Minutes of the best-track change committee meeting, 08 August 2002.

• Jack Beven
• Brian Jarvinen
• Richard Pasch
• Ed Rappaport
• Jim Gross
• Herbert Saffir (observer)
• chair (Colin McAdie) present

The committee met to continue considering the re-analysis of Hurricane Andrew (1992) and associated changes to the best-track file. As noted previously, Herbert Saffir agreed to observe the work of the committee, and was present for the entirety of committee business. He was provided copies of all correspondence related to the re analysis, all of the background materials provided to the committee itself, and has given his comments upon the same.

The committee wishes to thank Mr. Saffir for his perceptive remarks and helpful suggestions during these proceedings. The committee also wishes to thank the pre senters of the 1 August meeting for their comments, additional analyses, and responses to written questions (Pete Black, Chris Landsea, and Mark Powell of NOAA/AOML/HRD, and Jimmy Franklin of our office). As is true of previous meetings, the minutes of the 1 August and 8 August meetings, all of the written summaries, and electronic cor-respondence will become part of the permanent record of the committee business, and will be available for public inspection.

It should be noted at the outset that this discussion has not been prompted by the introduction of new data, but rather by the re-interpretation of existing data. Some newly derived wind estimates using existing radar data were provided to the committee and have been considered, however.

In response to a comment by Mr. Saffir the committee agreed that, in better conformity with past practice, the initial point of U.S. landfall should be given as a location on the barrier islands, rather than 'Homestead' as had been used originally. The decision was made that Elliott Key, Florida was the point of initial U.S. landfall, on 24 August 1992, 0840 UTC. The committee agreed that in conformance with past practice, this should be interpreted as the time the geometric center of Andrew's eye passed over the designated point. The strongest winds at this time were primarily to the north, over open water near the entrance to Biscayne Bay.

It was also decided that because Elliott Key is near the northern extremity of a barrier island chain, with an extensive area of open water to the north, a subsequent (mainland) landfall point be specified. The point chosen was Fender Point. The time at which the geometric center passed over this point was 24 August 1992, 0905 UTC.
Fender Point is about 8 n mi east-northeast of Homestead, Florida, and will be referenced as such.

The intensities were then considered. By agreement, the committee members had entered the meeting prepared with individual opinions on the intensities along the entire track, based upon their own analysis, the presentations and discussions of the previous week, and answers from the presenters. As a starting point, the intensities at 23 August 1992, 1800 UTC (near the lifetime peak intensity of the hurricane), 24 August 1992, 0840 UTC (at the time of initial Florida landfall), and 24 August 1992, 0905 UTC (at the time of second Florida landfall) were considered.

The committee sought agreement on what winds should be assigned to these times. After considerable discussion, the committee estimated that the maximum sustained 1-min wind at 23/1800 was 150 kt, an increase of 15 kt over the original estimate of 135 kt (see Table 1, attached). This change makes Andrew a category 5 hurricane on the Saffir/Simpson hurricane scale at that time. This increase in intensity is in agreement with the track revision for 23/1800 submitted by Franklin and Landsea (the other presenters submitted analyses focused primarily on times surrounding Florida landfall).

This decision was based on a re-examination of the original 700-mb flight-level winds, and the use of a 90% reduction of those winds to the surface (10-m). This is the reduction that would be applied in similar cases operationally, using current guidelines. The scientific basis for these guidelines appears in Franklin, et al. (Weather and Forecasting, submitted) and is reiterated in his written summary. A substantial body of GPS dropsonde data supports the use of these guidelines.

The committee notes that the 90% reduction to 10-m (or any other reduction) cannot be applied blindly in all circumstances. Factors that must be considered include the presence of deep convection, the strength of the system, the altitude of the observations, position within the vortex, and representativeness of the observations. The committee carefully reviewed the original reconnaissance data and found that the 90% reduction was appropriate based on these factors.

