ASCension Island Initiative (ASCII)

The ASCension Island Initiative (ASCII) is a project to study cloud and smoke interactions over the southeast Atlantic Ocean.

The ASCension Island Initiative (ASCII) is a project to study cloud and smoke interactions over the southeast Atlantic Ocean. A ground-based depolarising UV-lidar is operated on Ascension Island (8◦S, 14◦W) to monitor aged smoke from the African continent (a distance of about 2500 km) during the dry monsoon and cloud property changes under influence of the smoke.

The interaction between clouds, aerosols and radiation is currently one of the most uncertain components in global climate models. The region where differences between climate model simulations of aerosol radiative effects is most prominent is the south Atlantic Ocean. In this area interactions between clouds, aerosols and radiation is very complex and not represented well in climate models.

In order to better constrain models simulations, a consortium of research institutes coordinated various observational campaigns to study the influence of smoke from the African continent on cloud development over the ocean. In September 2016 an Intensive Observation Period (IOP) was defined when the dry period of the African Monsoon is at its peak and vegetation burning in Africa is wide-spread. Several airplanes were base in Walvis Bay, Namibia (23◦S, 14◦E), sampling the atmosphere off the west coast of the African continent, while ground-based stations were fitted on Ascension island, St. Helena island (15◦S, 5◦W), and near Walvis Bay to study the smoke and cloud properties at various places along the route of the smoke travelling west from the heart of the African continent to the Atlantic Ocean. The in situ and ground-based observations are complemented by satellite observations over the region from a consortium of research institutes from the US, UK, France and The Netherlands. Airplanes, ground stations on islands and satellite instruments are combined to investigate the radiation–clouds–aerosol interactions over the region.

ASCII comprises the Dutch contribution to the observation campaign in the Atlantic, by placing a depolarising UV-lidar on Ascension Island. The UV-lidar is capable of measuring smoke properties over the island, while at the same time inverting cloud droplet size and number from overhead water clouds. The primary object was to use a single instrument to quantify the first aerosol indirect or Twomey effect: cloud droplets will be smaller and more numerous when the number of cloud condensation nuclei is increased, given a fixed amount of moisture. The Twomey effect is a well-known aerosol effect on clouds, and has been demonstrated in several observational studies and is usually simulated in climate models. However, the effect is very difficult to isolate from other effects on cloud development, like meteorological changes and other aerosol indirect and semi-direct effects. Furthermore, the effect is strongly dependent on the aerosol microphysical and chemical properties, which can be very different for different aerosol types around the globe. Therefore, new and more accurate observations are valuable for constraining simulations of aerosol-cloud interactions.

KNMI UVLidar at Ascension.
KNMI UVLidar at Ascension.