.,,
Us.
UCRL-ID-138871
Reduction of Tropical
Cloudiness by Soot
A. S. Ackerman, O.B. Toon, D.E. Stevens, A.J. Heymsfield,
V. Ramanathan and E.J. We/ton
kpmtment ofEnergy
I&
Lawrence
Liverrnore
National
Laboratory
May 8,2000
Approved for public release; further dissemination unhmited
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Reduction of tropical cloudiness by soot
A. S. Ackerman*, O. B. Toon2, D. E. Steverrs3, A. J. Heymstleld4, V. Ramanathan5,
E. J. Weltonb
Measurements and models show that enhanced aerosol concentrations can augment cloud albedo
not only by increasing total droplet cross-sectional area, but also by reducing precipitation and
thereby increasing cloud water content and cloud coverage. Aerosol pollution is expected to exert
a net cooling influence on the global climate through these conventional mechanisms. Here we
demonstrate an opposite mechanism through which aerosols can reduce cloud cover and thus
significantly offset aerosol-induced radiative cooling at the top of the atmosphere on a regional
scale. In model simulations the daytime clearing of trade cumulus is hastened and intensified by
solar heating in dark haze (as found over much of tire northern Indian Ocean during the northeast
monsoon).
lNASA Ames Research Center, Moffett Field, CA, 94035, USA.
2University of Colorado, Boulder, CO, 80309, USA.
3Lawrence Livermore National Laboratory, Llverrnore, CA, 94551, USA.
4Natiorral Center for Atmospheric Research, Boulder, CO, 80301, USA.
5Scripps Institution of Oceanography, La Jolla, CA, 92093, USA.
%cience Systems and Applications, Greenbelt, MD, 20771, USA.
*To whom correspondence should be addressed. E-mail: ack@sky.arc.nasa. gov
A primary objective of the Indian Ocean Experiment (INDOEX) was to quantify the
indirect effect of aerosols on climate through their effects on clouds (1). Conventionally,
increased aerosol concentrations are expected to increase cloud droplet concentrations, and hence
total droplet cross-sectional area, thereby causing more sunlight to be reflected to space (2).
Furthermore, model simulations of marine stratocumulus (3, 4, 5) and observations of ship tracks
(6, 7,8) suggest that increased aerosol concentrations can enhance cloud water content, physicrd
thickness and areal coverage by decreasing precipitation. Deep layers of dark (solar absorbing)
haze were observed over much of the tropical northern Indian Ocean in February-March of
1998-1999 during INDOEX (9, 10). The clouds observed in the northern hemisphere were
typicrdly embedded in the haze, as seen in Fig. 1. In contrast to the conventional expectation that
aerosols augment cloud depth and coverage, very sparse cloud cover is found in that region
during that time of year (11). These INDOEX observations suggest a new mechanism by which
aerosols impact clouds, in which a dark haze can significantly reduce meal coverage of trade
cumulus (the predominant cloud type expected at that latitude and season).
Model simulations of marine stratocumulus indicate that intense absorption of solar
energy can desiccate an opticafly thick stratocumulus cloud layer (12). Here we show that
significantly less intense aerosol-induced solar absorption, of the magnitude observed in the 1998
INDOEX measurements, can drastically after the properties of trade cumulus, which are drken
by dynamics that differ greatly from stratocumulus (13). hr well-mixed stratocumulus-topped
boundary layers found over cool subtropical water, convection is driven by downdrafts that are
generated by radiative cooling near cloud top. Enhanced solar heating can offset the longwave
cooling enough to reduce convective mixing and effectively cut off the cloud layer from its
source of moisture, thereby dksipating the cloud w). Trade-cumulus are found over warm
tropicaJ water in boundary layers typically -1.5-3 km deep (compared to less than -1 km for
stratocumulus-topped mixed layers) and appear in the conditionally unstable zone between the
well-mixed surface layer and the trade inversion capping the boundaxy layer. In the trade-wind
boundruy layer, particularly energetic updrafts rise far enough and release enough latent heat by
condensation to become buoyant, accelerating upwards until resisted by the stability of the trade
inversion. Detrained cloud water spreads cnrtbelow the trade inversion as an anvil, evaporating
as it mixes with its environment. Trade-cumulus cloud cover is typicafly dominated by these
remnants of convection, which evaporate more rapidly with decreasing humidity (14). Infrared
cooling from anvils can drive local turbulence, which mixes moisture up from below (& 15).
The INDOEX observations suggest a conceptual model in which dark haze amplifies the
radiatively-driven diurnaf cycle of cloudiness by increasing solar heating in the boundary layer.
Composite time series of cloud coverage from trade-cumulus field projects rrflshow sinusoidal
variation, with maximum coverage between 7 and 10 AM, and a minimum between 4 and 10PM
(IQ. The daytime clearing has been attributed to two complementary mechanisms: (i) solar
heating directly reduces relative hurnidhies, thereby accelerating evaporation of the anvils, and
(ii) solar heating maximizes near the top of the boundary layer, where cloud coverage is greatest,
thereby stabilizing the boundary layer and suppressing convection ~.
We focus hereon the amplification of daytime clearing due to aerosol-induced solar
heating. Rather than attempting to cover the many possible combinations of meteorology and
pollution, our strategy is to adopt a representative trade-cumulus scenario and compare a
sequence of model simulations subject to varying degrees of aerosol-induced solar heating. The
tool we use is a large-eddy simulation model (17,)with pammeterized precipitation (U) ~d
plane-parallel radiative transfer (18).
For a meteorological context we use measurements averaged over the fust five days of
the Atlantic Trade-Wind Experiment (ATEX), characterized as “nearly classic” trade-cumulus
(19). The model is initialized with surface conditions and soundings from the upstream ship in
the ATEX flotilla (20). As in previous studies (&l, U D, we ignore my ~lm’rr~v~ation in
sea-surface temperature (21). Large-scafe advective forcings are pararneterized to represent the
net influx of cooler, drier air in the equatorward flow through the model domain (22).
We specify varying degrees of absorbing aerosol pollution as follows (23). For the
baseline case, cloud droplet and haze concentrations are fixed at 250 and 1200 cm-3, respectively
(24); in this case the haze is non-absorbing and has an optical depth of 0.17 (at 0.5 Lm). We
idealize the 1998 INDOEX measurements with the same concentrations, embedding a soot core
of 0.06 ~m radius within each haze particle and resulting in a 0.5-~m single-scattering albedo of
0.88 and optical depth of 0.20. The aerosol-induced diurnal-average solar heating of the
cloudless boundary layer for this haze is 0.5 K/d (25). We idealize the more polluted conditions
measured during the 1999 INDOEX campaign by doubling the concentration of haze particles,
which all have soot cores.
Fig. 2 reveals a number of salient features of the baseline simulations (26). Cumulus
convection, with cloud-base atop the surface mixed-layer, arises an hour into the simulation and
penetrates the 250 m deep trade-inversion starting at
-1600 m. Subsequent convection is
sporadic, owing to our limited model domain (6.4x 6.4 km). A local maximum of relative