A NUMERICAL CASE STUDY OF AN OROGRAPHICALLY ENHANCED FRONTAL SYSTEM IN CENTRAL CHILE

The interaction of a frontal system with the extreme orography of the Andes mountain range is examined for a case of intense precipitation in central Chile during June 2002. The heavy rainfall was associated with a synoptic scale rainband that moved slowly over central Chile during a 48 hour period. Numerical simulations with the WRF mesoscale model show that both the intensification of precipitation within the rainband, and its semi-stationary character, were fundamentally determined by interaction with the topography. It is suggested that the intensification of otherwise weak frontal zones by orographic flow deformation may be an important precipitation mechanism in central Chile

A NUMERICAL CASE STUDY OF AN OROGRAPHICALLY ENHANCED FRONTAL SYSTEM IN CENTRAL CHILE

The interaction of a frontal system with the extreme orography of the Andes mountain range is examined for a case of intense precipitation in central Chile during June 2002. The heavy rainfall was associated with a synoptic scale rainband that moved slowly over central Chile during a 48 hour period. Numerical simulations with the WRF mesoscale model show that both the intensification of precipitation within the rainband, and its semi-stationary character, were fundamentally determined by interaction with the topography. It is suggested that the intensification of otherwise weak frontal zones by orographic flow deformation may be an important precipitation mechanism in central Chile

Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

This is the publisher's version, also available electronically from http://www.atmos-chem-phys.net/10/4491/2010/acp-10-4491-2010.html.Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds

Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability

This is the publisher's version, also available electronically from http://www.atmos-chem-phys.net/10/4507/2010/acp-10-4507-2010.htmlIn the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. In the prognostic local MBL height equation the height change, the horizontal MBL height advection, and the large scale vertical velocity at MBL top are calculated explicitly from the simulation. The entrainment velocity is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes

The Low-Level Atmospheric Circulation near Tongoy Bay–Point Lengua de Vaca (Chilean Coast, 30°S)

This is the publisher's version, also available electronically from http://journals.ametsoc.org/doi/abs/10.1175/MWR-D-11-00059.1.Strong southerly, terrain parallel winds often occur along the coast of north-central Chile (25°–35°S) embedded in the marine atmospheric boundary layer and the lower part of the capping temperature inversion. Their offshore structure and variability have received considerable attention because of the effect on open-ocean processes and connection with the southeast Pacific cloud layer. Mesoscale low-level circulations linked to the coastal topography (e.g., coastal jets and sea breeze) are less studied in Chile, but are particularly relevant as they alter the upper-ocean circulation and the cloud pattern in the nearshore strip. Surface, radiosonde, and airborne meteorological observations near point Lengua de Vaca (LdV)–Tongoy Bay (TB) at 30°S are used alongside numerical modeling to understand the local circulation near a prominent upwelling center. Most observations were gathered during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Chilean Upwelling Experiment (VOCALS-CUpEx) during two weeks in late spring 2009. The regional topography resembles other major capes, but south of TB and east of LdV there is a low (100–300 m), dry marine terrace bounded by high elevation at the coast (~600 m) and farther inland. Coastal soundings 25 km upstream of LdV revealed a southerly wind maximum near the surface and another at 900 m separated by a destabilized layer, deviating from the two-layer model often applied to coastal flow. In the morning a shallow sea breeze penetrates from TB to the marine terrace, but is overridden by southerly flow in the afternoon. Furthermore, between 400 and 900 m, warm continental air is advected from over the marine terrace creating a residual boundary layer over TB. Concurrent with slower changes offshore, the low-level warming over TB leads to a marked cross-shore pressure gradient enhancing the coastal jet just north of LdV

Impacts of atmospheric rivers on precipitation in Southern South America

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.Fil: Viale, Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientifícas y Tecnológicas; Argentina. Universidad de Chile; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Valenzuela, Raúl. Universidad de Chile; ChileFil: Garreaud, René D.. Universidad de Chile; ChileFil: Ralph, F. Martin. University of California at San Diego; Estados Unido

