The general area of interest for ASTEX was a region of the eastern Atlantic extending southeast from Santa Maria (37°N, 25°W) in the Azores to Porto Santo (33°N, 16°W), a small island just north of Madeira This region was selected since (1) the climatology (Hanson, 1990) in this area was favorable for low-level cloud conditions ranging from solid stratocumulus decks to broken trade cumulus; (2) the islands in this region provided suitable sites for surface measurements and aircraft operations; and (3) we were able to coordinate our efforts with an ONR oceanographic investigation called the Subduction Accelerated Research Initiative (SARI) that was conducted in this same region. Aircraft operations were be made from Santa Maria and Terceira. Island-based observations were made from Santa Maria and Porto Santo.
A key strategy was the measurement of the budgets of mass, thermodynamic energy, and moisture within an Eulerian 'grid column' located in the ASTEX observing region in order to provide datasets that could be used to drive and validate climate models. Some of the observations were needed as inputs to climate model codes; good examples were the large-scale pressure gradient force, the large-scale vertical motion field, and the tendencies of temperature and moisture due to horizontal advection. A single-column version of a climate model was used to simulate observed ASTEX cases, since external forcings, which no single-column model can determine, were provided as input. Additional ASTEX observations were compared directly with results produced by a single-column climate model; examples were the cloudiness, the convective and radiative fluxes, and the vertical profiles of temperature and water vapor. The observations were also used to test, decisively, the physical assumptions underlying the parameterizations used in a climate model. For example, models typically parameterize the entrainment rate in terms of the convective flux profiles; ASTEX provided observations of both the flux profiles and the entrainment rate, allowing direct tests of such closure assumptions.
A second strategy was to obtain atmospheric, chemical, and cloud measurements within a moving, or Lagrangian, air mass. By following a "tagged" parcel of air for a day or two, it was possible to directly observe the conversion of precursor gases into aerosols, thus strengthening the link between trace gas emissions and the CCN that influence the life cycles of clouds. A Lagrangian experiment also permited us to make much better estimates of the fluxes of these aerosols and gases into the boundary layer where the clouds form and dissipate. Atmospheric chemists launched several "smart" constant-level balloons (which could adjust their own buoyancy to maintain a preset height) from the UNOLS Oceanus research vessel as a surrogate "tag" of a mass of boundary layer air for extensive airborne measurements on two successive days. Trajectory forecasts were made using ECMWF and NORAPS analyses. Two 48-hour Lagrangian experiments were conducted during ASTEX. The experiments started about 500 km upwind of the aircraft staging area and traversed into the ASTEX triangle 24-48 hours later.
These Lagrangian measurements were of great help in understanding the dynamics associated with the transition from stratocumulus to cumulus. Process models of the CBL predicted physical variables following columns of air moving with the mean CBL wind velocity. Direct measurements of turbulence, microphysics, and radiation parameters provided an excellent comparison test for such models, and provided an excellent opportunity to directly observe a transition between cloud types in a given air column.
In summary, a large number of platforms, instruments, and research organizations participated
in ASTEX. Surface observations were made from the islands of Santa Maria, Azores, and Porto
Santo, Madeira, and from the UNOLS ship Malcom Baldrige. Instruments used included Doppler
radars, wind profilers, microwave radiometers, short and longwave radiometers, spectrometers,
and rawinsondes. Other island measurements included a Doppler sodar, lidar, and tethered
balloons. The German ship Valdivia was located about 900 km equidistance from the two islands
so as to make an equilateral triangle. Three other research vessels made surface and oceanic
measurements within and around the ASTEX triangle. Rawinsondes were released on a 3-hourly
basis from the two islands and from three ships. Seven aircraft were used to make observations
above, within, and below the low-lying clouds. A network of stationary and drifting buoys was
deployed in the ASTEX area. The observations were made in concert with overpassing satellites.
ECMWF, NMC, and mesoscale model analyses will be combined with the ASTEX observations to
define the large-scale dynamic and thermodynamic fields. Other collaborative experiments
participating were SARI, the Surface of the Ocean Fluxes and Interaction with the Atmosphere
(SOFIA), and the Marine Aerosol and Gas Exchange (MAGE). Close to 300 researchers from
over 50 research institutions from the U. S., United Kingdom, France, Russia, Spain, Germany,
Netherlands, and Portugal participated.
FUTURE PLANS
The data analysis phase for ASTEX will include both individual and multi-investigator analyses. End data products for the field experiment are expected to be quantitative measurements of the radiative, physical, microphysical, optical, dynamical, thermodynamical, and meteorological properties of marine stratocumulus clouds.