scholarly journals Dynamics of the Low-Level Jet off the West Coast of Subtropical South America

2005 ◽  
Vol 133 (12) ◽  
pp. 3661-3677 ◽  
Author(s):  
Ricardo C. Muñoz ◽  
RenéD. Garreaud
Keyword(s):  
Boundary Layer ◽  
South America ◽  
Pressure Gradient ◽  
West Coast ◽  
Marine Boundary Layer ◽  
Force Balance ◽  
North Central ◽  
Central Chile ◽  
Low Level ◽  
Coastal Jet

Abstract The subtropical west coast of South America is under the influence of the southeast Pacific anticyclone year-round, which induces persistent southerly winds along the coast of north-central Chile. These winds often take the form of a low-level coastal jet, in many aspects similar to the coastal jet existing off the California coast. Extensive diagnostics of mesoscale model results for a case in October 2000 are used here to describe the mean momentum budget supporting the coastal jet. The jet appears to occur when midlatitude synoptic conditions induce a northerly directed pressure gradient force along the coast of north-central Chile. The very steep coastal terrain precludes the development of a significant easterly low-level wind that would geostrophically balance the pressure gradient. Instead, the meridional flow accelerates until turbulent friction in the marine boundary layer balances the meridional pressure gradient. The resulting force balance is semigeostrophic, with geostrophy valid only in the zonal (cross shore) direction. At higher levels, the topographic inhibition of the easterlies relaxes, and a small easterly flow ensues, which turns out to be very important in the temperature and stability budgets of the layer capping the marine boundary layer.

scholarly journals Recent Changes in the Low-Level Jet along the Subtropical West Coast of South America

Atmosphere ◽  
2021 ◽  
Vol 12 (4) ◽  
pp. 465
Author(s):  
Catalina Aguirre ◽  
Valentina Flores-Aqueveque ◽  
Pablo Vilches ◽  
Alicia Vásquez ◽  
José A. Rutllant ◽  
...  
Keyword(s):  
South America ◽  
West Coast ◽  
Southern Boundary ◽  
Central Chile ◽  
Low Level ◽  
Long Term Trends ◽  
Low Level Jet ◽  
Wind Events ◽  
Ocean Environment ◽  
June July

Surface winds along the subtropical west coast of South America are characterized by the quasi-weekly occurrences of low-level jet events. These short lived but intense wind events impact the coastal ocean environment. Hence, identifying long-term trends in the coastal low-level jet (CLLJ) is essential for understanding changes in marine ecosystems. Here we use ERA5 reanalysis (1979–2019) and an objective algorithm to track anticyclones to investigate recent changes in CLLJ events off central Chile (25–43 °S). Results present evidence that the number of days with intense wind (≥10 ms−1), and the number and duration of CLLJ events have significantly changed off central Chile in recent decades. There is an increase in the number of CLLJ events in the whole study area during winter (June-July-August; JJA), while during summer (December–January–February; DJF) a decrease is observed at lower latitudes (29–34 °S), and an increase is found at the southern boundary of the Humboldt system. We suggest that changes in the central pressures and frequency of extratropical, migratory anticyclones that reach the coast of South America, which force CLLJs, have played an important role in the recent CLLJ changes observed in this region.

Some Insights into the Characteristics and Dynamics of the Chilean Low-Level Coastal Jet

2010 ◽  
Vol 138 (8) ◽  
pp. 3185-3206 ◽  
Author(s):  
Qingfang Jiang ◽  
Shouping Wang ◽  
Larry O’Neill
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Mesoscale Model ◽  
Primary Role ◽  
Pressure System ◽  
Low Level ◽  
The Andes ◽  
Coastal Jet ◽  
Southeast Pacific ◽  
Baroclinic Zone

