Articles published for Earthzine’s Extreme Weather theme (March 21-June 21, 2011) examine aspects of weather…
Death From a Clear Blue Sky: Extreme Non-Convective High Winds
- Published on Saturday, 04 June 2011 00:01
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John A. Knox
Department of Geography, University of Georgia, Athens, GA
John D. Frye
Department of Geography & Geology, University of Wisconsin-Whitewater, Whitewater, WI
Joshua D. Durkee
Meteorology Program, Department of Geography and Geology, Western Kentucky University, Bowling Green, KY
Matthew C. Lacke
Air and Radiation Protection Division, Jefferson County Department of Health, Birmingham, AL
Corresponding author address: Dr. John A. Knox, Department of Geography, University of Georgia, 210 Field Street, Room 204, Athens, GA 30602. E-mail: email@example.com
Introduction: a tragedy at football practice
It was a completely clear, breezy late afternoon in South Bend, Indiana on October 27, 2010. Severe thunderstorms and tornadoes the previous day, associated with an intense low-pressure system over northern Minnesota, had pushed out of the northern Indiana area ahead of a cold front. The University of Notre Dame football team was practicing outdoors in preparation for a home game with the University of Tulsa the following Saturday [1,2].
Like many prominent football programs, Notre Dame records its practices from multiple overhead angles. On this day, three hydraulic lifts carried student videographers high above the field to capture the “Fighting Irish” as the team practiced its plays and ran drills. Despite sunshine and relatively warm autumn temperatures (near 65°F, or 18.3°C), the weather conditions were of some concern to at least one of the videographers, Notre Dame junior Declan Sullivan. Based on an online weather forecast, Sullivan posted a message on Twitter at 3:22 pm EDT on the 27th, about 25 minutes before practice started, saying “Gust of wind up to 60 mph well today will be fun at work . . . I guess I’ve lived long enough :-/.” 
However, wind gusts reported at the National Weather Service (NWS) observing site at South Bend had remained below the 35 mph (15.6 m s-1) threshold used by Notre Dame to pull the videographers off the lifts. The NWS downgraded a High Wind Warning for the South Bend region to a Wind Advisory an hour before practice began, around the time that Notre Dame athletic staff last consulted weather data (but not forecasts) before heading to the field. The wind advisory called for “frequent wind gusts between 46 and 57 miles an hour” . Wind gusts exceeding these speeds had been experienced throughout the upper Midwest during the past two days (Fig. 1).
At 4:54 pm EDT, a sudden gust of wind from the southwest, estimated at 53 mph (23.7 m s-1), swept across the Notre Dame campus, strewing garbage, bags of footballs, water bottles and other debris across the practice field. The team’s defense coordinator called it a gust “of hurricane significance.” Sullivan’s hydraulic lift and platform, facing south, caught much of the brunt of the wind’s force. The lift tipped backward and collapsed across a nearby road, dropping Sullivan 40 feet (12 m) vertically to his death .
In the aftermath of Declan Sullivan’s death, Notre Dame focused blame on the “unusual wind conditions” that toppled the hydraulic lift . This article demonstrates how the circumstances of this event fit hand-in-glove with recent research on non-convective high winds, from the meteorological aspects to the university’s response to official advisories. We then turn to the future, and discuss the probability of these events in the changing climate of the 21st century as well as the problems inherent in raising public awareness of this type of extreme weather event.
Understanding non-convective “hurricane west winds”
From the days of ancient mariners to the modern era of remote sensing (Fig. 2), it has been known that it doesn’t take a hurricane, tornado or thunderstorm to create damaging and even deadly winds .
Nevertheless, it may come as a surprise to many Americans that in a typical year high winds due to non-convective processes cause about as many deaths in the U.S. (approximately 25) as tropical cyclone winds or thunderstorm winds . These non-convective winds also typically lead to more property and crop damage annually (roughly $800 million) than either thunderstorm winds or tornadoes . As seen in Fig. 2, wind gusts can easily exceed hurricane force (74 mph, or 33 m s-1; red arrows in Fig. 2) over water. Hurricane-force gusts are rarer over land, but do occur . The ferocity of these non-convective winds was on display on October 26-27, 2010, during which the highest wind gusts reached at least 78 mph (34.9 m s-1) and were stronger than nearly all gusts reported over Indiana due to thunderstorms (see Fig. 1).
