Submitted to: Scanning
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/20/2003
Publication Date: 5/1/2003
Citation: RANGO, A., FOSTER, J., JOSBERGER, E.G., ERBE, E.F., POOLEY, C.D., WERGIN, W.P. RIME AND GRAUPEL: DESCRIPTION AND CHARACTERIZATION AS REVEALED BY LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY. SCANNING. 2003. V. 25. P. 121-131. Interpretive Summary: The characterization of snow crystal form is of special importance in meteorology, hydrology, avalanche warnings and interpreting remote sensing data. Photography of the snow crystal form has been conducted since around 1900 using light microscopy. Because of limited resolution and depth of field, light microscopy can't be used to characterize such small snow crystals. Using a field procedure developed over several years and low temperature scanning electron microscopy, this technique is able to display a category of crystals called rime and graupel. Although light microscopy is not able to provide a clear picture of these crystals, low temperature scanning electron microscopy shows for the first time the detail of rimed snow crystals and the microstructure of graupel. The use of low temperature scanning electron microscopy allows for the precise measurement of the frozen droplets that form rime (10-50um) on the surface of snow crystals for the first time. In the case of graupel, this study shows clearly the structure of the frozen droplets and internal air spaces. The low temperature scanning electron microscope provides the only means to quantify the size and structure of these very small crystals.
Technical Abstract: For nearly 100 years, investigators using light microscopy have vaguely alluded to unique types of snow crystals known as rime and graupel. However, the limited resolution and depth of field of the light microscope have prevented investigators from characterizing these crystals. In this study, a field emission scanning electron microscope equipped with a cold stage was used to document the structural features, physical associations and atmospheric metamorphosis of rime and graupel snow crystals. Rime and graupel form in clouds having both snow crystals and super cooled cloud droplets. This study shows that the frozen droplets range from 10-50um in diameter. With the low temperature scanning electron microscopy approach, it is possible to distinguish the original snow crystal types until the accretion of droplets become thick enough to obscure the original crystal and graupel is the result. With continued riming, lump graupel can reach a diameter of 1-3mm. This study confirms the light microscopy conclusion that the structure of graupel includes considerable air spaces. The low temperature electron microscope approach is the only method that allows measurement of the accreted cloud droplets on rimed crystals and graupel.