Mesoscale Convective Systems over Southeastern South America and their relationship with the South American Low Level Jet

PAOLA SALIO; MATILDE NICOLINI; EDWARD ZIPSER

Monthly Weather Review

American Meteorological Society

Lugar: Boston; Año: 2006 p. 1 - 17

1520-0493

Prior studies have shown that the low level jet is a recurrent characteristic of the environment during the initiation and maturity stages of mesoscale convective systems (MCSs) in 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-year MCS sample in South America, south of the Equator, and its related geographical distribution of convection frequency. The other is to advance into the knowledge of the evolution of favorable environmental conditions for the development of large MCSs, specifically those which mature under SALLJ conditions. High horizontal and temporal resolution satellite images are used to detect MCSs in the area, for the period September 1, 2000 to May 31, 2003. Operational one degree 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 per cent of the SALLJ days, whereas in the days with no SALLJ conditions this percentage dropped to 12 per cent. 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 maturity stage. The large-scale environment associated with large long lived MCSs during SALLJ events over SESA evolves under thermodynamic and dynamic forcings 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.