On May 24th, the VIIRS automated fire detection software identified seven possible fire points very close together in the Canadian Yukon Territory with the 1136 UTC SNPP satellite pass, however, the circumstances of the event suggest that these were probably not fires. Below is a VIIRS i04 (3.74um) shortwave IR image zoomed in over the Yukon with a circle showing the location of the fire points. This satellite channel is very sensitive to the shortwave energy emissions of fires and hot spots. The image shows a band of convective cumulus with a dark line to the northeast which is where the fire points were indicated. The colormap legend in the upper left part of the image shows that warm colors are on the left and cold colors are on the right. You can see that the dark grey or black indicates warmer Brightness Temperatures (TB) than the lighter grey and cyan shades. Hotter temperatures go from green to red to yellow.
But what is causing those warm temperatures north of the cloud band? Below is the VIIRS i05 (11.5um) longwave IR image over the same region. This channel is used for observing cloud top temperatures. A different colormap is used for the i05 image, but warm colors are again on the left (dark gray shades) with cold colors on the right. Comparing this longwave IR image below to the shortwave IR image above you can see the location of the dark line actually has very cold temperatures from high cloud tops.
So why is the 3.74 um shortwave IR seeing warm temperatures? One thing to remember about this particular infrared channel is that the emitted radiation it is sensing is close to the short wavelengths of reflected solar energy that occur during the day, so the sensor is affected by a combination of emitted radiation and reflected solar radiation. The image below is the VIIRS i04 (3.74 um) shortwave IR with VIIRS i01 (0.64 um) visible image overlaid on top. The visible image doesn’t cover the whole scene because the time of this pass at 1136 UTC (3:36 AM local time) is still night for part of this region. The dotted yellow line highlights where the visible data to the northeast is cut off due to decreasing sunlight, with only the shortwave IR data available to the southwest. The red circle marks the location of the cumulus band and the anomalous fire points. Although this location is outside of the delineated “full reflectance” area, the bright cloud tops in the sunlit visible region, imply that there is some reflected solar energy from the cumulus band reaching the satellite sensor. Daytime addition of reflected solar radiation at the 3.74 um wavelength is typically accounted for in the fire detection software, however the occurrence of this feature in the terminator transition zone between day and night made it difficult for the algorithm to account for.
The VIIRS Active Fires software is a powerful tool for automated monitoring of wildland fires but as this example illustrates the extended daylight in northern latitudes can cause special circumstances that need to be considered when assessing its output.