Instability vortices along a jet stream axis

September 28th, 2009

GOES-12 6.5 µm water vapor imagery

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

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

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

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 cross section

GFS40 model cross section

Wildfires in Oregon

September 23rd, 2009
GOES-11/GOES-12 water vapor composite image + GFS 500 hPa height

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

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

MODIS true color and false color images

GOES-12 visible 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 visible images

GOES-11 (GOES West) visible images

Dust storm over eastern Australia

September 23rd, 2009

MTSAT-1R visible images

MTSAT-1R visible images

One of the worst dust storms in the past 70 years swept across a large part of eastern Australia on 22 September23 September 2009 (Daily Mail Online photos). A sequence of MTSAT-1R visible images (above) showed the progression of the large dust cloud as it moved eastward during the daylight hours. Note the appearance of “lee waves” along the top of the dust cloud, as the strong winds interacted with the high terrain of the Great Dividing Range. An undular bore could also be seen forming out ahead of the cold front, over the offshore waters of the South Pacific Ocean.

The surface meteorogram for Brisbane, Australia (station identifier YBBN) is shown below; note that the surface visibility dropped to 0.2 km (0.1 mile) as the cold front passed, and following the frontal passage the dew point dropped from +16º C (61º F) to -16º C (+3º F).

Brisbane, Australia surface meteorogram

Brisbane, Australia surface meteorogram

A larger-scale view of the dust cloud feature could be seen using MODIS true color imagery from the NASA MODIS Rapid Response site (below, viewed using Google Maps). See also the NASA  MODIS Image of the Day.

MODIS true color image

MODIS true color image

Record rainfall in Madison, Wisconsin

September 22nd, 2009
GOES-12 10.7 µm IR image

GOES-12 10.7 µm IR images

Madison, Wisconsin received 3.63 inches (92 mm) of rainfall on 22 September 2009 — much of it within a 4 hour period — which set a record for the highest rainfall amount for any day during the month of September (additional details are available from the NWS Milwaukee/Sullivan). AWIPS images of the GOES-12 10.7 µm IR Window channel (above) showed that the cloud top IR brightness temperatures were rather unremarkable, with only a few areas exhibiting temperatures colder than -50º C (yellow color enhancement) during the heavy rainfall event. The 1-km resolution AVHRR Cloud Top Temperature values were only slightly colder, at -54º C — this corresponded to a AVHRR Cloud Top Height of 11-12 km.

GOES-12 visible images (below) did not show any pronounced cloud top texture or shadowing that would indicate strong convective elements over southern Wisconsin — in fact, there were no cloud-to-ground lightning strikes seen during the day, and no thunder was reported at any of the first order observing stations.

GOES-12 visible images

GOES-12 visible images

Blended Total Precipitable Water product

Blended Total Precipitable Water product

The Blended Total Precipitable Water (TPW) product (above) did reveal that an area of TPW values of 40 mm and greater (red colors) developed across northern Illinois and far eastern Iowa after about 08 UTC — this seemed to be associated with a disturbance that developed and moved northward along and ahead of a frontal boundary that was moving slowly eastward across the region. It should be noted that these TPW values were about 175-200% above normal.

However, about 5 hours earlier the MODIS TPW product displayed TPW values of 40 mm and higher (red colors) over parts of northwestern Illinois, at a time when the Blended TPW product was still only indicating TPW values of 30-35 mm (below). This area of higher TPW values over northern Illinois was then advected northward during the day, and lifted along and ahead of a developing warm frontal boundary. A cross section oriented north to south showed a deep column of upward vertical motion (omega values of 10 µbars per second, red colors) over southern Wisconsin.

Blended TPW + MODIS TPW products

Blended TPW + MODIS TPW products