Joint Seminar: Towards defining the low-cloud interaction with the smoky large-scale southeast Atlantic atmosphere

 The southeast Atlantic is home to one of the largest stratocumulus decks on the planet. It is also unique in that it is overlain by shortwave-absorbing aerosols during the months when the cloud deck is most pronounced. These originate from Africa, the world’s largest emitter of biomass-burning aerosols. Interest in the aerosol-cloud-radiation interactions of this climatically-important region is spurring new field deployments beginning in 2016. Towards providing context, this talk will provide an overview of the large-scale factors establishing the aerosol-cloud environment

Our work to date supports the idea that strong atmosphere-land coupling works in concert with the fire emissions to exert pronounced constraints on the response of the offshore stratocumulus. Radiosondes at St. Helena Island (15S, 5W) combined with satellite datasets reveal that stratocumulus cloud tops are lower when biomass-burning aerosols are present overhead, supporting stronger coupling to the surface. Simultaneously, the large-scale vertical velocity is reduced. While this may reflect the influence of the shortwave-absorbing aerosols in part, ERA-Interim reanalysis also articulates a secondary circulation capable of reducing the large-scale subsidence, associated with the strong free-tropospheric zonal winds that are instrumental to the offshore aerosol transport. These free-tropospheric zonal winds occur at the northern edge of a land-based anticyclonic circulation. HYSPLIT forward trajectories from MODIS-detected fire sources confirm the importance of a narrow latitudinal range centered on 10S for the offshore transport. Better-developed land-based anticyclones ultimately encourage more recirculation of the biomass burning aerosol back to southern Africa, further helping to distribute the aerosol offshore. Enhanced warm temperature advection off of the continent at 800 hPa independently strengthens the cloud-top inversion, more so when the land-based anticyclone is stronger. The aerosol outflow is also moist, which should reduce cloud-top longwave cooling, all else equal.




13:30 Uhr


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Paquita Zuidema, University of Miami


Bjorn Stevens

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