. We also analysed air temperature data from a station located outside the studied glacier. Then the ablation amount is estimated both from the surface energy balance (from supraglacial AWS data) and from T-index method (from daily air temperature measured outside the glacier) and the results were compared.