Hurricane Paloma

November 8th, 2008 |
GOES-12 10.7 µm IR imagery

GOES-12 10.7 µm IR imagery

GOES-12 10.7 µm IR imagery (above) showed Category 4 Hurricane Paloma just south of Cuba on 08 November 2008. A well-defined eye was present, surrounded by cold cloud top temperatures in the -70º to -80º C range (as cold as -81º C at 16:25 UTC). GOES-12 visible images from the CIMSS Tropical Cyclones site (below, with an overlay of QuikSCAT winds on the initial image) also displayed a nice eye structure during the morning hours.

GOES-12 visible images (with overlay of QuikSCAT winds)

GOES-12 visible images (with overlay of QuikSCAT winds)

CIMSS Advanced Dvorak Technique intensity estimate plot

CIMSS Advanced Dvorak Technique intensity estimate plot

A plot of the CIMSS Advanced Dvorak Technique tropical cyclone intensity estimate (above) indicated that Hurricane Paloma experienced a period of rapid intensification late in the day on 07 November — apparently the tropical cyclone was moving over a tongue of high Ocean Heat Content (below), which may have aided such intensification. On 08 November, Hurricane Paloma became the second strongest November hurricane on record (behind Hurricane Lenny in 1999) — and it is also interesting to note that 2008 now becomes the only year on record with a major hurricane occurring in 5 separate months (Bertha in July, Gustav in August, Ike in September, Omar in October, and Paloma in November).

Ocean Heat Content (courtesy of RSMAS)

Ocean Heat Content (courtesy of RSMAS)

November: a month for blizzards, gales, and tropical storms

November 6th, 2008 |
GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

AWIPS images of the GOES-12 6.5 µm “water vapor channel” (above) displayed 3 impressive storms during the 05 November06 November 2008 period: a powerful blizzard in the northern Great Plains states, a strong “gale” storm along the US East Coast, and Tropical Storm Paloma in the western Caribbean Sea. The Plains blizzard produced wind gusts to 85 mph at Rapid City and snowfall as high as 45.7 inches at Deadwood in western South Dakota; the East Coast storm was responsible for wind gusts in excess of 58 mph and wave greater than 18 feet at the Diamond Shoals buoy off the coast of North Carolina; and Tropical Storm Paloma was showing signs of entering a period of rapid intensification.

Note the presence of abnormally dry air on the water vapor imagery over southern Georgia, northern Florida, and the adjacent offshore waters of the western Atlantic — water vapor brightness temperatures were as warm as -10 º C (darker orange color enhancement) early in the day on 06 November. GOES-12 water vapor channel weighting functions (below) showed that the altitude of the layer being detected within the dry air mass  in place over Jacksonville, Florida was at a significantly lower altitude (centered near the 500 hPa pressure level) compared to what would be detected in a more “normal” US Standard Atmosphere (centered near the 325 hPa pressure level).

GOES-12 water vapor weighting functions

GOES-12 water vapor weighting functions

Taking a closer look at the Great Plains blizzard, AWIPS images of the MODIS 6.7 µm water vapor channel (below) revealed an intensifying dry slot that was moving northward across Nebraska and South Dakota on 05 November. CRAS model fields of the 850-700 hPa layer equivalent potential temperatures (red contours) suggested that a Trough of Warm Air Aloft (TROWAL) was in the process of forming over central South Dakota, which was helping to fuel a line of new convective development around 19 UTC.

MODIS 6.7 µm watr vapor images + CRAS model fields

MODIS 6.7 µm watr vapor images + CRAS model fields

Several hours later, the GOES sounder Total Column Ozone product (below) revealed a tongue of higher ozone values (around 350 Dobson Units, green colors) surging northward from Nebraska into central South Dakota.

GOES sounder Total Column Ozone product

GOES sounder Total Column Ozone product

This ozone feature was apparently associated with a potential vorticity (PV) anomaly (below), with the dynamic tropopause (taken to be the pressure of the 1.5 Potential Vorticity Unit surface, red contours) being brought downward to near the 500 hPa pressure level over north-central South Dakota at 06 UTC on 06 November. The approach of this PV anomaly may have played a role in further intensification of the blizzard later that day.

