GOES-16 RGB, 21 Nov 2019 13:20 UTC

Andes weather in November

20-21 November 2019

GOES-16 RGB, 21 Nov 2019 13:20 UTC
GOES-16 RGB, 21 Nov 2019 13:20 UTC

Andes mountain waves are frequent in November, with one or more convection lines associated with them.

Last Updated

10 May 2021

Published on

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.

Figure 1: GOES-16 ABI convection RGB, 21 November 14:40-17:20 UTC, showing the generation of  lee mountain waves East of Andes and their propagation to the Atlantic.

 

Figure 2: GOES-16 channel 7.3µm, 21 November 14:50-17:20 UTC
CPTEC analysis from 21 November 2019 at 0 UTC
Figure 3: CPTEC analysis from 21 November 2019 at 00:00 UTC. A powerful jet, typical of the month of November reaches 170 knots in its core. Credit: CPTEC/INPE

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.

Lee wave formation
Figure 4: Scheme of the lee wave formation under weak and strong turbulence for a mountain range like Andes. A clear gap shows under turbulent wind.

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.

GOES-16 0.6µm image at 14 UTC on 21 November 2019
Figure 5: GOES-16 ABI Visible (0.6µm) image at 16:00 UTC on 21 November. Letters indicate: C: Convective cloud near the Andes. R: Roll convection. T: Waves trapped at low level. B: Billows. S: Cloud streets. SB: Sea breeze. Also to notice gust winds in San Luis and cumuli in La Pampa region.