Andes weather in November
20-21 November 2019
Andes mountain waves are frequent in November, with one or more convection lines associated with them.
10 May 2021
14 December 2020
By Gabriela Ishikame (SMN Argentina), Fausto Polvorinos (Spain) and José Prieto (EUMETSAT)
Usually around November, warm air masses reach Argentina where the subtropical and sub-polar jets reinforce each other, at the same time as the Bolivian High develops. The upper-level anticyclone pushes south the position of the subtropical jet and affects the central region of Argentina.
This example of November 2019, using GOES-16 Convection RGB (Figure 1) and channel 7.3 µm (Figure 2) imagery, plus NWP analysis (Figure 3), shows the jets generated convection near the Andes along two or three lines. The waves propagated downstream, but parts of them were hidden by subsidence after the air dried up.
The scheme of the lee wave formation (Figure 4) shows that the turbulence created a gap on the lee side of Andes. Cirrus on the lee side was thicker and colder than on the wind side. On the wind side of Andes, gusts of more than 220 km/h (120 kt) at the 200 hPa level originated on the cold front, with a centre in the southern Atlantic. On the lee side, the cloud was parallel to the mountain range, with perturbations moving with the flow.
Several types of cloud can be best identified in the GOES -16 solar imagery (Figure 5): convective cloud near the Andes, roll cloud, billows on the wind side, and trapped waves on the lee side west of Neuquen, where the mountain range is lower. Billows are the result of Kelvin-Helmholtz instability and of shear between two separate air layers.
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