Radar-Observed Characteristics Of Precipitation In The Tropical High Andes Of Southern Peru And Bolivia

This study investigates precipitation delivery using the first detailed radar measurements of the vertical structure of precipitation obtained in the tropical Andes of southern Peru and Bolivia. A vertically pointing 24.1 GHz Micro Rain Radar in Cusco, Peru (3,350 m asl, August 2014-February 2015) and La Paz, Bolivia (3,440 m asl, October 2015-February 2017) provided continuous 1-min profiles of reflectivity and Doppler velocity during the respective time periods. Additional datasets collected include thermodynamic profiles from rawinsonde releases, hourly observations of various meteorological variables, and backward air trajectories from the NOAA HYSPLIT model. 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. Backward air trajectories for two stratiform case studies indicate that low-level flow originated in the Amazon basin three days prior to the events. Median melting layer heights were above the altitude of nearby glacier termini (~5,000 m) approximately 17% of the time in Cusco and 30% of the time in La Paz, indicating that some precipitation is falling as rain rather than snow on nearby glacier surfaces. Melting layer heights were highest in La Paz during the 2015-16 El Nino (47% above 5,000 m).