Lately there has been a bit of discussion about Flight Level Ash Advisories and how people interpret them visavi actual columnal height.
Some have been disappointed when it has turned out that the advisories have been up to 3 times as high as the confirmed numbers released by the various agencies at a later stage. So, I thought it was time to clear things up a bit.
So, I sent an email to London VAAC asking them about how they do, since they always tend to be spot on with their advisories in regards of ash height. And they answered in such a perfect manner that I will leave their answer ad verbatim, and then I will talk a bit in the end about difference with other VAAC-agencies.
Thank you for your enquiry.
As meteorologists for VAAC London, we have been aware of the Volcanocafé blog for a couple of years, and regularly enjoy the numerous informative articles and discussions on there.
To answer your question, there are a number of different sources that are used to determine the height level of an eruption, however the exact ones used will depend upon the location, and accessibility of equipment and observations at the time. It is also important to note that other volcanic ash centres may have a different approach, and I can only talk for VAAC-London.
Firstly, VAAC-London are fortunate enough to have a close working partnership with the Iceland Met Office (IMO), as this is where the vast majority of the volcanoes within our area of responsibility lie. This means that any eruption that occurs is assessed by them, through a mixture of ground based observation, pilot reports, and local radar. They then report the observed height of the eruption, and this is what we use in the first instance. However, this is a working partnership, and any information either of us have to ensure the correct values are used, is freely shared.
One way we can help guide IMO regarding the observed height of the eruption, is by combining satellite observations, the observed motions of the ash, and also our modelling of the atmospheric winds at different heights. These then have to match up with what we are observing. So for example, an eruption that reaches stratospheric heights could show ash in satellite imagery moving in a very different direction or speed to that at lower levels. This is also likely the best method to use for any remote volcanoes, for example, if we had to assess one from outside our region of responsibility in order to support another centre (e.g. Africa, which is the responsibility of VAAC-Toulouse)
Once the ash is airborne, we are then able to verify height levels through a mixture of radar and further ground based observations (e.g. LIDAR), as well as continuous monitoring on various satellite channels. Where possible, research flights and pilot reports would also help us here.
However there is no “correction” factor included, as our aim would be to model the ash cloud as observed, and track its progress, and any such factor would affect the accuracy of these forecasts. However, if there is a range of uncertainty, it is possible that we would assume the topmost level in a first instance, and reassess later.
Any safety decisions would then be down to the airlines themselves, as we simply forecast where the ash will be.
Finally, we do have some information on scenarios, especially for the major Icelandic culprits, based upon historical eruptions, and these can be used for “what-if” exercises. However each new eruption would be considered on its own, and assessed from the observations we have, and the information proved to us by IMO.
Not all agencies have the clear advantage of cooperating with the Icelandic Met Office. They have all sorts of equipment to help with discerning ash height, including specialized mobile ash-radars. This makes it possible for London VAAC to be as exact as is possible.
Other agencies might not have this advantage, they instead operate on other less exact data, against language barriers, and have other methodology.
Some seem to operate on the “better safe than sorry”-methodology. So, they go on the worst-case scenario giving ash heights that are initially quite high, and then dial them back as reliable figures come in.
For a VAAC agency safety is the paramount issue, and that is not necessarily the same as satisfying volcano-aficionados with metre-exact columnal heights. London VAAC will be useful, others not so much from our viewpoint.
I guess what I am trying to say is, know thine VAAC, and now we know London VAAC.
I would hereby like to graciously thank London VAAC for the essential work that they do, and their help clearing things up with an official answer.