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| Titel |
An aircraft case study of the spatial transition from closed to open mesoscale cellular convection over the Southeast Pacific |
| VerfasserIn |
R. Wood, C. S. Bretherton, D. Leon, A. D. Clarke, P. Zuidema, G. Allen, H. Coe |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 5 ; Nr. 11, no. 5 (2011-03-15), S.2341-2370 |
| Datensatznummer |
250009462
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| Publikation (Nr.) |
 copernicus.org/acp-11-2341-2011.pdf |
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| Zusammenfassung |
Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the
remote subtropical southeast Pacific (18° S,
80° W). Data from two aircraft that took measurements
approximately twelve hours apart but in the same advected
airmass are used to document a remarkably sharp spatial
transition in marine boundary layer (MBL), cloud, and aerosol
structure across the boundary between a well-mixed MBL
containing overcast closed mesoscale cellular stratocumulus,
and a pocket of open cells (POC) with significantly lower
cloud cover. Long (~190–250 km) straight and level
flight legs at three levels in the marine boundary layer and
one level in the lower free troposphere permit sampling of the
closed cells, the POC, and a 20–30 km wide transition zone
with distinctly different structure from the two airmasses on
either side. The POC region consists of intermittent active
and strongly precipitating cumulus clouds rising and
detraining into patches of drizzling but quiescent stratiform
cloud which is optically thin especially toward its edges.
Mean cloud-base precipitation rates inside the POC are several
mm d−1, but rates in the closed cell region are not
greatly lower than this. This latter finding suggests that precipitation is
not a sufficient condition for POC formation from overcast
stratocumulus. Despite similar cloud-base precipitation rates
in the POC and overcast region, much of the precipitation
(>90%) evaporates below cloud in the overcast region,
while there is significant surface precipitation inside the
POC. In the POC and transition region, although the majority
of the condensate is in the form of drizzle, the integrated
liquid water path is remarkably close to that expected for
a moist adiabatic parcel rising from cloud base to top.
The transition zone between the POC and the closed cells often
consists of thick "boundary cell" clouds producing mean
surface precipitation rates of 10–20 mm d−1,
a divergent quasi-permanent cold/moist pool below cloud,
a convergent inflow region at mid-levels in the MBL, and
a divergent outflow near the top of the MBL.
The stratiform clouds in the POC exist within an ultra-clean
layer that is some 200–300 m thick. Aerosol concentrations
(Na) measured by a PCASP in the diameter range
0.12–3.12 μm in the center of the ultra-clean layer
are as low as 0.1–1 cm−3. This suggests that
coalescence scavenging and sedimentation is extremely
efficient, since Na in the subcloud layer, and
droplet concentration Nd in the active cumuli are
typically 20–60 cm−3. The droplet concentrations in
the quiescent stratiform clouds are extremely low (typically
1–10 cm−3), and most of their liquid water is in the
form of drizzle, which mainly evaporates before reaching the
surface. The cloud droplet concentration in the overcast
region decreases strongly as the transition region is
approached, as do subcloud accumulation mode aerosol
concentrations, suggesting that coalescence scavenging is
impacting regions in the overcast region as well as inside the
POC. Both flights show lower accumulation mode aerosol
concentration in the subcloud layer of the POC (Na ∼ 30 cm−3) compared with the overcast region
(Na ∼ 100 cm−3), but elevated (and mostly
volatile) total aerosol concentrations are observed in
the POC at all levels around 20–50 km from the transition
zone, perhaps associated with some prior nucleation event.
Despite the large differences in cloud and MBL structure
across the POC-overcast boundary, the MBL depth is almost the
same in the two regions, and increases in concert over the
12 h period between the flights. |
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