Intense thunderstorms in Argentina

By Klaus Sievers (Vereinigung Cockpit (German ALPA), Frankfurt), Jochen Kerkmann (EUMETSAT), Martin Setvak (CHMI) and Christian Garavaglia (Servizio Meteorologico Nacional, Argentina)

According to livescience.com, a global satellite survey of thunderstorm activity (based on TRMM data) has helped meteorologists pinpoint exactly where Earth's hotspots for intense thunderstorms are located: the American Midwest, Argentina , and some semi-arid regions such as the edges of the Sahara desert. According to the lead author Edward Zipser, from the University of Utah, the study, which appeared in the August 2006 issue of the Bulletin of the American Meteorological Society, was the first quantitative look at thunderstorms around the world (see Zipser et al., 2006).

According to this study, the strongest storms the satellite observed were in areas east of the Rocky Mountains in the United States and east of the Andes Mountains in Argentina, where geography plays a very important part in storm formation. In these regions, the mountains help to lift warm, moist air (from the Gulf of Mexico in the US, for example), which collides with cooler, drier air, promoting the formation of powerful storms. A collision of different types of air masses also forms intense storms in semi-arid regions, like the Sahel region.

At the edge of the Sahara, for example, very hot, dry air meets warm, moist air and produces lifting that forms intense storms. When the same phenomenon happens in the American Midwest or in Argentina, it can lead to another type of storm. These regions are essentially the tornado alleys of the World. Though places like the Amazon and parts of southeast Asia see considerable rainfall, they have few intense thunderstorms because the warm, moist air that covers those regions has no cooler, drier air to mix with.

The case below shows an example of the development of two intense thunderstorms in the border region of Argentina/Brazil, which were simultaneously observed from GOES-13 and Meteosat-9. The first storm developed at around 07:30 UTC (i.e. in the night) in the Misiones region of northern Argentina, the second one, one hour later more to the west, in the western part of the Corrientes region.

The IR imagery (see animation of Met-9 IR10.8 images, 07:00–12:00 UTC) reveals cloud top temperatures of well below -65°C for both storms. It also shows the explosive growth of the storms and the re-building of new convective cells on the western edge (upstream) of the storms (backbuilding storms). As discussed above, the collision of different types of air masses must have played a role in this case. This can be seen in the Meteosat-9 Dust RGB product of 9:00 UTC (source: EUMeTrain), where dry air appears with a more reddish colour than moist air*.

Surface and lightning observations in southern Brazil confirm the strong thunderstorm activity on 27 October 2012. Intense wind gusts were recorded at the airport of Chapeco in western Santa Caterina (gust of 104km/h). In Rio Grande do Sul, INMET recorded 26. mm of rain in São Borja between 12h and 13h local time. In the late afternoon, all areas of southern Brazil were subject to thunderstorms with heavy rain, lightning, wind gusts and even risk of hail damage (see web report). On the Argentinian side, no significant severe weather reports could be found (just normal thunderstorm activity). However, the 12:00 UTC sounding of Resistencia (source: University of Wyoming), capital of the Chaco province, shows the instability with a northerly low-level jet, which is typical for such situations.

Interestingly, a series of photos of the first (more to the east) storm was taken by Captain Sievers, a commercial airline pilot, enroute from Europe to Buenos Aires (see image below and slide show (Credit: Klaus Sievers www.vcockpit.de )).

Captain Sievers comments: "When flying southbound over Brazil, my crew and I observed a very beautiful sunrise to the east. Before the Sun could come up over the horizon, our attention was drawn to a very special looking cloud far ahead (see Figure 2 and corresponding slide show). It looked like a growing cumolunimbus (CB), and it´s anvil indicated the influence of winds from the west. The radar image (see Figure 3) confirmed the visual impression of a strong, active CB, as it showed a 40nm wide area of moderate (yellow) and strong (red) precipitation. This was clearly seen from more than 150nm, and the best strategy was to avoid the cloud as well as the turbulence created by the winds blowing around it.

"A track-change for a significant, early diversion from the planned route was requested from air traffic control. This happened where Brazil, Argentina and Paraguay are close together, near the Iguacu waterfalls. It required coordination with the air traffic control systems of these states. The approval for route-change was obtained in time, and so we could fly around the cloud. Coming closer, it was impressive to see the outflow from the top of the large cloud. It was much, much higher than our cruising level of 36,000ft. Aesthetically, it was a joy to see the development of the flower-like outgrowth near the end of our passage by the cloud. Lighting-conditions were superb, as the Sun was well hidden behind the large CB-cloud. The route of flight was free of any turbulence."

According to cloud top experts Martin Setvak (CHMI, Czech Republic) and Prof. Pao Wang (Univ. of Wisconsin, USA), there is no doubt that some of the images from Captain Sievers show so-called jumping cirrus (see slides 9, 11, 12, 13, 14 and 15), while some of the cirrus features rather resemble pileus clouds (see slides 16 to 21). The name 'jumping cirrus' comes from a phenomenon observed by Fujita in the 1980s who reported that "one of the most striking features seen repeatedly above the anvil top is the formation of cirrus cloud which jumps upward from behind the overshooting dome as it collapses violently into the anvil cloud". Some information on jumping cirrus and their simulation with models is presented in this powerpoint by Prof. Pao Wang.

*Note: to learn more about the colours of the Dust RGB product, see this case study and related links.

Corresponding Met-9 IR10.8 Image (8:15 UTC)
GOES-13 IR image 11:45 UTC (source: SMN-Argentina)
Animation Met-9 IR10.8 channel (07:00–12:00 UTC, animated )

Additional content

Met-9 Ice Particle Size (Convection) RGB, 12:00 UTC
Severe convective storms over Argentina in November 2009
Comparison of EUMETSAT MPE products and aircraft weather radar
Tropical cyclone in the Southern Atlantic close to the coast of Brazil (27 March 2004)