A quick-moving upper-level trough and associated cold front moved across the TN and lower OH valleys this morning, producing snow showers around the region. The Nighttime Microphysics RGB image below from the VIIRS instrument (Image 1), from around 0805 UTC, shows low/mid clouds associated with the trough. Notice most of the clouds appear light green/yellow and or green/red, indicating predominantly low/mid cloud types. A synthesis of radar and sounding data indicates these clouds were dominated by snow/ice crystals and perhaps some super-cooled liquid droplets. Reports of only light snow have been received at surface stations in the region this morning.
Image 1. Suomi NPP VIIRS Nighttime Microphysics RGB ~0805 UTC 18 Feb 2015
Notice the narrow swath of darker green colors stretching from NE Mississippi northeastward into northern middle Tennessee. Although it may not be apparent, the green color contribution (green represents the 10.8-3.9 µm channel difference in the RGB recipe) in this swath was actually lower than in adjacent clouds. In addition to lower contributions of green, red and blue color contributions were also lower in this narrow swath. The resulting interpretation is that this area was composed of lower, warmer clouds. Due to the variations in green color contribution across the cloud deck, it was immediately unclear whether there was a mix of super-cooled liquid water and ice/snow crystals. Radar imagery from the Nashville, TN and Columbus, MS radars from about the same time indicate mostly uniformity in falling hydrometeors. The image below (Image 2) shows Correlation Coefficient values from the KOHX and KGWX radars at ~0806 UTC.
Image 2. VIIRS Nighttime Microphysics RGB overlaid with Correlation Coefficient (CC) values from KOHX and KGWX radars
A small area of slightly lower CC values can be seen to the northwest of Nashville and an inspection of ZDR values (not shown) indicated slight/moderate wet snowflakes. Although, standard reflectivity imagery from the same time shows little in the way of precipitation in that area (Image 3). It should be noted that precipitation echoes were somewhat absent from the darker green swath. However, since these clouds and any resulting precipitation (if present) was relatively low, echoes were below the lowest scans of the regional radars.
Image 3. VIIRS Nighttime Microphysics RGB overlaid with 0.5 Reflectivity from KOHX and KGWX radars
Incidentally, the KHTX radar was down for a needed repair and thus not available during this time. One advantage of this type of satellite imagery is that it is safer to make inferences about the presence of precipitation, and in some cases, perhaps even precipitation intensity and type, given similar RGB cloud characteristics. During an event such as this where standard radar data may not be available at a given location, the value of the imagery becomes even more apparent.
The prospect for additional snow and mixed precipitation events over the upcoming week or so will offer more interesting observations of the Nighttime Microphysics RGB. More posts to follow soon!
Read Full Post »