Impacts of the Low Level Jets on the precipitation over southern South America

CARLOS FREDERICO ANGELIS; LUIZ AUGUSTO TOLEDO MACHADO; PAOLA SALIO

Foz de Iguazu - Brasil

Conferencia; 8th Inter. Conference on Southern Hemisphere Meteorology and Oceanography; 2006

la American Meteorological Society

The South American Low Level Jets (SALLJ) are northerly jets which develop along the eastern slopes of the Andes Cordillera and are defined according to wind magnitude and wind speed vertical profile. A jet is characterized when a maximum of wind speed at low level and a wind shear occurs. Criteria established by Bonner (1968) (listed in Section 3) to define LLJ over North America been applied to South America. The SALLJ cause impacts on the vertical and horizontal flows of temperature and humidity and it has been suggested that the development of deep convective systems over northern Argentina, parts of Paraguay and the west of southern Brazil is associated with the SALLJ events (Saulo et al., 2002; Salio et al., 2002). Studies carried out by Wang and Paegle (1996) also suggest that the SALLJ are the main source of moisture for most parts of the La Plata basin. SALLJ displays a large variability on daily, intraseasonal and interannual time scales and plays an important role in the genesis of severe weather systems and organized convective systems (Silva Dias et al., 1987; Velasco and Fritsch, 1987; Kleeman, 1989; Stensrud, 1996; Douglas et al., 1998; Nogués-Paegle et al., 1998; Saulo et al., 2002; Campetella and Vera, 2002). Observational measurements acquired in the eastern slopes of the Andes are rare and most of SALLJ events cases are reported from data assimilated by numerical models (Wang and Paegle, 1996; Nogués-Paegle and Mo,1997). According to Vera et al., (2006) the SALLJ extend over a large area of the South America and radio sondes data available are not enough to give a complete description of their three-dimensional structure or their diurnal cycle. Those deficiencies will not be minimized without a great number of observations which can be used to calibrate the diurnal cycle modelled by simulations. Numerical experiment to simulate LLJ was carried out by Berry and Izunza (1993) who analysed 10 cases of occurrence and 10 cases of non-occurrence identified in data from two stations located in Argentina. A mesoscale model was used to simulate the wind field with high resolution at low levels and results obtained from the assessment of the meridional transport of water vapour have shown that the LLJ is an efficient way of transporting humidity southwards. Another experiment performed by Cavalcanti and Souza (2002) who used NCEP/NCAR reanalysis data from 1979 to 2000 to study the SALLJ east of Andes shows that the highest frequency of SALLJ occurs during summer and winter, while the minimum frequency occurs in autumn. The wind magnitude at about 850 mb indicates the presence of the LLJ east of the Andes, with the highest intensity over the southeast of Bolivia (approximately at 20°S, 60°W of Figure 1). This area is located near the Andean elbow, confirming the important role of the Andes Mountains in defining and modulating the SALLJs. Elevated orography is also found around southeast Brazil, near the Atlantic coast, and in northern Amazon (Guyana Highlands). Although these mountains are much lower than the Andes, they also play a prominent role in creating the SALLJ (Nogues- Paegle and Paegle, 2000) once they contribute to confine the winds in the region where the low level jets occurThese features suggest that the Amazon coast line located in eastern South America might be the region through which part of the atmospheric moisture enters the continent. The penetration of humidity inland is enhanced by the SALLJ stream that flows westwards. After meeting the Andes, the winds are deflected southwards and channelled along the eastern slopes of the Cordillera. Near 20° area is located near the Andean elbow, confirming the important role of the Andes Mountains in defining and modulating the SALLJs. Elevated orography is also found around southeast Brazil, near the Atlantic coast, and in northern Amazon (Guyana Highlands). Although these mountains are much lower than the Andes, they also play a prominent role in creating the SALLJ (Nogues- Paegle and Paegle, 2000) once they contribute to confine the winds in the region where the low level jets occurThese features suggest that the Amazon coast line located in eastern South America might be the region through which part of the atmospheric moisture enters the continent. The penetration of humidity inland is enhanced by the SALLJ stream that flows westwards. After meeting the Andes, the winds are deflected southwards and channelled along the eastern slopes of the Cordillera. Near 20° °S, 