A leeside cold frontal gravity wave in the Plains, and a strong polar jet stream over the Rockies

January 14th, 2021 |

GOES-16 Upper-level Water Vapor (6.2 µm) images, with plots of hourly surface wind barbs and gusts [click to play animation | MP4]

GOES-16 Upper-level Water Vapor (6.2 µm) images, with plots of hourly surface wind barbs and gusts [click to play animation | MP4]

GOES-16 (GOES-East) Upper-level Water Vapor (6.2 µm) images (above) displayed the signature of a leeside cold frontal gravity wave (reference) as it moved rapidly southward across the Plains (surface analyses) on 14 January 2021. Post-frontal wind gusts in the 50-70 knot range were seen, with a peak gust to 91 knots (105 mph) in eastern Wyoming.

In addition, an anomalously-strong (170-180 knot) meridional branch of the upper-tropospheric polar jet stream was progressing southward over the Rocky Mountains — and GOES-16 Water Vapor images with plots of Derived Motion Winds and contours of RAP40 model maximum wind speeds (below) revealed that the highest satellite-tracked Derived Motion Wind (DMW) speed was 182 knots over southern Montana at 1401 UTC. Some of the DMW speeds were nearly 10 knots faster than the RAP40 model maximum wind (for example, this 170-knot DMW over Wyoming at 1616 UTC).

GOES-16 Upper-level Water Vapor (6.2 µm) images, with plots of 6.2 µm Derived Motion Winds and contours of RAP40 model maximum wind speeds [click to play animation | MP4]

GOES-16 Upper-level Water Vapor (6.2 µm) images, with plots of 6.2 µm Derived Motion Winds and contours of RAP40 model maximum wind speeds [click to play animation | MP4]

Cold air advection in the Bering Sea

January 5th, 2021 |

GOES-17

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 (GOES-West) “Red” Visible (0.64 µm) images (above) displayed cloud streets across the Bering Sea — cloud features that frequently occur in areas with a strong flow of cold air over warmer water. This northerly flow of cold air across the Bering Sea was due to a strong pressure gradient between high pressure over Siberia and broad low pressure centered over the Gulf of Alaska (surface analyses).

In a GOES-17 Visible image with plots of ASCAT scatterometer surface winds from Metop-A (below), ASCAT sampled winds with speeds as high as 33 knots (although the instrument did not adequately sample the western portion of the Bering Sea, where the strongest winds likely existed).

GOES-17 "Red" Visible (0.64 µm) image, with plots of Metop-A ASCAT winds [click to enlarge]

GOES-17 “Red” Visible (0.64 µm) image, with plots of ASCAT winds from Metop-A [click to enlarge]

A sequence of Suomi NPP VIIRS Day/Night Band (0.7 µm) images (below) provided higher-resolution views of the cold air advection cloud streets.

Suomi NPP VIIRS Day/Night Band (0.7 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) images [click to enlarge]

A toggle between Suomi NPP VIIRS Day/Night Band (DNB) and GOES-17 Visible images around 2320 UTC (below) highlighted the advantage of  VIIRS DNB imagery at high latitudes, particularly during low-light periods of the winter season.

Suomi NPP VIIRS Day/Night Band (0.7 µm) and GOES-17

Suomi NPP VIIRS Day/Night Band (0.7 µm) and GOES-17 “Red” Visible (0.64 µm) images [click to enlarge]

Hurricane Force low moves into the Bering Sea

December 31st, 2020 |

GOES-17 Air Mass RGB and Mid-level Water Vapor (6.9 µm) images [click to play animation | MP4]

GOES-17 Air Mass RGB and Mid-level Water Vapor (6.9 µm) images [click to play animation | MP4]

GOES-17 (GOES-West) Air Mass RGB and Mid-level Water Vapor (6.9 µm) images (above) showed an anomalously-deep Hurricane Force low pressure system as it approached the western Aleutian Islands and moved into the Bering Sea on 31 December 2020 (surface analyses). The peak wind gust at Shemya (PASY) was 72 knots (83 mph) at 09 UTC.

A closer view using a GOES-17 Water Vapor image at 0910 UTC (below) included plots of Metop ASCAT surface scatterometer winds — the highest ASCAT wind speeds were 66 knots north of the storm center, and 78 knots south of the storm center.

