A EUMETSAT blog post discusses in detail the winter weather that impacted the southern US in mid-Feb 2021. Specifically, it highlights satellite tools for differentiating snow cover vs ice cover.
Satellite channels in the Near-Infrared can be used to differentiate ice cover vs snow cover since ice absorbs more strongly than snow at these wavelengths. This difference in absorption is apparent in the ABI 1.6 μm band and the lower resolution 2.25 μm band. Similarly, snowpack characteristics can be gleaned from analysis of the near-IR bands. Fresh, clean snow will reflect more energy compared to old, melting, dirty snow.
The VIIRS 1.24 μm band is even more sensitive to absorption differences among snowpack characteristics and between snow and ice. A VIIRS “Snowmelt RGB”, developed by Curtis Seaman at CIRA, combines the 1.6 μm (red), 1.24 μm (green), and 0.64 μm (blue) bands from VIIRS to capture the varying characteristics of snow cover, and snow vs ice cover. From the Mid-Feb event, and specifically the significant 17 Feb snow/ice storm, the zone of ice cover (relatively dark blue) that fell along the southern extent of the winter storm can easily be differentiated from the snow cover (relatively light blue) to the north in the VIIRS Snowmelt RGB (Fig 1). Also note the cloud development over the areas where rain/freezing rain fell vs snow covered grounds (see EUMETSAT blog post for more details on this).
In the absence of the 1.24 μm band on ABI, a similar (experimental) GOES Snowmelt RGB can be created using the 1.6 μm, 2.25 μm, and 0.64 μm bands (Fig 2). A west-east oriented corridor of snow fell across the already snow covered upper Midwest on 27 Feb. As clouds cleared during the day on 28 Feb, the relatively light blue of the newer snow corridor contrasts with the darker blue (lower reflectance in near-IR) of the older snow to the north in Minnesota and south in Iowa.
Comparing with VIIRS Snowmelt RGB with a mostly cloud free scene on 01 Mar, we see that both products capture the differing snowpacks, with the VIIRS product showing even greater contrast between new and old snow, and in higher spatial resolution (Fig 3). Note another band of (light) snow fell overnight on the 28th, and can be observed in the imagery extending from North Dakota, southeast across central Minnesota into southeast Minnesota.
If we loop through a period from the 28th through the 2nd of March using both the VIIRS snowmelt and ABI Snowmelt RGBs (Figs 4 and 5), we can see that the snowpack gradually gets darker as the snow melts and absorption in the near-IR increases.
Figure 6 compares three near-IR bands from VIIRS, VIS, and the Snowbelt RGB for the Midwest scene on 01 March. Note the difference in reflectance between the new snow and old snow becomes greater from 2.25 um and 1.61 um to 1.24 μm. The difference in snowpacks is unrecognizable in the VIS alone. The RGB allows one to differentiate the fresh snowpack from the older snowpack as well as other features such as bare ground, open waters, and clouds.
Bill Line, NESDIS and CIRA; Curtis Seaman, CIRA
Satellite Liaison Blog 