Orographic precipitation gradient over the west slope of the Andes at 30 degrees south

The Elqui valley around 30°S in Chile is located within a semi-arid region in which the mean annual precipitation (80-180 mm) accumulates in austral winter in connection with mid-latitude weather disturbances: fronts and cut-off lows. Given the steep topography of the Andes in this region (0 - 5000 m in ~ 200 km) the flow and precipitation are strongly influenced by the Andes. Typically, the precipitation increases with elevation due to the forced ascent over the topography in a well-mixed atmosphere. However, it has been observed that some particular storms produce an inverse orographic precipitation gradient (OPG). An eventual increase in the frequency of this type of storms would lead to decreased water availability during the warm, rainless season and consequently to the damming capacity of the watershed. Therefore, clarifying the mechanism that produce either positive or negative OPGs within individual storms may shed light on the issue of expected climate variability. In this work we characterize OPGs according to the Froude number and associated intensity and location of the barrier jet when orographic blocking occurs. The flow blocking and stability parameters will be estimated using CFSR Reanalysis winds and temperatures along the slope from surface weather stations. These results are contrasted with studies over the Sierra Nevada that show a negative correlation between the height of the barrier jet and OPGs, and also a positive correlation between Froude number and a OPG.