GOES-17 Mid-level Water Vapor (6.9 µm) image at 0910 UTC, with plots of Metop ASCAT surface scatterometer winds [click to enlarge]

GOES-17 Mid-level Water Vapor (6.9 µm) image at 0910 UTC, with plots of Metop ASCAT surface scatterometer winds [click to enlarge]

A toggle between Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images (below) showed the cloud circulation associated with the low at 1457 UTC — ample illumination from a Full Moon provided a superb visible image at night.

Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images [click to enlarge]

In another comparison of Suomi NPP VIIRS Infrared Window and Day/Night Band images about 10 hours later at 0053 UTC on 01 January (below), note the ship report approximately 300 miles west-northwest of Shemya: 50 knot winds, with blowing spray (which is plotted with the symbol normally used to indicate blowing sand).

Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) images [click to enlarge]

Model fields indicated that the height of the Dynamic Tropopause — taken to be the pressure of the PV1.5 surface — descended to the 600 hPa pressure level as the storm moved across the Shemya area (below).

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 pressure [click to play animation | MP4]

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 pressure [click to play animation | MP4]

Of particular significance was the fact that a new low pressure record for the State of Alaska was set when Shemya dropped to 924.8 hPa at 2159 UTC (below).

Plot of surface report data from Shemya [click to enlarge]

Plot of surface report data from Shemya [click to enlarge]



Mesovortex over Lake Superior

December 28th, 2020 |

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) revealed the formation of a mesovortex in northern Lake Superior on 28 December 2020. Mid-lake convergence — as depicted by RAP40 model surface winds — contributed to the development of this feature.

ASCAT surface scatterometer winds from Metop-A at 1537 UTC (below) provided a good view of the cyclonic flow of the mesovortex in its early stages, before it became organized enough to become obvious on satellite imagery.

GOES-16 Visible mage, with an overlay of Metop ASCAT surface scatterometer winds [click to enlarge]

GOES-16 Visible mage, with and without an overlay of Metop ASCAT surface scatterometer winds [click to enlarge]

A toggle between ASCAT winds from Metop-A and Metop-B (source) is shown below.

ASCAT winds from Metop-A and Metop-B [click to enlarge]

ASCAT winds from Metop-A and Metop-B [click to enlarge]

VIIRS True Color and False Color RGB images from Suomi NPP and NOAA-20 (below) showed a higher resolution view of the mesovortex; the shades of cyan in the False Color images suggested that the tops of some of the cloud bands were becoming glaciated.

VIIRS True Color and False Color RGB images from Suomi NPP and NOAA-20 [click to enlarge]

VIIRS True Color and False Color RGB images from Suomi NPP and NOAA-20 [click to enlarge]


False color images from VIIRS as shown above combine bands M11, I2 and I1: 2.25 µm, 0.865 µm and 0.64 µm. Inclusion of the near-IR channel at 2.25 µm causes a color change – less red (blue and green make cyan) – in regions where ice crystals exist, because ice crystals absorb, rather than reflect, solar energy at that wavelength. A similar occurrence happens at 1.61 µm wavelengths.

Accordingly, the Day Cloud Phase Distinction RGB, shown below, highlights ice crystals in clouds (they are yellow or orange because there is less green in the RGB where ice crystals are present;  before sunrise, and after sunset, the RGB is only red:  green and blue contributions depend on solar reflectance). Solar reflectance is small at all wavelengths at this time of year over Lake Superior, but a definite color difference in the clouds of the vortex is apparent. Snow showers/squalls are more likely where the Day Cloud Phase Distinction RGB suggests ice crystals in the clouds, as shown in this blog post (and this one!).

Day Cloud Phase Distinction RGB, 1401 UTC – 2131 UTC, 28 December 2020 (Click to animate)

Mesoscale vortices over warm lakes owe their existence to sensible and latent heat fluxes from the (relatively) warm water into the colder atmosphere aloft. (Click here for a presentation of such an event over southern Lake Michigan). When the vortex moved over Ontario and lost the lake fluxes, it dissipated. Visible imagery from the morning of 29 December 2020, below, showed no circulation.

GOES-16 Visible Imagery (0.64 µm), 1301 – 1531 UTC on 29 December 2020 (Click to animate)