Previous conditions associated to a development of a mesoscale convective system under South American Low-Level Jet event: A case study

PALOMA BORQUE; RICARDO VIDAL; PAOLA SALIO; YANINA GARCIA SKABAR; MATILDE NICOLINI

Foz do Iguaçu, Brazil

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

American Meteorological Society

The objective of the present work is to study the prestorm environment prior to the development of a group of subtropical mesoscale convective systems (MCSs) during a South American Low-Level Jet (SALLJ) event. The evolution of the kinematics and thermodynamic structure of the atmosphere is studied in order to understand the gradual building of optimal conditions for the formation of organized convection during an intensive observation period. The work is centered in the study of a situation that occurs on 5 – 7 February 2003 included in SALLJEX (SALLJ Experiment). This case is characterized by an intense pre-frontal MCS and the presence of SALLJ conditions that penetrates up to subtropical latitudes during the whole period. IR satellite images with high temporal and horizontal resolution are used to detect subtropical convection, and determine the initiation, maximum extent and decay times. The environment associated with the systems is described using operational analyses (GDAS - Global Data Assimilation System) with 1 degree horizontal and 6-hours temporal resolution. SALLJEX data enhance the operational pibal and sounding network generating a unique dataset to study mesoscale processes. The period of study is included in a long intensive observation period, where in some stations soundings were launched every 6 hours, and pibal observation every 3 hours in the whole network. The subtropical area was dominated by a northerly flux that advected Humidity and high temperature values on February 5. The convection developed in the central area of Argentina and it moved towards the northeast of this country. Due to this movement, the northerly flux retreated up to 27S. At the beginning of Feb 6 the northerly flux developed close to the slope of the Andes, and propagated to the east during the following hours, this is observed in GDAS dataset as well as in the wind profiles.  February 6 was characterized by the absence of convection in all the area south of 20S and between just to the east of the Andes to 55W, in spite of showing SALLJ conditions in all the region. A new convective cell developed at 32S-64W at 18 Z, this system grew up to an enormous size that reached 350.000 km2 and it dissipated while moving northward. A convective regeneration was observed north ahead of the dissipation area. New isolated developments grow up mainly in northern Argentina and Uruguay 600 km away from the baroclinic zone. Sounding observations show the building of the environment generated by SALLJ conditions, this is evident by an increase in humidity and temperature values at low levels. Once maximum in specific humidity is attained close to the surface the changes by the low-level jet centers in an increase in the height of the mixing layer with an extreme constant specific humidity value. Santiago del Estero soundings display a deep mixing layer, that evolves from 2000 m to 3500 m (between Feb 6 18Z to Feb 7 6Z) before the passage of MCS.