Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: Snow Crystal Imaging Using Scanning Electron Microscopy: Part II - Metamorphosed Snow

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

Submitted to: Hydrological Sciences Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 4, 1996
Publication Date: N/A

Interpretive Summary: Microwave remote sensing data are increasingly being investigated for use in estimating areal water equivalent of the snowpack. As the resolution of these satellite microwave data improve and their use is more imminent, parameters in microwave radiative transfer models like grain size, shape, and structure must be estimated. There is no good remote sensing approach which allows measurement of these parameters, so we have started a study to determine the characteristic grain sizes, shapes, and structure under different snowpack conditions using a low temperature scanning electron microscope (SEM). The SEM images of metamorphosed snow crystals in the snowpack show an unbelievable variety of rounded and faceted shapes and sizes which shows that the microwave radiative transfer model assumption of spherical grains is false. The SEM can easily measure the size and shape of crystals in the pack which can be used to develop more representative model algorithms. These SEM data can also be fed into future algorithms for estimating snowpack water equivalent from passive microwave satellite data. Snowmelt runoff forecasters of action agencies will make use of this information as will microwave modellers and remote sensing experts with Federal agencies and universities.

Technical Abstract: Low-temperature scanning electron microscopy (SEM) was used to observe metamorphosed snow crystals and grains obtained in the field. Metamorphosed snow was obtained from seasonal snowpacks in the Colorado Rocky Mountains and in Alaska. The snow samples obtained in snowpits were mounted on modified SEM stubs, frozen in liquid nitrogen, transported in dewars to the SEM facility, sputter coated with platinum, and imaged with an electron beam. Analysis of a representative set of snow samples collected revealed examples of metamorphosed stellar crystals, fine snow grains with sintering, rounded and faceted crystals, several types of depth hoar, rounded grains of melt metamorphism, and an ice lens. Some of the crystals exhibiting both rounding and facets indicate that both equitemperature and temperature gradient metamorphism influenced the snowpack. The SEM methods developed are operable in the field and can be used to three-dimensionally quantify size, shape, and bonding characteristics of crystals. SEM appears to have direct application for better understanding of snow crystal metamorphism and snowpack processes, increasing knowledge of conditions leading to avalanche formation, and improving modelling of the transfer of microwave energy from the ground surface through the snowpack for eventual estimation of snow water equivalent.

Last Modified: 4/19/2014