GOES-13 10.7 µm IR images

Rapid Scan Operations (RSO) images of the GOES-13 10.7 µm IR channel (above) showed some interesting evolution of Tropical Storm Hanna on 05 September 2008. Strong bursts of convection were seen to develop near the center of Hanna, with periods of cloud top temperatures colder than -80º C (purple colors).

A comparison of AWIPS images of the 1-km resolution MODIS 11.0 µm IR and the 4-km resolution GOES-12 10.7 µm IR channel data (below) indicated that the coldest cloud top temperatures were -89º C with MODIS and -78º C with GOES.

AWIPS images of the MODIS 11.0 µm IR + GOES-12 10.7 µm IR chanels

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GOES-12 IR image + SSM/IS microwave image

Hurricane Ike intensified into a  Category 4 storm late in the day on 03 September 2008. A comparison of GOES-12 10.7 µm IR and SSM/IS 85 GHz microwave images from the CIMSS Tropical Cyclones site (above) showed a well-defined eye structure around 09:15 UTC on 04 September. A period of rapid intensification was seen on the CIMSS Advanced Dvorak Technique (ADT) intensity estimate plot (below), which was noted in the National Hurricane Center discussion:

HURRICANE IKE DISCUSSION NUMBER  12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL092008
1100 PM EDT WED SEP 03 2008

IKE HAS RAPIDLY INTENSIFIED THIS EVENING.  AN EYE BECAME APPARENT IN CONVENTIONAL SATELLITE IMAGERY SHORTLY AFTER 2100 UTC.  SINCE THAT TIME THE EYE HAS BECOME MORE DISTINCT WITH A RING OF VERY COLD CLOUDS TOPS SURROUNDING IT.  THE LATEST DVORAK DATA T-NUMBERS WERE T6.0 AND RAW ADT ESTIMATES FROM UW-CIMSS HAVE AVERAGED T6.2 SINCE 2045 UTC.  BASED ON THESE ESTIMATES THE INITIAL INTENSITY IS SET AT 115 KT…MAKING IKE A CATEGORY FOUR HURRICANE.

CIMSS Advanced Dvorak Technique (ADT) intensity plot

GOES-12 10.7 µm IR and GOES-12 visible images (below) showed that while Hurricane Ike maintained a well-defined eye during the morning hours on 04 September, the appearance of the eye did degrade somewhat as the day went on — and the intensity of Ike was reduced from 120 knots to 115 knots.

GOES-12 10.7 µm IR images

GOES-12 visible images

The deep layer wind shear product (below) indicated that Ike was moving into an environment of increasing  northerly shear, which may have contributed to the slight decrease in intensity noted during the day.

GOES-12 IR image + deep layer wind shear product

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GOES-13 visible images (Tropical Storm Hanna)

A series of low-level “outflow boundaries” was seen on GOES-13 visible imagery (above), propagating westward from the shield of high clouds associated with Tropical Storm Hanna, moving across the Bahamas on 03 September 2008. The air behind each of these pulses appeared to be quite dry, judging from the general lack of cloudiness in their wake.

AWIPS images of the GOES sounder 7.4 and 7.0 µm water vapor channels, the GOES sounder Total Precipitable Water product, and the GOES imager water vapor channel at 16 UTC  (below) indicated that a tongue of dry air existed over the Bahamas, sandwiched in between Hanna and the clouds/moisture over Florida. The rawinsonde report from Nassau in the Bahamas showed that the air mass was indeed quite dry within the 700-500 hPa layer.

AWIPS images of GOES sounder WV and TPW, with GOES imager WV channel

The MIMIC Total Precipitable Water (TPW) product (below) suggested that a large pocket of dry air (TPW values around 35 mm, cyan colors) was situated east of Hanna on 01 September, with part of this dry air then moving westward around the northern periphery of the tropical storm on 02-03 September.

MIMIC Total Precipitable Water (TPW) product

The Saharan Air Layer (SAL) product (below) displayed a weak SAL signal (yellow to red colors) over the Bahamas region, supporting the idea that the pocket of dry SAL air had indeed moved around to the northwestern quadrant of Hanna. In addition, MODIS Aerosol Optical Depth values were slightly elevated over that region, suggesting the presence of African dust. Some of this dry mid-tropospheric SAL air may have been mixed downward behind the westward-propagating outflow boundaries noted on the GOES-13 visible imagery above.

Saharan Air Layer (SAL) product

Farther to the east, an even more pronounced “outflow boundary” was seen moving westward away from Tropical Storm Josephine, as shown on GOES-12 visible imagery (below). A strong SAL signature was depicted on the MIMIC TPW and SAL products, covering a large portion of the eastern Atlantic to the north of Josephine.

GOES-12 visible images (Tropical Storm Josephine)

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GOES-13 10.7 µm IR images

Tropical Storm Hanna exhibited some interesting satellite imagery features on 02 September 2008. GOES-13 IR images (above) displayed a period of transverse banding during the 01-05 UTC period — this transverse banding is a satellite signature of potential turbulence (although  no aircraft reports of turbulence were received from that region, since pilots generally try to avoid flying over tropical storms). Very cold cloud top brightness temperatures were observed prior to the period of banding, with values in the -80º to -89º C range (light purple to dark purple colors).

GOES-13 10.7 µm IR images

Later in the morning, an interesting “shear axis” or “deformation zone” signature was noted in the vicinity of Hanna, which was marked by a thin band of clouds oriented SW-NE. GOES-13 IR imagery (above) and visible imagery (below) showed that this shear axis was subsequently deformed  and eroded by a burst of  convective development near the center of Hanna.

GOES-13 visible images

A deep-layer wind shear analysis from the CIMSS Tropical Cyclones site (below) showed that strong northwesterly outflow from the remnants of Hurricane Gustav were impinging upon Tropical Storm Hanna, helping to reinforce the shear axis / deformation zone.

GOES-12 water vapor image + wind shear

An AVHRR false-color composite image (below; viewed using Google Earth) showed that the tropics were becoming fairly active, with Tropical Depression Karina in the East Pacific and Tropical Depression Gustav over the continental US, and Tropical Storm Hanna, Invest Area 90L, Tropical Storm Ike, and Tropical Storm Josephine in the Atlantic basin on 02 September 2008.

AVHRR false color composite image (viewed using Google Earth)

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