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These tropical storm systems can damage infrastructure and disrupt aviation.

By Karsten Shein
Comm-Inst Climate Scientist

Hurricane Dorian on September 2, 2019 as seen from the International Space Station. Tropical cyclones come in many sizes and strengths, but all pose a significant hazard to aviation.
Tropical cyclones are among the more destructive and unpredictable weather systems we face. Fortunately, their extended life and slow movement relative to other damaging weather phenomena allow us some ability to prepare and avoid these powerful storm systems.

Called tropical storms and hurricanes in the Atlantic, Caribbean, and northeast Pacific, typhoons in the northwest Pacific, and cyclones in the south Pacific and Indian Ocean, all are generically known in meteorology as tropical cyclones in that they are organized, rotating (cyclonic) storm systems that develop in the tropics, although they may move into extratropical latitudes.

All tropical cyclones form as tropical depressions, but not all depressions become cyclones. As its name suggests, a tropical depression is a low-pressure region in the easterly tropical flow of the atmosphere. As with all low pressure centers, surface air is drawn inward and upward, kicking off an area of towering cumuli and thunderstorms.

Most depressions remain just that, disorganizing and dissipating after a few days. It takes a very specific set of circumstances for a tropical depression to grow into a tropical storm or hurricane.

The depressions themselves normally begin as a short wave travels along the tropical easterly jet out over the warm ocean. When this upper-level disturbance meets the favorable conditions, a tropical depression may form.

Formative factors

Criteria for tropical cyclone formation include the easterly wave, and a warm ocean. Normally, the ocean surface temperature down to a depth of around 50 m must exceed 26.5° C (80° F).

This heat is necessary to ensure that the overlying air gains enough heat energy to destabilize and sustain convective lifting. This warm, maritime environment also supplies the water vapor that provides the energy necessary to sustain the system as it matures.

Other formative factors include high humidity that extends into the middle levels of the troposphere, and a strong environmental lapse rate. The atmosphere normally cools with height, but, in the tropics, the rate of cooling is often insufficient to support strong convection, instead favoring the typical tropical cumuli that bring passing showers and the occasional rumble of thunder.

Meanwhile, a strong environmental lapse rate will ensure that rising air remains warmer than its surroundings, even as it is releasing heat through condensation – the heat that powers the storm. The last 2 factors are lack of windshear, and a position that’s far enough from the equator.

The former is critical to ensure that the rising convective currents and accompanied towering cumuli are not toppled and disorganized by strong winds aloft. They must be able to continue to rise uninterrupted through the 50,000–60,000 ft levels of the tropical troposphere to the base of the stratosphere, where currents allow an exit region for the rising air.

Tropical cyclones also don’t form within ~5 degrees latitude (~560 km or 350 miles) of the equator, because the Coriolis effect from the rotating Earth, which is zero at the equator, must be large enough to deflect airflow toward the low pressure center of the storm.

Damaged aircraft poke out from a collapsed hangar at BCT (Boca Raton FL) in the aftermath of Hurricane Wilma in October 2005. Strong tropical cyclones can have sustained winds well over 100 kts and may inundate coastal airports with storm surge that exceeds 15 ft.

Hurricane hotspots

Tropical cyclones occur across most of the tropical ocean, with key exceptions of the south Atlantic and eastern south Pacific, where atmospheric and oceanic conditions rarely favor organized tropical convection for long enough to sustain a tropical cyclone.

Only 1 such storm has been recorded in the south Atlantic. In the north Atlantic and Caribbean, storm season begins on June 1, reaching a peak around September 10, and closing on November 30. The season in the eastern Pacific starts a bit earlier – May 15 – but follows a similar behavior.

Storm season is reversed for the Australian and southern Pacific regions, beginning November 1 as they enter southern Hemisphere summer, peaking around late February, and ending by April 30. A few extraseasonal storms have occurred in those basins, but they are infrequent and weak.

The western North Pacific and Indian Ocean have a year-round hurricane threat. However, like their part-year cousins, they tend to see peak activity corresponding to the warmest ocean waters – that is, late February in the southern hemisphere and early September in the northern.

The majority of tropical cyclones begin within 20º latitude of the equator, with just a few forming between 20º–30º latitude. Nearly all of them form along the intertropical convergence zone (ITCZ), which moves between the northern and southern hemispheres following the sun, and acts as a region of convergence for warm and humid air on both sides of the convergence zone.

Anatomy of a tropical cyclone

Once an atmospheric disturbance creates a surface low around which thunderstorm activity can organize, the warm and humid surface air flows toward the central low. The Coriolis effect deflects the incoming wind to where it spirals inward.

As it approaches the center, its curvature increases into a tightening spiral and its velocity increases proportionately. Near, but not in, the center, the air begins to rise, slowing its inward progress and reducing the Coriolis effect. This is why the strongest winds occur in the inner eyewall, but never reach the calm eye.

The eye itself is largely a column of cooler air descending from aloft, frequently sufficient to suppress cloud development. Around this eye is the eye wall, a more or less continuous circular squall line of strong thunderstorms. Storms here can achieve tops above FL500, with their outflow radiating from the eye as a cirriform shield.

Extending from the eyewall is a pinwheel of rain bands. These are curved squall lines that can contain strong storms as well, but as distance increases from the cyclone’s center, the decreasing concentration of energy and the sinking outflow from the system’s center conspire to reduce cell height and strength.

The entire cyclone structure acts as a heat pump with an internal circulation drawing in energetic air at the surface and expelling cooler, drier air aloft. This vertical/horizontal flow is what drives the hurricane, while its spiral motion is what translates into the cyclone’s destructive force.

