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Snowfall Characterization by Field Observation and Atmospheric Sounding
AuthorFrancisco, Dianna M.
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The snowfall rate is related to microphysical and dynamical processes in weather systems and is further specified by choice of measurement time, related to the statistics of individual falling ice crystals. A snowflake is definable as a single ice crystal having long range order in the crystal lattice or as a polycrystalline having a multitude of individual single crystals, frozen together as graupel or held together though interlocking shapes as an aggregate snowflake. Snowflakes are collected on a black cloth, next to a scale, and photographed using a macro magnification. Snowflakes captured at the surface are characterized by their appearance from the habit, shape, size, symmetry, thickness, concentration, and fall of the individual ice crystals and of aggregates. Individual ice crystals are identified, counted, and related to the snowfall rate for a calculation of ice crystal number flux. The snowfall rate is characterized as a concentration and an inferred flux of individual nucleation events, to be related to possible direct nucleation and secondary ice formation (such as rime splintering) aloft. Identifying the number of ice crystals requires a degree of persistence and skill, is not readily automated, and is capable of providing key information on the growth history of ice crystals, not obtainable by other means. Surface data is collected through a surface weather station within one mile of the Observation Site. Atmospheric soundings produced by the National Weather Service, less than two miles distance from the Observation Site, aids in the approximation of the ice crystal's life history.