The Chilean Orographic and Mesoscale Precipitation Study (CHOMPS): Field Program Description and Initial Results

The Andes Cordillera, which runs the length of South America and rises up to 5,000 m MSL within 200 km of the Pacific coast, dramatically influences the distribution of winter precipitation and snowpack over Chile and Argentina. The study of orographic precipitation processes, particularly along the western slopes of the Andes, is important to improve forecasts of severe flooding and snowpack in a region that depends on snowmelt for water resources. While orographic effects have been investigated on synoptic scales in the Andes, the lack of operational radar coverage and high-elevation, long-term precipitation records have, before the present study, precluded an in-depth investigation into the mesoscale and microphysical processes that affect the distribution of precipitation in the region.

This presentation introduces the Chilean Orographic and Mesoscale Precipitation Study (ChOMPS), which, from May-October 2016, investigated the evolution of precipitation amounts, dropsize distribution, and the vertical profile of radar echoes along an east-west transect that stretched from the Pacific coast to the windward slope of the Andes. The transect, at ~36°S, was made up of a coastal site upstream of the coastal mountain range (Concepción), a central valley site (Chillán), and a mountain site (Las Trancas). Instrumentation along the transect included three vertically pointing Micro-Rain-Radars, two Parsivel Disdrometers, and several meteorological stations. This novel in-situ data will also be complemented with atmospheric reanalysis and remotely-sensed precipitation and cloud data.

Preliminary results find a general decrease in radar echo frequency and intensity from the coast to the central valley and an increase in frequency, particularly of shallow, convective features, over the cordillera. The dataset also documents the evolution of Doppler velocity and reflectivity profiles with inland extent during early, middle, and late storm sectors. Additionally, the transect provides a season-long record of the inland evolution of melting layer height as well as the prevalence and structure of shallow non-brightband rain and the characteristics of its inland penetration to the central valley. This study, the first of its kind in the Chilean Andes, provides unique insight into mesoscale and orographic precipitation processes that also have applicability to the west coast of the United States and other mountainous regions.