High-altitude waves over the Arctic

March 27th, 2020 |

GOES-17

GOES-17 “Ozone” (9.61 µm) images, with rawinsonde sites plotted in yellow [click to play animation | MP4]

GOES-17 (GOES-West) “Ozone” (9.61 µm) images (above) revealed waves propagating northwestward over northern Alaska, northern Yukon and the adjacent Beaufort Sea during the pre-dawn hours on 27 March 2020. That area was too illuminated by either aurora borealis or the rising sun — so Suomi NPP VIIRS Day/Night Band (0.7 µm) imagery could not confirm the presence of mesospheric airglow waves (see this blog post for some examples).

A plot of the GOES-17 “Ozone” spectral band weighting function — calculated using 12 UTC rawinsonde data from Fairbanks, Alaska — showed a peak contribution from within the stratosphere at the 39 hPa pressure level, corresponding to an altitude around 21 km (below).

Plot of GOES-17

Plot of GOES-17 “Ozone” (9.61 um) weighting function, calculated using 12 UTC rawinsonde data from Fairbanks, Alaska [click to enlarge]

The curious aspect of these waves was their northwestward propagation — rawinsonde data from 3 sites across the region (below) indicated that the winds aloft within the upper troposphere and throughout the stratosphere were strong northwesterly, which meant the waves were moving against the ambient flow. Lacking a coherent, science-based explanation for these wave features, this blog post earns its place in the “What the heck is this?” category.

Plots of rawinsonde data from Fairbanks, Alaska [click to enlarge]

Plots of rawinsonde data from Fairbanks, Alaska [click to enlarge]

Plots of rawinsonde data from Utqiagvik (formerly Barrow), Alaska [click to enlarge]

Plots of rawinsonde data from Utqiagvik (formerly Barrow), Alaska [click to enlarge]

Plots of rawinsonde data from Inuvik, Northwest Territories [click to enlarge]

Plots of rawinsonde data from Inuvik, Northwest Territories [click to enlarge]

Bore-like feature over Lower Michigan

February 13th, 2020 |

GOES-16 Advanced Baseline Imager (ABI) “red” visible imagery (0.64 µm), 1435 – 1840 UTC on 13 February 2020 (Click to enlarge)

TJ Turnage, the Science and Operations Officer (SOO) at the National Weather Service forecast office in Grand Rapids, noted today the presence of smooth, curving bands over Lake Michigan. The animation above shows their development — and the smooth appearance of the bands (just offshore of Ottawa Co, and curving into Allegan Co) is in marked contrast to the north-south oriented lake-effect bands over central Lake Michigan. This falls into the “What the Heck is this?” Blog Category.

An hourly animation that includes surface conditions sheds little light. The bore-like feature seems to arise out of an interaction of the atmospheric flow with Big and Little Sable Points, and surface winds at Muskegon (just north of Ottawa Co) and Holland (in Allegan Co) change as the feature moves over — but no snow is observed at those stations during the bore passage.

GOES-16 Advanced Baseline Imager (ABI) “red” visible imagery (0.64 µm) and surface METARS hourly from 1300 – 2100 UTC on 13 February 2020 (Click to enlarge)

Radar imagery (from the College of Dupage) also shows little return associated with the bore-like features.  (Click to see images from 1720 and 1800 UTC, when the bands were on shore).

NEXRAD Composite Radar Imagery (Composite Reflectivity) centered on MI, 1655-1820 UTC on 13 february 2020 (Click to enlarge)

 

Water vapor imagery, below, suggests that the stable layer that is trapping the energy and causing the bore-like feature originated near Big and Little Sable Points, around 1600 UTC.  The enhancement also suggests the bore-like feature is higher than the tops of lake-effect bands in the middle of Lake Michigan.  (Click here for a rocking animation of the water vapor imagery;  the rocking allows for better tracking of the impulse back to the source near the Sables, its earliest hint is at 1610 UTC — vs. about 1635 UTC in visible imagery).

GOES-16 ABI Band 10 (7.34 µm, low-level water vapor) infrared imagery, 1520 to 2015 UTC, 13 February 2020 (Click to play animated gif)

GOES-16 ABI Band 2 (0.64 µm) visible imagery, 1520 to 2015 UTC, 13 February 2020 (Click to play animated gif)

Bore-like features require stable layers.  The Gaylord Michigan sounding at 1200 UTC — upstream from the region out of which the bore emerged — shows several inversion layers.  The weighting function for the sounding (from this site) shows peak contributions for 7.34 µm (indeed, from all water vapor channels) from above 500 mb.  The coldest brightness temperature in the bands is -28 º C;  based on the Gaylord sounding, that’s a pressure level near 560 mb.  These Bore-like features are not Lake-Effect snow bands, despite having the correct aspect ratio — their width and length both suggest Lake-effect bands, but their height suggests otherwise.

NOAA-20 overflew this region shortly after 1700 UTC, and a NUCAPS sounding is close to the Michigan shoreline, just east of Holland, where the cloud band is coming onshore. The sounding from NUCAPS at that point/time is below.  The very smooth sounding does bear a passing resemblance to the Gaylord Sounding, but the smoothness of the NUCAPS profile — sampling a volume of air that in this case is about as wide as a county, makes identification of sharp inversions difficult.

