NOAA-NESDIS
Regional And Mesoscale Meteorology Team
Daily Satellite Discussion
Monday September 29, 1997
Figure 1
The discussion today concerns itself tropical cyclone intensity estimates using a digital infrared Dvorak technique developed onsite which builds on the work of Dvorak and Wright (1978). For our purposes Hurricane Guillermo (See Figure 1) which formed on the 30th of July 1997 and became an intense hurricane on the 2nd of August is used as an example. This storm reached maximum intensity on August 4th with maximum sustained winds of 140 knots until 12Z on August 5th. The time series of six hourly running means of the current intensity numbers (T-numbers, maximum in light blue, average light green) obtained from this scheme along with the six hourly intensity estimates (knots) from the National Hurricane Center (black numbers) and rough estimates obtained from this technique (blue numbers) are shown in Figure 2. Note both the relatively good agreement between the NHC's estimate and the estimate from this method as well as the areas where they differ. A particularly large disagreement is shown between the digital technique and the NHC estimate during the period 12Z August 3rd to 12Z August 4th.

Figure 2
GOES-9 IR-4 (10.7 micrometers), Aug. 5, 1997 at 1200 UTC
The digital method shows clear weakening while NHC keeps their estimate constant with time. Note also that at least in this case the digital estimate gives greater intensities. These discrepancies could be due to the use of additional data received from aircraft reconnaissance and/or ship reports, but the exact reasons for these differences for now remain unknown.

John Knaff

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

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