Regeneration of Tropical Storm Gabrielle

September 10th, 2013 |
POES AVHRR 0.84 µm visible channel and 12.0 µm IR channel images (with overlays of surface buoy, surface analysis, and ASCAT winds)

POES AVHRR 0.84 µm visible channel and 12.0 µm IR channel images (with overlays of surface buoy, surface analysis, and ASCAT winds)

After a four and a half day hiatus, Tropical Storm Gabrielle began to regenerate south of Bermuda on 10 September 2013. AWIPS images of 1-km resolution POES AVHRR 0.86 µm visible channel and 12.0 µm IR channel data at 13:41 UTC (above) showed overlays of surface buoys, surface analysis, and ASCAT surface scatterometer winds. A number of these ASCAT winds exhibited speed values of 45-46 knots, prompting NHC to adjust the intensity of Gabrielle upward:

TROPICAL STORM GABRIELLE SPECIAL DISCUSSION NUMBER   9
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072013
130 PM AST TUE SEP 10 2013

A PAIR OF ASCAT PASSES FROM 1342Z AND 1436Z INDICATE THAT GABRIELLE 
IS STRONGER THAN PREVIOUSLY ESTIMATED. BOTH PASSES SHOWED SEVERAL 
45-KT WIND RETRIEVALS...AND THAT IS THE INITIAL INTENSITY FOR THIS
SPECIAL ADVISORY. THE INTENSITY FORECAST HAS BEEN ADJUSTED UPWARD
AT 12 AND 24 HOURS TO 50 KT BASED ON THE INCREASE IN THE INITIAL
INTENSITY. THE ANALYZED 34-KT WIND RADII HAVE ALSO BEEN ADJUSTED
OUTWARD BASED ON THE ASCAT DATA...AND THEIR FORECAST HAS BEEN
MODIFIED AS WELL TO REFLECT THE LARGER WIND FIELD. AN AIR FORCE
RESERVE AIRCRAFT WILL INVESTIGATE GABRIELLE LATER THIS AFTERNOON TO
PROVIDE MORE DETAILED INFORMATION ON THE STRENGTH AND STRUCTURE OF
THE CYCLONE.
850 hPa relative vorticity product (04-10 September) - click image to play animation

850 hPa relative vorticity product (04-10 September) – click image to play animation

A sequence of 6-hour interval 850 hPa relative vorticity product images from the CIMSS Tropical Cyclones site (above; click image to play animation) showed that the lower-tropospheric vorticity feature associated with Tropical Storm Gabrielle remained fairly intact during the 06-10 September period (between the times when Gabrielle had exhibited tropical cyclone organization and intensity).

GOES-13 0.63 µm visible channel images (click image to play animation)

GOES-13 0.63 µm visible channel images (click image to play animation)

McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (above; click image to play animation) revealed a well-defined central dense overcast (CDO) convective burst early in the day, followed by the emergence of the low-level circulation of Gabrielle as the middle and high-altitude cloud layers were sheared off to the northeast.

GOES-15 Navigation Anomalies

September 10th, 2013 |
GOES-15 3.9 µm shortwave IR images on 10 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 10 September (click image to play animation)

GOES-15, in service as GOES-West over the Equator at 135 W, has experienced navigation anomalies on each of the past three mornings, September 8, 9 and 10, 2013 (as noted here, for example). Navigation anomalies occur approximately between 0830 and 1100 UTC. GOES Engineers are investigating the source of the anomaly, which can be on the order of 40 km. The animation above is the shortwave infrared from Tuesday 10 September. Animations for 6, 7, 8 and 9 September are below. Note that even the 6 September and 7 September have small navigation issues.

(Update: From SATOPS: NOTE: GOES-15 (West) Image registration (alignment of images to earth grid coordinates) was shifted by approximately 40 kilometers on DOY251 (ed. note: 8 September) from 08:50z to 10:50z and shifted by approximately 25 kilometers on DOY252 & DOY253 (ed. note: 9 and 10 September) from 9:30z to 10:40z. The earth coordinate grid shift was due to a lack of available valid star data for predicting the proper attitude pointing profile around eclipse. The root cause is under investigation.) In other words, the Star Pointer was unable to find enough targets to produce accurate navigation. (Link)

Update, 11 September: The anomalies continued on 11 September, starting after the 0845 UTC image, peaking at the 1000 UTC image, and subsiding by 1045 UTC. Link.

Update, 17 September: Large navigation anomalies were not present overnight.

GOES-15 3.9 µm shortwave IR images on 6 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 6 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 7 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 7 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 8 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 8 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 9 September (click image to play animation)

GOES-15 3.9 µm shortwave IR images on 9 September (click image to play animation)