Upstream Orographic Enhancement of a Narrow Cold-Frontal Rainband Approaching the Andes

Abstract Upstream orographic enhancement of the rainfall from an extratropical cyclone approaching the Andes from the Pacific is investigated using the Weather Research and Forecasting (WRF) Model and the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar. The main precipitation from the cyclone over central and coastal Chile fell when a narrow cold-frontal rainband (NCFR) interacted with a midlevel layer cloud deck formed from the orographically induced ascent of the prefrontal “atmospheric river” upstream of the Andes. Model output indicates that low-level convergence enhanced the NCFR where it met low-level blocked flow near the mountains. The NCFR had stronger updrafts with decreasing distance from the mountains, and the NCFR produced larger rain accumulations over the land region upstream of the Andes than over the open ocean. A sensitivity simulation with a 50% reduction in the Andes topography, for comparison to various west coast mountain ranges of North America, demonstrates that the extreme height of the real mountain barrier strengthens frontogenesis and upstream blocking, which produces stronger frontal lifting and a slower progression of the frontal system. The model and the satellite data suggest that the larger precipitation rates upstream of the Andes resulted from a seeder–feeder effect connected with the orographically invigorated NCFR updrafts, when they penetrated the orographically enhanced midlevel stratiform clouds forming as a result of the upstream orographic ascent of the atmospheric river. The supercooled water of the NCFR updrafts formed a feeder zone for the snow particles in the midlevel stratiform cloud just upstream of the Andes.