Characteristics of flow upstream of the central Chilean Andes: an examination of radiosonde data from 2007

Central Chile, located between 32°S and 35°S, is a mountainous and densely populated strip of land between the South American Pacific Coast and the main divide of the Andes Cordillera. Mountain heights in the Andes range from 1000-5000 m above sea level in the mid-latitudes to over 5000 m above sea level in the subtropical and equatorial latitudes. The Cordillera produces significant disruptions on a variety of scales in the weather systems that affect central Chile. While frontal interactions with mountain ranges are complex, one common characteristic of these interactions is blocking of low-level atmospheric flow. As a result of this blocking, precipitation in central Chile has been shown to deform along a north-south axis.

Radiosonde data from NCDC's Integrated Global Radiosonde Archive for stations at Quintero and Santo Domingo was examined to determine the interactions between atmospheric flow at various levels and precipitation in central Chile. This study examined data from 2007 and analyzed the relationships between wind direction, dew point temperature, and wind speed at 925 hPa, 850 hPa, 700 hPa, and 500 hPa. Wind rose plots of 850 hPa data showed a bimodal distribution of northwesterly and southwesterly winds. This bimodal distribution was likely caused by low-level blocking of atmospheric flow of the Andes Cordillera. At 700 and 925 hPa, the majority of the wind for the entire data record was from the southeast. Qualitative analysis of the radiosonde data for rainy days in Santiago, Chile revealed that significant precipitation events were strongly related to atmospheric moisture content and wind direction. On average, precipitation occurred on days where the winds at 925 and 850 hPa were from either the northwest or southeast and wind at 700 hPa was from the northwest. Winds at these levels, especially at 700 hPa also tended to have higher velocities and be moister than winds from the other directions at that level. Thus, precipitation in Central Chile occurred most often when strong, moist winds were from the northwest or southeast.