“Superstorm” Sandy will go down as one of the most destructive storms in the history of the United States. Total losses may exceed $50 billion, and estimated losses for the airline industry are near $500 million. More than 20,000 flights were canceled starting Sunday, Oct. 28, 2012, and continuing through Wednesday, Oct. 31.
Particularly hard hit was the New York City area. LaGuardia Airport, which is located on a waterfront, suffered significant damage, with the tarmac flooded, and did not reopen until Nov. 1. In anticipation of Sandy’s strong winds, New York City’s three major airports closed Sunday night. Weather conditions steadily deteriorated overnight. Winds were gusting over 40 kt by morning. In the evening, wind gusts near 70 kt were recorded, and at times, wind-driven rain severely lowered visibility.
The magnitude of the storm can be seen by looking at the peak wind gusts reported at various airports along the U.S. East Coast: Dulles International Airport outside Washington, 47 kt; Philadelphia International Airport, 59 kt; John F. Kennedy International Airport in New York, 69 kt; Boston Logan International Airport, 52 kt; Portland (Maine) International Jetport, 48 kt. All of these peak gusts occurred within several hours on the evening of Oct. 29.
Even after winds subsided on Tuesday, Oct. 30, and flying conditions improved, widespread power outages and a lack of surface transportation impeded airport operations. Structural damage to buildings and significant damage to aircraft on the ground were reported.
Throughout its duration, the storm was referred to as Sandy, the name given when it first reached tropical storm status on Oct. 23. As Sandy was ravaging New York and New Jersey on Oct. 29, meteorologists stopped calling it a hurricane, even though it maintained the same intensity. Technically, just as Sandy was coming ashore near Atlantic City, New Jersey, it became a midlatitude or extratropical cyclone, losing its tropical characteristics. The U.S. National Weather Service continued to use the name Sandy to avoid public confusion. The results — the wind, the rain and the massive storm surge — were the same, regardless of the nature of the storm.
So, why should we be concerned if a storm is tropical, extratropical or something in between? Consider these elements of Sandy: The storm was 1,000 mi (1,609 km) across, more than twice the size of the large and extremely destructive Hurricane Irene that affected this same region in August 2011. When Irene came up the East Coast, it weakened considerably over the cooler waters, typical of a true tropical system. Sandy did not weaken, even though it traversed the same waters nearly two months later in the year. In fact, it strengthened. The central pressure fell to 940 millibars (mb; 27.76 in Hg), 20 mb lower than the famed superstorm that moved up the East Coast in March 1993.
What are the differences between tropical cyclones such as hurricanes and extratropical cyclones, the typical winter storms?
Tropical cyclones only develop over warm waters, usually in the lower latitudes. Extratropical cyclones can develop over land or water where the air is colder and have even occurred in Arctic regions. Extratropical cyclones require a temperature contrast to develop. They usually form along fronts that separate warm and cold air masses. Tropical cyclones develop within a single warm, humid air mass with no fronts involved. Tropical cyclones get their energy from the warm ocean water. Evaporation puts vast amounts of water vapor in the air. When the air is lifted in the storm’s circulation, the water vapor condenses in the towering cumulonimbus clouds, releasing latent heat that drives the storm. Extratropical cyclones derive their energy from the temperature contrast between warm and cold air masses. Energy is released as the warm air is lifted over the cold.
An earlier article (ASW, 2/12) described how a surface low pressure area is produced. Air is removed from above in the process called divergence. Air is lifted by the low pressure and then spreads over a larger area above the surface low. This removal of air lowers the surface pressure. For extratropical cyclones, the divergence aloft is produced on the east side of a pre-existent upper-level trough of low pressure. When this upper trough moves over a surface front, cyclogenesis — the process by which the low pressure area develops — occurs. For tropical cyclones, there are no pre-existent upper features. However, over time, the towering cumulonimbus clouds release enough heat aloft to develop a high pressure area over the low-level cyclone. This self-developed high, miles above the surface, provides the divergence aloft needed to maintain the surface storm.
It is not unusual for tropical cyclones to become extratropical. If the tropical system moves into higher latitudes, especially in the late fall, it can merge with a midlatitude frontal system and its attendant upper trough. Some of these converted storms can be very strong. The “textbook case” was Hurricane Hazel, which moved up the East Coast in 1954. Hazel came ashore along the extreme southern coast of North Carolina on Oct. 15. It was a powerful Category 4 (ASW, 7/12) hurricane with maximum sustained winds of 110 kt and a minimum central pressure of 937 mb (27.67 in Hg). It almost immediately joined a strong cold front and began to accelerate to the north. Cold air poured into the system from the west, quickly transitioning the storm into an extratropical system. Unfortunately, weakening was limited, and by the next day, it passed Washington with a central pressure of 970 mb (28.64 in Hg) and the transformed Hazel produced a gust at Washington National Airport (now Ronald Reagan Washington National Airport) of 98 mph, a record that still stands. Hazel continued hundreds of miles northward into Canada, still producing hurricane force winds, although its track was well inland.
Sandy made the transition to fully extratropical, probably a few hours before landfall on Oct. 29. Of more interest was what was happening with Sandy in the days prior to this. Sandy started as a pure tropical system. It formed in the Caribbean Sea on Oct. 22. Three days later, as it was coming ashore on the south coast of Cuba, Sandy was at its maximum strength as a purely tropical system — a strong Category 2 hurricane with maximum sustained winds of 100 kt and a central pressure of 954 mb (28.17 in Hg; Figure 1). Tropical storm or gale force winds (34 kt or greater) spanned a diameter of 200 mi (322 km). By the next day, a weakened Sandy continued to move northward toward the Bahamas. The central pressure had risen to 969 mb (28.61 in Hg), and maximum winds were barely hurricane force (64 kt), but the storm had doubled in size with gale force winds covering 400 mi (644 km). And by late in the day on Oct. 26, Sandy no longer looked like a true tropical system on satellite imagery. It was still warm core with convection near the center, but it now had a long frontal-looking cloud band associated with it. Forecasters at the National Hurricane Center said in their technical discussion that they were dealing with a “hybrid cyclone,” part tropical system, part extratropical system.