NOAA-20 NUCAPS Profile points over Lake Michigan and lower Michigan, ca. 1730 UTC on 13 February 2020 (Click to enlarge)

NUCAPS Profile of temperature and moisture, 17 UTC on 13 February 2020 (Click to enlarge)

Gravity waves over the Gulf of Mexico and Florida

January 22nd, 2020 |

GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with pilot reports of turbulence [click to play animation | MP4]

GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images, with pilot reports of turbulence [click to play animation | MP4]

GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed a packet of gravity waves over the eastern Gulf of Mexico and southern Florida on 22 January 2020. Later time in the time period, there were isolated pilot reports of moderate turbulence in the vicinity of the waves (though it’s uncertain whether the gravity waves were directly responsible).

What caused these gravity waves to form and slowly propagate southeastward is also uncertain — earning this example its place in the “What the heck is this?” blog category. The SPC Mesoscale Analysis at 07 UTC (below) did show weak convergence of 300 hPa ageostrophic winds (dark blue oval) in the entrance region of a secondary jet streak “J” over the Gulf of Mexico — this convergence could have played a role in the gravity wave development.

SPC Mesoscale Analysis valid at 07 UTC, showing 300 hPa height, isotachs and ageostrophic winds [click to enlarge]

SPC Mesoscale Analysis valid at 07 UTC, showing 300 hPa height, isotachs and ageostrophic winds [click to enlarge]

GOES-16 Derived Motion Winds (calculated using 6.9 µm imagery) in the vicinity of the gravity waves (below) had velocities in the 50-60 knot range at pressure levels of 370-380 hPa (0916 UTC).

GOES-16 Water Vapor (6.2 um) Derived Motion Winds [click to enlarge]

GOES-16 Water Vapor (6.9 µm) Derived Motion Winds [click to enlarge]

Also of note was the fact that the surface of southern Florida was sensed by GOES-16 Low-level Water Vapor imagery (below).

GOES-16 Low-level (7.3 µm) Water Vapor images, with pilot reports of turbulence [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) Water Vapor images, with pilot reports of turbulence [click to play animation | MP4]

With an unseasonably cold, dry air mass moving southward over the peninsula, the 7.3 µm water vapor weighting functions were shifted to lower altitudes at Miami and Key West (below) — this allowed the thermal contrast between relatively cool land surfaces and the surrounding warmer water to be seen in the 7.3 µm imagery.

GOES-16 weighting functions for the 7.3 µm (violet), 6.9 µm (blue) and 6.2 µm (green) Water Vapor spectral bands, calculated using 12 UTC rawinsonde data from Miami, Florida [click to enlarge]

GOES-16 weighting functions for the 7.3 µm (violet), 6.9 µm (blue) and 6.2 µm (green) Water Vapor spectral bands, calculated using 12 UTC rawinsonde data from Miami, Florida [click to enlarge]

GOES-16 weighting functions for the 7.3 µm (violet), 6.9 µm (blue) and 6.2 µm (green) Water Vapor spectral bands [click to enlarge]

GOES-16 weighting functions for the 7.3 µm (violet), 6.9 µm (blue) and 6.2 µm (green) Water Vapor spectral bands, calculated using 12 UTC rawinsonde data from Key West, Florida [click to enlarge]

In fact, at Key West the Total Precipitable Water value of 0.3 inch calculated from 12 UTC rawinsonde data (below) was a new record for the date/time (the previous record minimum value was 0.36 inch).

Climatology of Total Precipitable Water for the Key West, Florida rawinsonde site [click to enlarge]

Climatology of Total Precipitable Water for the Key West, Florida rawinsonde site [click to enlarge]

Stationary linear boundary over the Pacific Ocean

May 2nd, 2019 |

GOES-17 Low-level Water Vapor (7.3 µm), Mid-level Water Vapor (6.9 µm), Upper-level Water Vapor (6.2 µm) and

GOES-17 Low-level Water Vapor (7.3 µm), Mid-level Water Vapor (6.9 µm), Upper-level Water Vapor (6.2 µm) and “Clean” Infrared Window (10.3 µm) images [click to play MP4 animation]

In a comparison of GOES-17 (GOES-West) Low-level Water Vapor (7.3 µm), Mid-level Water Vapor (6.9 µm), Upper-level Water Vapor (6.2 µm) and “Clean” Infrared Window (10.3 µm) images (above), the Water Vapor imagery revealed an interesting stationary linear boundary — oriented NNW to SSE, near 152-154ºW longitude — over the North Pacific Ocean on 02 May 2019. In addition, note the other linear boundary that propagated from E to W, moving right through the aforementioned stationary boundary (best seen in the 6.19 um Upper-level Water Vapor imagery). There was no evidence of either of these linear features in the corresponding GOES-17 Infrared imagery, or in Visible imagery (not shown). A perfect candidate for the “What the heck is this?” blog category.

One possible explanation for the curious stationary feature was that it resulted from a convergence of flow around the cutoff low to the east and a digging trough approaching from the west. GOES-15 Infrared cloud-tracked Derived Motion Winds from the CIMSS Tropical Cyclones site (below) did show evidence of some converging flow in that region. Derived Motion Winds from GOES-17 were still in the Beta stage, and were not available for display in AWIPS.

GOES-15 Infrared cloud-tracked Derived Motion Winds [click to enlarge]

GOES-15 Infrared cloud-tracked Derived Motion Winds [click to enlarge]