Episodes of strong flow down the western slope of the subtropical Andes

ABSTRACT Nocturnal flows down the narrow Andean valleys within the western slope of the subtropical Andes (central Chile) are episodically enhanced by easterly downslope winds that flow into the Santiago basin over the radiatively cooled air above the surface. Local, regional, and large-scale data have been used here to characterize the mean features of these episodes. About 80% of easterly downslope flow episodes in austral winter are forced by a reversal in the sea level pressure gradient along the coast of south-central Chile, when a midlatitude cold high migrates from southern Chile eastward across the Andes under midtroposphere SW winds associated with a warm ridge aloft. Under these circumstances low-level, easterly (offshore) flow sets in, producing a compensating downslope flow that subsides over central Chile. The remaining cases are associated with prefrontal conditions under a midlatitude trough with NW winds aloft. Since in most of these cases the easterly low-level flow occurs beneath westerly flow higher above, these episodes classify as strong windward downslope flows. Within the Andean valleys and canyons, the near-surface air experiences a sensible warming and drying at night and early morning during these episodes, as the strong downvalley winds tend to destroy the surfacebased radiative inversion and mix down warmer air from aloft. At the exit region of these valleys into the central basin, these downslope flows in austral winter are not able to flush the cold air pool there. Hence, dawn surface temperatures over the basin tend to be lower than average as clear skies and dry subsiding air aloft favor surface radiative cooling. The resulting enhancement of the near-surface static stability hampers the subsequent development of the mixed layer, leading to severe air pollution events in Santiago and other cities in central Chile. A comparative discussion on governing mechanisms with respect to apparently similar phenomena, as gap flow and shallow foehn, is included

Lightning in Western Patagonia

On the basis of 8 years (2005-2012) of stroke data from the World Wide Lightning Location Network we describe the spatial distribution and temporal variability of lightning activity over Western Patagonia. This region extends from ~40°S to 55°S along the west coast of South America, is limited to the east by the austral Andes, and features a hyper-humid, maritime climate. Stroke density exhibits a sharp maximum along the coast of southern Chile. Although precipitation there is largely produced by cold nimbostratus, days with more than one stroke occur up to a third of the time somewhere along the coastal strip. Disperse strokes are also observed off southern Chile. In contrast, strokes are virtually nonexistent over the austral Andes -where precipitation is maximum- and farther east over the dry lowlands of Argentina. Atmospheric reanalysis and satellite imagery are used to characterize the synoptic environment of lightning-producing storms, exemplified by a case study and generalized by a compositing analysis. Lightning activity tends to occur when Western Patagonia is immersed in a pool of cold air behind a front that has reached the coast at ~40°S. Under these circumstances, midlevel cooling occurs before and is more prominent than near-surface cooling, leading to a weakly unstable postfrontal condition. Forced uplift of the strong westerlies impinging on the coastal mountains can trigger convection and produces significant lightning activity in this zone. Farther offshore, large-scale ascent near the cyclone's center may lift near-surface air parcels, fostering shallow convection and dispersing lightning activity.publishedVersionFil: Bürgesser, Rodrigo E. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Ávila, Eldo E. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Física Enrique Gaviola; Argentina.Fil: Bürgesser, Rodrigo E. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Ávila, Eldo E. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Garreaud, René D. Universidad de Chile. Department of Geophysics and Center for Climate and Resilience Research; ChileFil: Nicora, M. Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Departamento de Investigaciones en Láseres y sus aplicaciones; Argentina.Fil: Nicora, M. Gabriela. Ministerio de Defensa. Instituto de Investigaciones Científicas y Técnicas para la Defensa. Departamento de Investigaciones en Láseres y sus aplicaciones; Argentina.Meteorología y Ciencias Atmosférica