Meteorologists have known about hybrid storms for years. They have even classified one type of hybrid storm, the subtropical cyclone. These low pressure areas develop only over ocean areas and have characteristics of both extratropical and tropical cyclones. Most subtropical cyclones develop from midlatitude, deep upper-level troughs or closed lows. They actually develop downward and eventually produce a surface low. The cloud pattern resembles a comma, very noticeable on satellite imagery. The strongest winds, which can exceed hurricane force, are found well away from the center of the storm, unlike tropical systems. If this system sits over warm water, convection may develop near the center. The storm can become warm-core and tropical in nature. The convection and warm core are often confined to a small central region, surrounded by the extratropical part of the storm. So you can have a tropical cyclone embedded within a larger subtropical or even extratropical system. Subtropical cyclones are not limited to the Atlantic. The “Kona storms” that sometimes affect the Hawaiian Islands in winter are subtropical. Other subtropical cyclones have occurred in the Mediterranean Sea and the Indian Ocean.
Prior to Sandy, probably the most famous of the hybrids was the so-called Perfect Storm of 1991. Developing in the North Atlantic, south of Nova Scotia, Canada, in late October, this cyclone had peak sustained winds of 65 kt and a minimum central pressure of 972 mb (28.70 in Hg). Although it never came ashore, severe beach erosion occurred from the Canadian Maritimes to North Carolina. A buoy in the open ocean measured a wave height of 100 ft. At one point, convection developed near the storm center, and the inner core took on the structure of a tropical cyclone.
Sandy began to resemble a subtropical cyclone (Figure 2), but Sandy was a tropical system that was acquiring extratropical characteristics, not vice versa. It featured a warm, tropical core embedded within a much larger non-tropical cyclone. It had two wind maxima, one near the center and one over 100 mi (161 km) north of the center. The surface map for 0000 coordinated universal time (UTC) on Oct. 28, 2012 (Figure 3) shows Sandy off the Southeast coast. The front it will eventually merge with is to the west. The 500 mb (~18,500 ft, 5,500 m) chart for the same time (Figure 4) depicts Hurricane Sandy as a warm core low off the Southeast coast. A powerful trough (cold core) is located in the middle of the United States. Not only is the trough steering Sandy to the north, but the divergence on the trough’s eastern side is causing the pressure to fall in the storm, 10 mb in one day. Cooler water and increased wind shear should have weakened the storm. Sandy’s peak winds remained the nearly the same, but the storm continued to grow in size.
Sandy moved parallel to the coastline on Oct. 28, while its central pressure continued to fall and the storm grew. The 1200 UTC Oct. 29 surface chart (Figure 5) shows Sandy well off the Virginia coast. The 500 mb chart for the same time (Figure 6) shows a large upper-level high over the Canadian Maritimes, blocking Sandy’s northward march. At the same time, a closed, cold core low has formed over North Carolina. Sandy is being pulled to the west into the stronger system. Convection near Sandy’s center continued to develop (Figure 7). The central pressure also continued to fall, reaching 940 mb by 2100 UTC. Sandy’s hybrid nature was a double-edged sword. Had Sandy been a pure tropical hurricane, with such low pressure, it would have been a Category 4 hurricane with maximum sustained winds of 114 to 135 kt. Instead, winds were still holding near 70 kt, but the wind field was huge, with gale force winds now covering nearly 1,000 mi.
Sandy likely became a true extratropical cyclone just before it came ashore in southern New Jersey with maximum sustained winds of 70 kt and a central pressure of 946 mb (27.94 in Hg). The surface map for 0000 UTC on Oct. 30 (Figure 8) shows a fully transformed Sandy now associated with an array of fronts. In terms of pressure, Sandy was the strongest storm ever to make landfall this far north. Although winds of 70 kt ordinarily wouldn’t produce an excessive storm surge, because of Sandy’s huge size, it brought devastatingly high tides to the New Jersey and New York shorelines. The 0000 UTC 500 mb chart (Figure 9) shows that the two 500 mb lows have basically merged over eastern Maryland. Sandy’s residual pool of warm air can be seen over eastern Pennsylvania and New Jersey.
Can these hybrid or transitioning cyclones be forecast? In the case of Sandy, the answer is yes. Computer models accurately forecast Sandy’s intensity and even its point of landfall days ahead. Advance warnings saved lives. However, the property destruction and disruptions to airline service were unavoidable.
The precise forecasts helped prevent flight incidents associated with Sandy. It was easy to cancel flights and even close airports based on the accurate predictions. But these hybrid storms may not always be forecast that well. Errors in track or intensity predictions could result in little warning of dangerous flying conditions.
How is climate change involved in all this? Obviously, the earth is getting warmer. As the air and water warm, there will be more energy available for all types of storms. Another way to look at this: The purpose of storms (cyclones) is to transport energy on the earth, basically from the equator, where it’s hot, to the poles, where it’s cold. A warmer earth would mean more storms and potentially stronger storms. Sandy could just be a harbinger of things to come.
Edward Brotak, Ph.D., retired in 2007 after 25 years as a professor and program director in the Department of Atmospheric Sciences at the University of North Carolina, Asheville.