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Archived: The Science of Hurricane Juan - Classifying Hurricane Juan

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We have archived this page and will not be updating it.

You can use it for research or reference.

Prepared by Peter Bowyer, October 10, 2003

These are just a few of the questions that have been asked in the countless emails that I have received since Hurricane Juan hit the Maritimes at the end of September. A power-denied and hurricane-wary public is looking for someone or something to blame and the storm that hit us, "had better be more than just a Category 1 hurricane!"

So what is the classification of Juan at landfall? In many ways, the answer is straightforward. The Saffir-Simpson scale is a 1-5 rating based on a hurricane's present intensity. The scale is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Note that all winds are a 1-minute average.

The extensive wind damage (especially to trees) seems to indicate that something stronger than a category 1 hurricane hit the Maritimes. However, it is difficult to use trees to make an assessment because their vulnerability to toppling depends on their type and the depth of their roots. As well, the severe wind shear (strong changes in wind through small changes in elevation) associated with tropical cyclones in higher latitudes could result in winds that are 20-30 km/h higher at the tops of trees than at their base. Wind shear of this magnitude can be a large factor in significant tree blow-downs. In keeping with these findings, the United States National Hurricane Centre cautions that wind speeds at the top of tall office buildings may be as much as 1 Saffir-Simpson category higher than at the surface. In addition, small-scale vortices and wind maxima within the storm can create areas of locally higher winds, and greater damage.

Wind Data

Surface wind reports, whether they be from land stations, ships, rigs or buoys, are invaluable in measuring the strength of a severe storm like a hurricane.

In the absence of wind data from near the surface, satellite imagery is diagnosed and the hurricane compared to analogous satellite signatures from known storms of the past. This diagnostic technique, known as the Dvorak technique, becomes more unreliable as hurricanes leave the tropics and move into our latitudes.

Doppler radar data provides another perspective as it remotely estimates the winds throughout the core of the storm. Radar echoes reflect back from the rain within the storm and a comparison of subsequent echoes allows for a calculation of rainfall motion, and hence, the wind. While Doppler radar gives only an estimate of wind speeds, it provides a glimpse at winds throughout the entire storm all at once.

In addition to surface, satellite, and radar data, reconnaissance aircraft may also be sent into a storm to help determine its intensity. Flight-level winds are converted to representative surface winds through known reduction factors. During three of the last four hurricane seasons, Environment Canada's Meteorological Service of Canada (MSC), in collaboration with the National Research Council (NRC), has used the Convair 580 aircraft to fly into tropical cyclones to deploy data-gathering instruments called dropsondes. These instruments, analogous to radiosondes (weather balloons), are released from the aircraft while it flies through the storm. As the dropsonde descends, it measures temperature, pressure, humidity and wind, and transmits these data back to the ground station.

Regardless of the data source, winds must be converted to a 1-minute average at the 10-metre level (the standard level established by international standards). These converted winds are referred to by meteorologists as "surface winds."

The Strongest Winds in Juan

The Convair aircraft flew into Juan just prior to landfall and deployed a number of dropsondes. The most notable winds were from the dropsonde released about 28 kilometres south southeast of Clam Harbour, Nova Scotia, at 11:54 p.m. ADT (taking 8 minutes to reach the surface). It is likely that this was just east of the strongest winds in Juan. The dropsonde reported winds of 154 km/h at 240 metres above the surface, 174 km/h at 630 metres, and 182 km/h at just over 1 kilometre. Conversion of the strongest of these winds to surface winds gives 143 km/h.

The Doppler radar at Gore, Nova Scotia, estimated the strongest winds between 206 and 213 km/h about 1 kilometre above the surface. Using a similar rate of change of winds in the lower level of the atmosphere recorded by the dropsondes gives surface winds of 133 km/h.

The McNab's Island autostation (anemometer located atop the 19-metre high lighthouse on Maugher's Beach) reported a 2-minute sustained wind of 151 km/h with gusts to 176 km/h. This converts to a surface wind of 152 km/h.

McNabs Island NS - Wind Speed and Direction - Hurricane Juan

The Eirik Raude (a semi-submersible drill ship) located about 37 kilometres south of Lawrencetown, Nova Scotia, reported a wind of 183.35 km/h (the anemometer was pinned at the top of its range for 30-60 seconds, indicating that the winds were actually higher). This converts to a surface wind of 147 km/h.

The Earl Grey recorded a peak wind gust (at 20 metre elevation) of 234 km/h while at anchor in the Bedford Basin (the head of Halifax Harbour). This converts to surface winds of 159 km/h.

There were also a number of anemometers which were destroyed during the hurricane and were, therefore, unable to capture the highest winds. The Marine Traffic Control Centre for Halifax Harbour reported that both of their instruments broke (Shannon Hill and Chebucto Head). The MacDonald Bridge between Halifax and Dartmouth recorded a sustained wind of 91 km/h with gusts to 126 km/h before it stopped reporting at 11:40 p.m. ADT (perhaps a power failure). The Sambro Light Boat Station anemometer was torn out by the storm even though it had survived a previous storm in which it had logged a peak wind of 198 km/h.

Other notable wind reports include:

Conclusions

Juan was a Category 2 hurricane at landfall

The available wind data objectively suggests that the storm was right on the line between category 1 and 2. The fact that it carried sustained hurricane force winds all through Nova Scotia and was possibly still at marginal hurricane strength as it was about to enter Prince Edward Island, indicates that it was a very strong storm at landfall. The storm surge data also suggest that Juan was more likely a category 2 than a category 1.

As is the practice within meteorology, re-analysis of the data from Juan will be ongoing as after-the-fact information becomes available. The methodologie for converting upper level winds to surface values in a hurricane is well documented for more southerly latitudes but clearly requires refinement for storms approaching Atlantic Canada. Also, the practice of categorizing hurricanes by 10-metre winds (which is done for the purpose of assigning an expectation of damage) may be problematic for hurricanes in our latitudes: damaging winds at 20-30 metres may be considerably higher than those at the 10-metre level of classification. This is true everywhere, but even more pronounced in more northern latitudes where the underlying coastal water temperatures are much lower.

Hurricane categorization can be very complex; Hurricane Andrew (1992) was upgraded to a Category 5 hurricane this past year, ten years after making landfall in Florida. It is hoped that we will be aided by wind/structural engineers regarding the damage from Juan to buildings and man-made structures and that this will help to polarize our findings to a single confident conclusion.

As to our certainties

Acknowledgements

I would like to thank the following people for helping pull together this report:

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