January 25, 2022 6:05 pm

The astonishing meteorological repertoire of the La Palma eruption

Jose Miguel Viñas 10 min
La Palma volcano plume
Plume from the La Palma eruption photographed from the Roque de los Muchachos, one day in which there was a thermal inversion that acted as a cover.

The eruption of Cumbre Vieja, on the island of La Palma, started on September 19, 2021, not only is it allowing us to know many volcanological aspects thanks to scientists from different public bodies, such as IGME-CSIC (Geological and Mining Institute of Spain) or INVOLCAN (Canary Islands Volcanological Institute). TIt also gives us the opportunity to observe the close relationship that volcanic activity has with Meteorology, through a wide repertoire of atmospheric phenomena, which we will review in the following lines.

The comings and goings of the plume

The two most striking elements of a volcanic eruption are the plume that comes out of the crater and the lava flows.. In the particular case of what is happening on the island of La Palma, as it is a fissure eruption, different mouths have emerged, which present variable explosiveness and effusiveness.

The eruption is strombolian in type, with a VEI (Volcanic Explosion Index) that was initially set at 2 (on a scale of 0 to 8), but was later raised to 3, due to the large volume of materials expelled to date. In this type of eruption, the columns of gases and pyroclasts do not reach the stratosphere.. The plumes of Cumbre Vieja have not reached an altitude of more than 6000 meters, evolving at the mercy of the winds from the lower and middle troposphere.

The changing appearance of the main plume of the La Palma eruption offers us visual information both on the local wind regime at a given moment, and on the characteristics of the lower atmosphere in the vertical (conditions of stability or atmospheric instability). In the more than two months that the eruptive process has taken place, the regime of trade winds (NE winds) has dominated La Palma, with presence of a thermal inversiona that for many days the rise of the plume has stopped abruptly, expanding horizontally at the level where said investment is located. Wind convergences near the ground, along the coast, alter the flow and favor the dispersion of gases and ash in the Aridane valley without a fixed pattern, easy to predict.

Volcanic rays
Rays formed in the mouth of the main crater of Cumbre Vieja, during the eruption of the island of La Palma. Photograph by Iván Vietio García.

Ash rain and volcanic lightning

In the most explosive phases, the plume has acquired a larger size, managing to incorporate gases and ash above the lower atmospheric layer., about 2,000 m thick, where the trade winds blow, favoring their long-distance dispersion. Large emissions of sulfur dioxide (SO2) are monitored at all times via satellite.

In the days that westerly winds have dominated the island, the ash cloud has moved to the east, forcing the island’s airport (next to Santa Cruz de La Palma) to close and, more sporadically, the from La Gomera and Tenerife North. Yes OK the deposition of large amounts of ash (occasionally with lapilli, pyroclasts with diameters ranging from 1 mm to 5 cm) concentrates on ground zero (mouths and lava flows) and surroundings, also reach the east of the island, favored by these changes in the direction of movement of the plume.

The great density of pyroclasts that the volcanic plume has presented in certain eruptive phases, has come to generate electric shocks, like the ones we can see in the night photograph of the main crater, which accompanies these lines. These volcanic rays are the result of the friction of the pyroclasts contained in the dense eruptive column., which causes a separation of electric charges of different signs and finally discharges. It has been at the times when the eruption has shown a greater explosiveness, when the conditions conducive to the formation of rays have been given, similar to those that occur in thunderstorms.

Gravity waves
Striking bands of circular clouds linked to gravity waves, which generated a violent column of gases ejected by the eruption of Cumbre Vieja, on the island of La Palma, on October 1, 2021. Image captured by the MODIS sensor of the Aqua satellite. © NASA Earth Observatory.

In some of these explosive peaks, sudden pressure changes atmospheric generated by gases and ashes thrown violently upwards, have generated horizontal gravity waves, which have propagated radially, generating layers of concentric clouds, shaped like rings, captured both by photographers on the island and from satellite (see attached image).

Like when we throw a stone in a lake a train of waves forms on the surface of the water, which expands from the point of impact outwards, in this case, the upward “cannon shot” of a violent ejection from the crater, causes a similar ripple at the base of the reversal layer of the trade wind, spreading these circular cloud bands in all directions, with the result that we observe in the image that accompanies these lines.

Micrometeorology in lava flows

The fiery lava flows (with temperatures that, when flowing, are around 1,000 ºC) generated by the overflows of magma in the different eruptive mouths, apart from destroying everything in its path, favor the formation of different microscale meteorological phenomena, which modify the prevailing weather on a local scale. Violent gusts of wind have formed, due to the great thermal (and therefore pressure) contrast between the air above the runoffs and the surrounding areas.

In areas buried by ashes in the surroundings of the mouths, there have been no shortage of dust devils or dust collectors., similar to those that form on bare, dusty ground subjected to strong sunlight. In the lava flows, the lava itself – in the process of solidification, but still hot – on which the ash has been deposited, acts as a source of heat, promoting the formation of the whirlwind.

On days of marked atmospheric instability, the plume has formed a pyrocumulus, while other clouds of great vertical development have formed around it. At least one of those days, a tuba was dropped from the base of one of them. Chen it rains, the droplets on impacting the hottest springs evaporate, creating the false sensation that the volcanic terrain is smoking. In fact, a kind of evaporation mist is formed, due to the condensation of that water vapor contributed by the rain.

In the case of the wastes that have reached the sea, the violent evaporation of seawater has created dangerous acid clouds, shaped like columns, in which the cloud droplets that form them contain hydrochloric acid and to a lesser extent sulfuric and hydrofluoric acid, generated from sodium chloride and sulfates present in sea salts.


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