October 9th, 2013
850-hPa relative vorticity product (click image to play animation)
A sequence of images of the satellite-wind-derived 850 hPa relative vorticity product at 6-hour intervals covering the period 04 October to 09 October 2013 (above; click image to play animation) showed that the vorticity associated with Tropical Storm Karen over the Gulf of Mexico on 04 October could be followed northeastward as the storm made the transition to an extratropical low as it movved inland and interacted with a frontal boundary, then eventually organizing into a slow-moving coastal low along the US East Coast.
A comparison of 1-km resolution MODIS 0.65 µm visible channel, 11.0 µm IR channel, and 6.7 µm water vapor channel images (below) showed the organization of the coastal low at 18:10 UTC on 09 October, which featured deep offshore convection with IR cloud-top temperatures as cold as -75º C.
MODIS 0.65 µm visible channel, 11.0 µm IR channel, and 6.7 µm water vapor channel images (09 October)
===== 10 October Update =====
A comparison of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images at 17:59 UTC on 10 October (below) revealed that some strong convective elements had moved inland over southeastern Virginia and northeastern North Carolina, producing heavy rainfall.
Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR chanel images (10 October)
September 19th, 2013
MTSAT-2 10.8 µm IR images (click image to play animation)
McIDAS images of MTSAT-2 10.8 µm IR channel data (above; click image to play animation) showed Super Typhoon Usagi in the West Pacific Ocean as it continued to move northwestward across the Philippine Sea. Note the slight amount of “trochoidal wobble” seen in the path of the eye. The coldest IR brightness temperature seen on the MTSAT-2 IR images was -92º C at 12:32 UTC. At the time of these images, advisories issued by the Joint Typhoon Warning Center listed the maximum sustained winds at 140 knots, with gusts to 170 knots; at its peak intensity, Usagi had an estimated lowest pressure of 882 hPa, making it the most intense tropical cyclone so far in 2013.
TMI and SSM/I 85 GHz microwave brightness temperature images
From the CIMSS Tropical Cyclones site, a comparison of 85 GHz microwave images from the TRIMM Microwave Imager (TMI) at 10:35 UTC and the DMSP SSM/I at 19:13 UTC (above) displayed a very small ring of high brightness temperatures surrounding the “pinhole eye”.
MIMIC-TC morphed microwave images (below; click image to play animation) showed a well-defined closed eyewall during much of the early part of the day on 19 September.
MIMIC-TC morphed microwave imagery (click image to play animation)
MTSAT-2 IR image with AASCAT scatterometer surface winds
Scatterometer surface winds from the ASCAT instrument at 13:17 UTC (above) and the OSCAT instrument at 15:10 UTC (below) showed the large areal coverage of strong winds around the center of Usagi.
MTSAT-2 IR image with OSCAT scatterometer surface winds
===== 20 September Update =====
MTSAT-2 0.67 µm visible channel images (click image to play animation)
MTSAT-2 0.67 µm visible channel images (above; click image to play animation) showed the detailed structure of the compact eye of Super Typhoon Usagi.
September 16th, 2013
GOES-13 10.7 µm IR channel image (with track of Humberto)
A GOES-13 10.7 µm IR channel image with the track of Humberto from the CIMSS Tropical Cyclones site (above) showed that the storm (which became the first hurricane of the season in the Atlantic Basin on 11 September) had been meandering over the eastern North Atlantic Ocean for several days, eventually weakening to a post-tropical cyclone on 14 September before re-organizing into a tropical storm early in the day on 16 September (NHC advisories archive).
GOES-13 0.63 µm visible channel image (with ASCAT surface scatterometer winds)
A GOES-13 0.63 µm visible channel image with an overlay of ASCAT surface scatterometer winds (above) showed evidence of tropical storm force winds (yellow wind barbs) north of the storm center at 13:17 UTC. Note the presence of a well-defined cloud vortex located southwest of the 18 UTC center fix position.The NHC discussion #26 mentioned that several smaller-scale vortices had been rotating around the mean circulation center of Humberto — and a close-up view using GOES-13 0.63 µm visible channel images (below; click image to play animation) revealed one of these exposed vortices to the southwest of the center of Humberto.
GOES-13 0.63 µm visible channel images (click images to play animation)
See From the Lee Side for a discussion on the role of vertical wind shear on tropical cyclones such as this.
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)
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
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)
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.