Enhanced Dust Forecasting Capability
August 2011 - In late July 2011 the Met Office extended its operational dust storm forecasting capability to give global coverage, and to produce longer range forecasts extending out to 5 days.
The Met Office has been producing operational dust forecasts since 2008, in support of British and NATO Forces operating in the Middle East and South Asia using a limited area model configuration of the Unified Model.
While Saharan dust occasionally reaches the UK, it rarely causes anything more noticeable than a vivid sunset or dust deposits on cars; dust is a much more significant issue in arid and semi-arid regions of the world. Dust storms vary in size and intensity from small dust devils, to very intense but localised dust storms, known as Haboobs, which are severe enough to bring almost all activity to a standstill. The largest dust events can extend over thousands of miles, and while these are not usually as severe as Haboobs, they can reduce visibility enough to significantly impair civil and military aviation over large areas. Exposure to atmospheric dust can also have impacts on human health; outbreaks of meningitis in Africa are linked to dust storms, although the exact mechanism is not well understood.
The Met Office dust forecasting capability is an application of the dust model developed for Earth System Science research using the Met Office Unified Model. Dust is emitted in the model according to a set of equations based primarily on wind tunnel observations; dust is emitted when strong winds blow over dry bare soil surfaces. This dust is then transported by the model's 3-dimensional winds and removed by gravitational settling, turbulent mixing and falling rain. In order to correctly represent the emission, transport and settling of dust it is important to represent the variability of the sizes of different dust particles; we do this by emitting and transporting up to 6 different dust tracers, each with different representative sizes.
During the initial testing of the global dust forecasting system a dust event reached the South West of England, including Met Office Headquarters in Exeter, providing an ideal case study to demonstrate the new dust forecasting capability. On the 4th April, a group of thunderstorms formed over the Atlas Mountains in Morocco, generating a small cyclonic circulation which lifted a large amount of dust. This dust was then blown over Spain and into the Atlantic, reaching the South West of England on the 7th April.
Verifying the model forecasts against satellite imagery and ground-based observations showed that the event was very well represented by the model. An animation, comparing satellite observations of dust with Aerosol Optical Depth (AOD) from the model, is available using the 'Enlarge' button of the satellite image on this page.
The animation demonstrates that in this case:
- The model produced a convective event in Morocco at the right place and time and the associated cyclonic winds correctly lifted a large amount of dust into the atmosphere.
- The large-scale winds within the model correctly transported the dust plume, producing spatial patterns of dust which match the observations as it was transported over Spain and into the Atlantic, reaching the UK 3 days later.
- The NASA observations of AOD from the ground-based AERONET project show the dust event over Paris at the same time, where the AOD values agree with the model values. Nearby AERONET sites in Holland are dust-free in the AERONET and Satellite observations, and the model.
During the dust forecasting system's final testing phase dust forecasts and more conventional meteorological fields were provided to scientists involved in flight planning for the FENNEC and ICE-T projects. The FENNEC project is a NERC funded observing campaign involving the Universities of Oxford, Leeds, Reading and Sussex with support from the Met Office. The campaign took the FAAM BAE146 aricraft and ground based instruments to observe the meteorology and aerosol composition over Sahara. The Ice Clouds Experiment - Tropics (ICE-T) campaign was an observational campaign in the Caribbean organised by the US National Center for Atmospheric Research, in collaboration with US Universities and the Met Office, which included aerosol observations of Saharan dust transported across the Atlantic.
Feedback and subsequent analysis from observing campaigns like these can provide extremely useful insights for developing forecast models, and provide data sets for validating the models which are always of the highest possible quality. For example the Met Office/NERC GERBILS flight campaign in June 2007 was crucial in the development of the Met Office dust forecasting system, with the analyses from the campaign described in the (July 2011) GERBILS special edition of the Royal Meteorological Society's Quarterly Journal.