Observations and Storm Characteristics of Maximum Liquid Equivalent Hourly Snowfall Rates in the Tropical Andes and Comparisons to other Regions

The upper limit of maximum hourly liquid equivalent snowfall rates in tropical and subtropical high-mountain regions is largely unknown. There are only a handful of stations above 5,000 m asl worldwide and there are several challenges to accurately measuring the liquid equivalent of solid precipitation. Quantifying liquid equivalent hourly snowfall rates is of substantial importance given the critical role high-mountains regions serve as the world’s water towers and sources of deadly avalanches and glacier-lake outburst floods. We analyze hourly rates of snow and graupel (i.e., snowflakes completely encased in rime ice) precipitation at three high-elevation sites in the tropical Andes of southern Peru (Laguna Sibinacocha, 4,895 m asl; Quelccaya Ice Cap, 5,650 m asl) and Bolivia (Chacaltaya, 5,160 m asl) with >5 years of data. Hourly observations of precipitation type and liquid equivalent precipitation were recorded by OTT HydroMet Parsivel present weather sensors and co-located Pluvio² weighing precipitation gauges surrounded by double-alter shields to minimize gauge undercatch in wind. We place the observations in a broader context of maximum hourly snowfall rates from the Nepal Himalaya, orographic precipitation in the US mountain west, and Great Lakes lake-effect snow. We also analyze the storm characteristics (e.g., temperature, timing, duration) and synoptic patterns associated with the heaviest (top 1%) events.

Maximum hourly liquid equivalent snowfall rates >14 mm hr^-1 were observed at all three locations in the tropical high Andes, including 20 mm hr^-1 at Lake Sibinacocha in Peru (4,895 m asl). Hourly rates peaking >10 mm hr^-1 were likely short-period convective cells within successive precipitation events and usually included at least one hour of graupel. In comparison, maximum hourly solid precipitation rates from the Khumbu (Everest) region in the Nepal Himalaya, across the U.S. Automated Surface Observing System network, and at the snowiest mountain sites in the Western US have rarely exceeded 10 mm hr^-1, even in the most intense coastal cyclones, lake-effect snow bands, and atmospheric rivers. The extreme rates registered in the Andes with surface elevations at or above 550 hPa raise interesting questions about the associated storm typologies. Available data suggest that the highest precipitation rates consisted mainly of graupel and occurred during late afternoon convective storms, where abundant cloud liquid water may result in rapid accretion of rime ice. Expanded observational networks and development of long-term climatologies of precipitation at high-elevation mountain sites are needed to better quantify the observed distributions of solid precipitation intensity.