Selected Results from Passive-Microwave Snow Studies at GSFC during the 1970s and '80s

Selected Results from Passive-Microwave Snow Studies at GSFC during the 1970s and '80s

Dorothy K. Hall
Oceans & Ice Branch, Code 971
NASA/Goddard Space Flight Center
Greenbelt, MD 20771

Theoretical work by Al Chang and others (Chang et al., 1976) in their landmark paper published in the Journal of Glaciology entitled, "Microwave Emission from Snow and Glacier Ice," spawned a multitude of studies about the remote sensing of snow and ice using passive-microwave sensors. This work, along with some earlier work, laid the groundwork that enabled algorithms to be developed to measure snowpack properties such as snow extent through darkness and cloud cover, snowpack "ripening," and to estimate snow depth and snow-water equivalent (SWE) (the amount of water in a snowpack), using microwave sensors from aircraft and satellites.

This work is of great practical interest for example, for hydrological modeling, and for estimation of the SWE. Improved prediction of snowmelt runoff permits more effective use of water which is in short supply in many areas of the world (such as the western U.S.). Thus the ability to determine SWE from space through the use of microwave remote sensors addresses the critical need to improve the prediction of snowmelt runoff and is the underlying reason for research on the microwave remote sensing of snow cover.

In the Chang et al. (1976) paper, the potential of passive-microwave data to measure snowpack properties is clearly elucidated by Al Chang and co-authors Per Gloersen, Tom Schmugge, Tom Wilheit and Jay Zwally. They described the development of an analytical model that considers the individual snow grains as the scattering centers of the microwave radiation and, for the first time, Mie extinction and scattering coefficients were employed to obtain an explanation of the low microwave brightness temperatures observed over snow fields. (Mie scattering predominates when the particles causing the scattering are comparable to the wavelengths of radiation in contact with them.) Volume scattering was shown to increase with snow grain size and internal layering and with an increase in snow depth. Radiation at wavelengths comparable in size to snow crystals (about 0.05 to 3.0 mm, or greater if depth hoar is present) is scattered in a dry snowpack according to Mie scattering theory.

As is discussed in the aforementioned paper, the microwave brightness temperature decreases as the snow depth increases because there is more scattering from the snow crystals and grains in a deeper snowpack as compared to in a shallower snowpack simply because there are more crystals and grains in a deeper snowpack. The size of the crystals in the snowpack is also important because larger crystals are more effective scatterers than are smaller crystals. And whether or not crystal shape affects scattering was an open question for quite a while.

Using the theoretical calculations from Chang et al. (1976), we hypothesized that constructive metamorphism was responsible for the lowering of the microwave brightness temperature as determined from spaceborne microwave sensors in northern Alaska (Hall et al., 1986). Constructive metamorphism occurs when snow grains grow at the expense of neighboring grains and crystals; this tends to occur in snowpacks that stay on the ground for many months such as those found in northern Alaska. Not only do the large grains in certain snowpacks in Alaska produce lower microwave brightness temperatures (at shorter wavelengths), but as we showed, microwave brightness temperatures in different regions of northern Alaska can vary due to depth-hoar development even when snow depth remains fairly constant.

Some depth-hoar crystals have extremely uncharacteristic "crystal" shapes. Later research in collaboration with the U.S. Department of Agriculture using microscopy showed that the shape of the snow grain is not important (Foster et al., 1999) in terms of its ability to scatter passive-microwave radiation. This was a groundbreaking finding that permits researchers to focus on other parameters in a snowpack for understanding the microwave brightness temperature from snow.

The situation in a snowpack is very complicated because of a number of factors including different size snow crystals, and snowpacks developed from destructive versus constructive snow metamorphism. Through a combination of many aircraft overflights, numerous field expeditions and modeling, Al, Jim and I produced a number of papers that followed on from the theoretical results of the Chang et al. (1976) paper, and showed how the microwave radiation at different frequencies responds in various geographic areas and under different surface conditions. This early work led to more studies that ultimately produced algorithms (e.g., Chang et al., 1987) to map snow extent and depth globally. Because the Earth is heterogeneous, global algorithms may not work well everywhere. Thus, attempts to adapt a global algorithm to specific regions where, for example, large areas of constructive metamorphism (depth hoar) and forest cover are found, are ongoing to this day and much progress has been made by a variety of researchers.

Finally, many other useful algorithms based on passive-microwave satellite measurements have also been developed for global and regional applications, and much research has been conducted and continues to take place outside of GSFC. Many of the scientific papers detailing that work also cite the Chang et al. (1976) paper as the theoretical underpinning of the work. The focus of my presentation will be on work done from my direct collaboration with Al and may therefore appear to be myopic.

References:

Chang, A.T.C., P. Gloersen, T. Schmugge, T.T. Wilheit and H.J. Zwally, 1976: Microwave emission from snow and glacier ice, Journal of Glaciology, 16(74):23-39.

Chang, A.T.C., J.L. Foster and D.K. Hall, 1987: Nimbus-7 SMMR derived global snow cover parameters, Annals of Glaciology, 9:39-44.

Foster, J. L., D. K. Hall, A. T. C. Chang, A. Rango, W. Wergin, and E. Erbe, 1999: Effects of snow crystal shape on the scattering of passive microwave radiation, IEEE Transactions on Geoscience and Remote Sensing, 37(2):1165-1168.

Hall, D.K., A.T.C. Chang and J.L. Foster, 1986: Detection of the depth-hoar layer in the snowpack of the Arctic Coastal Plain of Alaska, U.S.A., using satellite data, Journal of Glaciology, 32(110):87-94.

[back]