Abstract The characteristics and dynamics of the Chilean low-level coastal jet (CLLCJ) are examined here through diagnosing real-time mesoscale model forecasts in support of the Variability of the American Monsoon System (VAMOS) Ocean–Cloud–Atmosphere Land Study (VOCALS) and additional sensitivity simulations. The forecasted surface winds over the southeast Pacific compare favorably with available observations. According to the forecasts and sensitivity simulations, the Southeast Pacific high pressure system (SEPH) plays a primary role in driving the CLLCJ. The Andes significantly intensify the CLLCJ mainly through interacting with the SEPH and anchoring a baroclinic zone along the Chilean coast. The land–sea differential heating also enhances the CLLCJ by strengthening the coastal baroclinic zone. Based on the location of the SEPH center, the CLLCJ can be separated into two types: a strong-forcing jet, with the SEPH close to the central Chilean coastline; and a weak-forcing jet, with the SEPH centered far away from the coastline. The former is much more intense and associated with stronger interaction between the SEPH and the Andes. The CLLCJ is slightly supergeostrophic within the marine boundary layer top inversion, where weak easterlies develop, and subgeostrophic in the turbulent boundary layer below, where westerlies are present. The inversion easterlies induce strong subsidence along the coast, which contributes to the formation of the coastal low and the coastal baroclinic zone.

The Low-Level Jet off the West Coast of Subtropical South America: Structure and Variability

2005 ◽  
Vol 133 (8) ◽  
pp. 2246-2261 ◽  
Author(s):  
RenéD. Garreaud ◽  
Ricardo C. Muñoz
Keyword(s):  
Wind Speed ◽  
South America ◽  
West Coast ◽  
Inversion Layer ◽  
Surface Winds ◽  
The West ◽  
Subtropical Anticyclone ◽  
Central Chile ◽  
Low Level ◽  
Southeast Pacific

Abstract The subtropical anticyclone over the southeast Pacific drives low-level southerly flow along the west coast of South America. In turn, the alongshore flow induces coastal upwelling that supports a wealth of fishery resources. Within this region, satellite data, marine reports, and coastal observations indicate the existence of a southerly coastal jet (i.e., a maximum of wind speed) off central Chile (26°–36°S). The mean features and variability of this southerly jet is documented in this work using 4 yr of satellite-derived sea surface winds, complemented by satellite-derived cloud amount fields and atmospheric reanalysis. Furthermore, analysis of in situ data and model results of a well-defined jet event during October 2000 allows a preliminary description of the jet’s three-dimensional structure and a comparison with the northerly jet off the coast of California. Southerly jet events off central Chile occur year-round, but they are more frequent during spring–summer (over 60% of the time). The jet is characterized by an elongated maximum of surface wind speed (∼10 m s−1) with its axis at about 150 km off the coast and a cross-shore scale of about 500 km. The two Quick Scatterometer (QuikSCAT) fields per day (a.m. and p.m. passes) allow a rough estimate of the amplitude of the diurnal cycle of the surface winds, which appears to be remarkably small in the region of the jet. The jet events are associated with the passage of a midlatitude ridge over the southeast Pacific strengthening the subtropical anticyclone. Upstream and over the jet region the coastal deck of stratocumulus clouds tends to dissipate in contrast to an increase in cloudiness downstream of the jet. In the case study the jet core resides at the top of the marine boundary layer (MBL)/inversion layer. Weak offshore flow prevails above the jet axis, and even weaker onshore flow prevails in the MBL. Consistent with its subtropical location the jet is embedded in a region of large-scale subsidence; nevertheless a mesoscale area of mean upward motion is simulated just downstream of the jet core.

scholarly journals What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

2019 ◽  
Vol 19 (21) ◽  
pp. 13489-13506 ◽  
Author(s):  
Karmen Babić ◽  
Norbert Kalthoff ◽  
Bianca Adler ◽  
Julian F. Quinting ◽  
Fabienne Lohou ◽  
...  
Keyword(s):  
Boundary Layer ◽  
West Africa ◽  
Climate Models ◽  
Marine Boundary Layer ◽  
Monsoon Season ◽  
Onset Time ◽  
Data Set ◽  
Cold Air ◽  
Low Level ◽  
Stratus Clouds

Abstract. Nocturnal low-level stratus clouds (LLCs) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLCs in the state-of-the-art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region and gaps in understanding of underlying processes. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-aerosol-chemistry-cloud interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary-layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLCs during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection, and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to a reduced contribution from horizontal cold-air advection. This in turn results in weaker cooling, and thus saturation is not reached. Such deviation in the dynamics of the NLLJ is related to the arrival of a cold air mass propagating northwards from the coast, called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from, for example, central or south Africa, reduce the background moisture over large parts of SWA. Backward-trajectory analysis suggests that another possible reason for clear nights is descending air, which originated from drier levels above the marine boundary layer.

