Mesoscale Convective Systems over Southeastern South America and Their Relationship with the South American Low-Level Jet

2007 ◽  
Vol 135 (4) ◽  
pp. 1290-1309 ◽  
Author(s):  
Paola Salio ◽  
Matilde Nicolini ◽  
Edward J. Zipser
Keyword(s):  
South America ◽  
Diurnal Cycle ◽  
Large Scale ◽  
South American ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Low Level Jet ◽  
Convection Initiation ◽  
Heat And Moisture

Abstract Prior studies have shown that the low-level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems (MCSs) over the Great Plains of the United States. The South American low-level jet (SALLJ) over southeastern South America (SESA) has an analogous role, advecting heat and moisture from the Amazon basin southward into the central plains of southeastern South America, generating ideal environmental conditions for convection initiation and growth into MCSs. This research has two purposes. One is to describe the characteristics of a 3-yr MCS sample in South America, south of the equator, and its related geographical distribution of convection frequency. The other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large MCSs, specifically those that mature under SALLJ conditions. High horizontal and temporal resolution satellite images are used to detect MCSs in the area for the period 1 September 2000–31 May 2003. Operational 1° horizontal resolution fields from NCEP are used to examine the environment associated with the systems and for the same period. Differences between tropical and subtropical MCSs in terms of size, diurnal cycle, and duration are found. Tropical MCSs are smaller, shorter in duration, and are characterized by a diurnal cycle mainly controlled by diurnal radiative heating. Subtropical MCSs show a preference for a nocturnal phase at maturity over Argentina, which contrasts with a tendency for a daytime peak over Uruguay and southern Brazil. In all seasons, at least one subtropical MCS developed in 41% of the SALLJ days, whereas in the days with no SALLJ conditions this percentage dropped to 12%. This result shows the importance of the synoptic conditions provided by the SALLJ for the development of MCSs and motivates the study of the atmospheric large-scale structure that evolves in close coexistence between SALLJ and subtropical organized convection at the mature stage. The large-scale environment associated with large long-lived MCSs during SALLJ events over SESA evolves under thermodynamic and dynamic forcings that are well captured by the compositing analysis. Essential features are low-level convergence generated by an anomalous all-day-long strong low-level jet prior to the development of the system, overlapped by high-level divergence associated with the anticyclonic flank of the entrance of an upper-level jet streak. This provides the dynamical forcing for convection initiation in an increasingly convectively unstable atmosphere driven by an intense and persistent horizontal advection of heat and moisture at low levels. These processes act during at least one diurnal cycle, enabling gradual building of optimal conditions for the formation of the largest organized convection in the subtropical area. The frequency of convection culminates in a geographically concentrated nocturnal maximum over northeast Argentina on the following day (MCS–SALLJ day). The northeastward displacement and later dissipation of subtropical convection are affected by a northward advance of a baroclinic zone, which is related to horizontal cold advection and divergence of moisture flux at low levels, both contributing to the stabilization of the atmosphere.

scholarly journals Environment Associated with Deep Moist Convection under SALLJ Conditions: A Case Study

2010 ◽  
Vol 25 (3) ◽  
pp. 970-984 ◽  
Author(s):  
Paloma Borque ◽  
Paola Salio ◽  
Matilde Nicolini ◽  
Yanina García Skabar
Keyword(s):  
Frontal Zone ◽  
Mesoscale Convective Systems ◽  
South American ◽  
Moist Convection ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Low Level Jet ◽  
Deep Moist Convection

Abstract The present work focuses on the study of the environmental conditions preceding the development of a group of subtropical mesoscale convective systems over central and northern Argentina on 6–7 February 2003 during the South American Low Level Jet Experiment. This period was characterized by an extreme northerly low-level flow along the eastern Andes foothills [South American low-level jet (SALLJ)]. The entire studied episode was dominated by the presence of a very unstable air mass over northern Argentina and a frontal zone near 40°S. The SALLJ generated an important destabilization of the atmosphere due to the strong humidity and differential temperature advection. Orography provided an extra lifting motion to the configuration of the regional wind field, which was efficient in forcing the initiation of convection. Once convection developed, it moved and regenerated in regions where the convective instability was horizontally homogeneous and stronger.

