Future changes in the free tropospheric freezing level and rain–snow limit: The case of central Chile

Abstract

The freezing level in the free troposphere often intercepts the terrain of the world's major mountain ranges, creating a rain-snow limit. In this work, we use the free tropospheric height of the 0 degrees C isotherm (H0) as a proxy of both levels and study its distribution along the western slope of the subtropical Andes (30 degrees-38 degrees S) in present climate and during the rest of the 21st century. This portion of the Andes corresponds to central Chile, a highly populated region where warm winter storms have produced devastating landslides and widespread flooding in the recent past. Our analysis is based on the frequency distribution of H0 derived from radiosonde and surface observations, atmospheric reanalysis and climate simulations. The future projections primarily employ a scenario of heavy greenhouse gasses emissions (RCP8.5), but we also examine the more benign RCP4.5 scenario. The current H0 distribution along the central Chile coast shows a gradual decrease southward, with mean heights close to 2600 m ASL (above sea level) at 30 degrees C S to 2000 m ASL at 38 degrees S for days with precipitation, about 800 m lower than during dry days. The mean value under wet conditions toward the end of the century (under RCP8.5) is close to, or higher than, the upper quartile of the H0 distribution in the current climate. More worrisome, H0 values that currently occur only 5% of the time will be exceeded in about a quarter of the rainy days by the end of the century. Under RCP8.5, even moderate daily precipitation can increase river flow to levels that are considered hazardous for central Chile.