Middle and upper-atmospheric wave structures in the vicinity of a subtropical jet stream

April 4th, 2016 |

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images, with ECMWF model maximum wind isotachs [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images, with ECMWF model maximum wind isotachs [click to enlarge]

A strong (120-knot) subtropical jet stream was moving eastward across the Gulf of Mexico during the 03 April – 04 April 2016 period. During the overnight hours between these 2 days, a Suomi NPP VIIRS Day/Night Band (0.7 µm) image at 0753 UTC (above) revealed a large packet of arc-shaped mesospheric airglow waves south of the axis of the jet stream (as indicated by isotachs of the maximum tropospheric wind speed from the ECMWF model). Note how there were no cloud features which corresponded to these waves in the 0753 UTC VIIRS Infrared Window (11.45 µm) image; since the Moon was in the waning Gibbous phase (at 13% of Full), there was very little lunar illumination of cloud features, so airglow — essentially the “night glow” emitted from a variety of high-altitude (80-105 km) gases (primarily the sodium layer) near the mesopause — was allowing these high-altitude waves to be detected using the sensitive Day/Night Band (reference: “Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities”).

During the subsequent daytime hours on 04 April, more interesting (tropospheric) waves were seen in the vicinity of this subtropical jet stream — small packets of waves that were propagating westward, against the ambient flow –one over Florida/Georgia/South Carolina, and another over South Texas. Unfortunately, these features fall into the “What the heck is this?” blog category, so no coherent explanation of them can be offered at this time.

GOES-13 Water Vapor (6.5 µm) images, with ECMWF model maximum wind isotachs [click to play animation]

GOES-13 Water Vapor (6.5 µm) images, with ECMWF model maximum wind isotachs [click to play animation]

An interesting question from Shea Gibson:

Convective snow squalls in the Upper Midwest

April 2nd, 2016 |

GOES-13 Visible (0.63 µm) images, with hourly surface weather symbols [click to play animation]

GOES-13 Visible (0.63 µm) images, with hourly surface weather symbols [click to play animation]

A vigorous clipper-type shortwave moved rapidly southeastward across the Upper Midwest on 02 April 2016; there were widespread convective elements associated with this system as seen in GOES-13 Visible (0.63 µm) images (above), which produced moderate to heavy snowfall at times (and even thundersnow) creating brief white-out conditions (time-lapse video from the AOSS rooftop camera in Madison, Wisconsin). A sequence of visible images from the polar-orbiting MODIS, VIIRS, and AVHRR instruments (below) provided another detailed view of these convective elements. This disturbance produced strong winds and accumulating snowfall; more information can also be found here from the NWS Chicago.

MODIS, VIIRS, and AVHRR visible images [click to enlarge]

MODIS, VIIRS, and AVHRR visible images [click to enlarge]

A pronounced warm/dry signature of middle-tropospheric subsidence (yellow color enhancement) was evident on GOES-13 Water Vapor (6.5 µm) images (below), which appeared to be along or just ahead of the areas of stronger wind gusts at the surface.

GOES-13 Water Vapor (6.5 µm) images with hourly wind gusts in knots [click to play animation]

GOES-13 Water Vapor (6.5 µm) images with hourly wind gusts in knots [click to play animation]

This area of middle-tropospheric subsidence was located along the leading edge of a strong (110-120 knot) 500 hPa jet, as indicated by the NAM40 model isotachs (below).

GOES-13 Water Vapor (6.5 µm) images with METAR surface reports, surface fronts, and NAM40 500 hPa wind isotachs [click to play animation]

GOES-13 Water Vapor (6.5 µm) images with METAR surface reports, surface fronts, and NAM40 500 hPa wind isotachs [click to play animation]

The convective elements were relatively shallow, with cloud-top infrared brightness temperatures only in the -20 to -30º C range (cyan to darker blue color enhancement) as seen in 4-km resolution GOES-13 Infrared Window (10.7 µm) images (below) and also in 1-km resolution MODIS, VIIRS, and AVHRR infrared images.

GOES-13 Infrared Window (10.7 µm) images, with hourly surface weather symbols [click to play animation]

GOES-13 Infrared Window (10.7 µm) images, with hourly surface weather symbols [click to play animation]

The 24-hour snowfall amounts ending at 12 UTC on 02 and 03 April are shown below, from the NOHRSC site. There was a narrow swath of snowfall in excess of 3 inches just north of the track of the surface low (surface analyses), from northeast Minnesota across Wisconsin to southwest Lower Michigan.

24-hour snowfall amounts (in inches) ending at 12 UTC on 02 and 03 April [click to enlarge]

24-hour snowfall amounts (in inches) ending at 12 UTC on 02 and 03 April [click to enlarge]