Solar radiation, cloudiness and longwave radiation over low-latitude glaciers: implications for mass-balance modelling

Broadband radiation schemes (parameterizations) are commonly used tools in glacier mass-balance modelling, but their performance at high altitude in the tropics has not been evaluated in detail. Here we take advantage of a high-quality 2 year record of global radiation (G) and incoming longwave radiation (L down arrow) measured on Kersten Glacier, Kilimanjaro, East Africa, at 5873 m a.s.l., to optimize parameterizations of G and L down arrow. We show that the two radiation terms can be related by an effective cloud-cover fraction n(eff), so G or L down arrow can be modelled based on n(eff) derived from measured L down arrow or G, respectively. At n(eff) = 1, G is reduced to 35% of clear-sky G, and L down arrow increases by 45-65% (depending on altitude) relative to clear-sky L down arrow. Validation for a 1 year dataset of G and L down arrow. obtained at 4850 m on Glaciar Artesonraiu, Peruvian Andes, yields a satisfactory performance of the radiation scheme. Whether this performance is acceptable for mass-balance studies of tropical glaciers is explored by applying the data from Glaciar Artesonraju to a physically based mass-balance model, which requires, among others, G and L down arrow as forcing variables. Uncertainties in modelled mass balance introduced by the radiation parameterizations do not exceed those that can be caused by errors in the radiation measurements. Hence, this paper provides a tool for inclusion in spatially distributed mass-balance modelling of tropical glaciers and/or extension of radiation data when only G or L down arrow is measured.