The Chilean Coastal Orographic Precipitation Experiment Pilot Project (CCOPE-2015): Overview and Preliminary Results

The Nahuelbuta Mountains are a coastal range of modest height (peak elevation 1.3km ASL) in Southern Chile (38 deg. S). Rain gauge observations reveal a strong orographic enhancement of rainfall over the Nahuelbuta. The interior of the mountains receives up to 4.0 m annual rainfall, in contrast to nearby lowland and offshore locations that receive about 1 m. The small horizontal scale of the Nahuelbuta (~40km) indicates that orographic precipitation enhancement is potentially sensitive to the microphysical efficiency of orographic clouds.

The Chilean Coastal Orographic Precipitation Experiment Pilot Project (CCOPE-2015) was conducted during the Austral winter of 2015 (May-Aug) to better understand the mechanisms of orographic enhancement in the Nahuelbuta. Observations collected include: (1) surface rainfall from a network of 10 data logging rain gauges, (2) vertical precipitation profiles from a pair of K-band Micro Rain Radars, (3) rain drop size distributions from an optical disdrometer, (4) upstream wind and thermodynamic profiles from radiosonde launches, and (5) aerosol number concentration and size distribution measurements from a condensation particle counter and an ultra-high sensitivity aerosol spectrometer.

An overview of observations collected during CCOPE-2015 is presented. The character of precipitation over the Nahuelbuta is contrasted between periods of ice-initiated and warm rain, which typically occur within the same frontal passage. Thirty-four percent of rainfall fell during warm rain periods, while fifty-two percent of rainfall fell during ice-initiated periods. Warm rain drop size distributions are characterized by many more and relatively smaller drops than ice-initiated drop size distributions. Both the portion and properties of ice-initiated and warm rainfall compare favorably with observations of coastal mountain rainfall at a similar latitude in California. Orographic enhancement is consistently strong for rain of both types. The mechanisms controlling the variability of this enhancement in each rain regime will be discussed.