With this week's set of satellite greenness maps, the last vestiges of snow cover influence have been removed. Most areas should now have accurately calculated Relative Greenness (RG) values, so the output from the Oklahoma Fire Danger Model for areas previously affected can now be trusted (given the fuel model assigned each 1-km pixel of land).
Dr. J. D. Carlson
Fire Meteorologist
Oklahoma State University
Stillwater, OK
jdc@okstate.edu
405-744-6353
In our fall 2009 OK-FIRE workshops, we talked about the importance of relative greenness (RG) - the weekly level of greenness that the satellite detects for each 1-km square of land in Oklahoma.
The last two weeks of RG maps clearly show the effects of snow cover in Oklahoma. Look, for example at the Jan 4 2010 RG map in the FIRE section, under "Current/Recent Fire Danger" and "Satellite Greenness Maps". You can see a large band of super-low RG values (around 1%) running SW to NE across Oklahoma. Snow cover is extremely "non-green" and thus masks the true RG values of the surface fuels.
As a result of this, the fire model (e.g., Burning Index) is from time to time showing unrealistically high fire danger values in these areas of very low RG (the snow cover areas) and thus the fire model output in these areas (e.g., Stillwater) should be disregarded. Until the snow cover finally melts in these areas and we finally get a snow-free RG map, the fire model products will tend to overpredict fire danger in these areas. A short-term fix is to select the closest Mesonet stations having more realistic RG values (e.g., 10-30%) for this time of year and look at the fire model output (e.g., site-specific charts or tables) for these sites.
Note that you can determine the specific RG value being used at a given Mesonet site by going to one of two locations - (1) on the home page at the bottom of the "Fire Danger" data box on the left, or (2) in the FIRE section under "Current/Recent Fire Danger" and "Site-Specific Fire Danger" (bottom line of that data box).
Dr. J. D. Carlson
Fire Meteorologist
Oklahoma State University
Stillwater, OK
jdc@okstate.edu
405-744-6353