EFFECTS OF SNOW CRYSTAL SHAPE ON THE SCATTERING OF PASSIVE MICROWAVE RADIATION

Authors: FOSTER, JAMES - NASA; HALL, D - NASA; CHANG, A - NASA; Rango, Albert; Wergin, William; Erbe, Eric

Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Proceedings
Publication Acceptance Date: 8/13/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Much of the water used by agriculture in the western part of the United States 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. One problem with existing models is that their accuracy is not perfect. To improve this accuracy, a special type of microscope, called a scanning electron microscope, was used to examine the individual snowflakes in the winter snow pack. The snowflakes were found to exist as a variety of shapes including spheres, ovals, cylinders, cubes and six sided figures. The results indicated that the shape of the snowflake did not affect or scatter the microwaves as they pass through the snow cover. Therefore, in their models scientists can assume that all snowflakes act as spheres in affecting the passage of microwaves. 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 developing and tuning passive microwave algorithms, which are used to estimate snow extent and depth, much of the effort has been towards better accounting for the effects of snow crystal size on the microwave response, and relatively little effort has been given to the role crystal shape plays in this regard. However, crystal size alone does not account for all of the scattering or energy distribution. In this study, a discrete dipole scattering model is used to measure the passive microwave radiation scattered by spheroids, ellipsoids, cylinders, cubes, tetrahedrons, and hexahedrons of sizes ranging between 0.1 mm to 0.9 mm in radius. In conjunction with the modeling, snow crystals were collected in the field and analyzed in detail with an electron microscope in order to assess the variation and range of crystal shapes encountered in different geographic areas. The model results demonstrate that the shape of the snow crystal is insignificant in scattering microwave energy in the 37 GHz region of the spectrum. Therefore, the assumption used in radiative transfer approaches, where snow crystals are modeled as spheres, is adequate to account for the transfer of microwave energy emanating from the ground and passing through a snowpack