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type: Concept

ASTEX
type: Concept
in scheme: projects
definition: The Atlantic Stratocumulus Transition Experiment (ASTEX) was conducted in June 1992 off North Africa in the area of Azores and Madeira Islands. ASTEX was based on two islands and several ships in an area where the total cloud cover (mostly stratocumulus) ranges from 50 - 60%. The region is dominated by low-level clouds with moderate optical thicknesses, from about 1 to 10 on average. The optically thinner (more highly broken) clouds generally have cloud tops below the 800 mb level). The optically thicker clouds have lower top pressures down to about 700 mb. The region is characterized by broken low cloudiness and strong gradients of low level cloud amount. Satellite studies show cloud conditions ranging from solid stratocumulus decks to broken trade cumulus. The region is not directly influenced by continental effects, and islands provide suitable sites for surface observations and aircraft operations. ASTEX was thus able to address issues related to the stratocumulus to trade-cumulus transition and cloud-mode selection. ASTEX involved intensive measurements from several platforms and was designed to study how the transition and mode selection are affected by 1) cloud-top entrainment instability, 2) diurnal decoupling and clearing due to solar absorption, 3) patchy drizzle and a transition to horizontally inhomogeneous clouds through decoupling, 4) mesoscale variability in cloud thickness and associated mesoscale circulations, and 5) episodic strong subsidence lowering the inversion below the lifting condensation level. From a broader perspective ASTEX was designed to provide improved dynamical, radiative, and microphysical models and an improved understanding of the impact of aerosols, cloud microphysics, and chemistry on large-scale cloud properties. From a broader perspective ASTEX was designed to provide improved dynamical, radiative, and microphysical models and an improved understanding of the impact of aerosols, cloud microphysics, and chemistry on large-scal…