Coupling a Decade of Surface Observations and Satellite Data to Reveal Convective Forcing of Tropical Glacier Mass Balance in the Peruvian Andes

Hourly observations from paired weather stations embedded along Andean valley slopes reveal increases in freezing-level height and greater warming at higher altitudes suggestive of valley fluxes that impact precipitation formation and mass balance of glaciers during the wet season. We analyze more than a decade of meteorological data from two ground-based weather stations anchoring an array of six air temperature and humidity sensors spanning over 1200 m to 4700 m in the Llanganuco Valley, Peru (9°S). We find a connection between diurnal and seasonal behavior of lapse rates and freezing levels and observed nocturnal rainfall maximum. We provide evidence of the valley influenced boundary layer air decoupling from the free troposphere based on our high-resolution climate model derived lapse rates. The freezing level is consistently higher during the day and exhibits an average diurnal range between 150 and 420 meters during the wet season and between 200 and 420 meters during the dry season. The average near-surface lapse rate ranges from 5.9 to 7.1 °C km^-1 during the wet season and from 6.2 to 8.0 °C km^-1 during the dry season. Analysis of real-time precipitation from the Tropical Rainfall Measurement Mission during the period 2004-2012 indicates preference for rainfall to occur between 22:30 to 5:30 (62%) across the Peruvian Andes. This suggests a consistent pattern of enhanced longwave radiation from cloud formation and high humidity during the day and increased accumulation at night. We discuss the implications of these meteorological dynamics for the regime of tropical glaciers.