Low-level jet causes heavy rainfall over southern Brazil

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Meteosat-10 imagery can be used to investigate the connection between a low-level jet and heavy precipitation over southern Brazil.

Date & Time
10 October 2015 10:00 UTC

More information and detailed analysis of the feature can be found in the In Depth section.


In Depth

by Humberto Alves Barbosa (LAPIS) and Carlos Pinto Sa Silva Neto (SIMEPAR)

Figure 1: Met-10, 10 October 2015, 10:00 UTC
IR 10.8µm
Full Resolution
Figure 2: Met-10, 10 October 2015, 10:00 UTC
IR 10.8µm image and NWP (pressure and wind) fields
Full Resolution

In the colour-enhanced Meteosat-10 IR 10.8µm image on 10 October (Figure 1) there is a widespread area with very cold cloud tops (191 K). In this example, a couple of severe subtropical Mesoscale Convective Systems (MCSs) are observed over southern Brazil.

The lower level jet is channelled by the Andes, with moisture extending across northern Chile-Paraguay to southern Brazil. This flow is associated with a subtropical jet (upper level jet) from the south, which enhances the low level convergence and helps with the development of convection.

A maximum wind speed of about 25–30 ms-1 (90–110 km/h) at 1016 hPa is centered near 30°S-55°W. It is a consequence of the subtropical MCS activity (Figure 2).

The thermodynamic and wind fields structure of the Chaco Jet enhanced the convective instability by transporting heat and moisture over southern Brazil. Heavy precipitation is triggered at the exit region of the low level jet.

The subtropical jet, as one of the primary sources of upper divergence, is found between 250–200 hPa.

Figure 3
Figure 3: NWP vertical cross section along transect depicted in Figure 1.
Full Resolution

Model vertical structure of the low-level flow shows a maximum wind speed of about 35–40 ms-1 (120–140 km/h) (Figure 3).

Overall the South American Low-level Jet (SALLJ) is a northerly wind current with a wind maximum located immediately to the east of the Andes.

This maximum is positioned between 10°S and 20°S, near Santa Cruz de la Sierra (18°S) in Bolivia. The SALLJ often produces a large transport of atmospheric water vapour from tropical to extratropical latitudes.

Previous case study

Heavy hail in northern Uruguay (21 September 2015)