Severe Local Storms in Southern Brazilian Non-Line Convective Systems

Evolution of 22 radar observed strong Non-line Mesoconvective Systems (NMS) with refletivities Z >55 dBZ occurred in Pelotas region of Southern Brazil during summer of 1993-1997 is studied. In contrast to systems like frontal bands or squall lines occurred in strong baroclynity conditions, these NMS occur in conditions of weak baroclinity as complexly evolving arrangement of uni-, multi- and super-cellular thunderstorms that move with passive translation velocity (PTV) mainly less than 10 m/s, intensify between 15 and 18 h and dissipate at 18 - 20 h. Moving with velocity about of 30 km/h, the typical severe storm lasts more than 1,5 hour extending vertically at 14 km and horizontally at 30 km with overhang of 6 km. The 60% (30%) of thunderstorms were left (right)- moving and deviated from PTV direction for angle of 40 - 500 due to propagation of 20 km/h. The daily averaged velocity of 178 storms that has been observed is a linear function of PTV in spite of velocities of severe storms has been influenced by PTV just slowly. All of 44 strongest storms have been initiated on the land with eventual penetration and rapid dissipation above sea/lake surface. Some preference to storm initiation on the hills slope with continuos descent to three principal regional valleys was observed.

The composite image of NLMS evolution was derived by accumulation of strong echoes with reflectivity > 40 dBZ during it´s life time in coordinate system moving with passive translation velocity. In contrast to “chaotic” distribution of intense echoes on instant radar images, their time-integrated distribution is self-organized in three hierarquially subordinated mesoscale ensembles. The small linear and circular meso -b ensembles with dimension of 28-33 km form one large-b ensemble ~100 km and 4-5 hours in duration. The b ensembles mainly of linear form were observed. From one to four large b-ensembles constitute one small meso-a cluster of 350 km in extension that is active during 7-8 hours. The a-cluster intensity oscillates with periods of 1 and 3 hours due to cyclical appearance of new small and large ensembles.