GOES-12 visible image

Late on October 4th, the weather system in the far northeast Atlantic acquired sufficient tropical characteristics to be classified as a Tropical Storm, and Grace was named. The visible image from GOES-12 above shows the counterclockwise swirl of clouds. GOES-12 is over the Equator at 75 degrees W Longitude, and Tropical Storm Grace at the time of the image above was at 45 degrees North latitude and 16 degrees W Longitude; consequently, the view angle is very oblique. Indeed, the visible image shows a convective spiral band that lies beneath the cirrus shield that covers the system. Note that no overshooting tops penetrate the cirrus overcast over the tropical system. The system sits over sea surface temperatures near 70 degrees Fahrenheit (see the Sea Surface Temperature analysis here, and those temperatures are yielding insufficient CAPEs to produce overshooting tops.

Grace developed underneath a decaying upper-level low. The low was able to draw north modestly high values of precipitable water, as shown in the MIMIC analysis here. Grace is associated with the very small region of enhanced precipitable water that is at 40 N, 20 W at the start of the loop, then moving northeastward towards Ireland.

A comparison of Terra MODIS visible and 11.0 µm IR images (below) showed that Grace exhibited a fairly well-defined banded structure and some semblance of an eye at 11:40 UTC.

MODIS visible and IR images

(Added: Jesse Ferrell at AccuWeather notes that Grace was almost the farthest-east forming tropical system on record! Link).

(Added, 6 October: Grace merged with/was absorbed by a front southwest of Ireland late in the day on the 5th.) AMSU microwave data from early on the day on the 5th clearly show a warm core to the system, one of the hallmarks of a tropical Storm. For example, data from the AMSU-A instrument in NOAA-18 at 0413 UTC on 5 October show a region of warmth at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7); the 89-GHz channel on AMSU-B also shows warmth at the center of the storm. These warm signals were critical in determining that system was tropical in nature. The warmth persisted; AMSU-A data from NOAA-19 at 1406 UTC on the 5th also showed a warm core at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7), as well as in the 89-GHz channel on AMSU-B. (More imagery is available here).

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MODIS visible, 2.1 µm near-IR, and 3.7 µm shortwave IR channels

AWIPS images of the MODIS visible channel, the 2.1 µm near-IR “snow/ice” channel, and the 3.7 µm shortwave IR channel (above) displayed a swath of snow cover on the ground in far northern Quebec, Canada on 04 October 2009. The Environment Canada snow cover analysis at 06 UTC placed a maximum of 21 cm (8 inches) in that area. The area of snow cover appeared bright on the visible image, and darker on the 2.1 µm near-IR snow/ice image (due to the strong absorption of snow at that wavelength) — however, there was a patch of supercooled water cloud over the northern portion of the snow cover, which appeared brighter white on the snow/ice image and darker (warmer) on the shortwave IR image (due to increased solar reflection off the supercooled water droplets).

A comparison of the MODIS visible image and the corresponding MODIS false color Red/Green/Blue (RGB) image constructed using the visible and the near-IR snow/ice channels (below) shows the value of RGB imagery for helping to distinguish between snow cover (which appears darker red on the false color image) and supercooled water droplet clouds (which appear as cyan to white shades on the false color image). Note the semi-transparent nature of this particular cloud deck: surface features (such as rivers, and the edges of the snow cover) can be seen through the thin cloud feature. Farther to the south, glaciated clouds that are composed primarily of ice crystals also appear as varying shades of red on the false color image. The ability to display these types of false-color RGB images will hopefully be available to forecasters using the next generation  of AWIPS II software.

MODIS visible and false color RGB images

The MODIS Land Surface Temperature product (below) indicated that LST values were only in the 30s F (darker green color enhancement) in the region of snow cover, compared to much warmer 40s and 50s F (lighter green to yellow color enhancement) in the surrounding bare ground areas.

MODIS Land Surface Temperature product

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GOES-12 6.5 µm water vapor imagery

AWIPS images of the GOES-12 6.5 µm “water vapor channel” (above) revealed a pair of vortices immediately poleward of a well-defined jet stream  axis that was moving over the southeastern US  on 28 September 2009. It was initially thought that these vortices may have represented either a type of  Kelvin-Helmholtz instability (which can occur when there is sufficient velocity difference across the interface between two fluids) or a type of Rayleigh-Taylor instability (which can occur along an interface of two fluids of different densities) — however, a more likely explanation might be that these vortices were a result of barotropic instability, where the waves grew by extracting kinetic energy from the shear flow from which they were embedded.