scholarly journals Impact of enriched analyses on regional numerical forecasts over southeastern South America during SALLJEX

2014 ◽  
Vol 29 (3) ◽  
pp. 315-330
Author(s):  
Yanina García Skabar ◽  
Matilde Nicolini
Keyword(s):  
Data Assimilation ◽  
South America ◽  
Atmospheric Modeling ◽  
South American ◽  
The South ◽  
Low Level ◽  
Positive Effects ◽  
Accumulated Rainfall ◽  
Low Level Jet ◽  
The Impact

During the warm season 2002-2003, the South American Low-Level Jet Experiment (SALLJEX) was carried out in southeastern South America. Taking advantage of the unique database collected in the region, a set of analyses is generated for the SALLJEX period assimilating all available data. The spatial and temporal resolution of this new set of analyses is higher than that of analyses available up to present for southeastern South America. The aim of this paper is to determine the impact of assimilating data into initial fields on mesoscale forecasts in the region, using the Brazilian Regional Atmospheric Modeling System (BRAMS) with particular emphasis on the South American Low-Level Jet (SALLJ) structure and on rainfall forecasts. For most variables, using analyses with data assimilated as initial fields has positive effects on short term forecast. Such effect is greater in wind variables, but not significant in forecasts longer than 24 hours. In particular, data assimilation does not improve forecasts of 24-hour accumulated rainfall, but it has slight positive effects on accumulated rainfall between 6 and 12 forecast hours. As the main focus is on the representation of the SALLJ, the effect of data assimilation in its forecast was explored. Results show that SALLJ is fairly predictable however assimilating additional observation data has small impact on the forecast of SALLJ timing and intensity. The strength of the SALLJ is underestimated independently of data assimilation. However, Root mean square error (RMSE) and BIAS values reveal the positive effect of data assimilation up to 18-hours forecasts with a greater impact near higher topography.

The diurnal cycle can trigger convective self-aggregation 

2021 ◽  
Author(s):  
Gorm Gruner Jensen ◽  
Romain Fiévet ◽  
Jan O. Haerter
Keyword(s):  
Surface Temperature ◽  
Diurnal Cycle ◽  
Large Scale ◽  
Horizontal Resolution ◽  
Cold Pool ◽  
Atmospheric Sciences ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Spatio Temporal ◽  
Self Aggregation

<p>Convective self-aggregation (CSA) is an established modelling paradigm for large-scale thunderstorm clusters, as they form in mesoscale convective systems, the Madden-JulianOscillation or tropical cyclo-genesis [1]. The onset of CSA is characterized by the spontaneous formation of persistently dry patches with suppressed deep convective rainfall. Recently another type of spatio-temporal pattern formation was observed in simulations where the diurnal cycle was mimicked by a sinusoidally varying surface temperature [2]. This diurnal aggregation (DA) is characterized by clusters of intense rain that correlate negatively from one day to the next. </p><p>Here we demonstrate that the diurnal cycle can also act as a trigger of persistently dry patches resembling the early stages of CSA. When the surface temperature is held constant, CSA has been shown to occur within simulations of coarse horizontal model resolution, but not when the resolution was increased [3]. We show that, when a temporally periodic surface temperature forcing is imposed, persistently convection free patches occur even faster when the spatial resolution is increased. The failure to achieve CSA at high horizontal resolution has so far been attributed to the more pronounced cold pool effects at such resolution. In our simulations these cold pools in fact play a key role in promoting CSA. Our results have implications for the origin of persistent convective organization over continents and the sea — and point a path towards achieving such clustering under realistic conditions.</p><p><br>[1]  Christopher S Bretherton, Peter N Blossey, and Marat Khairoutdinov.  An energy-balance analysisof deep convective self-aggregation above uniform SST.Journal of the Atmospheric Sciences, 62(12):4273–4292, 2005.<br>[2]  J. O. Haerter, B. Meyer, and S. B. Nissen.  Diurnal self-aggregation.npj Climate and AtmosphericScience, 3:30, 2020.<br>[3]  Caroline  Muller  and  Sandrine  Bony.   What  favors  convective  aggregation  and  why?GeophysicalResearch Letters, 42(13):5626–5634, 2015.  doi:  https://doi.org/10.1002/2015GL064260.</p>

scholarly journals The Nature of a Heat Wave in Eastern Argentina Occurring during SALLJEX

