Cloud reflectivity is a function of cloud liquid water content and droplet
number concentration. Since cloud droplets form around pre-existing aerosol
particles, cloud droplet number concentration depends on the availability of
particles that can serve as cloud condensation nuclei. Given constant liquid
water amount, increased availability of cloud condensation nuclei leads to
clouds with a greater droplet number concentration, greater total droplet
surface area and consequently, greater reflectivity. The change in cloud
reflectivity resulting from the increased availability of condensation nuclei is
known as the aerosol indirect effect. The aerosol indirect effect ranks as one
of the largest sources of uncertainty in current estimates of global climate
change, largely due to difficulties in measurement. Changes in cloud
reflectivity resulting from the aerosol indirect effect are typically much
smaller than the natural background variability observed in clouds. As a
result, the modification signal is very difficult to detect against the
background noise. Additionally, since atmospheric aerosols are ubiquitous, it
is difficult to find polluted and nonpolluted clouds that are sufficiently alike
for reasonable comparison. However, ship tracks seen in satellite images
present one opportunity to study the aerosol indirect effect in relative
isolation. Ship tracks are regions of enhanced reflectivity in marine stratus,
resulting from the addition of aerosols from ship exhaust plumes to
preexisting clouds. Ship tracks are a common feature of satellite images of
the North Pacific. Since the marine atmosphere has comparatively low
background aerosol concentrations, the addition of ship exhaust particles can
lead to distinct increases in cloud reflectivity. Ship tracks allow for sampling
of polluted and nonpolluted clouds from adjacent regions with similar solar
and viewing geometry, cloud temperatures and surface properties, and
consequently provide a unique opportunity to study the effects of aerosol
modification of cloud reflectivity. Using satellite images of the North Pacific
in July 1999, over 1000 ship tracks were identified, logged and analyzed,
yielding 504 sets of radiance data matching polluted clouds with nearby
nonpolluted clouds. It was expected that increasing the size of the region for
selection of nonpolluted clouds would increase the variability in observed
reflectivity, and make detection of the modification signal more difficult. In
order to study this potential effect of domain size for selection of nonpolluted
clouds on measurements of the aerosol indirect effect, three data sets were
collected, using domain sizes for selection of nonpolluted clouds of 15, 50
and 100 km. Analysis of retrieved optical depth and droplet effective radius
for modified and control pixels shows evidence of a 1-5% increase in visible
optical depth of marine stratus following modification by addition of ship
exhaust particles, but unexpectedly, shows only slight increases in uncertainty
with increasing domain size. A subsequent study revealed that
autocorrelation lengths of radiances and retrieved cloud properties were only
8-15 km. This indicates that even the 15 km control domain captured much of
the background variability present. Domain sizes smaller than 15 km are
difficult to sample automatically while avoiding the inclusion of polluted
clouds in the nonpolluted cloud sample. As a result, it remains necessary to
analyze large numbers of ship tracks to separate the aerosol modification
signal from the background variability. / Graduation date: 2003
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28779 |
| Date | 23 May 2002 |
| Creators | Walsh, Christopher D. |
| Contributors | Coakley, James A. Jr |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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