Meteorological observations on the northern Chilean coast during VOCALS-REx
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Rutllant Costa, José
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Meteorological observations on the northern Chilean coast during VOCALS-REx
Abstract
Surface coastal observations from two automatic
weather stations at Paposo ( 25 S) and radiosonde observations
at Paposo and Iquique ( 20 S) were carried out
during VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-
Land Study Regional Experiment). Within the coastal marine
boundary layer (MBL), sea–land breezes are superimposed
on the prevailing southerlies, resulting in light northeasterly
winds from midnight to early morning and strong
southwesterlies in the afternoon. The prevailing northerlies
above the MBL and below the top of the Andes are modulated
by the onshore–offshore (zonal) flow forced by the diurnal
cycle of surface heating/cooling along the western slope
of the Andes. The daytime phase of this diurnal cycle is consistent
with an enhanced afternoon coastal subsidence manifested
in afternoon warming near the top of the subsidence
inversion ( 1.8K at 800 hPa), lowering ( 130 m) of its base
(top of the MBL), and clearing of coastal Sc (stratocumulus)
clouds. Results from a numerical simulation of the atmospheric
circulation in a mean zonal cross section over
the study area capture the afternoon zonal wind divergence
and resulting subsidence of about 2 cm s−1 along a narrow
( 10 km) coastal strip maximizing at around 800 hPa. Dayto-
day variability in the MBL depth during VOCALS-REx
shows sub-synoptic oscillations, aside from two major disruptions
in connection with a deep trough and a cutoff low, as
described elsewhere. These oscillations are phase-locked to
those in sea-level pressure and afternoon alongshore southerlies,
as found in connection with coastal lows farther south.
From 24-h forward trajectories issued from significant
points at the coast and inland at the extremes of the diurnal
cycle, it can be concluded that the strong mean daytime
Andean pumping prevents any possibility of continental sulfur
sources from reaching the free troposphere above the Sc
cloud deck in at least a one-day timescale, under mean conditions.
Conversely, coastal sources could contribute with sulfur
aerosols preferentially in the morning, provided that the
weak daytime inland flow becomes partially blocked by the
coastal terrain.
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Atmos. Chem. Phys., 13, 3409–3422, 2013
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