Impacts of an Explosive Cyclone over Southern Brazil from Two Detection Methods

Explosive cyclones are weather events with high destructive power, mainly near coastal areas. In this work, we intend to analyze one explosive cyclone episode on the east coast of the Southern Region of Brazil, which took place between June 30th and July 1st, 2020, under the perspective of two detection schemes.

In the south-central region of South America, weather conditions are heavily influenced by weather systems. Particularly, extratropical cyclones and their associated fronts are the meteorological systems that most impact the region because they are in mid and high latitudes.

The rapid intensification of extratropical cyclones has been the subject of meteorological research in recent decades. This is due to the damage they cause and the ability of the models to predict this type of phenomenon. Explosive cyclones, commonly referred to as “bombs,” are defined as negative relative vorticity systems in the Southern Hemisphere in which the central pressure drops at an average rate of at least 1 Bergeron.

The outputs from both the Climate Forecast System Reanalysis (CFSR) and a simulation with the MPAS-A (Model for Prediction Across Scales - Atmosphere) numerical simulation model are used to understand the explosive cyclone’s behavior. For that, this work uses two detection methods: the Observational Method (OBSM) and the Automatic Method (AUTM). The first uses the visual analysis of the Mean Sea Level Pressure (MSLP) fields combined with functions to identify the local minimums using the Grid Analysis and Display System (GrADS) software. The second uses a function called “mfhilo” from the OpenGrADS software, which shows the minimum MSLP in the grid using Laplacian, Magnitude, and Percentile. The MSLP minimums are analyzed to track the cyclone's path. Then, the Normalized Deepening Rate is computed to determine if the cyclone is explosive.

Meteorological analysis shows that this explosive cyclone was associated with a squall line. This system crossed the Rio Grande do Sul and Santa Catarina states, significantly damaging property and people. The dominant synoptic conditions were: i) lower-troposphere wind convergence in the cyclone region; ii) unstable atmosphere; iii) hot (cold) advection at the front (rear) of the system; and iv) heat and moisture transport from the adjacent ocean and the low-level jet. The southern regions of Brazil suffered significant impacts resulting from the combination of these meteorological conditions.

Based on the findings, the OBSM methodology successfully detected the cyclone at 15Z30JUN2020, as it rapidly intensified over southern Brazil, transitioning from a continental low near northern Argentina. The explosive phase of the cyclone ended over the Southwest Atlantic Ocean. It reached 2.11 Bergeron and is recorded as one of the deadliest weather events in Southern Brazil. On the other hand, AUTM did not detect this case as the MSLP minimums were only identified during the mature stage, on 06Z01JUL2020. It is worth mentioning that OBSM and AUTM used distinct spatial and temporal resolutions - OBSM utilized ERA5 with a 3-hour temporal resolution, while AUTM relied on CFSv2 and MPAS with a 6-hour temporal resolution.

It is important to emphasize that the direct comparison between the methods should not be considered since they use different datasets with different attributes. However, this set of visually obtained systems serves as “truth” for validating the automatic method. Each method has its advantages; therefore, it is up to the user to choose the best way to analyze the cases. Although AUTM did not detect this case, there are a lot of advantages to use this method for other cases. The automatic method, after updates, emerges as a tool that can assist in decision-making and can even be used operationally and as a method to identify the transition of transient and semi-permanent anticyclones.