What causes these extreme winds? Surface wind strength is generally proportional to the near-surface pressure gradient, and very tight pressure gradients can be found in the vicinity of intense extratropical low-pressure systems. Several studies correlate non-convective high winds climatologically with proximity to mid-latitude cyclones [10,12,13,14,15,16]. These large storms have been connected to some of the most high-profile non-convective high wind events in modern American history, from the sinking of the Great Lakes iron ore freighter Edmund Fitzgerald on Lake Superior in November 1975 to the “Perfect Storm” off the New England coast in October 1991 .
A seemingly chronic feature of non-convective high winds is their directional preference. Climatological studies performed in the U.S. [10,12] and case studies and conceptual models developed in Europe  generally indicate that non-convective high winds will predominantly come from the southwest quadrant of the compass. Fig. 3 depicts results adapted from our 44-year climatology for cold-season (November-April) non-convective high winds events across the Great Lakes region . Approximately 70% of all hourly observations with sustained winds of at least 40 mph (18 m s-1) for one hour were from the south-through-west direction, with a peak from the west-southwest.
A shift in the climatology of non-convective high wind events to northwest wind directions occurs in the northern Great Plains [10,13]. This suggests that the wind direction is related to one’s location vis à vis the climatological storm tracks. Locations to the south of the storm tracks typically experience non-convective high winds in both the Great Lakes and the Northeast . It is reasonable to assume that the direction of these winds is related in some way to the dynamics of mid-latitude cyclones.
One possible dynamical explanation for these strong winds is the transport of high-speed winds down to the ground in the “dry slot” of these cyclones [11,18,19]. The dry slot is a nearly cloud-free region to the south of the storm center that separates the “comma head” (the rounded portion of a comma cloud system) of a mature cyclone center from the “comma tail” along its cold front. It is composed of air that has been dragged down from the very dry upper troposphere and lower stratosphere by the three-dimensional circulation of the cyclone. Dry slots are easily identifiable on satellite imagery, as in Fig. 4 taken 24 hours before the Notre Dame tragedy. The dry slot on that day extended southwest-to-northeast across Indiana and Michigan and then wrapped into the low center over Minnesota. (The cyclone was quasi-stationary during the next day, placing the dry slot over virtually the same region on October 27.) Because of the counterclockwise circulation of mid-latitude cyclones in the Northern Hemisphere and the general west-to-east direction of jet streams, the winds aloft in dry slots are usually from the southwest direction. If some mechanism is present to transport these winds to the surface, then this is one plausible mechanism for creating the predominantly southwest-quadrant high winds found in previous climatological work. (Other mechanisms may also cause non-convective high winds that are not necessarily associated with clear skies; this is an area of ongoing research . See  for a comprehensive review of the subject of non-convective high winds.)
Taken together, the research described above has made strides toward explaining why non-convective high winds can be described as, in the memorable words of songwriter Gordon Lightfoot about the Edmund Fitzgerald storm, “a hurricane west wind.” But why are they as deadly as winds from thunderstorms and hurricanes, which can reach even higher speeds? Next we turn to recent research on non-convective high wind fatalities.
Identifying how and where non-convective high winds can kill
Unlike winds from tornadoes, thunderstorms or hurricanes, the risk from non-convective high winds is not immediately apparent from a glance at the sky. The phrase “high wind” also does not convey the same sense of terror that the word “tornado” does in American culture. As a result, it appears, a portion of fatalities due to non-convective high winds occur because people are out and about rather than sheltered from the effects of high winds. A 26-year climatology of U.S. non-convective high wind fatalities  revealed that 43% of deaths occurred in vehicles, another 25% in boats, and 23% outdoors. This is in stark contrast to convective wind fatalities; for example, over 70% of tornado deaths occur within buildings or other structures, with a small percentage outdoors . Almost one-fifth of the outdoor fatalities examined in  occurred at construction sites, where equipment or buildings under construction fell or collapsed due to wind.
In addition, at least one-third of all non-convective high wind fatalities are associated with falling trees . Non-convective high winds, in turn, accounted for 35% of all wind-related tree-failure fatalities in the U.S. from 1995-2007, mostly in the months of October-April; only thunderstorm winds (41%) caused more deaths, by a small margin . This helps explain the spatial distribution of fatalities, as shown in Fig. 5. The heavily forested and densely populated regions of the Northeast and Pacific Northwest experience the most deaths. However, the Great Lakes region is also at elevated risk in a band extending from Wisconsin to western New York state. The windy Great Plains, in contrast, experience fewer deaths, partly because of the lack of trees. The spatial distribution is also related to the location of cyclone tracks across the U.S.; 83% of non-convective high wind fatalities were associated with extratropical cyclone phenomena .
