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Extreme Summer Convection in South America

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  • 1 University of Washington, Seattle, Washington, and University of Vienna, Vienna, Austria
  • | 2 University of Washington, Seattle, Washington
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Abstract

Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data are used to indicate mechanisms responsible for extreme summer convection over South America. The three-dimensional reflectivity field is analyzed to define three types of extreme echo, deep convective cores, wide convective cores, and broad stratiform regions. The location and timing of these echoes are sensitive to midlatitude synoptic disturbances crossing the Andes. At the leading edges of these disturbances the nocturnal South American low-level jet (SALLJ) transports moisture along the eastern edge of the Andes from the tropical to the subtropical part of the continent. Where the SALLJ rises over lower but steep mountains on the east side of the southern central Andes, deep and wide convective cores are triggered in the evening. When the SALLJ withdraws to the north as the disturbance passes, nocturnal triggering occurs in the northeastern foothills of the central Andes. Extreme convection over the Amazon basin takes the form of broad stratiform regions that evolve from systems with wide convective cores moving into the center of the region from both the southwest and northeast. The systems from the northeast form at the northeast coast and are likely squall lines. Along the coast of the Brazilian Highlands, diurnal/topographic forcing leads to daytime maxima of deep convective cores followed a few hours later by wide convective cores. Wide convective cores and broad stratiform regions form in the South Atlantic convergence zone (SACZ) with a diurnal cycle related to continental heating.

Corresponding author address: Ulrike Romatschke, Atmospheric Sciences, P.O. Box 351640, University of Washington, Seattle, WA 98195-1640. Email: ulli@atmos.washington.edu

Abstract

Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data are used to indicate mechanisms responsible for extreme summer convection over South America. The three-dimensional reflectivity field is analyzed to define three types of extreme echo, deep convective cores, wide convective cores, and broad stratiform regions. The location and timing of these echoes are sensitive to midlatitude synoptic disturbances crossing the Andes. At the leading edges of these disturbances the nocturnal South American low-level jet (SALLJ) transports moisture along the eastern edge of the Andes from the tropical to the subtropical part of the continent. Where the SALLJ rises over lower but steep mountains on the east side of the southern central Andes, deep and wide convective cores are triggered in the evening. When the SALLJ withdraws to the north as the disturbance passes, nocturnal triggering occurs in the northeastern foothills of the central Andes. Extreme convection over the Amazon basin takes the form of broad stratiform regions that evolve from systems with wide convective cores moving into the center of the region from both the southwest and northeast. The systems from the northeast form at the northeast coast and are likely squall lines. Along the coast of the Brazilian Highlands, diurnal/topographic forcing leads to daytime maxima of deep convective cores followed a few hours later by wide convective cores. Wide convective cores and broad stratiform regions form in the South Atlantic convergence zone (SACZ) with a diurnal cycle related to continental heating.

Corresponding author address: Ulrike Romatschke, Atmospheric Sciences, P.O. Box 351640, University of Washington, Seattle, WA 98195-1640. Email: ulli@atmos.washington.edu

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