Differences between SALLJ and No SALLJ events and its impact on the development of mesoscale convective systems over subtropical South America.

PAOLA SALIO; MATILDE NICOLINI; EDWARD ZIPSER

Foz de Iguazu - Brasil

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

American Meteorological Society

Abstract   Prior studies have shown that a low-level jet is a recurrent characteristic of the environment during the initiation and maturing of mesoscale convective systems (MCS) in the Great Plains of the United States. The South American low-level jet (SALLJ) has an analogous role, advecting heat and moisture from the Amazon basin southward into the central plains of  southeastern South America (SESA), generating ideal environmental conditions for convection initiation and growth into MCSs.   High frequency of MCSs is evident in the subtropical area under conditions SALLJ, but during situations without SALLJ the presence of organized convection is still significant in this region.   The purpose of this research is to describe the environment associated with the subtropical MCSs over SESA and their strong relationship to the SALLJ. The differences between the environment associated to MCS during days without SALLJ are studied in order to advance in the knowledge of the conditions that precede the MCS development.   IR satellite images with high temporal and horizontal resolution are used to detect large subtropical MCSs during the period 2000-2003. A definition of MCS similar to that used by Cotton et al. (1989) and by Nicolini et al. (2002) is applied in order to study deep and organized convection in a sample of systems with features similar to those studied by other authors in different parts of the world. The initiation stage of the systems is defined when the area enclosed by the -55 ºC (218 ºK) isotherm exceeds 50,000 km² (3,125 pixels). The mature stage is attained when the above area reaches its maximum extent, whereas the dissipation stage is defined when the enclosed area again crosses the 50,000 km² threshold.   The environment associated with the systems is described using operational analyses (GDAS - Global Data Assimilation System) with high spatial and 6-hours temporal resolution. SALLJ and No SALLJ days are detected using GDAS, a criteria similar to Bonner (1968) is used to identify this events. .   During the 3-year period 645 MCSs were detected satisfying the MCS criteria. These systems are distributed during the seasons studied as follows: 286 in summer, 202 in spring and 157 in the fall. Systems which achieve their maximum extent north of 23ºS were considered as tropical, and the remainder as subtropical. Subtropical systems represent 33.2 per cent of the sample, the tropical systems 66.8 per cent.   During spring, summer and fall at least one subtropical MCS developed in 41 per cent of the SALLJ days, whereas in the days without SALLJ this percentage dropped to 12 per cent.   The environment associated with the large subtropical MCSs exhibits a strong SALLJ during the previous day, and persisting during the whole life cycle of the system. The dissipation stage of the MCS is influenced by the advance of a baroclinic zone that moves towards the north.   The percentage of subtropical MCS (35%) that develop under No SALLJ conditions present smaller size and shorter life spam, and the position at the maximum extent time is closer to the Andes.   Corresponding First author address: Paola Salio: Centro de Investigaciones del Mar y la Atmósfera. Ciudad Universitaria, Pabellón II, 2do piso. (1428) Buenos Aires, Argentina. e-mail: salio@cima.fcen.uba.ar