NOAA-NESDIS
Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Monday March 30, 1998
Hurricane Guillermo, which formed in August of 1997 in the eastern Pacific Ocean had nearly constant intensity (130 knots to 140 knots) for a couple of days (Aug. 4 - Aug6). This offered the chance to study the convective asymmetries due to motion alone, since the environmental shear was low and the sea surface temperatures nearly constant. What is shown in Fig. 1 below is the storm relative, rotated channel four average of Hurricane Guillermo for August 4 00 UTC to August 6 00 UTC. Using the MAKNAV and REMAP commands in MCIDAS the individual storm relative images (created by the SR command) were rotated in a RADAR projection so that the storm's motion is always upward toward the top of the image. Note that during this period the storm motion was mostly toward the west varying from 84 to 109 degrees.

Fig. 1 (GOES-8 Channel 4, 10.7 micrometers)

click picture to enlarge

More interesting, however, are the results of this analysis. The eyewall of Guillermo shows clear asymmetries, with the coldest cloud tops occurring to the forward side and to the left side of the storm and warmest tops to the back and right side of the storm. It would appear that convection is most strongly forced in the forward region of the storm and is advected around to the left side. Could the forward motion of the storm be forcing convergence in the front flank of the eyewall? This analysis would say yes, but this is just one case. More analyses of this type (which are planned) are needed before any conclusions concerning eyewall asymmetries with respect to tropical cyclone motion can be made. At outer radii, this analysis shows that the colder cloud tops seem to be ahead of the storm. This asymmetry likely indicates that weak wind shear from front to back of the storm is occurring, which in this case would be easterly which is consistent with the climatology of this region and the motion of this particular storm.

John Knaff

We welcome your comments and discussion at ramsdis@comet.ucar.edu

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