A discrepancy in simulation

The recent airline chaos may have been excerbated by reliance on a faulty simulation, according to press reports

According to the Daiy Mail, the ban on flights over the UK was imposed by NATS on the basis of advice from the Met Office, which has “eight scientists in the Met Offices London Volcano Ash Advisory Centre (LVAAC).” There was little data to go on, the model forecast more ash than inn fact arrived, and no-one now has aircrft available to sample the atmosphere at the heights involved. This, coupled with an over-zealous safety culture, led to the ban, according to the Mail. The Times has a more balanced account.

The Mail adds “As the Met Office is responsible for forecasting ash for Europe, air traffic controllers across the continent soon followed the UK lead, closing down aviation” (which seemms to ingore the role of the Toulouse VAAC, which covers most of Europe and all of Africa.)

The Toulouse website links to a long report about the methods used. According to this, “Météo-France has developed a global three-dimensional Chemical Transport Model, MOCAGE, dedicated to the numerical simulation of the interactions between dynamical, physical and chemical processes in the lower stratosphere and in the troposphere.” The Met office said an “atmospheric dispersion model, NAME, is used to generate volcanic ash forecasts up to 48 hours ahead, and can be run up to 6 days ahead. It produces graphical output showing the forecasted concentration of the plume. Recent improvements to NAME include the use of the New Dynamics version of the Unified Model (UM), which gives better vertical resolution, especially near the tropopause.” The US use “the HYbrid-Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for volcanic ash modelling developed at the NOAA Air Resources Laboratory (ARL) and upgraded from collaboration between ARL and the Australian Bureau of Meteorology (ABM).”

The Met office modestly says of NAME “This world-renowned atmospheric pollution dispersion model is an invaluable and versatile tool for accident and episode analysis, and for pollution forecasting…. NAME lies at the heart of the Met Offices air quality forecasting system, and is widely used by industry and government to help solve pollution problems”. It is shortly to be replaced by Name 2. There is a more thorough description on Wikipedia. (Wikipeida also has a long list of other air dispersion models, which claims that the NAME models name is short for “Nuclear Accident ModEl”. The Met Office also goes into more detail about its own Unified Model (Why does it worry me that this is written in Fortran?) and their New Dynamics upgrade, and theres a fuller account here. Its not clear how this relates to NAME, though.

The Met Ofice has become something of a target for people who dont accept current alarm about climate change -= see this blog entry, and this latest incident will not help. See also this blog posting, with a large number of irate comments from the public.

In summary the problem seems to be
1. the model is not 100% accurate
2. but nobody now has aircraft able to go up and measure the true levels of ash
3. nobody knows what level of ash is safe for jets to fly through
4. so the European authorities decided that no risk could be tolerated.

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