The Structure and Behaviour of Supercell Storms in the State of São Paulo, Brazil
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ERAD 2010 - THE SIXTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY The structure and behaviour of supercell storms in the State of São Paulo, Brazil G. Held1, A. M. Gomes1, K. P. Naccarato2 1Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista, Bauru, S.P. 2 Grupo de Eletricidade Atmosférica, Instituto de Pesquisas Espaciais, São José dos Campos, S.P. Gerhard Held 1. Introduction In the State of São Paulo, located in the south-east of Brazil, severe convective storms frequently cause local flooding in towns and/or devastation of crops and property due to hail or wind storms (microbursts, tornadoes), resulting in many millions of US Dollar damage annually, loss of lives and leave many persons injured (Gomes et al., 2000; Held et al., 2001; Held and Nachtigall, 2002). The Bauru-based Meteorological Research Institute (IPMet) has been observing and analyzing the three-dimensional structure of thunderstorm and rain events in the central and western region of the State of São Paulo, using S-band Doppler radars, since 1992. The first supercell storm recorded by the Bauru radar occurred on 14 May 1994 (Gomes et al., 2000). However, no similar event was observed during the following years and therefore it was believed, that supercell and especially tornadic storms were rather rare events in Southeast Brazil, since very few of those reported had been observed within radar range. However, this changed when the first severe tornadoes, most of them associated with supercells, were observed by IPMet´s radars in May 2004 (F1 and F2, according to the Fujita scale), followed by an F3 in May 2005 (Held et al., 2005, 2006a, 2006b). These latter events also prompted the analysis of lightning flashes relative to the storm areas, from 2004 onwards, and in collaboration with the Lightning Research Group (ELAT) of the Institute for Space Research (INPE). 2. Data and Methods IPMet’s radars are located in the central and western State of São Paulo, viz. in Bauru and Presidente Prudente, which is 240 km west of it (275 azimuth; Figure 1). Both have a 2° beam width and a range of 450 km for surveillance (0 or 0,3 PPI every 30 or 15 min), covering the entire State of São Paulo, but when operated in volume-scan mode every 15 or 7,5 minutes it is limited to 240 km, with a resolution of 1 km radially (250 m since March 2006) and 1° in azimuth, recording reflectivities, spectral width and radial velocities. The Brazilian Lightning Detection Network (BrasilDat) currently comprises 47 sensors in total (most are outside the area shown in Figure 1), with a detection efficiency of 80-90 % (CG = Cloud-Ground strokes only) and a location accuracy of 0,5 - 2,0 km (Pinto Jr., 2003). Reliable lightning observations are only available from 1999 onwards. The analysis of the various cases was performed with Sigmet/IRIS/Analysis, the propriety Software of IPMet’s radars, and NCAR’s (National Center for Atmospheric Research) TITAN (Thunderstorm Identification, Tracking, Analysis and Nowcasting; Dixon and Wiener, 1993) Software, which had been implemented at IPMet and adapted for local requirements in 2005/2006. TITAN produces a variety of important parameters for a chosen reflectivity and volume threshold throughout the lifetime of storms, such as Area, Volume, Precipitation Flux, VIL (Vertically Integrated Liquid water content), Maximum Reflectivity, Hail Metrics, speed and direction of propagation, etc, per volume scan, as well as cell tracking, including splits and mergers of cells. It also has the facility to superimpose flashes with the radar echoes, including a separation into positive and negative strokes. For this analysis TITAN was running with a resolution of 1 km in the horizontal and 750 m in the vertical. A reflectivity threshold of 40 dBZ with a volume of >16 km3 was chosen for tracking the cells. 3. Case Studies In this paper, all known cases of supercell occurrences will be discussed, but this does not imply that there are not more cases, which have not yet been detected in the vast amount of 18 years of Doppler radar observations. The days which have been studied so far are: 14 May 1994 (one supercell lasting >5 hours), 17 October 1999 (hail swath extending for 200 km during 4 hours), 21/22 May 2004 (one supercell lasting >5,5 hours), 25 May 2004 (two supercells moving on parallel tracks for >8,5 hours and >5 hours, respectively, but only the shorter one produced an ERAD 2010 - THE SIXTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY F2 tornado), 24 May 2005 (two supercells moving on parallel tracks for >3 hours and almost 4 hours, respectively, but only one produced an F3 tornado) and 24 July 2007 (hail swath extending for about 300 km during 5 hours). It