Geology of part of a long-lived dynamic plate margin: the coastal cordillera of north-central Chile, latitude 30°51′–31°S

1988 ◽  
Vol 25 (4) ◽  
pp. 603-624 ◽  
Author(s):  
James J. Irwin ◽  
Carlos García ◽  
Francisco Hervé ◽  
Maureen Brook
Keyword(s):  
South America ◽  
Tierra Del Fuego ◽  
First Episode ◽  
North Central ◽  
Central Chile ◽  
Igneous Complex ◽  
Plate Margin ◽  
Rock Formations ◽  
Facies Metamorphism ◽  
Basaltic Lavas

A complex composed of ultrabasic and basaltic lavas, chert, arkose, and conglomerate was assembled in the coast of north-central Chile (lat. 30°30′–31°S) prior to 200 Ma. The character of, and relationships between, the rock formations exposed here are consistent with an autochthonous evolution of this part of Chile in the last 200 Ma. Three major episodes of deformation and metamorphism have been observed in this area. The first episode (F1) produced a compositional layering (S1) and amphibolite-facies metamorphism coeval with the intrusion of an extensive igneous complex between 220 and 200 Ma. A second episode of deformation (F2) locally formed reverse faults and tight, recumbent folds in S1. Units in which F2 folds are well developed yield K–Ar and Rb–Sr ages between 163 and 140 Ma. At between 140 and 126 Ma, upright, open to tight folds (F3) formed with sharp hinges and axial planes that strike north and dip steeply east. Structures of similar age, style, and orientation have been reported as far south as Tierra del Fuego. The timing of the F3 deformation appears to correspond to the opening of the South Atlantic Ocean and accelerated westward motion of South America.

Simulation of Seasonal Variation of Marine Boundary Layer Clouds over the Eastern Pacific with a Regional Climate Model*

2011 ◽  
Vol 24 (13) ◽  
pp. 3190-3210 ◽  
Author(s):  
Lei Wang ◽  
Yuqing Wang ◽  
Axel Lauer ◽  
Shang-Ping Xie
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Seasonal Cycle ◽  
Marine Boundary Layer ◽  
Eastern Pacific ◽  
Cloud Base ◽  
Low Level ◽  
Surface Latent Heat Flux ◽  
Cloud Regime ◽  
Low Cloud

Abstract The seasonal cycle of marine boundary layer (MBL) clouds over the eastern Pacific Ocean is studied with the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM). The results show that the model is capable of simulating not only the overall seasonal cycle but also the spatial distribution, cloud regime transition, and vertical structure of MBL clouds over the eastern Pacific. Although the modeled MBL cloud layer is generally too high in altitude over the open ocean when compared with available satellite observations, the model simulated well the westward deepening and decoupling of the MBL, the rise in cloud base and cloud top of the low cloud decks off the Peru and California coasts, and the cloud regime transition from stratocumulus near the coast to trade cumulus farther to the west in both the southeast and northeast Pacific. In particular, the model reproduced major features of the seasonal variations in stratocumulus decks off the Peru and California coasts, including cloud amount, surface latent heat flux, subcloud-layer mixing, and the degree of MBL decoupling. In both observations and the model simulation, in the season with small low-level cloudiness, surface latent heat flux is large and the cloud base is high. This coincides with weak subcloud-layer mixing and strong entrainment at cloud top, characterized by a high degree of MBL decoupling, while the opposite is true for the season with large low-level cloudiness. This seasonal cycle in low-cloud properties resembles the downstream stratocumulus-to-cumulus transition of marine low clouds and can be explained by the “deepening–decoupling” mechanism proposed in previous studies. It is found that the seasonal variations of low-level clouds off the Peru coast are mainly caused by a large seasonal variability in sea surface temperature, whereas those off the California coast are largely attributed to the seasonal cycle in lower-tropospheric temperature.

scholarly journals Impact of nucleation on global CCN

2009 ◽  
Vol 9 (3) ◽  
pp. 12999-13037 ◽  
Author(s):  
J. Merikanto ◽  
D. V. Spracklen ◽  
G. W. Mann ◽  
S. J. Pickering ◽  
K. S. Carslaw
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Environmental Changes ◽  
Control Strategies ◽  
Cloud Condensation Nuclei ◽  
Free Troposphere ◽  
Carbonaceous Particles ◽  
Upper Troposphere ◽  
Low Level ◽  
Primary Emissions

