Wayward meteorology student on a journey back to the promise land. Aggie. Extreme Weather Junkie. Precipitation Snob. Catholic. Musician. Texan. Nerd.
By: 1900hurricane , 3:51 PM GMT on September 14, 2010
It appears that Igor did have an Eyewall Replacement Cycle yesterday/last night, and it did catch the NHC off guard. It was a rather subtle one, but it affected Igor's strength the same. Here are some 85GHz Microwave passes to show it's evolution:
SSMIS Pass @ 2245 UTC September 12th: Hurricane Igor is rapidly intensifying and screams past the threshold for a category 4 hurricane and threatens category 5. At this time, Igor's core is amazingly consolidated and probably in it's most efficient form, which is what allowed the rapid intensification in the first place. Igor is displaying the features of a mature hurricane.
SSMIS Pass @ 11:19 UTC September 13th: Hurricane Igor has reached maximum intensity. At this time, he is just 10 kts shy of the category 5 threshold. However, Igor's intensification levels off here. In the pass above, it can be seen that his core is not as perfectly structured as before and the thickness of the return maximum has increased. This is a sign of some type of internal change, and the widening of the return maximum means that individual storms close to the eyewall have increased in intensity. With the diffusion of the strength in his core, Igor can no longer strengthen.
AMSRE Pass @ 16:40 UTC September 13th: Igor's core has continued to undergo changes, and by this time, a secondary return maximum has developed outside the eyewall. This signifies that the intense storms close to Igor's eyewall have continued to develop and formed a secondary eyewall. I estimate that an Eyewall Replacement Cycle (ERC) has started to begin at this point. This can also be hinted at in infrared imagery as Igor's cloud tops began to cool, especially those directly adjacent to his eye.
However, at this time, the NHC seemed to disregard the dual return maxima and possible ERC when they stated in their forecast discussion:
WHILE A 1640 UTC
AMSR-E PASS DOES NOT SHOW ANY EVIDENCE OF A SECONDARY EYEWALL
YET...IT IS POSSIBLE THAT AN EYEWALL REPLACEMENT CYCLE WILL OCCUR
IN THE FIRST COUPLE OF DAYS...WHICH COULD RESULT IN SOME INTENSITY
FLUCTUATIONS NOT REPRESENTED IN THE OFFICIAL FORECAST.
However, the trends continued with the next couple of passes:
TMI Pass @ 20:36 UTC September 13th: This pass basically shows an extension of the earlier AMSRE pass and continues to show the evolution of the dual return maxima and progress of the ERC. At this time, it appears that the outer eyewall is becoming the dominant.
SSMIS Pass @ 22:27 UTC September 13th: Once again, the trend continues and it appears that the outer eyewall is circling in and trying to take the same place as the collapsing inner eyewall. By now, the ERC is entering its final stages.
After these couple of passes as well as the continuing trend of Igor's cooling cloud tops, the NHC had this to say in their forecast discussion:
MICROWAVE AND GEOSTATIONARY SATELLITE IMAGERY SUGGESTS THAT AN
EYEWALL REPLACEMENT CYCLE MAY HAVE BEGUN. TRMM...WINDSAT...AND
SSMIS PASSES FROM ABOUT 5-6 HOURS AGO ALL INDICATED THAT A
SECONDARY EYEWALL WAS BEGINNING TO FORM...AND MORE RECENTLY
CONVECTIVE TOPS AROUND THE EYE HAVE BEEN WARMING.
It appears that they had finally caught on.
TMI Pass @ 04:48 UTC September 14th. By now, the inner eyewall has collapsed and the outer eyewall has taken over, which can be seen in the reconsolidation of a single return maximum at the eyewall. Also of note is the enlarged eye this pass. Igor's ERC is complete and subsequently, his cloud tops began to cool once more.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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