is noteworthy, that all, but one, of these cases occurred during the month of May, which is the austral autumn, and thus a transition period of the atmosphere from summer conditions, with predominantly moist tropical air being advected from Amazônia, to the winter regime, when occasional baroclinic systems (cold fronts), coming from south-west, affect the western and central interior of the State. A similar seasonality was observed for the occurrence of confirmed tornados, which have a maximum frequency in autumn, followed by a secondary spring maximum (Held et al., 2006a). All times in the case studies are in Local Time (LT = UT-3h), unless otherwise stated (TITAN uses UT). FIG. 1. IPMet’s Radar Network (BRU = Bauru; PPR = Presidente Prudente), showing 240 and 450 km range rings. Lightning sensors are marked as red stars. 3.1 14 May 1994 A frontal system, extending from an Atlantic Ocean Low across the southern States of Brazil, and an associated cut-off low over the State of São Paulo induced strong convergence at the surface, which triggered and maintained a supercell storm with radar reflectivities exceeding 60 dBZ for five hours along its atypical southward path. It caused extensive damage to property (US $ 11 million) and life, killing three people and injuring 130 along its path of destruction. Numerical meso-scale modeling of the vorticity budget for this day indicated that the divergence term was dominating the low-level vorticity tendency and that the horizontal advection of vorticity was working against the cyclonic vortex evolution (Silva Dias et al., 1996). During the early afternoon on this day, radar observations already indicated the development of isolated precipitation cells in the central and far north-eastern area of the State of São Paulo. The convective activity intensified from 15:00 onwards with the majority of storms developing during the late afternoon and early evening. Maximum reflectivities exceeded 55 dBZ and echo tops were on average above 8 km. In contrast to the supercell, they generally moved in south-easterly directions with average speeds of 50 km.h-1, producing severe hail falls and strong surface winds along their paths. Significant damage to houses and people, as well as great damage to coffee plantations in the central and southern parts of the State, was reported as a result of these storms. Several of these severe storms have been described in detail by Gomes et al. (2000), but the emphasis in this discussion will be on the supercell storm only. Radar surveillance indicated very strong convective activity over southern Minas Gerais at 16:30, moving southwards along an unusual north - south trajectory at an average speed of 62 km.h–1 (Gomes et al., 2000). At least one cell of this long-lasting storm complex could be classified as a supercell storm, because radar reflectivities exceeding 60 dBZ were recorded for five hours. Figure 2a shows the tracks of all cells for a 24-hour period, starting 09:00 on 14 May 1994, the majority being long-lived, but not in the same category as this superell, visible on the far right. When the reflectivities of this particular cell were analyzed in conjunction with the radial velocity fields, rotation signatures that could be related to the observed severity were identifiable. The supercell storm was first detected at 5,5 km (48 dBZ) at 21:47. For most of its five-hour life cycle the maximum reflectivity exceeded 60 dBZ and echo tops varied between 10 and 13 km. Figures 2b and 2c show the reflectivities and radial velocities of this storm at 23:02, respectively, clearly indicating an anticyclonic rotation, coincidental with the echo core (“couplet”). Considering that the storm was more than 150 km north-east of the radar, the rotation was evident between 0.3° and 1.7° elevation, corresponding to 4–6 km height, but most prominent at 4 km. It should be noted that in the south-eastern part of the rotation core radial velocities of up to +10 m.s-1 (outwards) were observed, while on the opposite side the radial velocities varied between –9 and –14 m.s-1 (towards ERAD 2010 - THE SIXTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY the radar). The centers of velocity maxima were about 6 km apart (Figure 2c), thus implying a local shear of –3,5x10- 3s-1. Such values are generally associated with meso-cyclones or meso-anticyclones and are precursors for the formation of tornadoes (Nielsen-Gammon et al., 1995). a) b) c) FIG. 2. 14 May 1994: (a) Cell tracks during the 24-hour period from 09:00 to 15 May 1994, 09:00; (b) Reflectivity of the supercell at 23:02 and (c) Corresponding radial velocity field. 3.2 17 October 1999 A similar frontal system as on 14 May 1994, with a surface trough extending over the south-south-eastern States of Brazil, resulted in a line of precipitation over the State of São Paulo.