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

“Southerly surge” of stratus along the coast of California

April 22nd, 2013 |
GOES-15 0.63 µm visible channel images (click image to play animation)

GOES-15 0.63 µm visible channel images (click image to play animation)

McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) revealed a well-defined “southerly surge” of stratus clouds, which were moving northward along the coast of the Big Sur region of central California on 22 April 2013.

AWIPS images of the GOES-R Marginal Visual Flight Rules (MVFR) product algorithm applied to GOES-15 data (below; click image to play animation) showed the northward progression of the narrow band of stratus clouds, as a mesoscale reversal in the boundary layer winds just off the coast (from northerly, to southerly) was seen in the buoy data.

GOES-15 Marginal Visual Flight Rules (MVFR) Probability product (click image to play animation)

GOES-15 Marginal Visual Flight Rules (MVFR) Probability product (click image to play animation)

A 1-km resolution MODIS 11-3.7 µm IR brightness temperature difference (BTD) “fog/stratus product” image at 05:34 UTC or 10:34 PM local time (below) showed the initial stages of the southerly surge stratus cloud feature, in the Santa Barbara Channel north of the Channel Islands off the coast of southern California (and south of Point Conception). Farther offshore, the BTD image also displayed a well-defined eddy and a few ship tracks embedded within the marine boundary layer stratus clouds.

MODIS IR brightness temperature difference

MODIS IR brightness temperature difference “fog/stratus product”

Several hours later, a 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) image at 09:56 UTC or 2:26 AM local time (below) indicated that the leading edge of the southerly surge stratus cloud feature had moved to the north of Point Conception. This demonstrates that the VIIRS DNB can provide a “visible image at night”, given sufficient illumination by the Moon (or other light sources). On this night the Moon was in the waxing gibbous phase, at 95% full.

Suomi NPP VIIRS 0.7 µm Day/Night Band image

Suomi NPP VIIRS 0.7 µm Day/Night Band image

A Suomi NPP VIIRS 0.64 µm visible channel image at 21:13 UTC or 2:13 PM local time (below) showed leading edge of the southward surge stratus feature as it was curving eastward into the Monterey Bay area. Not far to the north, San Francisco reached a record high temperature of 83º F.

Suomi NPP VIIRS 0.64 µm visible channel image

Suomi NPP VIIRS 0.64 µm visible channel image

===== 23 April Update =====

A comparison of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference (BTD) “Fog/stratus product” images at 09:28 UTC or 2:29 AM local time (below) showed that the leading edge of the southerly surge stratus clouds had progressed just to the north of Point Reyes. Once again, the BTD image revealed the presence of cloud eddies and ship tracks within the marine boundary layer stratus clouds.

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference

Suomi NPP VIIRS 0.7 µm Day/Night Band and IR brightness temperature difference “Fog/stratus product” images

 

Undular bore over Texas and the Gulf of Mexico

March 5th, 2013 |
MODIS IR brightness temperature difference

MODIS IR brightness temperature difference “fog/stratus product”, 3.7 µm shortwave IR, 11.0 µm IR window, and 6.7 µm water vapor channel images

A comparison of AWIPS images of MODIS 11.0-3.7 µm IR brightness temperature difference “fog/stratus product”, 3.7 µm shortwave IR, 11.0 µm IR window, and 6.7 µm water vapor channel images (above) showed the night-time (08:04 UTC or 3:04 AM local time) development of parallel cloud bands associated with an undular bore that was forming in advance of a strong cold frontal boundary which was  moving southward across Texas on 05 March 2013. Note how the cloud band features showed up with better clarity in the 3.7 µm IR image compared to the 11.0 µm IR image, since the shortwave IR channel is more sensitive to warmer temperatures (the IR brightness temperatures of the cloud bands averaged about +5º C).

After sunrise, the undular bore cloud bands coud be seen moving southeastward off the coast of Texas and across the adjacent offshore waters of the Gulf of Mexico on McIDAS images of GOES-13 0.63 µm visible channel data (below; click image to play animation).

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)

As the leading edge of the bore passed through Corpus Christi, Texas (CRP on the GOES-13 visible images), surface observations (below) indicated that winds gusted to 38 knots or 44 mph at 12:27 UTC.

Corpus Christi surface observations

Corpus Christi surface observations

The 12 UTC morning rawinsonde data from Corpus Christi (below) revealed a very strong and deep boundary layer temperature inversion (with a top around 962 hPa or 2460 feet), which was acting to duct the undular bore as it propagated southeastward.

Corpus Christi, Texas rawinsonde data

Corpus Christi, Texas rawinsonde data

MODIS 0.65 µm visible channel image and MODIS Sea Surface Temperature product

MODIS 0.65 µm visible channel image and MODIS Sea Surface Temperature product

It is interesting to note that in the wake of the undular bore passage, there appeared to be a signal of Gulf of Mexico water wave activity in both the MODIS Sea Surface Temperature product (above) and the Suomi NPP VIIRS 11.45 µm IR channel image (below). On each corresponding visible channel image, a subtle wave signature could also be seen, but these waves did not appear to be cloud features. Could the strong winds of the bore passage have created wave swells which then acted to mix the water surface enough to allow a small amount of cold water upwelling?

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images