Abstract. Cloud condensation nuclei (CCN) are derived from particles emitted directly into the atmosphere (primary emissions) or from the growth of nanometer-sized particles nucleated in the atmosphere. It is important to separate these two sources because they respond in different ways to gas and particle emission control strategies and environmental changes. Here, we use a global aerosol microphysics model to quantify the contribution of primary and nucleated particles to global CCN. The model considers primary emissions of sea spray, sulfate and carbonaceous particles, and nucleation processes appropriate for the free troposphere and boundary layer. We estimate that 45% of global low-level cloud CCN at 0.2% supersaturation are secondary aerosol derived from nucleation (ranging between 31–49% taking into account uncertainties primary emissions and nucleation rates), the remainder being directly emitted as primary aerosol. The model suggests that 35% of CCN (0.2%) in low-level clouds were created in the free and upper troposphere. In the marine boundary layer 55% of CCN (0.2%) are from nucleation, 45% being entrained from the free troposphere. Both in global and marine boundary layer 10% of CCN (0.2%) is nucleated in the boundary layer. Combinations of model runs show that primary and nucleated CCN are non-linearly coupled. In particular, boundary layer nucleated CCN are strongly suppressed by both primary emissions and entrainment of particles nucleated in the free troposphere. Elimination of all primary emissions reduces global CCN (0.2%) by only 20% and elimination of upper tropospheric nucleation reduces CCN (0.2%) by only 12% because of increased impact of boundary layer nucleation on CCN.

Backpressure propagation mode and balance property in a rectangular duct

2018 ◽  
Vol 233 (4) ◽  
pp. 1472-1489
Author(s):  
Yubao He ◽  
Hongyan Huang ◽  
Daren Yu
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Rise ◽  
Adverse Pressure Gradient ◽  
Force Balance ◽  
Propagation Mode ◽  
Rectangular Duct ◽  
Local Boundary ◽  
Starting Point ◽  
Balance Property

The backpressure propagation mode accompanied by shock-train evolution is investigated numerically in a rectangular duct with an open space. On this basis, the balance mechanism and parametric effects of heat transfer and skin friction for backpressure propagation are revealed to understand the nature of force competition better. As a result, the backpressure propagation mode can be classified into two different flow processes with increased backpressure. In addition, balance property mechanism reveals that both the momentum inside the boundary layer and the shear force which transfers the momentum from the outer core flow to boundary layer are combined to resist the adverse pressure gradient. Further, parametric effect indicates that varying wall temperatures and roughness heights lead to different degrees of changes in balance property. According to quantitative results, both wall temperature and roughness height decrease the local boundary-layer momentum at the starting point of original pressure rise and thus the local adverse pressure gradient wins the force competition. In the subsequently continuous flow, the adverse pressure gradient continues to propagate upstream and then is retarded gradually by the boundary layer with a fuller velocity profile until a new force balance is generated.

scholarly journals What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

2019 ◽  
Author(s):  
Karmen Babić ◽  
Norbert Kalthoff ◽  
Bianca Adler ◽  
Julian F. Quinting ◽  
Fabienne Lohou ◽  
...  
Keyword(s):  
Boundary Layer ◽  
West Africa ◽  
Climate Models ◽  
Marine Boundary Layer ◽  
Monsoon Season ◽  
Onset Time ◽  
Data Set ◽  
Cold Air ◽  
Low Level ◽  
Stratus Clouds

Abstract. Nocturnal low-level stratus clouds (LLC) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLC in the state–of–the–art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-Aerosol-Cloud-Chemistry Interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLC during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to reduced contribution from horizontal cold-air advection. This in turn results in a weaker cooling and thus saturation is not reached. Such deviation in the dynamics of NLLJ is related to the arrival of cold air mass propagating northwards from the coast called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from e.g. central or south Africa, reduce the background moisture over the large parts of SWA. Based on the backward trajectories analysis, another possible reason for clear nights is descending of air originating from drier levels above the marine boundary layer.

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