Extreme Orographic Precipitation Events in the Central Andes: Examining the Role of Cold Air Intrusion

On October 4-5 2013, one raingauge located at 1.5 km elevation along an orographic envelope transect in the Central Andes registered 180 mm in six hours (roughly 30% of monthly totals) in the early morning with nearby gauges (~1 km) reporting 100-30 mm (decreasing with height) in half the duration. Total accumulations over a 24hour period were about 25 -30 cm, similar to the magnitude of hurricane Katrina at landfall. Synoptic scale conditions were dominated by a Cold Air Intrusion (CAI) event. High-resolution NWP simulations were conducted to elucidate storm dynamics. Model results suggest that an unstable air mass associated with easterly and northeasterly flow blocked at low levels by orography forms a deep pool of moist warm air along the foothills of the Andes behind the cold front. Forced lifting along the eastern slopes of the Andes results in the establishment of stationary terrain-following mountain waves that pull moisture upslope and within inner valleys resulting in the development of precipitation cells spatially organized by the updrafts. Depletion of the moist air pool weakens upslope moisture convergence, and thus the moisture supply at high elevations first, and eventually shuts down precipitation down the topographic gradient. Rainfall duration is determined by elevation, unstable moist air supply, and maintenance of orographic blocking behind the cold front and how it evolves in time. The follow-up question concerns the role of CAI in extreme weather events in the Central Andes.

To address this question, a climatology of CAI events was derived from ERA Interim from 1979 to 2016, and the spatial organization and temporal frequency of CAI over the Central Andes was examined in detail with a focus on detecting extreme events that are currently missing from observations and, or are severely underestimated from satellite records. A CAI event includes a high-pressure anticyclone in the high latitudes moving eastward over the Andes. Due to the blocking effect of Andes, a low-level ageostophic circulation develops near the Andes maintaining southerly advection of cold air. The cold front creates a well defined band of enhanced convection and blocks the South Atlantic Convergence Zone (SACZ) causing extreme precipitation especially in the warm season. In the event of October 2013, the SACZ is blocked about three days prior to the event by another system of high pressure trapping larger amount of moist air than usual events. Analysis of a second extreme event observed during the monsoon season in January 2015 also associated with CAI synoptic conditions shows differences consistent with CAI seasonality. Extreme events over the TRMM-GPM period of record are examined toward developing of a physically based conceptual model of orographic precipitation extremes in the Central Andes.