A Marina Takes A Hit
August 20, 2009
IntroductionWhen thunderstorms threaten the Great Lakes, forecasters at the National Weather Service (NWS) traditionally conclude their marine forecasts with the following phrase -- Winds and Waves Higher In And Near Thunderstorms. Both an observation and a warning, the statement is intended to alert mariners to the potential for localized areas of much stronger winds than those anticipated in a non-storm environment. Such warnings are justified, as even a garden-variety thunderstorm is capable of producing winds of 50 knots or more.
Thunderstorm Damage
Spitzer Lakeside Marina is located on the southern shore of Lake Erie approximately twenty miles west of Cleveland. Several strategically placed breakwalls shelter the marina from the occasionally tempestuous conditions encountered on Lake Erie, but they offered little protection from a thunderstorm that moved through the area on the afternoon of August 20, 2009.
The photographs above are representative of the damage produced by the thunderstorm. While the dock on the left shows a visible curve caused by the force of the wind, it pales in comparison to the mangled dock on the right. Prior to the storm, the dock on the right extended straight into the background of the photograph. In addition to bending the outer portion of the dock's heavy metal frame into the shape of a reverse "C", the wind altered the angle of the dock to the main pier from 90 degrees to approximately 45 degrees. The dock, one of several that were a total loss, was also listing at a dangerous angle. Fortunately, both of the boats remained firmly attached to these docks and managed to avoid significant damage during the storm.
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The individual docks are attached to a much larger floating concrete pier with heavy-duty interlocking metal fittings. This hardware is engineered to withstand the rigors of the marine environment, but several were compromised by the storm.
The photograph on the left shows the attachment of my dock to the main pier. The force of the wind snapped a large bolt that passed through the bottom of the metal bracket and large timber to secure the fitting to the main pier. When I arrived the morning after the storm, the bracket was flexing steadily back and forth near the point where it passed over the top of the timber.
Fortunately, the storm and its strong winds passed quickly, because it is unlikely this bracket would have survived this constant flexing for a prolonged period. The failure of this fitting would have been disastrous for the two boats attached to the dock, as well as adjacent boats.
Observations
In the aftermath of the storm, there was considerable speculation among the dockers regarding the wind speed associated with the storm. Unless you encounter strong winds on a regular basis, it is very difficult to make an accurate observation of wind speed in the absence of a measuring device. However, there is no need to rely upon speculation in this instance as a nearby weather station maintained by the Lorain Sailing and Yacht Club (LSYC) recorded the storm's winds at 5-minute intervals.
LSYC is the oldest continuously operating yacht club on Lake Erie and is located just southwest of Spitzer Lakeside Marina (click here for a map of the harbor ). The weather station is affixed to a tower on the club's grounds at a height of approximately 20 feet. It was in a perfect spot to intercept the wind as it sped from the storm toward the marina. Other than damage to a few sails and the overturning of a couple of trailerable sailboats, there was no damage at LSYC.
The graph below shows the maximum observed wind speed (in knots) from 11 am to 7 pm on August 20, 2009. A review of the graph indicates that the maximum winds were rather brisk for several hours preceding the event, ranging from 20 to 30 knots. At 4:40 pm the station observed a gust of 42.6 knots (49 mph), followed just five minutes later (4:45pm) by 62.6 knots (72 mph) -- the highest observed wind speed associated with the storm. Although meteorologists make a distinction between sustained wind and gusts, this gust was very close to the initial hurricane threshold of 64 knots.
The force of the wind on an object can be expressed in pounds per square foot (psf). While the speed of the wind increases incrementally, its force on an object increases exponentially. For example, a wind of 34 knots produces a force of approximately 6.1 psf on a flat surface, such as a wall. However, a 91% increase in wind speed from 34 knots to 64 knots results in a nearly 250% increase in force to 21.9 psf.
The peak wind from the storm on August 20 was 62.6 knots, just slightly below the 64 knot example mentioned above. A 40 foot dock at Spitzer Lakeside Marina has approximately 60 square feet of exposed surface and therefore would have been subjected to approximately 1,300 pounds of force from the gust.
The side of an average 30-foot sailboat presents approximately 100 square feet to the wind. Using this estimate, a 64 knot gust creates slightly more than 2,100 pounds of force. As the 62.6 knot gust hit the side of a boat tied to the windward side of a dock, a significant portion of this force would have been transferred to the dock. This transfer would have been focused on a small area of the dock as the widest part of the boat made contact with it. Considering the large amount of force produced by the 62.6 knot gust, it isn't surprising that several docks were damaged during the storm.
Downdrafts
The wind danger related to thunderstorms is associated with the downdraft, a storm-scale flow of air descending towards the ground from several thousand feet above the surface. Particularly strong downdrafts are referred to as downbursts and are further sub-categorized based upon the size of the affected area and the duration of the peak wind. Microbursts affect the smallest area (less than 2.5 miles in length) and have peak winds lasting less than five minutes. On a broader scale, macrobursts impact an area greater than 2.5 miles in length and contain peak winds that persist for as long as twenty minutes.
Two incidents that occurred at Andrews Air Force Base during the 1980s serve as an example of the remarkable winds associated with a downburst. The first, containing a gust of 113 knots (130 mph), occurred on August 1, 1983 just minutes after Air Force One (and the President) had touched down. In 1986, the second, and much stronger, downburst peaked at 137.3 knots (158 mph). While downdrafts may occasionally be quite strong and produce a surprising amount of damage, they are a natural part of a thunderstorm's life cycle.
