| Numerical experiments were carried out using the Tel-Aviv University 2-D
cloud model to investigate the effects of increased concentrations of Cloud
Condensation Nuclei (CCN), giant CCN (GCCN) and Ice Nuclei (IN) on the
development of precipitation and cloud structure in mixed-phase sub-tropical
convective clouds. In order to differentiate between the contribution of the
aerosols and the meteorology, all simulations were conducted with the same
meteorological conditions.
The results show that under the same meteorological conditions, polluted
clouds (with high CCN concentrations) produce less precipitation than clean
clouds (with low CCN concentrations), the initiation of precipitation is
delayed and the lifetimes of the clouds are longer. GCCN enhance the total
precipitation on the ground in polluted clouds but they have no noticeable
effect on cleaner clouds. The increased rainfall due to GCCN is mainly a
result of the increased graupel mass in the cloud, but it only partially
offsets the decrease in rainfall due to pollution (increased CCN). The
addition of more effective IN, such as mineral dust particles, reduces the
total amount of precipitation on the ground. This reduction is more
pronounced in clean clouds than in polluted ones.
Polluted clouds reach higher altitudes and are wider than clean clouds and
both produce wider clouds (anvils) when more IN are introduced. Since under
the same vertical sounding the polluted clouds produce less rain, more water
vapor is left aloft after the rain stops. In our simulations about 3.5 times
more water evaporates after the rain stops from the polluted cloud as
compared to the clean cloud. The implication is that much more water vapor
is transported from lower levels to the mid troposphere under polluted
conditions, something that should be considered in climate models. |