Improved Forecast for aviation safety: The mixture does it
| Fig. 1: Transport of volcanic ash cloud over the north Pacific on June 23, 2019 as seen by Terra MODIS |
On June 21, 2019, the Raikoke volcano erupted. Raikoke is located on a Russian island in the northwest Pacific. It emitted 1.5 Mt SO2 and 1.9 Mt of very fine ash particles with diameters smaller than 32 µm over the course of 9 h. These ash particles are small enough to be transported in the atmosphere by wind over distances of several thousand kilometers and stay airborne for several months or even years.
As volcanic ash jeopardizes air traffic, an accurate and reliable forecast of volcanic ash dispersion is vital for aviation safety. Such forecast can only be achieved when all responsible physical and chemical processes, that influence dispersion, are represented in the forecast models. These processes comprise sources, sinks and interaction mechanisms of particles in the atmosphere. Over the last decade, the parametrization of volcanic sources has been enhanced by various research groups. This includes for example descriptions on how much ash is emitted in which height over which period of time and what is the fraction of very fine particles that are small enough for long range transport. In contrast, sinks and interactions of particles in the atmosphere have received less attention. As a consequence, aerosol dynamic processes and aerosol-radiation interaction are neglected so far in forecast models for volcanic ash dispersion. Aerosol dynamic processes comprise the formation of non-ash particles out of gaseous precursor substances, such as SO2, and the interaction of these new particles with volcanic ash, forming so called aged ash particles. Such aged particles are usually larger and therefore, experience a larger gravitational force. As a result, they sediment faster which shortens their atmospheric lifetime. On the contrary, the interaction of (aged) particles with solar and thermal radiation heats them which can result in a lofting effect. This can increase the particles’ atmospheric lifetime.
In our “Aerosols, Trace Gases and Climate Processes” research group we extend the ICON-ART modeling system with the aim to improve its forecast quality in order to prepare for the next big volcanic eruption in Europe. Therefore, we implemented the described processes in our model. In order to show the benefit of their representation for the forecast of volcanic ash transport we investigated the Raikoke eruption, that was one of the largest volcanic eruptions of the last 30 years. For this eruption exist a variety of satellite measurements which can be compared to our model results.
Our results show that around 50 % of very fine volcanic ash mass is removed due to particle growth and aging. Furthermore, the maximum volcanic cloud top height rises more than 6 km over the course of 4 days after the eruption due to aerosol-radiation interaction. These two effects would not be visible in the model without our newly implemented model extension. This is the first direct evidence that shows how cumulative effects of aerosol dynamics and aerosol-radiation interaction lead to a more precise forecast of very fine ash lifetime in volcanic clouds. Consequently, ICON-ART allows a more accurate forecast of volcanic aerosol dispersion for aviation safety in the future.
Fig.2: Eruption of Raikoke on June 21, 2019. The photograph has been taken by an astronaut onboard the International Space Station (ISS)
Workinggroup Aerosols, Trace Gases and Climate Processes