Vertical Structure and Character of Precipitation in the Tropical High Andes of Bolivia and Southern Peru

Glaciers in the tropical high Andes provide critical freshwater to surrounding areas and preserve information on regional and global paleoclimatology. These glaciers are currently threatened by rising temperatures and changing precipitation patterns. In this study, we investigate the meteorological processes associated with precipitation delivery by examining the first detailed radar measurements of the vertical structure of precipitation obtained in the tropical Andes of southern Peru and western Bolivia. A vertically pointing 24.1 GHz Micro Rain Radar (MRR) in Cusco, Peru (3,350 m asl, August 2014-February 2015) and La Paz, Bolivia (3,440 m asl, October 2015-present) provided continuous 1-min profiles of reflectivity and Doppler velocity during the respective time periods. MRR observations during 1,084 hours (Cusco) and 1,121 hours (La Paz) of precipitation were collected for analysis. These data enable determination of melting layer heights (MLHs) using a Matlab algorithm, and convective and stratiform precipitation characteristics. Thermodynamic profiles were collected from several balloon launches during precipitation events in La Paz. Hourly observations of various meteorological variables were collected from stations at the Cusco International Airport (3,350 m asl) and the Universidad Mayor de San Andres (3,440 m asl), on the Quelccaya Icecap (5,650 m asl) and Chacaltaya mountain (5,160 m asl), and from Murmurani Alto (5,050 m asl). European Centre for Medium-Range Weather Forecasts model reanalysis data (ERA-Interim) were also used to analyze the predominant synoptic patterns during both convective and stratiform precipitation. The vertically-pointing radar time-height data reveal a bimodal diurnal cycle in precipitation with cellular convection predominant in the afternoon and stratiform precipitation predominant overnight. Preliminary comparison of the radiosonde data and results from the Matlab algorithm reveal good agreement between the altitude of the 0°C isotherm and the derived altitude of the MLH. Median MLHs were above the altitude of nearby glacier termini (~5,000 m) approximately 8% of the time in Cusco and 19% of the time in La Paz, indicating that some precipitation may be falling on glaciers as rain rather than snow.