Climate and Climate Change
Large Scale Dynamics
Radiation and Remote Sensing
Cloud and Atmospheric Physics
Cloud & Precipitation Physics and
Chemistry Physical and chemical processes in the atmosphere
are often intertwined and influencing each other. One
of the most obvious examples of this mutual influence
is the formation of clouds. It is commonly thought
that clouds form when the air is saturated with water
vapor, but the real process is much more complicated.
Studies show that without the help of condensation
nuclei or ice nuclei, small particles in the air that
promote the formation of cloud droplets or ice crystals
respectively, it would require several hundred percent
relative humidity to initiate the condensational process.
Different chemicals have different efficiencies in
achieving nucleation. Hence the chemical type of particles
may have great influence on cloud and precipitation
Once formed, clouds will have significant impacts
on the atmosphere within which they form. These are
the cloud feedback mechanisms. For example, the formation
of a thick cloud shields the underlying surface from
solar radiation, resulting in a local cooling. Formation
of widespread persistent cloud cover may cause a significant
reduction of solar heating on the surface and influence
the global climate process. Recently, it has been found
that not only thick clouds such as cumulonimbus, but
even thin clouds such as cirrus, have great impact
on the radiation budget of the earth- atmosphere system.
Another example is the cleansing of aerosol particles
in the atmosphere. Some of the aerosol particles serve
as condensation nuclei to initiate the cloud formation.
The formation of clouds and precipitation, in turn,
carries the particles back to the earth's surface,
thus resulting in the removal of these particles from
the atmosphere. One of the burning questions in this
regard is how efficiently clouds can cleanse the atmosphere,
in light of the increasing loading of man-made chemicals
due to industrial activities.
The following is a partial list of several research
projects that we are engaged in, relating to this area:
- We study how, and how fast, ice particles (pristine
ice crystals, snow, graupel, and hail) grow in clouds.
We solve the Navier-Stokes equations to determine
the flow fields around falling ice particles. We
determine the water vapor distribution around falling
ice particles and the collision (riming) efficiencies
of cloud droplets by ice crystals.
- We developed a three-dimensional nonhydrostatic
cloud model with detailed microphysics packages that
can be used to simulate the cloud formation process
realistically. We use this model to study how sensitive
the cloud formation is to a particular microphysical
mechanism (for example, nucleation, riming, or melting).
- The cloud model is currently used to study the
cirrus plume phenomenon above thunderstorm anvils.
This phenomenon may be an important conduit of the
transport process between the troposphere and stratosphere.
- We determine, both experimentally and theoretically,
the efficiency with which aerosol particles are removed
from the atmosphere by rain and snow.
- We also use the cloud model to simulate the interaction
of SO2 with cloud and precipitation particles, and
the formation and transport of sulfates in a deep
- In order to understand the impacts of cirrus clouds
on the radiative budget and climate, we developed
a cirrus model with both radiation and microphysics
packages that can simulate the evolution of cirrus
- Other areas of research include the effect of
electricity on the cloud microphysical processes,
the microphysical structure of narrow cold-frontal
rainbands, and the partitioning of hydrometeors by
particle type in deep convective clouds at various
Larissa Back, Steve Ackerman, Tristan L'Ecuyer, Grant Petty, Pao Wang
Research Groups Home Pages:
Physics Group Web Page