Explaining the Notre Dame tragedy climatologically
Now let’s return to the scene of the October 2010 accident at Notre Dame and examine how the circumstances of Declan Sullivan’s death correlate with what is known about non-convective high winds and the fatalities that result from them. The high winds occurred:
• In northern Indiana, a region known for non-convective high wind events and fatalities;
• During the last week of October. October is a month of somewhat elevated risk due to non-convective high winds in northern Indiana , and October 27 is just two weeks before the November 10 anniversaries of notable non-convective high wind events in the Great Lakes region in 1913, 1940, 1975 and 1998 ;
• To the south of a mid-latitude cyclone that set an all-time record low pressure of 955.2 mb for the state of Minnesota on October 26, creating tight pressure gradients behind the cold front to the south of the cyclone center where non-convective high wind events often occur;
• In the vicinity of the dry slot of the cyclone, which are noted for windy conditions;
• With clear sunny skies, which promote vertical mixing via dry thermal convection;
• Shortly before 5 pm, when vertical mixing would have been nearing its peak due to afternoon heating. Research in progress on high wind events suggests a late-afternoon peak for non-convective events . The atmospheric mixed layer at the time of the accident was nearly 9600 feet (2.9 km) in depth, permitting high-speed winds of at least 85 mph (38 m s-1) to be conveyed downward close to the surface. Peak wind gusts at South Bend, just upwind of the Notre Dame campus, increased from about 2:30 pm to 4:30 pm EDT, consistent with this mixing hypothesis ;
• From a direction just west of southwest, with a compass value of 230-235° , which is climatologically a dominant direction of non-convective high winds in the Great Lakes region (see Fig. 3).
Turning to the circumstances of the fatality itself, it occurred:
• Outdoors, as with nearly a quarter of all non-convective high wind fatalities ;
• High above the ground using construction-type equipment that toppled over, as with nearly 20% of all outdoor non-convective high wind fatalities ; and
• Without any special preventive actions being taken despite a High Wind Warning and later a Wind Advisory being in effect for the area, illustrating the “lack of urgency and awareness [that] may result in people placing themselves in harm’s way during non-convective wind events” .
This discussion illustrates that while the tragedy at Notre Dame was unusual from the perspective of everyday weather and life, it also closely fits the climatological profile of an extreme event: non-convective high winds and the fatalities that they cause.
Speculating on possible developments in the 21st century
Few weather-related deaths receive the amount of national publicity and post-mortem scrutiny (including fines from the Indiana Occupational Safety and Health Administration) that Declan Sullivan’s has . More such deaths may occur in this century, all other things being equal, given projected increases in population and population density, and even initiatives to stave off global warming by planting more trees. The two variables that may determine the future risk the most, however, are the frequency of strong mid-latitude cyclones and the societal response to forecasts of high winds.
More high winds in the future?
Scientific projections of recent and future mid-latitude cyclone trends in the 20th and 21st centuries are a mixed bag. A rapidly growing body of research focuses on Europe, where public awareness of non-convective high winds is much higher than in the United States. Some studies identify trends already in progress leading to more intense cyclones in the mid-latitudes of the Northern Hemisphere [27,28] that may be related to upward trends in extreme winds over the ocean waters of this region .
Are these trends part of a longer-term trend owing to global climate change? No one is sure. Several studies project more extreme winds in the 21st century Northern Hemisphere mid-latitudes [30,31]. These extreme winds are predicted to occur to the south of cyclone centers , as is often the case for non-convective high winds. A different study  does not project any trends in 21st century extreme winds for the same region, however. The influential IPCC modeling simulations suggest fewer, but more intense, mid-latitude cyclones and stronger winds in the mid-latitude storm track regions in a warming climate in the 21st century [34,35], but a recent study  disputes this finding. Even fewer-but-stronger cyclones is no guarantee of more extreme winds in the future; trends in extreme winds may differ markedly from one region to another, and deeper cyclones may not lead to tighter pressure gradients and higher winds because of a general decrease in surface pressure in the mid-latitudes .
