Cloud-top “warm trench” infrared signature over Colorado

July 22nd, 2018 |

GOES-16

GOES-16 “Clean” Infrared Window (10.3 µm) images, with plots of surface reports [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.3 µm) images (above) revealed a well-defined “warm trench” signature (ring of brighter red enhancement) surrounding the cold overshooting top (cluster of pixels enhanced as black to lighter shades of gray) of a thunderstorm in far eastern Colorado during the nighttime hours on 22 July 2018. This warm trench appears to be a ring of compensating subsidence immediately surrounding the vigorous overshooting top; the cold/warm (overshooting_top/warm_trench) “delta-T” on the 0412 UTC image was 16.4ºC (-85.5ºC / -67.1ºC). Just to the south, at 0453 UTC there was a northerly peak wind gust to 45 knots or 52 mph at KITR (Burlington Colorado: plot | text) as the updraft supporting the overshooting top collapsed — but no other SPC storm reports were seen in that area.

A 250-meter resolution Terra MODIS Infrared Window (11.0 µm) imageat 0402 UTC (below) with a slightly different color enhancement showed similar delta-T values (-81ºC/-67ºC) with the overshooting top / warm trench. The diameter of the warm trench was approximately 30-40 miles.

Terra MODIS Infrared Window (11.0 µm) image, with plots of surface reports [click to enlarge]

Terra MODIS Infrared Window (11.0 µm) image, with plots of surface reports [click to enlarge]

A plot of 00 UTC rawinsonde data from North Platte, Nebraska (below) showed a tropopause temperature of -73ºC at an altitude of 15.7 km or 51,500 feet — so the much colder infrared brightness temperatures seen on GOES and MODIS imagery were indicative of a very robust overshooting top that penetrated the tropopause a significant distance.

Plot of 00 UTC rawinsonde data from North Platte, Nebraska [click to enlarge]

Plot of 00 UTC rawinsonde data from North Platte, Nebraska [click to enlarge]

The GOES-16 Cloud Top Height product at 0412 UTC (below) indicated values of 54,000 ft / 49,000 ft for the cold overshooting top / warm trench features — however, note that the resolution of this infrared-derived product is 10 km (and the accuracy is within 1500 feet).

GOES-16 Cloud Top Height derived product at 0412 UTC [click to enlarge]

GOES-16 Cloud Top Height derived product at 0412 UTC [click to enlarge]

Another interesting (and yet-to-be-explained) feature was an arc of warming cloud-top infrared brightness temperatures that was seen propagating southwestward toward the overshooting top / warm trench signature. A larger-scale view (below) showed this wave feature moving from southwestern Nebraska at around 02 UTC to southern Colorado/Kansas by 10 UTC.

GOES-16

GOES-16 “Clean” Infrared Window (10.3 µm) images, with 11.2 µm infrared Derived Motion Winds [click to play animation | MP4]

This cloud-top wave feature was also apparent on GOES-16 Upper-level Water Vapor (6.2 µm) images (below) — at times the wave was tracked by Derived Motion Winds at speeds of 20-30 knots (0252 UTC | 0337 UTC | 0922 UTC). According to rawinsonde data from Dodge City, Kansas (plot | data) as well as North Platte, winds with a northerly to easterly component were only found at altitudes of 20 km or higher!

GOES-16 Upper-level Water Vapor (6.2 µm) images, with 6.2 µm water vapor Derived Motion Winds [click to play animation | MP4]

GOES-16 Upper-level Water Vapor (6.2 µm) images, with 6.2 µm water vapor Derived Motion Winds [click to play animation | MP4]

Special thanks to NWS Grand Rapids forecasters Brett Borchardt and TJ Turnage for bringing this case to our attention!

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