Gravity waves over Cumbre Vieja volcano

By Ivan Smiljanic (CGI), HansPeter Roesli (Switzerland), Miguel-Angel Martinez Rubio (AEMET)

Gravity waves, similar to those produced by stone thrown in a pond, form regularly in the atmosphere. They are most frequently triggered by deep moist vertical motions of big clouds systems which oscillate stable atmospheric layers. These oscillations are only visible if there is associated cloud formation.

On 1 October almost persistent ejections from the Cumbre Vieja volcano were responsible for vertical displacement of the interface between the boundary layer (seat of the trade winds) and the middle troposphere, which in turn started to oscillate and produce the concentric water vapour condensation regions, i.e. concentric clouds (Figure 1, 2a and 2b).

Figure 1 is a MODIS snapshot that clearly show the concentric cloud rings. It also shows two cloud banks that partially engulf the eastern flanks of La Palma and signal the presence of the (north-east) trade wind regime.

Figure 2a gives an overview of the situation during daylight with rapid-scan imagery (5-minute frame rate) from the HRV band of Meteosat-10. Note the bow-wave-like behaviour of the stratocumuli that cover the most eastern parts of the movie — the island and its eruption signatures appear to plough eastward through this cloud deck like a ship.

Figure 2b focuses on the dynamics of 'ringing' phase on this day., which lasted for at least five hours. What is remarkable is the almost perfect rotational symmetry of the cloud rings. The absence of notable distortions indicates a relatively weak wind field at the level of the interface between the boundary and upper layer (see also Figure 3 around the 500-600hPa levels.)

The gravity waves could also be clearly seen from the ground, as this video from JuanMa Hernández (on Twitter) shows.

Gravity waves, with estimated wavelength of around 3km, were produced on the troposperic temperature inversion at around 500hPa, associated to the trade winds regime. This is confirmed through temperature and wind profiles from a nearby Tenerife radiosounding (Figure 3).

The radiosounding also reveals the fact that the ambient temperatures were between 0 and -10°C at the oscillating level. The MODIS IR11.0 temperature channel confirms similar cloud top temperatures over La Palma and Tenerife islands. Theoretically, those temperature would have enabled the clouds to form from either supercooled water particles, or ice crystals. To see which water phase it was the Natural Color RGB and Cloud Phase RGB products were inspected.

Looking at the widely-used Natual Color RGB, one could be mislead about the phase of the oscillating clouds. In general, the cyan colour in this RGB represents the ice phase of the cloud, but in some instances mixed phase clouds or clouds with large water particles exhibit the same coloring.

A wider view from the VIIRS Cloud Phase RGB reveals that these clouds actually consisted of water droplets, especially when comparing with the clouds capping the Tenerife island in the east, at similar height. Contrary to what the purple shades of the Cloud Phase RGB would normally suggest (large water particles), these concentric clouds likely had smaller water particles, due to locally enhanced number of condensation nuclei, i.e. ejected smoke content from the volcano.

The blue component is dominant because smoke reflects far less in the near-IR spectral region (NIR2.25 and NIR1.6 channels), than in shorter, visible wavelengths — the blue component of both the Cloud Phase and Natural Color RGBs. This is vividly illustrated in the VIIRS imagery, from a few days earlier when there was a stronger episode of smoke ejection (Figure 6) — with smoke in blue, and cyan shades, respectively (Figure 6).

NOAA-20 image comparison

Cloud Phase RGB Natural Color RGB

 

Figure 7 shows animated GOES-16 view on the gravity waves, through combined information of VIS0.6 channel and Cloud Phase RGB or sandwich product.

For comparsion, ash appears in more green/brow hues in the Cloud Phase RGB (Figure 8) because the highest reflectance for ash particles happens at longer wavelengths, and the green beam (NIR2.25) gives most contribution, followed by the red (NIR1.6) and blue beams (VIS0.48).