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UK scientific aircraft researches desert dust and its effect on convective clouds


The Sahara emits large amounts of dust every day, which gets transported by winds across the Atlantic and occasionally reach the UK. Dust can affect visibility, impairing aviation over the areas affected, and has an impact on health. Dust modifies the radiation budget, and thus affects weather and climate forecasts, and it can interact with clouds affecting their formation and evolution.

Scientists at the Met Office, in collaboration with the National Centre for Atmospheric Science (NCAS), Manchester, Leeds, Reading and Hertfordshire Universities, and the British Antarctic Survey, have led the ICE-D field campaign, which was held in Cape Verde during August 2015. The main aim of the campaign was to characterise the properties of Saharan dust as ice nuclei (IN) and cloud condensation nuclei (CCN), and their impact on cloud microphysical processes, precipitation, and the formation of both convective and stratiform clouds. Another component of the experiment has been the characterisation of dust on its own, with the aim to improve dust remote sensing from space and from the ground and to validate the dust predictions in the Unified Model (UM).

FAAM aircraft and X-band RADAR during the ICE-D campaign

The experiment has seen the UK FAAM BAe-146 deployed at the island of Santiago in Cape Verde, carrying out 16 research flight for a total of 71 flying hours. Ground-based measurements were also performed in Santiago, including an X-band radar operated by the University of Leeds, a container dedicated to aerosol in situ measurements operated by the University of Manchester, a POM sun-photometer operated by the University of Valencia (Spain), and a CIMEL 8-wavelength sun-photometer operated by the Met Office. Moreover, a dustsonde-launching facility has been operated in the island of Sal by the University of Hertfordshire and Penn State University, in collaboration with the Cape Verde meteorological service, Instituto National Meteorological Geophysical (INMG).
Mineral dust with sources from the Saharan region have been identified as efficient ice nuclei in laboratory and field studies, but the effect on cloud evolution has not been measured. Ten research flights during ICE-D focused on the measurement of properties of mineral dust lifted from the Sahara, and on the dust-cloud interactions and subsequent evolution of towering cumulus clouds near Cape Verde. Given an initial well-characterised dust distribution, a key objective was to determine if the number concentration of initial ice could be predicted.
Three research flights on-board the BAe-146 have been dedicated to the mapping of dust gradients over long distances, encountering medium to large dust loads. The data from these flights will be exploited to validate the dust retrievals from satellites, in the perspective of their potential use in the UM global model forecasts. Moreover, one research flight was conducted under the flight path of the international space station, where the new space lidar CATS is being operated by NASA; the aim of this flight being to validate the retrievals from this new instrument. Finally, two research flights for the EUFAR Transnational Access project SAVEX-D have been conducted near the islands of Santiago and Sal, where ground-based sunphotometers were simultaneously operated.

Overall, the campaign has been successful in its scientific aims. The scientists involved will now focus on the scientific analysis of the data, which will yield to improvements in our understanding of the role of desert dust in the weather and climate system, and ultimately contribute to the constant improvement of the forecasts.Dust plume observed with the lidar remote sensing instrument on-board the BAe-146 scientific aircraft. This instrument permits mapping dust with a great accuracy.

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