In the afternoon of 27 October 2017, a severe thunderstorm was responsible for flash flooding and wind damage in Mogi Mirim, Itapira and Serra Negra (https://g1.globo.com/sp/campinas-regiao/noticia/chuva-provoca-queda-de-arvores-em-itapira-mogi-mirim-e-serra-negra.ghtml ; https://g1.globo.com/sp/campinas-regiao/noticia/apos-temporal-cerca-de-13-mil-moradores-continuam-sem-energia-eletrica-em-itapira-sp.ghtml).Since this storm occurred about 50 km from the SOS-Chuva X-band polarimetric radar, this event was a good case to analyze.
The synoptic-scale environment at 1800 UTC 27 October 2017 was characterized by a low-pressure system centered near the coast of southern Brazil, with the associated cold front moving north over the Paraná state (Fig. 1a). This setup caused strong 850-hPa west-northwesterly winds (20-30 kt) over MS and SP. At 500 hPa (Fig. 1b), the shortwave trough axis was located over Paraguay and favored synoptic-scale ascent over PR, southern SP and MS.
Figure 1: (a) 850-hPa and (b) 500-hPa synoptic charts at 1200 UTC 27 October 2017 (CPTEC/INPE).
The soundings launched at São Paulo-SP and Campo Grande-MS are shown below. The strong flow in the lower levels causes the storm-relative helicity to be high (< 200 m²/s² in the 0-3-km layer), which indicated a wind profile suitable for mesocyclone formation. Also, the temperature profiles had steep lapse rates, leading to CAPE greater than 2000 J/kg in most of the region before convection began.
Figure 2: São Paulo and Campo Grande 1200 UTC 27 October 2017 soundings.
The most intense thunderstorm formed in the afternoon of 27th is shown below. The storm presented very high reflectivity (~ 60 dBZ) over the Mogi Mirim, Mogi Guaçu and Itapira region, with a northwest-southeast orientation at 2130 UTC. Give the strong background wind shear in the environment, this storm was likely organized.
Figure 3: X-band radar reflectivity (dBZ) at 2120 UTC 27 October 2017.
The Dopper radial velocities confirms that this storm was in fact organized. Rotation is observed in association with the storm, indicating the presence of a mesocyclone. Th radial speeds were not very strong (wind dipole of ~10 m/s), but seemed to be sufficient to organize the storm and cause severe weather.
Figure 4: Doppler radial velocities (m/s) at 2125 UTC 27 October 2017.
The high amount of precipitation within the storm is evident in the ZDR cross section (Figure 5). High values were observed in the lower part of the storm, which were likely associated with the flash flooding observed in the region.
Figure 5: RHI of the ZDR along the storm at 2120 UTC 27 October 2017.
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