Orographic Precipitation and Water Vapor Fractionation over the southern Andes (Invited)

The climatological nature of orographic precipitation in the southern Andes between 40S and 48S is investigated primarily using stable isotope data from stream water. In addition, four precipitation events are examined using balloon soundings and satellite images. The MODIS images taken during precipitation events reveal complex patterns of upstream open-cell convection over the ocean, stratus and/or convective clouds over the mountains, sharp lee-side clearing and roll convection over the steppe. Using the water vapor bands on MODIS reveals a sharp drop in column water vapor from about 1.4 to 0.7 cm across the mountain range.

Seventy one water samples from streams across the southern Andes provide deuterium and oxygen-18 isotope data to determine the Drying Ratio (DR) of airstreams crossing the mountain range and to constrain free parameters in a mathematical model of orographic precipitation. From the strong isotope fractionation associated with orographic precipitation, we estimate that DR~ 50%, the highest value yet found for a mountain range. The cloud delay parameters in a high resolution linear model were optimized to fit the streamwater isotope data. The model agrees well with the data when the cloud delay time (i.e. elapsed time from condensation to precipitation) is about 1700 seconds. The tuned model is used to discuss the small scale spatial pattern of precipitation.

The isotope data from streams is also compared with that from sapwater. The good agreement suggests that future isotope mapping could be done using trees.