60°W of Figure 1). This area is located near the Andean elbow, confirming the important role of the Andes Mountains in defining and modulating the SALLJs. Elevated orography is also found around southeast Brazil, near the Atlantic coast, and in northern Amazon (Guyana Highlands). Although these mountains are much lower than the Andes, they also play a prominent role in creating the SALLJ (Nogues- Paegle and Paegle, 2000) once they contribute to confine the winds in the region where the low level jets occurThese features suggest that the Amazon coast line located in eastern South America might be the region through which part of the atmospheric moisture enters the continent. The penetration of humidity inland is enhanced by the SALLJ stream that flows westwards. After meeting the Andes, the winds are deflected southwards and channelled along the eastern slopes of the Cordillera. Near 20°° S, where the Andes form an “elbow”, the maximum SALLJ’s episodes occur (Salio et al., 2002; Cavalcanti and Souza, 2002). According to Liebman et al. (2003) the maximum rainfall is centred in Southern Amazon, extending Northwest-Southeast oriented towards the SACZ. Between 15°S and 20 °S the northerly winds over the eastern continent are associated with the South Atlantic high, while a separate region of northerly near continent are associated with the South Atlantic high, while a separate region of northerly near 20 °S the northerly winds over the eastern continent are associated with the South Atlantic high, while a separate region of northerly near continent are associated with the South Atlantic high, while a separate region of northerly near °S and 20 °S the northerly winds over the eastern continent are associated with the South Atlantic high, while a separate region of northerly near continent are associated with the South Atlantic high, while a separate region of northerly near °S the northerly winds over the eastern continent are associated with the South Atlantic high, while a separate region of northerly near 961 the Andes are a reflection of the low-level jet system. Liebman et al also show that the region located near the jet exit (area centred at about 25°S, 57°W) present a maximum of rainfall. According to them when the jet is strong there is more than twice of the daily average precipitation just south of this point. The jets are believed to be the main source of moisture for southern South America (Wang and Paegle 1996) and seem to play an important role in rainfall regime over the subtropical South America. Although much attention has been paid to the SALLJ, only a few studies have focused in the impacts on the spatial and temporal rainfall variability over the area located in the jet’s exits (Central-Southern South America). Therefore, the main goal of this assignment is TO investigate the occurrence of three different types of SALLJ and quantify their influence on the observed rainfall at the jet’s exits region. An improvement in the knowledge of the rainfall regime over the area under the influence of the SALLJ is expected According to them when the jet is strong there is more than twice of the daily average precipitation just south of this point. The jets are believed to be the main source of moisture for southern South America (Wang and Paegle 1996) and seem to play an important role in rainfall regime over the subtropical South America. Although much attention has been paid to the SALLJ, only a few studies have focused in the impacts on the spatial and temporal rainfall variability over the area located in the jet’s exits (Central-Southern South America). Therefore, the main goal of this assignment is TO investigate the occurrence of three different types of SALLJ and quantify their influence on the observed rainfall at the jet’s exits region. An improvement in the knowledge of the rainfall regime over the area under the influence of the SALLJ is expected °S, 57°W) present a maximum of rainfall. According to them when the jet is strong there is more than twice of the daily average precipitation just south of this point. The jets are believed to be the main source of moisture for southern South America (Wang and Paegle 1996) and seem to play an important role in rainfall regime over the subtropical South America. Although much attention has been paid to the SALLJ, only a few studies have focused in the impacts on the spatial and temporal rainfall variability over the area located in the jet’s exits (Central-Southern South America). Therefore, the main goal of this assignment is TO investigate the occurrence of three different types of SALLJ and quantify their influence on the observed rainfall at the jet’s exits region. An improvement in the knowledge of the rainfall regime over the area under the influence of the SALLJ is expected