Squall line transformation from asymmetric-type to symmetric-type

In this study we analyze a Mesoscale Convective System (MCS) which was presented as the evolution of three squall lines. This MCS occurred over the central part of the State of São Paulo (Brazil) on 05 May 1993. The analysis was performed using a relative coordinate system which moves with the passive translation velocity (PTV) of this MCS. The use of this coordinate system permit the elimination of the MCS’s passive translation making possible the observation of its pure evolution. The detailed analysis of one Squall Line (SL) transformation from asymmetric-type to symmetric-type is presented. The radar echo accumulation of this SL with reflectivity values Z>40 dBZ during 6 hours showed that the spatial distribution of the convective areas was discrete with a tendency to be concentrated in parallel bands 25 km far from each other. The appearance of a new band occurred every hour when the previous one had reach the mature stage. The echo accumulation of the convective clouds in relative coordinate system for a 3 hours period, showed that these echoes occupied a triangular area. The superposition of the instantaneous image of the SL at the end of the accumulation interval with the obtained accumulative image showed an impressive coincidence of the areas, which have completely different natures. This means that the Stratiform Region (SR) of this SL occupied the area where few hours before the deep convection took place. In other words, the SR was the result of the past convective activity of this SL. To explain the triangular pattern of an asymmetric-type SL a conceptual model is proposed. The triangular pattern is associated with the relation between the Along-line Evolution Velocity (AEV) and the Cross-line Evolution Velocity (CEV) of the SL (AEV and CEV were obtained in the relative coordinate system). If the AEV is greater than the CEV, it will result the development of a symmetric-type SL, otherwise a triangular-pattern will be observed, which corresponds to the asymmetric type. In order to verify this model the 9 asymmetric-type SLs, which occurred over the States of Rio Grande do Sul and Paraná (Brazil), were analyzed. The observed angles of the triangular region varied from 21 to 50. The difference between the calculated angles (obtained in according to this model) of the region with triangular pattern and the observed angles was small. The triangular pattern was conserved during a period varying from 2 to 3 hours. The CEV showed values changing from 20 to 40 km/h while the AEV showed twice these values.