SATELLITE
INTERPRETATION
DISCUSSION

NOAA/NESDIS  Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University   Fort Collins, Colorado

January 22, 2001

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Hurricane intensity is expressed as the associated maximum surface wind speed. The minimum sea-level pressure (MSLP) in the hurricane eye is also used as a measurement of intensity.  Observations of MSLP are easier to obtain than the maximum wind speed, particularly with the use of aircraft, either by extrapolation of flight level pressure or with dropsondes into the eye.  Even though the relationship between MSLP and maximum wind speed is somewhat variable, the MSLP change for a particular hurricane gives an excellent measurement of intensification rate.

Center locations and intensities at 6-hour intervals comprise the so-called “Best Track” files, based on routine post-analysis of all available information.  With Hurricane Keith, the “Best Track” data showed intensities of 55 knots (28 m s-1) at 06 UTC 30 September 2000 and 120 knots (62 m s-1) at 06 UTC 1 October.  During this 24-hour period the MSLP decreased 46 hPa from 987 to 941 hPa.  This intensification rate using the Best Track data for a 24-h period, ranked 4th out of the 23 Atlantic “intense” hurricanes during 1995-2000, surpassed only by Mitch (54 hPa / day, in 1998), Opal (1995), and Edouard (1996). 

Aircraft observations with Keith indicated MSLP of 977 hPa at 1808 UTC / 30 Sep and 943 hPa at 0555 UTC / 1 Oct, and then to 939 hPa about an hour later.  Using those observations and interpolating to a 12-hour period, gives an intensification rate of  69 hPa / day decrease.  Using the same approach to find the greatest intensification rate for a 12-hour period, Keith was exceeded only by Opal  (80 hPa / day over a 12 hr period).

GOES-8 IR images at the beginning and end of Keith’s rapid intensification period are shown in Fig. 1.  During this period, Hurricane Keith was in the Caribbean drifting slowly to the west toward Belize.  The cloud pattern takes on the familiar symmetric cold circular ring surrounding a well defined eye, and does that very quickly.
 

Figure 1

It is well known that rapid intensification events are difficult to predict, and those hurricanes’ intensities are nearly always under-forecast, even with short term (e.g. 24 hour ) forecasts.  A preliminary study of IR satellite images associated with “intense” hurricanes and “rapid intensification” events for Atlantic hurricanes during 1995-2000, has been completed.  It was found that the average intensity at the onset of  “rapid intensification” is 87 knots with 970 hPa MSLP.  For this study, “rapid intensification events” are defined as those that have a 42 hPa /day or greater MSLP intensification rate over a 12-hr period. “Intense” is defined as Saffir-Simpson Category 3 or greater, i.e. 100 knots or greater maximum wind (1-min average at 10 m). 

An evaluation of the IR images prior to onset of rapid intensification, when compared with hurricanes at the same intensity that do not rapidly intensify, shows the following characteristics are associated with rapid intensification:
1) unusually cold IR temperatures near the center 
2) symmetric cloud patterns
3) cold IR clouds becoming more concentrated about the center
4) ragged eye becoming well-defined.

Experimental IR image products for use in hurricane analysis and forecasting are being developed and evaluated. Two examples of animated IR image products are shown for  Hurricane Keith's rapid intensification. The images were derived with 30-min interval data but are shown here at 1.5-hr interval.  Loop 1 is a center relative 6-hour average IR image loop.  Loop 2 is derived from the 30-min time differences of those images.  The enhancement shades those areas with IR temperatures cooling with time in blue and depicts those areas that are warming in red. 
 

Loop 1

Click on images  to start loops

Loop 2