Radiative frosts are a major hazard to agriculture in the tropical Andes of Peru, but there are very few studies of their physical controls. In this study we focus on identifying and approximately estimating the effect that physical variables have on both the downward surface longwave flux (LW↓) and the minimum temperature (Tmin). Through a combination of case studies and statistical analysis of in situ data in the IGP Huancayo Observatory, we found that low cloud cover (CC), surface specific humidity (q), and soil moisture are key factors controlling the day-to-day variability of Tmin, which is more pronounced in the dry/cool season. We found that all frost days had q < 7 g/kg in the dry season and q < 5 g/kg in the wet season, although it should be emphasized that q covaries with CC and soil moisture. We successfully validated a numerical soil heat diffusion model with data from a field campaign in July 2010 and we used it, together with a radiative transfer model, to estimate the sensitivities of Tmin and LW↓ to atmospheric and soil variables. With these results we estimated the partial contributions of these variables to the overall day-to-day variability in Tmin and LW↓. We found that low cloud cover is the dominant factor, although specific humidity has a comparable role in the wet season. Lack of information on the cloud liquid water path is an important source of uncertainty. Enhanced soil moisture has a strong mitigating effect on frosts, although strong variability of soil moisture in the wet season could contribute substantially to the development of frosts.
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