GOES sounder Total Comumn Ozone + CRAS model fields

GOES sounder Total Comumn Ozone + GFS model fields

Around this time, the NWS forecast office at Rapid City noted that the TROWAL (below, as seen in GFS model 300-310 K layer equivalent potential temperatures, red contours) was also becoming better defined, stretching southwestward across  North Dakota into western South Dakota:

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
229 AM MST THU NOV 6 2008

.DISCUSSION…DEEP CYCLONE CENTERED ACROSS CENTRAL SOUTH DAKOTA
WITH WIDESPREAD BLIZZARD CONDITIONS ACROSS WESTERN SOUTH DAKOTA.
WINDS GUSTING TO 78 MPH IN THE RAPID CITY AREA. MOST OTHER
LOCATIONS GUSTING TO 60-65 MPH…EVEN IN CUSTER. TROWAL WRAPPED
ACROSS NORTH DAKOTA INTO WESTERN SOUTH DAKOTA…AIMED AT THE BLACK
HILLS.

GOES-12 6.5 µm water vapor image + GFS model fields

GOES-12 6.5 µm water vapor image + GFS model fields

Finally, Tropical Storm Paloma: GOES-12 IR images with an overlay of ASCAT winds from the CIMSS Tropical Cyclones site (below) showed a series of convective bursts having cold cloud temperatures along with a well-defined wind field associated with the intensifying tropical cyclone.

GOES-12 IR images + ASCAT winds

GOES-12 IR images + ASCAT winds

Mountain wave clouds over Alberta and Montana

November 4th, 2008 |
GOES-11 and GOES-13 water vapor images

GOES-11 and GOES-13 water vapor images

GOES-11 6.7 µm and GOES-13 6.5 µm “water vapor channel” images (above) showed the formation of mountain wave clouds over parts of southern Alberta and western Montana on 03 November 2008.  Note how the mountain wave clouds — which formed immediately downwind of the highest terrain of the Rocky Mountains –  dissipated as pockets of drier air aloft (darker blue enhanced areas) passed through the region. Not long after the initial area of mountain wave clouds disappeared, a second patch of mountain wave clouds formed very quickly over western Montana around 18:00 UTC (11 am local time); there were 3 reports of moderate (intensity level 4) turbulence in the general vicinity of that mountain wave cloud. While these features  could certainly be followed adequately using the 8-km resolution GOES-11 water vapor imagery, they appeared much clearer on the 4-km resolution GOES-13 water vapor imagery (also aided by the more direct satellite viewing angle of GOES-13, which was positioned at 105º W longitude).

AWIPS images of the MODIS visible channel, 11.0 µm IR window channel, 3.7 µm shortwave IR channel,  Cloud Top Temperature product, and Cloud Phase product (below) were useful to further characterize the mountain wave cloud shortly after its formation over western Montana. The cloud top temperatures appeared to be quite low (as cold as -52º C on the IR window channel data, and as cold as -52.8º C on the Cloud Top Temperature product), and the Cloud Phase product classified the feature as an ice cloud (salmon-colored enhancement). However, note the rather warm brightness temperatures on the 3.7 µm shortwave IR image: cloud top temperatures were in the +5 to +10º C range. These warmer brightness temperatures were due to reflection of solar radiation by the rather small ice crystals (having diameters which were likely in the 50-60 micrometer range) that made up the mountain wave cloud. The larger patch of cirrus clouds located farther to the west (over the Idaho/Montana border region) exhibited significantly colder 3.7 µm brightness temperatures (close to -20º C), since the typical larger (around 100 micrometers or greater in diameter) cirrus ice crystals are not efficient scatterers of light.

MODIS visible + IR window + shortwave IR + Cloud Top Temperature + Cloud Phase

MODIS visible + IR window + shortwave IR + Cloud Top Temperature + Cloud Phase

Further confirmation of the small ice crystal particle sizes within the mountain wave cloud is seen by examining the MODIS 11.0-12.0 µm (channel 31-32) IR difference product (below). IR difference values were in the 3-5º K range (cyan to blue color enhancement) over a good deal of the mountain wave cloud feature — but IR difference values over the cirrus clouds found farther to the west and southwest were negligible. Of particular interest was the thin filament of even higher IR difference values (7-8º K, darker blue color enhancement) seen along the leading (western) edge of the mountain wave cloud.

MODIS 11.0 µm IR and 11.0-12.0 µm IR difference images

MODIS 11.0 µm IR and 11.0-12.0 µm IR difference images

A MODIS IR window image about 90 minutes later (below) indicated that the mountain wave cloud had grown in size somewhat, with the coldest cloud top brightness temperature at -53º C (placing the cloud top at about the 32,300 foot level, not far below the tropopause according to the Great Falls, Montana rawinsonde report). There was one aircraft report of moderate turbulence (intensity level 4) at an altitude of 33,000 feet (close to the southern portion of the mountain wave cloud) near the time of the 19:56 MODIS IR image.

MODIS 11.0 µm IR window image (with Great Falls MT rawinsonde data)

MODIS 11.0 µm IR window image (with Great Falls MT rawinsonde data)