Atlantic Stratocumulus Transition Experiment

Project Description
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-scale cloud properties.

A telescoping approach was used in ASTEX to investigate connections
between scales ranging from microns to thousands of
kilometers. Satellites and upper- level aircraft provided a
description of large-scale cloud features, and instrumented aircraft
flying in the boundary layer and surface- based remote sensing systems
provided a description of the mean, turbulence, and mesoscale
variability in cloud microphysical properties of boundary layer
clouds. A major deficiency of the FIRE observations, however, was an
inadequate definition of the large-scale fields of temperature,
moisture, and winds. This deficiency was removed for ASTEX by making 4
- 8 soundings per day from the surface sites and ships, and including
many of these upper-air observations on the Global Telecommunications
System (GTS) for assimilation into the ECMWF and NMC
analyses. Furthermore, based on the demonstrated utility of
surface-based remote sensing during FIRE (Albrecht et al. 1990), the
use of such systems was expanded during ASTEX.

For more information, link to
http://kiwi.atmos.colostate.edu/scm/astex.html