Mechanisms associated with post-sunset convective initiation close to mountain ranges in Central Argentina

PAOLA SALIO; LUCIANO VIDAL; EDWARD ZIPSER; DANIEL VILA

San Francisco

Conferencia; AGU Fall Meeting; 2016

AGU

Many thunderstorms in mid-latitude South America stand out in satellite observations as being among the strongest anywhere on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, prevalence for extremely tall, wide convective cores and broad stratiform regions. Knowing when and where strong convection is initiated is of great importance because of the close relationship between convective storms, heavy precipitation, and severe weather phenomena.This paper seeks to identify the most salient features associated with mesoscale and synoptic scale that condition the environment in which deep convection develops in mid-latitude South America region close to principal mountain barriers (e.g. the Andes). A specific region of complex terrain in Central Argentina, known as Sierras de Córdoba (SCBA), tend to show active initiation near the end of the afternoon (between 21Z and 00Z) during the warm season. These may become very long-lived systems with lifetimes that can exceed 24 hours and even in extreme cases can reach 60 hours. They have associated cloud shields, with large horizontal extent at the time of maturity (>250,000km2), more than twice the average size of MCSs born outside this particular region. These events are strongly favored by the presence of a mid-level trough with a surface cold front and a westerly jet stream aloft, which along with an intense low-level jet (SALLJ) and local circulations, combine to pre-condition the environment and develop deep moist convection in the region of interest in the afternoon on the east and west side of the topography. MCSs tend to detach from the topography and develop in the valley between the Andes and SCBA, or east away from SCBA, where convergence reaches a maximum in low levels as a consequence of the zonal shift of SALLJ. While the circulation associated with the SALLJ plays an important role in strengthening convergences on the mountain and provides moisture and heat from low latitudes, mountain-valley breeze circulations and the presence of a semi-stationary frontal zone provides the necessary convergence. The frontal system generates subsidence in the lee side of the Andes, thus inhibits convection close to the range and provide the vertical wind shear to spread MCSs towards the east northeast away from SCBA.