Even if such windstorms become more common in and near the U.S. during this century, there is little research on what the implications might be in terms of deaths and damage. For Europe, where the impact of non-convective high winds is more widely researched, studies suggest that storm-related losses could be up to 37% higher in Great Britain and Germany , or up to 43% higher in Germany . But the jury is still out; the uncertainties in both the science and the societal impacts are too large for much confidence at this time.
Better public awareness?
What are the prospects for heightened awareness of non-convective high winds in the U.S. in the future? A first step might be in a more sophisticated appreciation of “unusual” and “extreme” events. The official Notre Dame investigation emphasized the “unusual” nature of the winds on October 27, 2010. However, the return interval for a 52.5-mph (23.5 m s-1) wind gust was estimated in that same report as 2 years, i.e., a 50% chance of occurring in a given year . The highest gusts on that day qualify as “extreme” by the IPCC definition  because they place in the highest 10% of all observed wind speeds at South Bend. But a 53-mph gust at South Bend is not astoundingly “unusual” in a long-term sense because it can happen once every few years, and higher non-convective wind gusts have occurred in South Bend at least three times since 1973 . In short, extreme events can and do happen; their consequences must be anticipated and planned for.
As noted earlier, there is circumstantial and anecdotal evidence that the High Wind Warnings and Wind Advisories issued by the National Weather Service are not triggering adequate societal responses to protect life and property. How can this impediment to preventive action be overcome? The word “tornado” allegedly inspired such strong public reactions that the U.S. Weather Bureau forbid its use in official statements for decades . In stark contrast, the phrase “non-convective high winds” stimulates no similar levels of awareness, understanding or reaction in the general public. Even long-lived straight-line winds in thunderstorms have attained some cultural visibility via the use of the Spanish moniker “derecho” by researchers [43,44], a word that is now making its way into public discourse. Unlike, say, amyotrophic lateral sclerosis, which became universally known as “Lou Gehrig’s Disease” when the New York Yankee baseball great was afflicted with it in the late 1930s , the victims of non-convective high wind events are usually not famous. As in the case of Declan Sullivan, the victims are more likely to be videographers, not star quarterbacks. It remains to be seen whether the research community and the operational meteorological forecasting community can convey the threat from these winds in ways that resonate with the public in the future.
Perhaps the most potent message that can be conveyed on this subject is that for Declan Sullivan, and for many (but not all) victims of non-convective high winds, death came from a clear blue sky.
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John A. Knox is an associate professor of geography in the atmospheric sciences program at the University of Georgia. His research interests focus on atmospheric and climate dynamics, including non-convective high winds, clear-air turbulence forecasting and inertial instability. He received a B.S. in mathematics from the University of Alabama at Birmingham and earned a Ph.D. in atmospheric sciences from the University of Wisconsin-Madison, and was a post-doctoral fellow at Columbia University in conjunction with the NASA⁄Goddard Institute for Space Studies (GISS). Knox has served as an associate editor of the Journal of Geophysical Research-Atmospheres, the National Weather Digest, and the Journal of Geoscience Education. He was the 2010 recipient of the T. Theodore Fujita Research Achievement Award from the National Weather Association.
John D. Frye is an assistant professor of geography and geology at the University of Wisconsin-Whitewater. His research interests are land surface–atmosphere interactions, weather impacts on society, climate variability, and remote sensing applications in meteorology and climatology. He earned a B.S. in journalism and an M.S. in geography from Ball State University and a Ph.D. in geography from the University of Georgia.
Joshua D. Durkee is an assistant professor in the meteorology program at Western Kentucky University’s Department of Geography and Geology. He received a B.S. in geography from Western Kentucky University, and M.S. and Ph.D. degrees in geography from the University of Georgia. His research interests include synoptic and mesoscale meteorology ⁄ climatology, hydroclimatology, atmospheric circulation and teleconnections, remote sensing and GIS applications in the atmospheric sciences, and education assessment and reform in the atmospheric sciences.
Matthew C. Lacke is a meteorologist in the Air and Radiation Protection division at the Jefferson County Department of Health in Birmingham, AL. He received a B.S. in meteorology from Northern Illinois University and a M.S. degree in geography from the University of Georgia. His research interests are aerosol influences on precipitation and influences of meteorological variables on pollution.