It should be noted at this point that each of the four presenters (Black, Franklin, Landsea, and Powell) were in agreement that Andrew was a category 5 hurricane over open water, between the Bahamas and Florida.

The committee then considered the winds at the times of both the initial and second Florida landfalls. As a procedural matter, it was noted that by long-standing practice, the winds given in the best-track file at a particular time represent the highest winds found anywhere within the surface circulation of the system. The committee agreed to adhere to this practice. Regarding the winds at the time of landfall, it was again decided that the 90% reduction was appropriate, in that the necessary conditions were met. The maximum winds were consistently found in the northern eyewall, and this area was still substantially over open water. After much further discussion (described below) it was concluded that the winds at the time of initial U.S. landfall at Elliott Key (24/0840) were 145 kt. It was also concluded that these winds had not significantly changed 25-min later, and thus the maximum sustained 1-min winds at Fender Point (24/0905) should also be given as 145 kt. The committee also considered the fact that surface pressure and satellite data indicated the system was clearly strengthening as it made landfall, after a 162 kt flight-level wind was recorded at 0810 UTC about 10 n mi north of the center. (This was the maximum flight-level wind recorded near the time of Florida landfall.)

There was much discussion about the use of the 90% reduction in close proximity to the coastline. It is noted here that the committee readily acknowledges that a boundary layer transition zone exists at some point between the open water surface and the rigid land surface. The width, nature, and exact location of this transition zone are, however, currently not well-known. It is the conclusion of the committee that further research in this area should be strongly encouraged, and that our current knowledge is insufficient to formulate any guidelines which could be used operationally at this time - and by extension - applied to this case. The committee recommends that field experiments be conducted specifically to document changes in the boundary layer before, during, and after hurricane landfall, using GPS dropsondes within the near off-shore environment.

The uncertainty in the transition zone complicates the estimation of the maximum winds that might have been experienced over land. The few reliable observations available (pursued exhaustively by Rappaport in his original report) did not record the maximum sustained wind, due to instrument failure. This is compounded by the fact that dropsondes, which could give a detailed near-surface wind profile, cannot be dropped over populated areas. As is often the case in the severe conditions encountered during hurricane landfall, the precise wind speeds experienced over the interior portions of Miami-Dade County may never be known with certainty.

Given this uncertainty, and pending further research, the committee defers to the assumption, reflecting current operational practice, that the core winds (145 kt) did affect a small area on the immediate coast having open exposure to Biscayne Bay. This assumption is supported by a strong convective maximum (eyewall) to the north of the center impacting the coastline at the time of landfall, in a strengthening system, as noted. This decision makes Andrew a category 5 hurricane at the time of Florida landfall.

Having agreed upon the intensities at the above times, the committee examined the remaining portions of the track as submitted by Franklin and Landsea. Based on a re-examination of the flight-level data, the committee found that winds at 23/0600 should be increased to 130 kt, and the winds at 23/1200 should be increased to 145 kt. Immediately following the peak wind, it was decided that the winds at 24/0000 should remain at 125 kt, supported by the observation at Harbour Island, Bahamas. The 120 kt recorded by the anemometer there was the maximum that could be registered, according to the report by Rappaport.

Revised winds suggested by Franklin and Landsea for the portion of the track over the Gulf of Mexico were accepted, after review. Winds during final landfall at Pt. Chevreuil, LA were then considered. Based on a review of the flight-level winds, it was decided that the winds at the time of landfall (26/0830) should be reduced 5 kt to 100 kt. This retains the category 3 designation.

The meeting concluded with a discussion of the need for anemometers which can survive extreme winds. Mr. Saffir indicated that such instruments do exist. The committee recommends that a pilot project be established at the Tropical Prediction Center, with the help of Mark Powell and others, to correctly install an anemometer with optimum exposure that will survive the extreme conditions encountered in a hurricane, and accurately record the winds throughout an event.

submitted by:
Colin McAdie

15 August 2002