Enhanced temperature-index distributed models for snowpack simulation, incorporating air temperature and a term for clear sky potential solar radiation, are increasingly used to simulate the spatial variability of the snow water equivalent. This paper presents a new snowpack model (termed TOPMELT) which integrates an enhanced temperature index model into a lumped basin scale hydrological model by exploiting a statistical representation of the distribution of clear sky potential solar radiation. This is obtained by discretising the full spatial distribution of clear sky potential solar radiation into a number of radiation classes. The computation required to generate a spatially distributed water equivalent reduces to a single calculation for each radiation class. This turn into a potentially significant advantage when parameter sensitivity and uncertainty estimation procedures are carried out. The model includes a routine, which accounts for the variability of clear sky radiation distributions with time, ensuring a consistent temporal simulation of the snow mass balance. Thus, the model resembles a classical temperature-index model when only one radiation class for each elevation band is used, whereas it approximates a fully distributed model with increasing the number of the radiation classes (and correspondingly decreasing the area corresponding to each class). TOPMELT is applied over the Aurino basin at S. Giorgio, a 614 km extsuperscript{2} catchment in the Upper Adige river basin (Eastern Alps, Italy) to examine the sensitivity of the snowpack model results to the temporal and spatial aggregation of the radiation fluxes.