Tuesday, December 19, 2017

Case study: 18 December 2017

     In the afternoon of 18 December 2017, a severe thunderstorm caused flash flooding, hail and high winds in Taubaté-SP (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/chuva-em-taubate-alaga-ruas-derruba-arvores-e-deixa-familias-desalojadas.ghtml).
    The 1800 UTC 18 December 2017 250-hPa fields depict an anticyclonic circulation over Paraguay and the subtropical jet over central Argentina. A trough is located over the Atlantic Ocean at nearly 30°S, too far from SP state to cause lifting over the region where the storm occurred. The SP state locates in a ridge environment. At surface, a stationary front locates over southern Brazil, and a trough is observed over SP state extending towards the Atlantic Ocean. This trough is associated with low-level convergence and lifting.


     Figure 1: (a) 250-hPa and (b) 850-hPa synoptic charts at 1800 UTC 18 December 2017 (CPTEC/INPE).

     The São Paulo-SP sounding at 1200 UTC shows an unstable lower troposphere, with a deep moist layer. At this time, surface heating was ongoing due to clear skies, and CAPE was increasing. The lack of high winds below 400 hPa also indcate that the thermodynamic instability was the main source of energy of the thunderstorms in the region. Also, the quiescent winds favored slow storm motion, which is an important factor in flash flood events.

    Figure 2: São Paulo 1200 UTC 18 December 2017 sounding.

     The thunderstorms formed over the Mantiqueira mountains and moved over the Paraiba Valley, where Taubaté is located. The figure below shows the high reflectivity associated with the storms over Taubaté at 1840 UTC. The storms were predominantly stationary.

      Figure 3: São Roque radar reflectivity (dBZ) at 1840 UTC 18 December 2017.

Thursday, December 7, 2017

Case study: 27 October 2017

     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.

Friday, June 2, 2017

Case sudy: May 19th, 2017

In the São Paulo state, atmospheric instability favored the formation of storms. In Dracena, near Presidente Prudente, strong hail damaged cars and house roofs and leave the municipality in an emergency state with 40 families homeless after the event; wind gusts were also reported (news here , here, here, here and here).

Lightning strike as detected by Rindat network in the region can be seen below:

The following images show the CAPPI 3km from  Radars Presidente Prudente and Bauru (source CPTEC):

Satellite images from GOES can be seen below:

GOES visible channel images (source CPTEC):

GOES IR (highlited) channel images (source CPTEC):

GOES WV channel images (source CPTEC):

Preliminary, non-operational IR (highlited) satellite images from GOES 16 near the time of the event can be seen below (source CPTEC):