Mauricio Oliveira-April 20

A New Look at the Objective Identification of Low-level Jets in South America: Issues and Proposed Modifications

Low-level Jets (LLJs) play a major role in building up the atmospheric conditions conducive to severe convective storms in subtropical South America since they significantly enhance low-level warm and moisture advection as well as vertical wind shear.

In both climatologies and case studies, objective criteria currently employed to identify LLJs in South America utilizing rawinsonde and gridded model data fail to detect a significant number of situations when LLJs are in place. Issues in such approaches are addressed and simple modifications to the objective criteria regarding layer depth for LLJ detection, wind direction and the use of fixed pressure/height levels are investigated. A sensitivity analysis of LLJ detection based on the choice of the objective criteria applied to rawinsonde and reanalysis data for the La Plata Basin reveals the advantages of modifying the existing criteria. The results indicate that the modified criteria are able to identify a broader spectrum of northerly LLJs over highly-active convective regions of South America. Furthermore, another advantage of the modified criteria is that it retains the identification of events that meet the well-established definition of the South American LLJ.

In addition to draw attention to issues in existing LLJ identification criteria, this study aims to be a first step in providing an expanded depiction of LLJ occurrence in South America.