Thunderstorm Life Cycle
The images below show the three stages of development associated with a garden-variety single cell thunderstorm. Thunderstorms develop where the atmosphere is unstable, generally typified by a warm, moist layer of air near the surface and relatively colder and drier air aloft. It is the difference in both temperature and moisture that provides the energy for thunderstorm development.
The first stage of development -- Towering Cumulus -- is characterized by the upward motion of warm, moist air parcels. These air parcels cool during their ascent; eventually the water vapor in the parcels condenses into very small water droplets and a cloud begins to form. The process of condensation releases heat, which warms the air column in which the thunderstorm is developing and paves the way for the water vapor in subsequent air parcels to condense at ever higher altitudes. As this process is repeated, the cloud associated with the storm's updraft grows taller. Given an abundance of upward motion and low-level instability, a thunderstorm may reach the upper limit of the atmosphere. During this period of initial development, no precipitation (downdraft) is associated with the storm.
As the storm's updraft develops upward into colder air (below 32° F), atmospheric processes promote the growth of larger precipitation particles (water droplets, ice crystals, hail, etc.). These particles eventually become too heavy to be suspended by the updraft and they begin to fall. The storm's downdraft now begins to form, and the storm reaches the Mature Stage, as the falling precipitation drags some of the surrounding air along the journey to the surface. During descent, the melting of ice crystals and the evaporation of water droplets serve to cool, and thereby strengthen, the speed of the downdraft. Eventually, this descending column of cool, dense air reaches the surface and spreads out in all directions. The leading edge of cool air from the storm is referred to as a gust front or outflow boundary, and is frequently described as a small-scale cold front.
The arrival of much cooler air from the storm's downdraft is evident in the observations of temperature and dew point from LSYC. At 3:50 pm, the temperature was 85.5° F, which was only slightly less than the day's highest temperature of 86.3°F. At 4:05 pm, in the wake of a 40 knot gust, the temperature dropped to 75.4°F -- a drop of 10.1°F in just fifteen minutes.
In the five minutes between 4:40 pm and 4:45 pm, the temperature dropped an additional 5.7°F as the 42.6 knot and then the 62.6 knot gust were recorded. The rapid decline in temperatures was accompanied by two dramatic increases in barometric pressure, a response to the arrival of colder, denser air (barometric pressure is simply the weight of the air above a location).
The falling precipitation, and associated cooling, diminishes the strength of the storm's updraft. In addition, the formation and growth of a cool, dense pool of air at the surface disrupts the ability of the storm's updraft to ingest warm, moist air. The persistence of the downdraft and the absence of an updraft marks the arrival of the Dissipating Stage. Although precipitation may persist for a while during this stage, the intensity will steadily diminish as the storm approaches the end of its lifecycle.
A single cell thunderstorm may transition from the Towering Cumulus to the Dissipating Stage in approximately sixty minutes. However, not all storms complete the process due to unfavorable atmospheric dynamics such as weak instability, a lack of moisture, or warm temperatures aloft. And not all thunderstorms are garden-variety single cell thunderstorms.
Supercell Thunderstorms
Based upon its appearance on Doppler radar and other observations, the thunderstorm responsible for damaging the marina was likely a supercell. Supercells are a special long-lived type of single cell thunderstorm, characterized by a rotating updraft . Supercell thunderstorms may last for several hours and are responsible for producing nearly 100% of all strong tornadoes (greater than EF2) and most reports of large hail. Supercells are also capable of producing downdrafts of 100 mph or more. Fortunately, only a small portion (< 5%) of severe thunderstorms in the lower Great Lakes are supercells.
The longevity of supercells is due to the separation of the storm's updraft and downdraft as shown on the schematic above. The storm's updraft is shown in green, while the storm's downdrafts, both rear and forward, appear in blue. In this configuration, the precipitation and rain-cooled air associated with the downdraft doesn't impede the updraft, ensuring a continued supply of warm, moist air into the storm. This steady state persists as long as the environment into which the storm travels is sufficiently unstable. Under the right conditions, a single supercell may persist for several hours and produce a lengthy trail of destruction.
Summary
Thunderstorms are regularly accompanied by strong winds. In some instances, this wind may exceed 50 knots and produce a surprising amount of damage. Boaters who venture onto Lake Erie (or any other body of water for that matter) when thunderstorms are in the forecast should remain vigilant for deteriorating weather conditions.
The specifications for my C&C 35 list the combined sail area of the mainsail and 100% jib at 571.9 square feet. Using the wind load calculator provided by U.S. Sailing, a 62.6 knot gust delivers approximately 9,600 pounds of force to the sails, mast, rigging, winches and miscellaneous fittings. Experiencing such a gust under full sail would be disastrous. A wise sailor lowers and secures their sails when an encounter with a thunderstorm is unavoidable. Loose lines should also be secured as they can be blown overboard and quickly foul a propeller.
Even a relatively small storm, such as the one that appears in the radar image above, may pack enough punch to disrupt an outing. This compact storm came ashore during the evening of August 5, 2010 and caused several junior sailboats to capsize in the Black River (click here for the news story). Personnel from the nearby Coast Guard station accomplished a quick rescue and no one was injured, but it is a reminder of the hazards associated with thunderstorm downdrafts. Even a small thunderstorm deserves respect.
Acknowledgments
I would like to thank Bobby Rhodes of the Lorain Sailing and Yacht Club for providing the weather data related to this event. The data made the publication of this paper possible.