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Seminar: Wednesday, March 11, 2009 at 3:30 p.m.

The life of a snowflake : Measuring snow from space
Dr. Benjamin Johnson
UMBC Joint Center for Earth Systems (JCET) and NASA Goddard Space Flight Center

On Earth, the vast majority of precipitation originates as snow particles in the atmosphere. At middle and high latitudes, snow falling on land contributes to snowpack; a critical source of freshwater for spring and summer months. Snow on the ground also provides an atmospheric cooling mechanism due to it's high visible-light albedo. In the middle and lower latitudes, falling snow often melts, becoming rain -- feeding our streams, rivers, and lakes. The growth and melting of snow also contributes to heating / cooling of the atmosphere through phase change, influencing atmospheric motions and stability. It is critical, especially given our changing climate, to be able to accurately measure atmospheric snow on a global basis for long time periods.

One of the key challenges to measurement is understanding how the physical characteristics a snowflake influence millimeter-wavelength observations made from a satellite-based remote sensing platform. On both the microscopic and synoptic scales, the physical aspects of the initiation, growth, and dissipation of an individual snowflake are reasonably well understood. However, when considering a precipitating cloud containing a wide variety of sizes, habits, orientations, phases, of snow, the problem of characterizing the 3-D scene in analytical fashion becomes troublesome.

In this talk I will focus the physical processes which contribute to the growth and eventual destruction of an individual snowflake, and I will describe how these individual processes are treated when considering a diverse ensemble of snow particles within, for example, a precipitating cloud. I will also briefly describe how the physical properties of snow particles influence incident electromagnetic waves at centimeter and millimeter wavelengths (~1-22 mm or 13 to 220 GHz), and the ramifications for inferring information about snow based on satellite observations at these wavelengths.

Physics Bldg., room 401