Frontal modification by complex topography in south-central Chile

Precipitation over the extratropical west coast of South America is largely produced by cold fronts, an integral part of the baroclinic waves that populate the midlatitudes. Here we focus our attention in South-central Chile (34-38°S), a densely populated area (> 7 million inhabitants) that also concentrate most of the country's economic activity. The region is also characterized by its prominent topography: a coastal range that in many places rises above 1000-m elevation and the Andes cordillera that rises sharply to its top within 200 km of the coastline. Precisely, in this range of latitude the Andes' mean elevation drops southward from 5000 m to 2500 m. In between these two mountain ranges lies the central depression (average elevation of 500 m) where some of the largest cities are located. Along the coast there is a pronounced, north-south gradient of the annual-mean precipitation (~300 mm at 33°S to >1000 mm at 40°S), although scatter evidence indicate a maximum of rainfall intensity between ~36-38°S. Seasonality is relatively weak in southern Chile but increases rapidly northward (>35°S), where more than 80% of the precipitation is concentrated in austral winter. On a continental perspective, there is also a significant west-east precipitation gradient with a relatively minimum about 300 km off the Chilean coast, wetter conditions over the western slope of the Andes and semiarid conditions to the east of the continental divide. In this work we present preliminary results from the AFEX project (Andes Cordillera – Frontal System Experiment, 2011-2013), including a network of rain gauges plus thermometers in the Nahuelbuta mountains, a coastal massif in the coastal zone of south-central Chile (37-38°S). The massif has a mean height of 600 m ASL and two prominent peaks of 1350 and 1280 m ASL. To the east of the massif, the central depression (300 m ASL) extends for about 100 km until reaching the Andes foothills that reaches 2000 m ASL in these latitudes. Half-hourly measurements during the 2011 winter (June-September) in the 15 AFEX stations, complemented with a similar number of daily records in conventional stations, show that in average the precipitation increase by a factor 2 from the coast to the massif foothills and then increases by a factor 1.6 toward the highest elevations, to sharply decrease downstream (factor 1.5 in less than 20 km). This mean pattern is rather typical of other mountain ranges at midlatitudes, but it shows strong variations in a storm-to-storm basis. The AFEX observations were also complemented by WRF simulation at 1-km horizontal resolution, that in average exhibit a very good correspondence with the measurements. The hourly WRF outputs and AFEX data reveal the orographic pattern dependence on the synoptic environment, from a weak increase/decrease under strong pre-frontal conditions to strong orographic pattern under post-frontal conditions or storms with mostly zonal flow aloft (weak synoptic forcing). The WRF-AFEX comparison also reveals that the model performance also depends on synoptic conditions. We also have used a Linear Theory Model (Smith and Barstad 2004) and verified that, in average, the linear solution captures most of the full (WRF) solution. The AFEX data allows us to evaluate the thermal gradient, revealing the important effect that small topographic features produce, leading to differences of more than 20°C between stations at same altitude but with different exposition to the winds.