Radiative properties of clouds over a tropical Bolivian glacier: seasonal variations and relationship with regional atmospheric circulation

Abstract

At low latitudes, strong seasonal changes in cloud cover and precipitation largely control the mass balance of glaciers. Measurements of shortwave and longwave radiation fluxes reaching Zongo glacier, Bolivia (16∘S, 5060 m asl), were analysed from 2005 to 2013 to investigate cloud radiative properties. Cloud shortwave attenuation and longwave emission were greater in the wet summer season (DJF) than in the dry winter season (JJA) probably because most DJF clouds were low warm cumulus associated with local convection, whereas JJA clouds were frequently altostratus associated with extra-tropical perturbations. Solar irradiance was high all year round and cloud radiative forcing on down-welling fluxes was strongly negative, with monthly averages ranging from -60 to -110 W m⁻² from the dry to the wet season, respectively. In the wet season, high extraterrestrial solar irradiance and low shortwave transmissivity caused very negative cloud forcing despite the high longwave emissivity of convective clouds. Reanalysis of wind and geopotential height anomalies and outgoing longwave radiation satellite data were used to characterize the regional atmospheric circulation causing thick cloud covers (10% thickest clouds) during the dry (JJA), transition (SON), and wet (DJF) seasons. Around 87% (80%) of cloud events in JJA (SON) occurred during the incursion of low-level southern wind from southern South America to the Bolivian Andes, which caused 2–3 days of cold surge episodes in the Cordillera Real. Around 13% of cloudy days in JJA were associated with high-level low-pressure conditions over the Chilean coast around 45∘S, including cut-off lows. In SON, 20% of cloudy days were associated with summer conditions, characterized by an active Bolivian High and moist air advection from the Amazon basin. In the wet season, only 46% of thick cloud events were associated with low-level southern wind incursions, the other events being associated with the South American Monsoon.