Chapter I: Characteristics of volcanic plumes
1. Characteristics of volcanic plumes
Figure 2: GOES-17 ABI VIS0.6, 13 January 2022. Loop starting at 15:10 UTC. © NESDIS
Large volcanic eruptions affect both weather and the global climate. The dimensions of this impact can be better understood when observed from space (Figure 2). Ejected ash and volcanic gases form plumes that have many commonalities with convective systems, at least in the early eruptive phase. At later stages, volcanic ash trails and gas plumes become their most characteristic feature as they adopt the shape of filaments. These ash and gas plumes can travel long distances.
However, some volcanic eruptions have peculiarities that are not observed in convective systems, such as shockwaves that propagate over long distances and which can be tracked by meteorological satellites. Moreover, volcanic plumes can reach altitudes never observed for convective systems.
In this chapter, we will look at examples of characteristic plume top features that can be observed in satellite imagery during volcanic eruptions.
1.1. Plume top features
Ash particles and gases form the eruption column as they are ejected from the vent at high velocity. The initial momentum from the eruption propels the column upwards. As surrounding air is drawn into the column, the bulk density decreases and it starts to rise buoyantly into the atmosphere. At a point where the bulk density of the column is the same as the surrounding atmosphere the column will cease to rise and starts moving laterally. Unlike with convective cells, that point is not necessarily the tropopause as the buoyancy energy involved is often much higher.
When the ash and gas clouds reach the level of equilibrium, lateral dispersion starts. The plume then forms an umbrella or mushroom-shaped ash plume in a low wind speed regime.
Figure 3: Sandwich product, Mount Kelud eruption, 13 February 2014 17:30 UTC. © Martin Setvak (CHMI)
The above sandwich product (Figure 3) uses a fine-scale temperature color table for the coldest temperatures. This color enhancement accentuates minor temperature variations that can be interpreted as a concentric wave pattern also known as gravity waves.
These gravity waves are initiated by the hot gases and tephra spewed out of the volcanic vent. Gas and ash eruptions are irregular and pulsating events rather than a continuous flow. When hot gas emissions reach a temperature inversion or finally the equilibrium level in a stable environment, they cause a disturbance that propagates horizontally. These individual pulses can be seen as overshooting tops (dark red central spot in Figure 3).
Gravity waves can occur at any level in the atmosphere, provided there is a stable layer with a temperature inversion. A stable stratification is common at levels above the tropopause but such temperature inversions are also found at much lower levels over seas. When the air is saturated, gravity waves produce water droplets and become visible when the air is lifted (Figure 4).
Figure 4: Gravity waves captured by the concentric cloud formation. Cumbre Vieja eruption seen from Aqua MODIS Natural Color RGB, 1 October 2021 at 14:41 UTC.
There is a particular kind of pockmarked or granular cloud top structure that occurs in DIBS (Dust Infused Baroclinic Systems) when mineral desert dust forms a characteristic pattern during nighttime cooling. A similar pattern, apparently based on the same physical principle, is observed at the top of volcanic ash plumes (Figure 5) at night and in the morning hours.
Figure 5: Day Night band SNPP, Mount Kelud eruption, 13 February 2014 17:30 UTC. © Martin Setvak (CHMI)
Quiz
The image below shows the eruption plume of Hunga Tonga-Hunga Ha`apai volcano on 15 January 2022 as seen from GOES-West satellite (10.35 µm channel).
Mark the overshooting top and the area where gravity waves are visible, then check the solution.
1.2 Shockwave propagation
Explosions that occur during volcanic eruptions can create atmospheric shockwaves that propagate at supersonic speed over long distances. In case of very strong eruptions, such as the Hunga-Tonga eruption in 2022, these shockwaves show up on pressure sensors at ground level (Figure 6) and are even visible from space (Figure 7).
Figure 6: Mean sea level pressure readings of the first shock wave at six MeteoSwiss stations, all south of the Alps. Note the slight time shift between the orange (northernmost) and blue (southernmost) traces. © MeteoSwiss
The impact of atmospheric pressure changes on mid-level water vapor resulting from shock-wave propagation is very small but noticeable when artificially amplified by increasing the water vapor image contrast (Figure 7). The water vapor absorption band at 6.9 µm reflects the mid-tropospheric water vapor content. The propagating shockwave slightly lifts the low-level moisture into higher levels, which results in a higher absorption of radiation emitted from the ground and which can be seen in this absorption band.
Figure 7: GOES-17 ABI mid-level water vapor (6.9 µm) time difference image of the shockwave. ©NOAA/NESDIS/ASPB
The above loop shows several shockwaves propagating from the volcano but only the initial wave has enough energy to travel around the globe.
1.3 The propagation of volcanic ash clouds and gases
The propagation of volcanic ashes and gases is wind driven. Depending on vertical wind shear, ash and gases can be transported in different directions at different heights (Figure 8).
Figure 8: GOES-16 Volcanic Ash RGB of Mount Fuego eruption, 1 February 2018 07:00-23:00 UTC, © NOAA
Because volcanic ash particles are either washed out or removed by gravitational deposition, SO2 is often the last remaining atmospheric component that indicates a major volcanic eruption. SO2 propagation has particular significance when it comes to climate change. It can reach the stratosphere where it interacts with water vapor to form small droplets of sulphuric acid (H2SO4). These droplets reduce incoming solar radiation by absorbing and reflecting light, which results in surface cooling. Figure 9 shows an SO2 plume (bright green band) from the Kasatochi eruption (2008) that circled the entire northern hemisphere over more than 20 days.
Figure 9: Meteosat-8 Cloud Microphysics RGB of Kasatochi eruption, 21 August 2008, 12:00 UTC - 22 August 2008, 06:00 UTC. © EUMETSAT
Quiz
Compare the orientation of the visible part of the volcanic plume with wind direction at various heights.
Which wind levels fits best? Click on an image and go through the gallery to determine the answer. Pay attention to the caption below the image.
Cumbre Vieja eruption (28 September 2021 at 12:00 UTC).