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Maximiliano Viale and René Garreaud

Abstract

Summertime [December–February (DJF)] precipitation over the western slopes of the subtropical Andes (32°–36°S) accounts for less than 10% of the annual accumulation, but it mostly occurs as rain and may trigger landslides leading to serious damages. Based on 13 year of reanalysis, in situ observations, and satellite imagery, a synoptic climatology and physical diagnosis reveal two main weather types lead to distinct precipitation systems. The most frequent type (~80% of the cases) occurs when a short-wave midlevel trough with weak winds and thermally driven mountain winds favor the development of convective precipitation during the daytime. The trough progresses northwest of a long-lasting warm ridge, which produces low-level easterly airflow that enhances its buoyancy as it moves over the arid land of western Argentina toward the Andes. The weak winds aloft facilitate the penetration of the moist easterly flow into the Andes. Midlevel flow coming from the west side of the Andes is decoupled from the low-level maritime air by a temperature inversion, and thus provides little moisture to support precipitation. The less frequent type (~20% of the cases) occurs when a deep midlevel trough and strong westerly flow produces stratiform precipitation. This type has a baroclinic nature akin to winter storms, except that they are rare in summer and there is no evidence of a frontal passage at low levels. The lifting and cooling ahead of the trough erode the typical temperature inversion over the Pacific coast, and thus allows upslope transport of low-level marine air by the strong westerlies forming a precipitating cloud cap on the western slope of the Andes.

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Maximiliano Viale and Federico A. Norte

Abstract

The most intense orographic precipitation event over the subtropical central Andes (36°–30°S) during winter 2005 was examined using observational data and a regional model simulation. The Eta-Programa Regional de Meteorología (PRM) model forecast was evaluated and used to explore the airflow structure that generated this heavy precipitation event, with a focus on orographic influences. Even though the model did not realistically reproduce any near-surface variables, nor the precipitation shadow in the leeside lowlands, its reliable forecast of heavy precipitation over the windward side and the wind fields suggests that it can be used as a valuable forecasting tool for such events in the region.

The synoptic flow of the 26–29 August 2005 storm responded to a well-defined dipole from low to upper levels with anomalous low (high) geopotential heights at midlatitudes (subtropical) latitudes located off the southeast Pacific coast, resulting in a large meridional geopotential height gradient that drove a strong anomalous cross-barrier flow. Precipitation enhancement in the Andes was observed during the entire event; however, the highest rates were in the prefrontal sector under the low-level stable stratification and cross-barrier winds exceeding 2.5 standard deviations (σ) from the climatological monthly mean. The combination of strong cross-mountain winds with the stable stratification in the air mass of a frontal system, impinging on the high Andes range, appears to be the major factor in determining the flow structure that produced the pattern of precipitation enhancement, with uplift maximized near mountaintops and low-level blocking upwindleading to the formation of a low-level along-barrier jet. Additionally, only the upstream wind anomalies for the 15 heaviest events over a 10-yr (1967–76) period were investigated. They exhibited strong anomalous northwesterly winds for 14 of the 15 events, whereas for the remaining event there were no available observations to evaluate. Thus, these anomalies may also be exploited for forecasting capabilities.

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Maximiliano Viale and Mario N. Nuñez

Abstract

Winter orographic precipitation over the Andes between 30° and 37°S is examined using precipitation gauges in the mountains and adjacent lowlands. Because of the limited number of precipitation gauges, this paper focuses on the large-scale variation in cross-barrier precipitation and does not take into account the fine ridge–valley scale. The maximum amount of precipitation was observed on the windward slope of the mountain range below the crest, which was twice that observed on the low-windward side between 32.5° and 34°S. Toward the east of the crest, precipitation amounts drop sharply, generating a strong cross-barrier gradient. The rain shadow effect is greater in the north (32°–34.5°S) than in the south (35°–36.5°S) of the low-lee side, which is probably due to more baroclinic activity in southernmost latitudes and a southward decrease in the height of the Andes enabling more spillover precipitation. The effect of the Andes on winter precipitation is so marked that it modifies the precipitation regimes in the adjacent windward and leeward lowlands north of 35°S. Based on the fact that ~75% of the wintertime precipitation accumulated in the fourth quartile, through four or five heavy events on average, the synoptic-scale patterns of the heavy (into fourth quartile) orographic precipitation events were identified. Heavy events are strongly related to strong water vapor transport from the Pacific Ocean in the pre-cold-front environment of extratropical cyclones, which would have the form of atmospheric rivers as depicted in the reanalysis and rawinsonde data. The composite fields revealed a marked difference between two subgroups of heavy precipitation events. The extreme (100th–95th percentiles) events are associated with deeper cyclones than those for intense (95th–75th percentiles) events. These deeper cyclones lead to much stronger plumes of water vapor content and cross-barrier moisture flux against the high Andes, resulting in heavier orographic precipitation for extreme events. In addition, regional airflow characteristics suggest that the low-level flow is typically blocked and diverted poleward in the form of an along-barrier jet. On the lee side, downslope flow dominates during heavy events, producing prominent rain shadow effects as denoted by the domain of downslope winds extending to low-leeward side (i.e., zonda wind).

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Maximiliano Viale, Robert A. Houze Jr., and Kristen L. Rasmussen

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.

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Maximiliano Viale, Raúl Valenzuela, René D. Garreaud, and F. Martin Ralph

Abstract

This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from 6-hourly CFSR reanalysis data over a 16-yr period (2001–16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27° to 55°S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38° and 50°S, averaging 35–40 days yr−1. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S. ARs contribute 45%–60% of the annual precipitation in subtropical Chile (37°–32°S) and 40%–55% along the midlatitude west coast (37°–47°S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2–3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.

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