Climatology of snow cover and snow water equivalent

• Chapter II: Physical snow characteristics determined from space
•  
• Satellite instruments' sensitivity
•  
• Snow detection in the visible spectrum
•  
• Snow detection in the infrared spectrum
•  
• Snow detection in the microwave spectrum
•  
• Factors which may lead to misidentification of snow

Snow detection in the microwave spectrum

Microwave (MW) instruments sense much longer wavelengths than infrared radiometers, and this energy can be expressed in units of frequency called gigahertz (GHz). The frequencies for meteorological observations fall in the range of about 5 to 200 GHz or 6 to 0.15 cm wavelength. The amount of microwave radiation emitted by the Earth's surface depends on the type of surface and its characteristics (roughness, moisture, vegetation, snow cover, etc.) (Fig. 3). Microwave emissions depend strongly on the physical properties (e.g. atomic composition and crystalline structure) of the object.

Figure 3: Surface features that have different characteristics in the microwave spectrum. Credit: COMET®.

Contrary to the visible and infrared spectrums, microwave energy is emitted not only from the surface of the snowpack, but also from deeper layers. It is sensitive to snow depth, snow water equivalent, snowpack temperature, wetness degree and soil conditions below the snowpack.

The two properties of emitting material that have a significant impact on emitted microwave radiation are polarization and the dielectric effect (relative permittivity). Polarization defines a group of simple waves comprising a radiation signal, while the dielectric constant is very sensitive to water and has a direct impact on absorption and emission efficiency. Each property is dependent on the wavelength and physical characteristics of the emitting and/or reflecting material. These properties of microwave radiation allow discriminating between solid, liquid, and frozen elements on both land and ocean surfaces (Fig. 4).

Figure 4: Microwave emissivity for common surface types. The differences in emissivity serve to differentiate different types of snow and to distinguish snow from other features. Credit: COMET®.

The physical characteristics of snowpack determine its microwave properties. Microwave radiation emitted from the underlying ground is scattered in many different directions by the snow grains within the snow layer, resulting in the microwave emission at the top of the snow being weaker than those from the ground (Fig. 5). Properties affecting a snowpack's microwave response include depth and water equivalent, liquid water content, density, grain size and shape, temperature and stratification as well as snow state and land cover (WMO CHy-14, 2012). These characteristics of snowpack and the snow's morphology are strongly correlated with climate.

Figure 5: The scattering of microwave radiation depends on snow depth, density and grain size. Credit: COMET®.

Vegetation emits microwave radiation of its own. In areas where vegetation covers at least part of a snow-covered surface, the brightness temperature sensed by the satellite increases (Fig. 6). Forest cover masks snowpack microwave emissions. The more vegetation, the less snow cover a satellite can observe directly. By using a multi-spectral approach to the analysis, the vegetation can be identified and its effects minimized.

Figure 6: Vegetation increases the emission of microwave radiation and thus masks the snow signal. Credit: : COMET®.

The 37 GHz microwave channel is often compared to the 19 GHz channel to differentiate dry and wet snow. The 37 GHz MW data is sensitive to snowpack scattering while 18-19 GHz data is relatively unaffected by snow (Dong et al., 2005). By taking the difference of the two channels, different snowpack characteristics and snow melt can be determined, which would not be possible with 37 GHz observations alone.

Snow cover characteristics sensed in the microwave spectrum (e.g. snow water equivalent) are difficult to retrieve in mountainous terrain due to the variability of snow cover and complex orography. Retrieval of snow water equivalent is also problematic over frozen ground, cold deserts, and ice sheets (Greenland and Antarctica). MW snow products are degraded along coastlines, in urban areas and other regions with highly variable surface types.

The big advantage of microwave instruments is that they can sense the ground despite cloud cover. MW channels also work during both day and night. Their main shortcoming is spatial resolution - it is 10 times less than for instruments working in the visible or infrared spectrum (Takala et al. 2011).