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United States Department of Agriculture

Agricultural Research Service

Title: Observations of Rime and Graupel Using Low Temperature Sem

Authors
item Wergin, William
item Rango, Albert
item Erbe, Eric

Submitted to: Scanning
Publication Type: Proceedings
Publication Acceptance Date: April 20, 1999
Publication Date: May 20, 1999

Interpretive Summary: Much of the water used by agriculture in the western part of the US comes from snow melt. To help farmers predict how much water may be available for agricultural use during a growing season, scientists are developing models that will predict the amount of water that exists in the winter snow pack. To build the models, scientists measure the amount of microwaves that are naturally emitted by the earth and then determine how this measurement changes when the earth is covered with snow. To improve the accuracy of the model, a special type of microscope, called a scanning electron microscope, was used to examine the individual snowflakes in the winter snow pack. Some snowflakes were found to be coated with supercooled cloud droplets that freeze to the surface of snowflakes. At this stage the snowflakes are referred to as rime. Eventually, further accumulation of frozen cloud droplets results in an aggregated mass known as graupel. The results indicated that the shape of rime and graupel will vary significantly and may affect or scatter the microwaves as they pass through the snow cover. Therefore, in their models scientists must compensate for how the shape of snowflakes that are found in a snowpack affect microwave transmission. This information will be used by scientists to further improve the models that are used to predict the amount of water in the winter snow pack.

Technical Abstract: In the northern climates, winter precipitation, commonly referred to as snow, can form by a processes known as accretion. In this process, forming and descending snow crystals encounter atmospheric supercooled cloud droplets. Contact between the snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystals. Crystals that exhibit frozen droplets on their surfaces are referred to as rime. When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting crystal is referred to as graupel. Rime and graupel are difficult to observe and photograph in the light microscope because the frozen cloud droplets cannot be resolved and the topography of a graupel particle is not easily recorded because of the limited depth of field in the light microscope. The current study evaluated use of low temperature SEM (LTSEM) to image rime and graupel. Results indicated that the accretion of supercooled cloud droplets on the surface of snow crystals could be easily observed with LTSEM. Problems, such as melting and sublimation, commonly associated with recording images of snow crystals with a light microscope were easily overcome. Finally, the topography characteristic of these types of crystals could be clearly resolved in the LTSEM.

Last Modified: 8/22/2014
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