On the Role of the Andes on Weather Patterns and Related Environmental Hazards in Chile

The complex topography and unique weather conditions of the Chilean territory, combined with emissions from highly populated urban areas, cause poor air quality episodes as well as vulnerability to flooding and other natural hazards in the country. The most rigorous available scientific tool for forecasting such hazards and evaluating ex-ante the effectiveness of preventive actions and mitigation strategies is the numerical modeling, appropriately downscaled to different scale ranges. However, at the present day, even state-of-the-science numerical weather prediction systems such as the Weather Research and Forecasting (WRF) model sometimes exhibits poor performances, one of the main reasons being the complexity of topography of the Chile region and the inability of the model to resolve it.

The Andes highlands along the western coast of South America, passing through Chile, represent the longest continental mountain range in the world. It is about 7000 km long, and about 200 km to 700 km wide (widest between 18° south and 20° south latitude). The average height is about 4000 m, rising from the sea level to this height in a distance less than 300 km. Given the topographic steepness therein, the horizontal grid resolution adopted in WRF plays a crucial role in model's ability to capture the flow patterns and phenomena at the mesoscale.

To further evaluate and to overcome these limitations, high resolution numerical simulations were performed for the areas of Temuco in southern Chile, Santiago in central Chile, and Atacama Desert in northern Chile. In particular, using WRF model outcomes and NCEP/NCAR reanalysis data, this study aims to investigate the role of the Andes and the meteorological conditions responsible for the following:

1) High pollution events in Santiago and Temuco, and the impact of Andes and mountain ranges in the proximity in forming such events. Several WRF model physics options were tested and the optimal set-up was selected for each domain. As the highest and most hazardous pollution events are usually recorded in winter and fall seasons, representative cases for those periods are presented.

2) The unique rainfall event of March 2015 over the Atacama Desert, one of the driest places on Earth, and ensuing catastrophic floods on 26 March 2015 over the northern regions of Atacama, Antofagasta and Coquimbo. Measurements at Pastillo pluviometric station (1000m Above Sea Level, ASL), south-east of Copiapo city, showed that the amount of rain over 3 days was equivalent to several years of cumulative rainfall, according to the climatology of the area. A number of human losses, failure of essential services and an economic loss of several million dollars were reported by the Chilean government authorities, with the cities of Chañaral (10m ASL), Copiapo (400m ASL) and Diego de Almagro (800m ASL) taking the brunt of the losses. The extreme rainfall event was the result of several concomitant factors, which allowed a cut-off low (COL) pressure system to move unusually further north–east, toward the Atacama Desert, and then trapping there by the Andes to the east and a ridge of high pressure system on the south-southwest of the area. This, combined with prior rapid warming of the eastern tropical Pacific and the strengthening of northwesterly winds, led to advection of moist air toward the coastline, thus triggering this record-breaking rainfall over the desert. The ability of WRF model in reproducing such event was evaluated. In particular, different model physics options, including several microphysics and cumulus parameterization schemes were tested to find the optimal model set-up that might be used as the backbone of a future multi-hazard warning system that is being developed by the Chilean National Research Center for Integrated Natural Disasters Management (www.cigiden.cl).

This research was funded by ND-PUC Seed Fund, and Cigiden, Conicyt/Fondap Grant 15110017.