Tropical cyclone movement is often difficult to forecast, and is sometimes described as resembling that of a cork in a stream. Cyclones are directed by 2 factors. The 1st is the surrounding atmospheric wind field, particularly mid-level (500 mb) winds. The easterly tradewinds on the equatorial side of the subtropical highs normally steer the storms westward.

Beyond environmental steering winds, there is also movement meteorologists call “beta drift.” Beta drift is a poleward and westward motion due to complex feedbacks from internal horizontal air movement and an ever-changing Coriolis effect. In this way, a cyclone might still move in the absence of strong environmental steering.

It is also a reason why storms will generally migrate poleward, eventually moving into a region of stronger westerly flow that ‘recurves’ the cyclones onto a more easterly track as they reach higher, extratropical latitudes.

Although all parts of a tropical cyclone are dangerous, the storm’s movement, coupled with its rotation, make certain regions more dangerous than others.

The right front quadrant of the storm (left front in the southern hemisphere) relative to the storm’s track is the most dangerous, because the rotation and movement work in the same direction. This means the strongest winds are found here, as is the greatest storm surge as those winds push the water out ahead of the storm.

Official forecast cone of uncertainty showing the probable path of Hurricane Irma in 2017 as it approached the eastern Caribbean. Cones represent the most likely area in which the cyclone will track, and with large systems, hazardous weather often occurs well outside of the cone.

Cyclone forecasting

Most modern weather forecast models can predict the formation and general movement of tropical cyclones with reasonable accuracy, but they still often have difficulty in estimating its track beyond a few hours.

Instead, each model gives its best guidance and forecasters predict the most likely track, placing the hurricane at the center of an expanding corridor within which it may move. The so-called “cone of uncertainty” in tropical cyclone forecasting closely follows a traditional mariners’ guide called the 1-2-3 rule.

Given the current location, radius of tropical storm force winds around the cyclone’s center, and the forward movement of the storm, a line is drawn on the map. At the storm’s expected position on that path 1 day out, the width of the cone is the storm’s radius + 100 nm.

At 2 days, the width is +200 miles, and at 3 days is +300 miles. Modern forecasting is not a simple linear trend, but rather uses forecast errors over the past 5 years to set the width of the cone at 67% of the greatest errors. Forecasters also take the forecast cone out 5 days, but even that sometimes proves inaccurate, which is why the forecasts are updated every 6 hours – or more frequently if the cyclone is threatening landfall.

Given the success of the European Centre for Medium-Range Weather Forecast (aka European) model at predicting the tracks of Atlantic tropical cyclones, many ask why we don’t just use this model. That’s because its success is largely due to chance, not better physics.

Most weather models are in very close agreement over the 1st day or 2, but tend to diverge with greater forecast lead times. So-called spaghetti plots (maps of the forecast tracks of many models) will similarly reveal a forecast cone. The narrower the cone, the more the models agree on a likely track.

Cyclones and aviation

Impacts on aviation from cyclones are normally not airborne. Given the ubiquitous satellite imagery of these storms, routine forecasting, and frequent alerts, there is no excuse for any pilot to accidentally penetrate a tropical cyclone.

A few intrepid pilots do so on purpose to collect scientific data that helps forecasters, but they’re experienced and well equipped to do so. The rest of us normally have ample time to see the storm coming, know where it is, and stay out of its way.

The greatest danger to aviation is ground based. Airports may close as a storm approaches, and aircraft and facilities may be damaged by the high winds, heavy rains, and even tornadoes that accompany these cyclones. Just as people living in hurricane-prone regions should have hurricane preparedness plans, pilots operating or based at airports in these areas should have a plan in place for evacuating aircraft to safety, and airports should have a plan to minimize impacts and damages so they can reopen quickly.

Aircraft evacuation plans do not have to be complicated, and moving an aircraft is far less expensive and disruptive than repairing or replacing one. Landfalling tropical cyclones quickly lose their punch after crossing over the coastline and being cut off from their fuel supply of warm ocean water.

Airports that are further inland, even less than 100 km (62 mi), and situated on higher ground, will usually be subject to less intense conditions than those experienced at coastal airports. It is best to avoid evacuating to inland airports that sit along a river, as cyclones can still drop over 1 m (39 in) of rain well inland, turning lazy rivers into dangerous flood hazards.

If you cannot evacuate your aircraft, the best course of action is to ensure it is securely tied down or hangared, but your aircraft may still be damaged in such a scenario. There is no point in trying to tie your aircraft down facing the wind, as winds during the passage of a tropical cyclone will come from several directions, and there may be tornadoes as well.

In addition, at low-lying coastal airports, storm surge may be 10–15 ft deep or more and will damage or destroy much in its path. You should also check whether you could get to an airport after the storm passes. As was made clear in Huston TX after Tropical Storm Allison in 2001 and Hurricane Harvey in 2017, substantial rain can flood out entire communities, making roads impassible – including airport access roads.

Even if roads are not flooded, downed power lines or emergency closures may restrict movement. It may be several days before aircraft owners would even be able to assess damage to aircraft left at the airport. Airports also may remain flooded for days to depths that may require a thorough inspection and perhaps overhaul of any aircraft partially submerged.

Most importantly, however, unless you are also planning to evacuate with the aircraft, an aircraft evacuation plan should be implemented well before the last minute, as that time should be set aside to ensure your own safety.

Give yourself time to move your aircraft and return to make final preparations either for sheltering in place or evacuating yourself and family.

Karsten Shein is co­founder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg Univer­sity and a climatolo­gist with NOAA. Shein holds a commercial license with instrument rating.