A severe thunderstorm traversing almost the full length of Nebraska from west to east just south of I-80 produced landspout tornados and numerous instances of hail > 1″ in diameter. The evolution of the storm during the afternoon into the overnight hours is captured well in a VIS/IR combo product, which transitions from the VIS/IR sandwich during the day to IR at night (Fig 1). This procedure is simple to make in AWIPS (feel free to reach out if you’d like it!) Apparent in the imagery is a persistent above anvil cirrus plume, indicative of a particularly threatening storm.
A keen eye will detect a trail of high reflectance (lighter gray compared to surrounding) in the VIS, and cold BT (lighter gray compared to surrounding) in the IR Window. This signature in satellite imagery is that of a accumulating hail, or a hail swath. NWS Hastings NE, was on the case, and shared GOES-East IR Window channel imagery of the storm and associated hail swath, with the colormap range adjusted to better highlight the cool BT signature at the surface (see below).
Alternatively, one can load two instances of ch13 IR (or Ch13 and Ch14), and adjust the lower layer as a grayscale with the range focusing around that of the surface, and the upper layer as a colored enhancement to highlight the clouds. This AWIPS procedure, shown in Fig 2, captures the hail swath while also maintaining the ability to diagnose storm top features.
Even better, we can use RGBs to really isolate the hail swath feature. During the day, the Day Cloud Phase Distinction RGB does the trick, isolating the hail swath as green (similar to snow cover; Fig 3)). Since the time period is toward sunset, we can make simple adjustments to the recipe to draw out the feature (green max to 20, blue max to 12).
After sunset, one can switch to the Nighttime Microphysics RGB, which captures the hail swath as a red magenta (~pink) compared to the blue magenta (~violet) of the surrounding clear sky surface (Fig 4).
Combining the two animations (in this instance, outside of AWIPS), we get a seamless day/night transition between the two RGBs over the hail swath (Fig 5). Such a transition product is available in real-time on the STAR Image Viewer.
During the rest of the overnight, the hail swath slowly faded in IR imagery, becoming difficult to discern in ABI (Fig 6).
Viewing 375 m VIIRS IR window imagery (band I5), the location of the remaining hail swath becomes more apparent again given the higher spatial resolution (Fig 7). The scene is captured in four consecutive passes between 0716 UTC and 0901 UTC (two from S-NPP and one from NOAA-20 and NOAA-21 each). The hail swath is noticeable in the first VIIRS image, is masked by clouds in the next two, and reappears in the final image. VIIRS imagery is available to view on Polar SLIDER, which will soon include a CONUS sector.
See previous posts on this blog for more about hail swath detection using satellites.
Bill Line, NESDIS/STAR