Low-level “barrier jet” along the southeast coast of Greenland

December 29th, 2013 |
GOES-13 6.5 µm water vapor images with Metop ASCAT scatterometer winds and surface METARs and surface analyses (click to play animation)

GOES-13 6.5 µm water vapor images with Metop ASCAT scatterometer winds and surface METARs and surface analyses (click to play animation)

AWIPS images of GOES-13 6.5 µm water vapor channel data with available overpasses of Metop ASCAT surface scatterometer winds (above; click image to play animation) revealed the presence of a low-level “barrier jet” along the southeast coast of Greenland on 29 December 2013. Maximum ASCAT wind speeds were 58 knots at 12:16 UTC, 62 knots at 13:57 UTC, and 62 knots at 22:09 UTC. It is interesting to note that a secondary area of low pressure was seen rotating around the primary low, and appeared to be rapidly intensifying judging from the quick development of a “corkscrew” appearance on the water vapor imagery near the end of the animation. ASCAT winds along the northwestern periphery of this secondary low were as high as 53 knots at 22:09 UTC.

The cyclonic circulation around the quasi-stationary area of low pressure located east of Greenland encountered the abrupt rise in topography of the island (below), which caused an acceleration of the flow known as a “barrier jet”.

Topography of Greenland, with Metop ASCAT scatterometer winds and surface METAR reports and surface analysis

Topography of Greenland, with Metop ASCAT scatterometer winds and surface METAR reports and surface analysis

Strong storm in the North Atlantic Ocean

December 12th, 2013 |
GOES-13 6.5 µm water vapor channel images, with surface analysis and METAR surface reports

GOES-13 6.5 µm water vapor channel images, with surface analysis and METAR surface reports

A large and strong cyclone was intensifying in the North Atlantic Ocean just south of Greenland on 12 December 2013. GOES-13 6.5 µm water vapor channel images with 06 UTC, 12 UTC, 18 UTC, and 00 UTC surface analyses and surface reports (above) showed that the central pressure of the storm deepened to 942 hPa at 18 UTC — and the strong pressure gradient between the low and a 1032 hPa high situated over northern Greenland was forecast to produce a broad region of hurricane-force winds.

EUMETSAT Metop ASCAT scatterometer surface winds at 22:41 UTC (below) highlighted 3 areas over the water which contained a number of remotely-sensed winds of 50 knots or higher (red barbs). The strongest wind velocity within this ASCAT swath was 54 knots, located in the largest area of red wind barbs south of the storm center.

GOES-13 6.5 µm water vapor image and ASCAT scatterometer surface winds

GOES-13 6.5 µm water vapor image and ASCAT scatterometer surface winds

Near the time of the 22:41 UTC ASCAT data, surface winds were a steady 50 knots from the north-northwest at Cape Dyer, Nunavut, Canada (station idendifier CWFD) — and the peak wind gust at that station was 56 knots several hours prior at 18 UTC. In southern Greenland, the peak wind gust at Narsarsuaq (station identifier BGBW) was 71 knots, occcurring earlier in the day at 13 UTC (below).

Time series of meteorological data at Cape Dyer, Nunavut, Canada

Time series of meteorological data at Cape Dyer, Nunavut, Canada

Time series of meteorological data at Narsarsuaq, Greenland

Time series of meteorological data at Narsarsuaq, Greenland

GOES-13 6.5 µm water vapor images at 30-minute intervals (below; click image to play animation) displayed an interesting range of signatures as moisture wrapped around the very large storm.

GOES-13 6.5 µm water vapor channel images (click to play animation)

GOES-13 6.5 µm water vapor channel images (click to play animation)

Formation of an “Otter Eddy” in Monterey Bay, California

May 13th, 2013 |
GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

Strong northwesterly winds along the California coast interacted with the complex terrain and orientation of Monterey Bay to promote the formation of a cyclonic coastal eddy (known locally as an “Otter Eddy”) early in the day on 13 May 2013. McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) showed the evolution of the eddy feature, which gradually dissipated by the early afternoon hours. “MRY” denotes the location of Monterey.

Farther to the north, an interesting type of “bow shock wave” formed downwind of Point Reyes (labelled “PR” on the images). Better detail of this feature could be seen in an AWIPS image of Suomi NPP VIIRS 0.64 µm visible channel data (below). At the time of this image, surface winds at the offshore buoy just to the north of Point Reyes were gusting to 33 knots (38 mph).

Suomi NPP VIIRS 0.64 µm visible channel image

Suomi NPP VIIRS 0.64 µm visible channel image

Diagnosing areas of light winds over water

April 25th, 2013 |
Suomi NPP VIIRS 0.65 µm visible channel image with overlays of surface reports and RTMA surface winds

Suomi NPP VIIRS 0.65 µm visible channel image with overlays of surface reports and RTMA surface winds

An AWIPS image of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel data (above) revealed a large patch of water within the eastern Gulf of Mexico “sun glint” region that exhibited a much darker appearance than the surrounding waters off the west coast of Florida on 25 April 2013. This patch of darker water generally corresponded to a region of very light to calm winds, as verified by an overlay of the Real-Time Mesoscale Analysis (RTMA) surface winds. As explained in a previous blog post, there is often a significant amount of sun glint off the wind-driven rough water surfaces below a polar-orbiting satellite overpass — due to scattering of light these areas of sun glint make the rougher water surfaces appear brighter on visible imagery. However, in an area of calm winds, the water surface becomes very flat; this flat water surface then reflects incoming sunlight like a mirror (with all the light being reflected back in one direction — but in this case, that one direction was not directly back toward the satellite).

Another interesting signature of the flat water surface is seen in a comparison of 1-km resolution Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or “IR window” channel images (below). The shortwave IR channel is very sensitive to reflected solar radiation, and will often exhibit a much warmer, darker signal over areas of sun glint. However, note that the areas of darker water seen in the visible image above appear significantly cooler (lighter gray enhancement) on the shortwave IR image. Again, in the case of smooth, flat water in light wind regions, the incoming solar radiation is reflected back in a direction that happens to be away from the satellite sensors. Since the 11.45 µm IR channel is not sensitive to reflected solar radiation, no such signature was seen in that particular image.

Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or

Suomi NPP VIIRS 3.74 µm shortwave IR channel and 11.45 µm longwave IR or “IR window” channel images

A night-time vs daytime comparison of the 1-km resolution MODIS Sea Surface Temperature (SST) product (below) showed that the SST values over the patch of calm water increased from the low to middle 70s F at 06:58 UTC (2:58 AM local time) to the upper 70s to 80º F at 16:22 UTC (12:22 PM local time). Such an increase in SST within a relatively short 10-hour period was possible due to the fact that the presence of very light winds also allowed the “skin temperature” of the water surface to warm very quickly (as we have prevously seen over Lake Michigan).

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product