Numerical modeling of volcanic ash clouds
Numerical dispersion models form a highly valuable tool for tracking volcanic ash clouds. The main advantage of numerical dispersion models such as the Lagrangian particle dispersion model Flexpart is their flexibility (as the name already indicates) to adapt to specific conditions. Given a particular set of initial weather conditions (NWP field), the height of the ash column as well as the initial time of eruption, the model provides the possible track of the ash cloud. The model output can then be compared to observations and finally warnings can be issued. However there is a range of uncertainties that should be considered when using the model output. On the one hand the model is based on a set of simplified equations and parameterizations and therefore the output cannot fully represent real future conditions. On the other hand the model input is also linked to a range of uncertainties.
The emitted rate of ash particles can show high temporal variations. Therefore the total rate of emitted material can only be roughly approximated. Moreover the identification of the height of the ash column is often based on pilot observations. Most often the only chance to mark the height of the ash column is to link it to a specific flight level.
Finally it should be noted that precipitation is another major source of uncertainty, since it can change the concentration of ash particles significantly. If the ash is subject to rain it can easily absorb considerable amounts of water reaching densities in excess of 1 400 kg/ m3 or more. Such wet ash has the consistency of wet cement and when deposited on top of hangars can cause buildings to collapse, as happened at Clark U.S. Air Force Base in the Philippines during the Mt. Pinatubo eruption in 1991 (see ICAO-Report). Knowing about the limitations of the model further assumptions concerning the track of the ash clouds can be taken.
Take a look at the animation generated by Flexpart Version5.0 which demonstrates the trajectory of the volcanic ash cloud.
Figure 4.1: Flexpart.
The colour range is linked to the expected distribution measured in g/m3. However it should be stressed again, that for starting the model a rough approximation of the total amount of particles was implemented in the models. Although there existed several raw concepts for the handling of volcanic ash clouds, there were no stringent regulations present at the beginning of April. Air space was closed whenever any notable trace of volcanic ash was expected. The effects of the eruption of Eyjafjallajökull forced the European Community to set up standard guidelines. In consequence the European air space was split into 3 regions. Zones containing ash clouds with lower concentration than 0.2 mg/m3 were considered to be open to air traffic. Air traffic in zones with ash concentrations ranging from 0.2mg/m3 to 2mg/m3 was bound to certain restrictions. The airplanes had to undergo regular additional inspections. Zones containing higher ash concentrations than 2mg/m3 were closed to air traffic (see fig. 4.2).
Figure 4.2: Zone-Setting Civil Aviation Authority.
After a few weeks and ongoing measurements the restrictions were finally completely removed.