Sistemas convectivos de mesoescala no sul da América do Sul: propagação em série contra o escoamento em baixos níveis

Abstract

Serial mesoscale convective system (MCS) events with lifetimes over 18 h and up to nearly 70 h are routinely observed over southeastern South America from infrared satellite imagery during the spring and summer. These events begin over the southern La Plata River basin, with individual convective systems generally moving eastward with the cloud-layer mean wind. However, an important and common subset of these serial MCS events shows individual MCSs moving to the east or southeast, yet the region of convective development as a whole shifts upstream to the north or northwest. Analyses of the composite mean environments from 10 of these upstream-propagating serial MCS events using NCEP/NCAR reanalysis data events indicates that the synoptic conditions resemble those found in mesoscale convective complex environments over the United States. The serial MCS events form within an environment of strong low-level warm advection and strong moisture advection between the surface and 700 hPa from the Amazonas region southward. One feature that appears to particularly influence the lowlevel flow pattern at early times is a strong surface anticyclone located just off the coast of Brazil. At upper-levels, the MCSs develop on the anticyclonic side of the entrance region to an upperlevel jet. Mean soundings show that the atmosphere is moist from the surface to near 500 hPa, with values of convective available potential energy above 1200 J kg-1 at the time of system initiation. System dissipation and continued upstream propagation to the north and northwest occurs in tandem with a surface high pressure system that crosses the Andes Mountains from the west. The 72 h simulated idealized numerical experiment reproduces the main conditions synoptic patterns in a upstream propagating mesoscale convective system, and generates a unique 18 h Upstream propagating MCS, which produces a significant precipitation event with more than 80mm/18h in some regions, it represents about 50% of the total monthly summer precipitation.A transient anticyclone drives the large scale low level circulaton at the early times, it produces moisture an war advection from Amazonia southward. A low thermal pressure system close to 22oS is observed and its interaction between with the south Atlantic anticyclone circulaton produces a moisture corridor transport promoted into a northerly-enhanced Low Level Jet. The numerical experiment evidences the importance of local storms to the development and triggering in the mesoscale convective environments, since it forms the first convective cluster what becomes organized and grow in size. After the MCS becomes mesoscale organized, its outflows dominates the convective environment interacting with the gravity waves pre-existing region, but the gravity waves propagation upstream still exerting a important hole since it generate new convective areas ahead of the outflow boundary. During the MCS life two main characteristics can be observed: Gravity waves propagate out ahead of the line and may initiate new convective development upstream of the existing convection. Once this new convection develops downdrafts, the density-current mechanism again dominates its evolution.