SSM/I Imagery of February 1996 Great Lakes Freeze Event

Comparison: 7 channels of SSM/I

What can SSM/I tell us about Lake Superior ice cover? What are the geophysical parameters of interest? A partial wish list would undoubtedly include ice presence/absence, ice concentration (areal fraction of surface covered by ice), ice type, thickness and motion.

Freshwater ice has a relatively high microwave emissivity () and fresh water a relatively low microwave emissivity () (both at 85 GHz vertical polarization).This results in ice being radiometrically bright and water being radiometrically dark. The higher frequencies (here operationally defined as 85 and 37 GHz) are effective at distinguishing ice from water. Figure 1 shows an example of an image and a brightness temperature profile running east-west across the image.

The upper portion of Figure 1 is an 85 GHz (h-pol.) image of Lake Superior. Land is the dark grey region surrounding Lake Superior. New, thin ice is the white area within Lake Superior, and the grey zone in eastern Lake Superior is a 'polynya', a persistent area of mostly open water. The grid units are pixels, so each 50 grid cell is approximately 50 km across. The profile in the lower portion of the figure runs east-west just below the position indicated by the text.

The lower portion of Figure 1 shows an example profile running across the lake. The brightest area is new ice, with TBs on the order of 250-255 K. Land appears as intermediate and radiometrically cold features, ranging from approximately 245-210 K. The polynya is the flat feature on the profile with a nearly constant 230 K signature.

What SSM/I frequencies are most useful for looking at Lake Superior ice cover?

The images in Figure 2 are arranged as follows (top to bottom):

Frequency (GHz; v,h = vertical, horizontal polarization)

19v 19h

22v 22v (repeated)

37v 37h

85v 85h

Table 1 lists SSM/I frequencies and their footprints. The SSM/I footprint is elliptical in shape. SSM/I data are typically rebinned to a grid of square cells, with dimensions smaller than the dimensions of the original ellipses.

Great Lakes Snow and Ice Web Page Passive Microwave Movie and Images of Great Lakes Ice