CIMSS Envrionmental Steering product

Jason Dunion (formerly at CIMSS, now at the NOAA/AOML Hurricane Research Division) penned an excellent summary of the utility of the CIMSS Environmental Steering (or Deep Layer Mean Wind) product that is available on the CIMSS Tropical Cyclones site. Satellite-derived Atmospheric Motion Vectors produced at CIMSS are in important inputs to these environmental steering flow products. With Hurricane Gustav, Tropical Storm Hanna, Hurricane Ike, and Tropical Storm Josephine all in the Atlantic basin, Jason noted the following interactions between the steering flow patterns and the motion of those 4 tropical cyclones during the first few days of September 2008:

03 September:
– Gustav has made landfall and helps build ridge #1 to its east
– big ridge #2 is located north-northeast of Ike

04 September:
– gigantic low over Canadian Maritimes lifts out;
– ridge #1 slides to the east, ridge #2 slides to the west

Late 04 September to early 05 September:
– ridge #1 & ridge #2 link to form a sprawling ridge to the north of Ike
– Ike is stuck in northwesterly steering flow until that “ridge linking”, and then starts a turn to the west

05 September:
– ridge #1 and ridge #2 consolidates and parks to the north-northwest of Ike; Ike then quickly comes into a southwesterly steering flow
– Hanna is forced “up the alley” between ridge #1/#2 and a trough approaching from the west
– Josephine is pushed to the northwest by the deep flow (shallow Deep Layer Mean is more of a west-southwesterly flow); Josephine then deteriorates as southwesterly  wind shear increases

The corresponding 3-hourly images of the AWIPS Northern Hemisphere water vapor channel composite are shown below, covering the period 01-05 September 2008.

AWIPS Northern Hemisphere water vapor composite images

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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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