The Synoptic Evolution of Mcss East of the Andes Mountains in Subtropical South America between Spring and Summer

Subtropical South America (SSA) east of the Andes Mountains is a global hotspot for deep convection, featuring frequent multi-day mesoscale convective system (MCS) outbreaks in the austral spring (SON) and summer (DJF). Typically, convection initiation involves a moisture flux from the Amazon via the South American low-level jet (SALLJ), a capping inversion formed from dry flow subsidence in the lee of the Andes, and a triggering mechanism combining synoptic forcing and terrain-induced lift by the Andes foothills or the Sierras de Córdoba (SDC), a secondary mountain range in northern Argentina. This study focuses on understanding the synoptic environments supporting wide convective cores (WCCs), which are convective echos of extreme horizontal dimension, because they are embedded within mature MCSs, contribute over 90% of SSA’s warm-season rainfall, and invoke hail, flooding, and tornadoes.

While previous work investigates conditions supporting WCCs in DJF, this study uses 16 years of the TRMM Precipitation Radar data to identify WCCs in SSA and composite environmental conditions associated with the largest and smallest WCCs in DJF and SON. Prior to the occurrence of large WCCs in both seasons, an anomalous mid-level trough impinges upon the Andes from the west, inducing a low-level lee trough and a moisture-rich SALLJ into SSA. Convection initiates near the SDC and grows upscale into WCCs in the presence of low-level convergence, as a result of the lee cyclone’s southerly return flow interacting with the northerly SALLJ. Interestingly, SON anomalies are greater in magnitude and further northeastward; this happens because the Andes block more flow in SON, leading to enhanced lee cyclogenesis and anomalies downstream. Small WCCs are associated with substantially less synoptic scale forcing, especially in DJF where topography is likely the primary triggering mechanism. These results are corroborated by a large WCC case observed during the RELAMPAGO field campaign (10-13 November 2018), in which convection grew upscale on three consecutive days. Mobile soundings in tandem with GOES-IR temperature data show that strong mid-level vertical wind shear induced by the juxtaposition of the SALLJ and southerly return flow contributes to the organization of elevated MCSs on three consecutive days. Many studies demonstrate the role of the SALLJ in SSA’s convective initiation, but this is the first study to exemplify the added impact of the lee cyclone’s southerly return flow using observations. This adds critical insight to the understanding of upscale growth and MCS development in SSA and would not have been possible without the extensive international field campaign.