Gravity Waves over Tierra del Fuego during my stay in Río Grande, May 2022

End of May 2022 I got the chance to visit Río Grande again. My main tasks were to do some maintenance on the CORAL system. On the 30th of May I took a nice timelapse video showing stripy patterns in the clouds southwest of the observatory.

Note that the clouds remain at their positions while the air is flowing northward. The extended wave field becomes more clear when looking from space. Below is a picture taken from the geostationary satellite MODIS. It reveals trapped lee waves downstream of the mountains of Tierra del Fuego.

What are the atmospheric conditions that allow for the propagation of those waves?

First of all, a sufficient forcing is needed meaning that the wind must be strong enough to pass over the mountains and not around them. I created an animation showing the forcing, i.e. horizontal wind speed, @700hPa (panel d). The data is taken from the Global Forecast System (GFS) and has a horizontal resolution of approximately 30km. On the 30th of May the southerly flow is quite strong (~30m/s). Strong enough to overflow the southernmost foothills of the Andes (see enhanced vertical wind in panel c)) and to excite mountain waves which I observed at Río Grande.

The subsequent question I'm usually interested in is: How does the excited mountain wave propagate vertically and at which altitude is it breaking?

To get first insights into what might be happening I've also animated the horizontal flow field @10hPa which is at about 30km altitude (panel b). The flow is now much more zonally oriented, i.e. we find southwesterly flow.

In order to propagate vertically, the mountain wave needs headwind. If the flow @10hPa were westerly the mountain wave would have reached a critical level already. The southerly component of the polar night jet keeps the mountain wave going. We can find its structure in the vertical wind field @10hPa (panel a). Additionally, the phaselines are elongated towards the East which is an indication for lateral or oblique propagation which takes place perpendicular to the mean flow.

Also, our CORAL measurement (not yet displayed) shows temperature perturbations that are most likely due to that mountain wave. However, 24h later, the temperature profile is rather smooth. The mountain wave is gone. What happened?

It becomes clear that even though the flow is still southerly, wind speeds @700hPa have tremendously weakened (panel d) and therefore, no mountain waves are excited anymore. Perturbations in the vertical wind field @10hPa (panel a) are uncorrelated to the topography.