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University of Wisconsin–Madison

Cloud and Atmospheric Physics

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 formation.

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 convective system.
  • 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 clouds.
  • 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 geographical locations.

Faculty Involved

Steve AckermanTristan L'EcuyerGrant PettyGreg Tripoli