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Clouds & AerosolsWhy?Clouds are important to climate because they strongly modulate incoming solar and outgoing thermal radiation. Clouds, as the source of precipitation, are also a key element in the hydrologic cycle. Clouds are currently under intense scrutiny by researchers to gain a better understanding of their role in our environment. This knowledge is needed for better predictions of climate change, to guide policy in ameliorating or adjusting to change, and to provide better stewardship of our depleting water resources. Aerosol particles are also important to climate, directly by scattering light and indirectly by serving as cloud condensation nuclei. Aerosol particles in high concentrations are health-endangering pollutants, and in lower concentrations also aid air quality observations as indicators of the movement and dilution of other pollutants. Lidar can reveal many characteristics of cloud and aerosol particles using light backscattered from them. Cloud ResearchSimple geometrical properties of clouds, such as fractional cover and cloud boundaries, are fundamental to radiative transfer. Microphysical parameters, such as phase (ice or water) and particle size, are also significant. Our lidar division has been developing novel methods to measure some of these basic cloud properties. Some methods utilize the particular attributes of infrared-wavelength lidar to infer average drop size and to discriminate between water and ice particles. Other methods use simultaneous lidar and radar measurements for profiling mean size, water content, and number density in cirrus clouds. The latter method is being extended to water clouds. We are also investigating the information content in Doppler lidar measurements of ice particle vertical motions. Another phenomenon under study is how often ice crystals orient with the long axis in the horizontal. Compared to random orientation, this alters radiative transfer and complicates interpretation of the lidar signal. Another topic of interest is the complementary nature of lidar and radar in providing a more complete description of cloud boundaries than either instrument alone. We apply these new methods, along with more traditional lidar techniques, in field campaigns to study cloud characteristics. These results eventually guide development of computer models for understanding and predicting climate. Numerical modeling rounds out the cloud research within our division. By including detailed microphysics in sophisticated dynamics models, the behavior of drops, precipitation, and aerosol particles in stratiform clouds are better simulated, providing improved understanding of cloud evolution, drizzle formation, cloud processing of aerosol, and cloud radiative properties. Aerosol ResearchStratospheric aerosol particles directly effect radiative transfer and can contribute to depletion of the ozone shield. Our lidars have observed enhancement of stratospheric particles from volcanic eruptions, and we developed a new two-lidar technique to profile the mean particle size. Cloud condensation nuclei (CCN) play a major role in cloud formation. We have conceived a number of aerosol retrieval methods to determine the fraction of aerosols that are hygroscopic. Specific Topics
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