2007 ◽  
Vol 135 (3) ◽  
pp. 1165-1174 ◽  
Author(s):  
S. Bibiana Cerne ◽  
Carolina S. Vera ◽  
Brant Liebmann
Keyword(s):  
South America ◽  
Heat Wave ◽  
Strong Impact ◽  
South American ◽  
Atmospheric Conditions ◽  
The South ◽  
Low Level ◽  
Central Argentina ◽  
Temperature Advection ◽  
Low Level Jet

Abstract This note describes the physical processes associated with the occurrence of a heat wave over central Argentina during the austral summer of 2002/03, during which the South American Low-Level Jet Experiment (SALLJEX) was carried out. The SALLJEX heat wave that lasted between 25 January and 2 February 2003 was punctuated by extreme conditions during its last 3 days, with the highest temperature recorded over the last 35 yr at several stations of the region. It was found that not only the activity of synoptic-scale waves, but also the intraseasonal oscillation variability, had a strong impact on the temperature evolution during this summer. During the weeks previous to the heat wave development, an intensified South Atlantic convergence zone (SACZ) dominated the atmospheric conditions over tropical South America. Temperatures started to increase in the subtropics due to the subsidence and diabatic warming associated with the SACZ, as depicted by SALLJEX upper-air observations. An extratropical anticyclone that evolved along southern South America further intensified subsidence conditions. By the end of January the warming processes associated with SACZ activity weakened, while horizontal temperature advection began to dominate over central Argentina due to the intensification of the South American low-level jet. This mechanism led to temperature extremes by 2 February with temperature anomalies at least two standard deviations larger than the climatological mean values. Intense solar heating favored by strong subsidence was responsible for the heat wave until 31 January, after which horizontal temperature advection was the primary process associated with the temperature peak.

Orogenic Convection in Subtropical South America as Seen by the TRMM Satellite

2011 ◽  
Vol 139 (8) ◽  
pp. 2399-2420 ◽  
Author(s):  
Kristen L. Rasmussen ◽  
Robert A. Houze
Keyword(s):  
South America ◽  
Tropical Rainfall Measuring Mission ◽  
The United States ◽  
South American ◽  
Convective Storms ◽  
Mountain Ranges ◽  
Low Level ◽  
Global Understanding ◽  
Low Level Jet ◽  
Downslope Flow

AbstractExtreme orogenic convective storms in southeastern South America are divided into three categories: storms with deep convective cores, storms with wide convective cores, and storms containing broad stratiform regions. Data from the Tropical Rainfall Measuring Mission satellite’s Precipitation Radar show that storms with wide convective cores are the most frequent, tending to originate near the Sierra de Cordoba range. Downslope flow at upper levels caps a nocturnally enhanced low-level jet, thus preventing convection from breaking out until the jet hits a steep slope of terrain, such as the Sierra de Cordoba Mountains or Andean foothills, so that the moist low-level air is lifted enough to release the instability and overcome the cap. This capping and triggering is similar to the way intense convection is released near the northwestern Himalayas. However, the intense storms with wide convective cores over southeastern South America are unlike their Himalayan counterparts in that they exhibit leading-line/trailing-stratiform organization and are influenced by baroclinic troughs more similar to storms east of the Rocky Mountains in the United States. Comparison of South American storms containing wide convective cores with storms in other parts of the world contributes to a global understanding of how major mountain ranges influence precipitating cloud systems.

The Amazonian Low-Level Jet and Its Connection to Convective Cloud Propagation and Evolution

2020 ◽  
Vol 148 (10) ◽  
pp. 4083-4099 ◽  
Author(s):  
Evandro M. Anselmo ◽  
Courtney Schumacher ◽  
Luiz A. T. Machado
Keyword(s):  
Convective Cloud ◽  
Moisture Flux ◽  
Time Of Day ◽  
Convective System ◽  
South American ◽  
Low Level ◽  
Convective Systems ◽  
Wind Speeds ◽  
Cloud Clusters ◽  
Low Level Jet