If a horizontal circulation  developed due to barotropic instability being forced by the horizontal wind shear, this could result in the formation of the vortex structures seen on the water vapor imagery. The warm/dry spot on the images (exhibiting brightness temperature values as warm as -11º C, darker orange color enhancement) was probably a pocket of warm/dry air that originated from the poleward edge of the moisture gradient — once the vortex formed, the warm/dry air in the center could not escape, and its properties would be preserved.  (Thanks to Jordan Gerth, Justin Sieglaff and Chris Rozoff at CIMSS…and Michael Morgan at UW-AOS for providing valuable inputs and helping to provide an explanation)

Overlays of parameters from the 45-km resolution CRAS model at 12:00 UTC  (below) showed the presence of a 50-60 knot jet axis just south of the primary dry-to-moist gradient on the water vapor image, along with a ribbon of 500 hPa vorticity and a 500 hPa wind shear axis over the region where the water vapor vortices were forming.

CRAS45 maximum wind speed, 500 hPa vorticity, and 500 hPa shear vectors

A comparison of the 1-km resolution MODIS 6.7 µm water vapor image and the 4-km resolution GOES-12 6.5 µm water vapor image (below) show the advantage of improved spatial resolution for displaying the structure and gradients associated with the leading vortex around 18:15 UTC.

1-km MODIS vs 4-km GOES-12 water vapor images

Examining the GOES-12 imager water vapor weighting function profiles at 00:00 UTC for Charleston SC (located in the “dry” portion of the sharp water vapor image gradient) and Jacksonville FL (located in the “moist” portion of the sharp water vapor image gradient) shows that there would be a pronounced downward shift in the altitude of features displayed on the water vapor image in the region of dry air located poleward of the jet stream axis.

GOES-12 water vapor weighting function profile for Charleston SC and Jacksonville FL

A northwest-to-southeast oriented vertical cross section using GFS40 model fields (below) displayed a minor intrusion of potential vorticity (the colored image portion of the cross section) downward into the upper troposphere immediately poleward of the jet stream core (which was located between the 200 and 250 hPa pressure levels). The wind speed shear axis was located at a much lower altitude (between the 400 and 500 hPa pressure levels), closer to the altitude peak of the water vapor channel weighting function in the region of drier air.

GFS40 model cross section

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GOES-11/GOES-12 water vapor composite image + GFS 500 hPa height

An anomalous ridge of high pressure developed across western North America on 23 September 2009, bringing hot and dry conditions to parts of the Pacific Northwest states — high temperatures at many locations in Oregon were in the 90s and low 100s F for two consecutive days. The effect of this large ridge could be seen quite well on an AWIPS composite image of the GOES-11 and GOES-12 water vapor channels (above). Stu Ostro at The Weather Channel pointed out that the 5950 meter geopotential height at Spokane, Washington at 00 UTC on 23 September is the record highest value for so far north in the US so late in the season (since the beginning of the NCEP reanalysis dataset, which goes back though 1948).

A pair of large wildfires were burning in southwestern Oregon — the “hot spots” from these 2 fires could be seen on MODIS 3.7 µm and GOES-11 3.9 µm shortwave IR images (below), located to the east of Roseburg (station identifier KRBG). The location and areal coverage of these wildfire hot spots was better depicted on the 1-km resolution MODIS image, compared to the 4-km resolution GOES-11 image; in addition, the leading edge of the marine fog/stratus that was moving inland was more accurately shown on the higher-resolution MODIS imagery.

MODIS 3.7 µm + GOES-11 3.9 µm shortwave IR images

250-meter resolution MODIS true color and false color images from the SSEC MODIS Today site (below) show even better details of the smoke plumes and the marine fog/stratus. There was also evidence of  some smoke remaining in a few of the valleys near the fire activity. The MODIS false color image also displays the larger active fire “hot spots” as pink-colored features at the source of the smoke plumes.

MODIS true color and false color images

GOES-12 (GOES East) visible images

The large plumes of smoke from these Oregon fires could be seen moving northward across western Oregon and western Washington, even drifting as far to the north as southern British Columbia and Alberta in Canada. Note that the leading (northern) edge of the smoke plume was easier to identify on GOES-12 (GOES East) visible imagery (above) compared to GOES-11 (GOES West) visible imagery (below) — this is a result of the more favorable forward scattering geometry with the GOES-12 satellite. However, the more direct viewing angle of GOES-11 made it easier to see the marine fog/stratus that was moving inland along coastal sections of Washington, Oregon, and California.

GOES-11 (GOES West) visible images

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