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ATHAM (Active Tracer High Resolution Atmospheric Model)

Research Topics: Explosive Volcanic Eruptions

Explosive volcanic eruptions influence the global climate on time scales of months to years depending on the amount of sulfur species injected into the stratosphere. The direct injection of volcanic gases modifies microphysical and chemical processes in the stratosphere which may lead to ozone destruction. Volcanic ashes determine the plume development and therefore the amount of volcanic gases reaching the stratosphere.

It is the actual weather condition at the time of the eruption as well as the mean meteorological state of the atmosphere that has a strong impact on the plume height and the amount of particles and volcanic gases injected into the stratosphere. Even diurnal variations in temperature can change the plume height by more than 10%.

Transport processes due to turbulence are as important as those due to advection. Turbulent processes determine the entrainment of ambient air, the plume development and height. The effect of microphysical processes on the plume dynamics is dominated by the amount of entrained water vapour. Under relatively humid conditions the release of latent heat can add up to 50% to the thermal energy released at the vent.

A mean cross wind is not only able to reduce the plume height significantly but can also substantially alter the horizontal distribution of particles and the horizontal extension of the plume. This may lead to misinterpretations of the sediment fan in terms of plume height. A fully 3d simulation is necessary for a realistic description of volcanic plumes.

The scavenging of gases and particles in an explosive volcanic eruption plume has been studied by numerical simulations with the plume model ATHAM. This study reveals the dominant role of hydrometeors in controlling many processes in the plume. By far the highest portion of condensed water freezes to ice in the eruption column.

An example for the hydrometeors in the plume resulting from the simulation of the Plinian Laacher See eruption in midlatitudes after 30 min. of eruption is shown below.

Figure 1 Figure 2

  • Frozen hydrometeors are highly dominant in the plume.
  • Liquid water exists only in the central rising zone (<1%).

The fast plume rise to regions, which are too cold for even supercooled liquid water to exist causes most particles to occur as ice-ash aggregates. The coating of volcanic ash with liquid water or ice results in growth of particles, which enhances the fallout velocity of ash. Precipitation of aggregates results in gas-particle separation, which increases the injection of volcanic gases into the stratosphere. In addition, it influences the stream pattern, which in turn influences the microphysics in the plume by lowering the supersaturation in the ascent zone.

We examined the scavenging of the most important volcanic gases, HCl, SO2 and H2S, by liquid and solid hydrometeors and by aggregates in the plume.

The figure below shows the plume of total HCl after 30 min. of eruption from the simulation of the Plinian Laacher See eruption in midlatitudes.

Figure 3

For the Plinian Laacher See volcano, which happened in a dry environment, all gases reach the neutral buoyancy height in the stratosphere. The injection into the stratosphere is > 99 % of the total eruption. HCl is partly scavenged by hydrometeors. The scavenging efficiency depends on:

– the individual solubility of the species,

  • HCl / HBr:
    up to 40% contained in ice
    high solubility in liquid water
    incorporation in ice
  • SO2 / H2S:
    up to 10% contained in ice
    low solubility in liquid water
    incorporation in ice

– the amount of hydrometeors in the plume,

  • In a wet environment the entrainment of humid ambient air increases the hydrometeor content. This might strongly enhance the scavenging of volcanic gases.

– the amount of liquid water.

  • A coignimbrite eruption with a slower vertical velocity might show a much higher content of liquid water, which could efficiently remove HCl from the atmosphere.

The final amount of volcanic gases in the stratosphere depends on the fate of the contaminated hydrometeors:

  • Precipitation would remove species from the stratosphere.
  • Sublimation releases species into the gas phase.

In our study, sublimation of ice releases the species back to the gas phase in the stratosphere at the end of the simulation time.

The stratospheric injection of volcanic gases is determined by the volcanic and the atmospheric conditions!

Low relative humidity in the troposphere in our simulations caused precipitation to reevaporate before it could reach the ground. As a consequence, no evidence of hydrometeor-ash interaction or gas scavenging could be found in the fallout of the eruption simulated here, although these processes occurred to a significant degree in upper parts of the plume.

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