AbstractWe describe the existence of an Amazonian low-level jet (ALLJ) that can affect the propagation and life cycle of convective systems from the northeast coast of South America into central Amazonia. Horizontal winds from reanalysis were analyzed during March–April–May (MAM) of the two years (2014–15) of the GoAmazon2014/5 field campaign. Convective system tracking was performed using GOES-13 infrared imagery and classified into days with high and weak convective activity. The MAM average winds show a nocturnal enhancement of low-level winds starting near the coast in the early evening and reaching 1600 km inland by late morning. Mean 3-hourly wind speeds maximize at 9–10 m s−1 near 900 hPa, but individual days can have nighttime low-level winds exceeding 12 m s−1. Based on objective low-level wind criteria, the ALLJ is present 10%–40% of the time over the Amazon during MAM depending on the location and time of day. The evolution of the ALLJ across the Amazon impacts the frequency of occurrence of cloud clusters and the intensity of the moisture flux. In addition, the ALLJ is associated with the enhancement of northeasterly flow in the midtroposphere during active convective days, when vertical momentum transport may be occurring in the organized cloud clusters. During the weakly active convective period, the ALLJ is weaker near the coast but stronger across the central Amazon and appears to be linked more directly with the South American low-level jet.

scholarly journals Origins of Vorticity in a Simulated Tornadic Mesovortex Observed during PECAN on 6 July 2015

2019 ◽  
Vol 147 (1) ◽  
pp. 107-134 ◽  
Author(s):  
Matthew D. Flournoy ◽  
Michael C. Coniglio
Keyword(s):  
Convective System ◽  
Vertical Vorticity ◽  
Low Level ◽  
Convective Systems ◽  
The North ◽  
Mesoscale Convective ◽  
Forecasting System ◽  
Convection Initiation ◽  
Inflow Jet ◽  
Gust Front

To better understand and forecast nocturnal thunderstorms and their hazards, an expansive network of fixed and mobile observing systems was deployed in the summer of 2015 for the Plains Elevated Convection at Night (PECAN) field experiment to observe low-level jets, convection initiation, bores, and mesoscale convective systems. On 5–6 July 2015, mobile radars and ground-based surface and upper-air profiling systems sampled a nocturnal, quasi-linear convective system (QLCS) over South Dakota. The QLCS produced several severe wind reports and an EF-0 tornado. The QLCS and its environment leading up to the mesovortex that produced this tornado were well observed by the PECAN observing network. In this study, observations from radiosondes, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event with a focus on the development of the observed tornadic mesovortex. All ensemble members simulated low-level mesovortices with one member in particular generating two mesovortices in a manner very similar to that observed. Forecasts from this member were analyzed to examine the processes increasing vertical vorticity during the development of the tornadic mesovortex. Cyclonic vertical vorticity was traced to three separate airstreams: the first from southerly inflow that was characterized by tilting of predominantly crosswise horizontal vorticity along the gust front, the second from the north that imported streamwise horizontal vorticity directly into the low-level updraft, and the third from a localized downdraft/rear-inflow jet in which the horizontal vorticity became streamwise during descent. The cyclonic vertical vorticity then intensified rapidly through intense stretching as the parcels entered the low-level updraft of the developing mesovortex.

scholarly journals Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

2009 ◽  
Vol 137 (4) ◽  
pp. 1272-1294 ◽  
Author(s):  
Brian Jackson ◽  
Sharon E. Nicholson ◽  
Douglas Klotter
Keyword(s):  
Diurnal Cycle ◽  
Rainy Season ◽  
Large Scale ◽  
Critical Role ◽  
Mesoscale Convective Systems ◽  
Convective Systems ◽  
Continental Scale ◽  
Equatorial Africa ◽  
Mesoscale Convective ◽  
The Right

Abstract This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

scholarly journals The Intensification of the Low-Level Jet during the Development of Mesoscale Convective Systems on a Mei-Yu Front

1998 ◽  
Vol 126 (2) ◽  
pp. 349-371 ◽  
Author(s):  
Chaing Chen ◽  
Wei-Kuo Tao ◽  
Pay-Liam Lin ◽  
George S. Lai ◽  
S-F. Tseng ◽  
...  
Keyword(s):  
Mesoscale Convective Systems ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Low Level Jet
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