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. Preliminary results find that the middle sector of mid-latitude storms produce the highest precipitation rates but limited orographic enhancement, while the late, or postfrontal, sector produces smaller precipitation rates but very high orographic enhancement. This high orographic enhancement is due to an increase in precipitating cloud frequency and longevity over the high terrain. Therefore, the postfrontal sector makes an important contribution to event-total, and perhaps climatological, orographic enhancement in the region. This study, the first of its kind in the Chilean Andes, provides unique insight into mesoscale and orographic precipitation processes that have applicability to the west coast of the United States and other mountainous regions.