Journal article

Observations of the boundary layer, cloud, and aerosol variability in the southeast Pacific near-coastal marine stratocumulus during VOCALS-REx

Zheng X, Albrecht B, Jonsson H, Khelif D, Feingold G, Minnis P, Ayers K, Chuang P, Donaher S, Rossiter D, Ghate V, Ruiz-Plancarte J, Sun-Mack S ...see all

Atmospheric Chemistry and Physics, vol. 11, issue 18 (2011) pp. 9943-9959

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Aircraft observations made off the coast of northern Chile in the
Southeastern Pacific (20 degrees S, 72 degrees W; named Point Alpha)
from 16 October to 13 November 2008 during the VAMOS
Ocean-Cloud-Atmosphere-Land Study-Regional Experiment (VOCALS-REx),
combined with meteorological reanalysis, satellite measurements, and
radiosonde data, are used to investigate the boundary layer (BL) and
aerosol-cloud-drizzle variations in this region. On days without
predominately synoptic and meso-scale influences, the BL at Point Alpha
was typical of a non-drizzling stratocumulus-topped BL. Entrainment
rates calculated from the near cloud-top fluxes and turbulence in the BL
at Point Alpha appeared to be weaker than those in the BL over the open
ocean west of Point Alpha and the BL near the coast of the northeast
Pacific. The cloud liquid water path (LWP) varied between 15 gm(-2) and
160 gm(-2). The BL had a depth of 1140 +/- 120 m, was generally
well-mixed and capped by a sharp inversion without predominately
synoptic and mesoscale influences. The wind direction generally switched
from southerly within the BL to northerly above the inversion. On days
when a synoptic system and related mesoscale costal circulations
affected conditions at Point Alpha (29 October-4 November), a moist
layer above the inversion moved over Point Alpha, and the total-water
mixing ratio above the inversion was larger than that within the BL.
The accumulation mode aerosol varied from 250 to 700 cm(-3) within the
BL, and CCN at 0.2% supersaturation within the BL ranged between 150
and 550 cm(-3). The main aerosol source at Point Alpha was horizontal
advection within the BL from south. The average cloud droplet number
concentration ranged between 80 and 400 cm(-3). While the mean LWP
retrieved from GOES was in good agreement with the in situ measurements,
the GOES-derived cloud droplet effective radius tended to be larger than
that from the aircraft in situ observations near cloud top. The aerosol
and cloud LWP relationship reveals that during the typical well-mixed BL
days the cloud LWP increased with the CCN concentrations. On the other
hand, meteorological factors and the decoupling processes have large
influences on the cloud LWP variation as well.

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