1. Severe convection in Brazil Nikolai Dotzek, Hartmut Höller (DLR-IPA)
Gerhard Held, and
Ana Maria Gomes
Institut für
Physik der
Atmosphäre
Motivation
IPMET research regions Bauru + Rondônia
The GOES8 band 4 enhanced image of deep convective clouds from the INCA campaign (17 Mar 2000) confirms that the Amazon basin is a region of widespread thunderstorm activity. To determine which fraction of the storms might become severe is important for TROCCINOX:
1) Severe storms often have significantly altered lightning activity compared to ordinary thunderstorms. Elevated plumes of small ice crystals are preferably being emanated from severe thunderstorms.
2) How representative of a “typical” South American storm are the CBs and their outflows observed during TROCCINOX?
3) Can severe storm phenomena occur near the campaign airport which could limit or endanger the TROCCINOX flight operations?
Topographic map of Brazil with LBA area (blue) in Rondônia in the Amazon region, and Bauru in the State of Sao Paulo (red). Note the more complex terran in southeast Brazil.
The two IPMET S-band Doppler radars at Bauru and Presidente Prudente (red circles show 240 km operational radar range) can give volume scans of storms up to every 7.5 min
Flash floods and Hail
Flood-producing storms have their highest frequency from January to March, while the general rainfall season is all summer. The Bauru flood of 8 Feb 2001 caused eight fatalities and about 1 MUS$ damage:
Severe weather: Definition and Threat
The definition of a severe thunderstorm is closely related to the perception of human beings affected by property or crop damage, casualties, or fatalities. Therefore, no rigorous definition of severe can exist, but the following criteria have gained some international acceptance:
1) hailstones larger than 2 cm in diameter
2) heavy, flash flood-producing rainfall
3) straight-line winds of more than 25 ms-1
4) tornadoes
Estimating occurrence, incidence, and threat of severe storm phenomena in the tropics is not easy due to very limited observational and climatological data.
Compared to mid-latitudes, severe storms are much less frequent in the tropics. Especially in the Amazon region, high thunderstorm number does not imply frequent severe events. This is due to, e.g.
* CAPE being distributed over deep layers
* smaller NCAPE in the lowest kilometers
* moist boundary layers, low cloud bases
* lack of synoptic forcing (trough, jet, shear)
Tropical severe weather apparently often results from orographic forcing, or intrusion of extratropical systems.
Climatological data on hail on the ground in Brazil shows a maxi-mum during spring and summer. A coupling of hail occurrence to oro-graphy favors SE Brazil.
Bauru radar analysis
a) Microburst signature (red oval) within the catchment of Rio Bauru. Watershed (black arc), point of strongest flood (X).
b) PPI and VCUT of Z and v at 19:01, half an hour before main flooding event
dBZ
ms-1
Downbursts and Tornadoes Z v Z v
Analysis of natural hazards in Brazil (Munich Re Group, 2000)
Conclusions
Data on severe storms in Brazil are still sparse, and even more so for the tropics in general. Future research must heavily rely on satellite and radar observations to overcome this situation. With the limited climatological information in mind, we can expect the following severe storm phenomena in Brazil during Jan/Feb:
S and SE Brazil:
* Heavy rain and probably flash floods
* Occasional mesoscale convective systems
* Tornadoes (extratropical waves / orographic forcing)
Amazon region:
* Almost no mesoscale convective complexes
* Very high cloud tops, often weak shear regime
* Apparently, severe weather occurs very seldom
General:
* Wet downbursts possible with heavy rain events
* Hail on the ground is more likely on higher terrain
* Aircraft cruise level: (large) hail may be encountered
Severe thunderstorms will likely be rare events.
Divergence signature
Meso vortex signature
dBZ
ms-1
Downbursts are hazardous to low-flying aircraft. On 22 Nov 1999, a microburst occurred from medium-sized multicell storms, with echo tops below 10 km and reflectivities <50 dBZ. Cell life cycle was ~30 min.
On 14 May 1994, Bauru radar detected the mesocyclone of a tornadic storm which exceeded 60 dBZ for 5 hours. Meso shear 3.5x10-3 ms-1; 11 MUS$ damage, 3 fatalities.
Tornadoes in southeastern Brazil could be more frequent than currently known. A tornado (dots) and damaging winds map (hatched area) for Argen-tina reveals a number of tornadoes and other wind hazards near the Brazilian border.
Lack of dense observation networks likely is one cause of the low number of tornado reports in Brazil
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