Journal article

Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx

Bretherton C, Wood R, George R, Leon D, Allen G, Zheng X ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 21 (2010) pp. 10639-10654

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Multiplatform airborne, ship-based, and land-based observations from 16
October-15 November 2008 during the VOCALS Regional Experiment (REx) are
used to document the typical structure of the Southeast Pacific
stratocumulus-topped boundary layer and lower free troposphere on a
transect along 20 degrees S between the coast of Northern Chile and a
buoy 1500 km offshore. Strong systematic gradients in clouds,
precipitation and vertical structure are modulated by synoptically and
diurnally-driven variability. The boundary layer is generally capped by
a strong (10-12 K), sharp inversion. In the coastal zone, the boundary
layer is typically 1 km deep, fairly well mixed, and topped by thin,
nondrizzling stratocumulus with accumulation-mode aerosol and cloud
droplet concentrations exceeding 200 cm(-3). Far offshore, the boundary
layer depth is typically deeper (1600 m) and more variable, and the
vertical structure is usually decoupled. The offshore stratocumulus
typically have strong mesoscale organization, much higher peak liquid
water paths, extensive drizzle, and cloud droplet concentrations below
100 cm(-3), sometimes with embedded pockets of open cells with lower
droplet concentrations. The lack of drizzle near the coast is not just a
microphysical response to high droplet concentrations; smaller cloud
depth and liquid water path than further offshore appear comparably
important. Moist boundary layer air is heated and mixed up along the
Andean slopes, then advected out over the top of the boundary layer
above adjacent coastal ocean regions. Well offshore, the lower free
troposphere is typically much drier. This promotes strong cloud-top
radiative cooling and stronger turbulence in the clouds offshore. In
conjunction with a slightly cooler free troposphere, this may promote
stronger entrainment that maintains the deeper boundary layer seen
Winds from ECMWF and NCEP operational analyses have an rms difference of
only 1 m s(-1) from collocated airborne leg-mean observations in the
boundary layer and 2 m s(-1) above the boundary layer. This supports the
use of trajectory analysis for interpreting REx observations. Two-day
back-trajectories from the 20 degrees S transect suggest that eastward
of 75 degrees W, boundary layer (and often free-tropospheric) air has
usually been exposed to South American coastal aerosol sources, while at
85 degrees W, neither boundary-layer or free-tropospheric air has
typically had such contact.

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  • C. S. Bretherton

  • R. Wood

  • R. C. George

  • D. Leon

  • G. Allen

  • X. Zheng

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