Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 309

the reduction of total water and cloud drops account for up to 20 % compared to a
control run without GCCN. Therefore, the incorporation of the GCCN accelerates
the hydrological cycle so that clouds precipitate faster (but not more).
Van den Heever et al. ( 2006 ) highlight the fact that the impacts of varying
GCCN and IN concentration are just as significant as those associated with CCN
in convective storms and their subsequent anvil development. With varying CCN,
GCCN, and IN concentrations, they found that all three nucleating aerosols affect
the depth, microphysical characteristics, water mass, and organization of the anvil.
Updrafts are consistently stronger as the concentrations of cloud-nucleating aerosol
are increased, with CCN having the greatest impact on updraft during the initial
stages and GCCN and IN concentrations having a greater impact on updraft
strength during the mature and dissipating storm stages. Cloud water increases
with increasing aerosol concentrations, with increases in GCCN concentrations
producing the most cloud water on average. Increasing either the GCCN or IN
concentrations produces the most rainfall at the surface, whereas enhanced CCN
concentrations reduce surface rainfall. The same model (RAMS) was also used by
Zhang et al. ( 2009 ) to investigate the mechanisms by which Saharan dust acting as
CCN can impact a tropical cyclone evolution. They showed that by adding CCN
within the initial environment, CCN directly affected the early eyewall evolution by
varying distributions of latent heat, therefore triggering variations in dynamic and
thermodynamic processes that ultimately modify eyewall intensity. Furthermore,
CCN indirectly affected the eyewall by modulating rainband development but in
a way that did not systematically depend on input CCN. Finally, Karydis et al.
( 2011 ) used the NASA Global Modeling Initiative (GMI), modular 3-D chemistry,
and transport model to assess the contribution of insoluble dust to global CCN
and CDNC. The predicted annual average contribution of insoluble mineral dust
to CDNC in cloud-forming areas was found to be up to 40 and 24 %, respectively,
with the results being sensitive to the level of hygroscopicity as well as the dust
size distribution. Coating of dust by hygroscopic salts during atmospheric aging
can substantially deplete in-cloud supersaturation during the initial stages of cloud
formation and therefore eventually reduce CDNC.
As mentioned earlier, IN in mixed-phase clouds tend to counteract the effect
of increased CCN. Lohmann and Diehl ( 2006 ) found that mineral dust in mixed-
phase stratiform clouds can have a significant impact on the liquid water path, cloud
lifetime, precipitation rate, and top of the atmosphere radiation on a global scale.
Although they based their IN parameterizations on laboratory data which has been
superseded, they show that stratus clouds are very sensitive to the parameterization
of ice nucleation. They show that precipitation is enhanced which reduces lifetime
of clouds, resulting in a warming due to reduced cloud cover. The decrease in
net radiation at the top of the atmosphere was up to 2.1 W m 2 , which is of a
similar magnitude but opposite direction to the anthropogenic CCN impacts on
cloud albedo. Storelvmo et al. ( 2011 ) modeled the impact of increased ice particle
concentrations on the radiative properties of clouds and showed that the greater
reflectivity largely counteracted the increased reflectivity from decreased cloud
lifetime.