During a brief period in mid-Janaury, when the “Polar Vortex” was safely docked in its home port of Alaska, temperatures over Alaska’s Interior plunged into the 40s and 50s below zero. Under clear skies and light winds, surface-based radiation inversions decouple a thin bottom layer of air from the free troposphere and produce extreme stratification of temperatures in the vertical over Interior Alaska’s complex terrain. Long-wave Infrared imagery from polar-orbiting satellites allows forecasters to “see” the topography during these events, since the coldest air settles into the valley bottoms and ground temperatures monotonically moderate with increasing height.
The 2013-2014 winter is the inaugural season for SPoRT’s RGB Night-time Microphysics product. Given that longwave IR contributes the “B” channel of the RGB NT Micro, one might wonder if topographic features might also be evident in the RGB NT Micro during a cold clear Alaska night.
This figure is a four-panel screen capture from an AWIPS workstation at National Weather Service WFO Fairbanks, Alaska. All four panels show imagery from the 1321Z (5:21am Alaska Standard Time) pass of the SNPP VIIRS instrument on Monday, January 13th, 2014. The area shown is zoomed into the “Upper Yukon Valley” of Alaska’s Interior. The village of Fort Yukon, on the Yukon River, is marked with the star in each panel. Conditions at this time in the Upper Yukon Valley were clear, calm, and cold, with surface temperatures around Fort Yukon in the 40s below zero.
The upper-left panel is the RGB NT Micro using the default color enhancement, the upper-right panel is the Day-Night Band, the lower-right panel is the 11 micron longwave IR, and the lower-left panel is the traditional channel differencing “fog product.” Topographic features are clearly evident in the DNB image in the upper-right, as well as in the longwave IR image in the lower-right. But interestingly, the topography is almost entirely non-evident in the RGB NT Micro in the upper-left, despite longwave IR being one of the three components of the RGB image.
Learning to use a new product includes discovering what that product is capable of depicting and *not* depicting under a variety of weather conditions. The Alaskan winter is an extreme and unusual environment, and this one example indicates that one should not expect to see terrain features in the RGB NT Micro under conditions of clear skies and extreme temperature stratification. Correspondingly, a “smooth” look to RGB NT Micro imagery in this case does *not* indicate a broad cloud deck covering the terrain.