The Power and Beauty of Hurricanes
A timely look at these magnificent heat engines
Hurricanes are the most powerful actors in the global climate system. The highest windspeed every recorded was for Tropical Cyclone Olivia at 253 MPH over Barrow Island, Australia. Hurricanes can live for weeks, can cover up to thousands of square miles at any one time and can release far more energy than even the most violent volcanic eruptions.
But there is also something awe-inspiring and massively beautiful about these products of the climate system's efforts to redistribute heat energy accumulated in the tropics - beautiful at least when viewed from above. At ground level, hurricanes elicit awe and fear from the power and destructive capacity they embody.
While we on the ground try to avoid these storms, hurricane hunters fly right through them and, amid the turbulence and danger, find moments of peaceful air and beautiful images like this one of the "stadium effect" at the top of the eyewall of Hurricane Katrina. Before the age of satellite remote sensing, these daredevil pilots were the major source of information on the three-dimensional structure of tropical storms.
I had been planning an essay on hurricanes for a couple of weeks, but the topic suddenly took on added interest as Hurricane Lee churned north toward the coast of New England. As I write this, Lee is passing us by with just a brush of winds up to 25 mph and maybe half an inch of rain. The story may be very different in Down East Coastal Maine and the Canadian Maritimes.
And it could have been very different here as well. The paths followed by hurricanes are difficult to predict, and Lee's history was no exception. At different times over the last week, The chance that coastal New Hampshire, where I live, would experience tropical storm-force winds (39 to 73 MPH) has varied from near zero to more than 30%.
Inside A Hurricane
A wealth of satellite- and Earth-bound instruments now track and measure hurricanes. What do they tell us about what is below that almost serene-looking swirl of clouds? What is inside this energy machine, how does that structure develop, and how does it decay?
The ingredients required to spawn a hurricane include excessively warm sea surface temperatures and an initial disturbance that begins to cause uplift of warm humid air over that warm ocean, bringing clouds, rain and thunderstorms. The rising air can begin to rotate through the Coriolis effect of the Earth's rotation. Hurricanes do not form at the equator where the Coriolis effect is minimal.
A forming storm can be "blown away" by wind shear - a strong, linear upper level wind flow that can interrupt the forming vortex, or suffocated by intense dust storms coming off the Sahara Desert.
But if the sea surface is warm enough (above 80oF), and surrounding atmospheric conditions are favorable, the forming vortex can be strengthened, with the structure becoming more organized, and wind speeds and rainfall increasing. Swirling outer bands of uplift, rainfall, thunderstorms, and even tornadoes are formed and spiral in towards the developing low pressure at the center (the "eye").
This static image belies the dynamic nature of a storm's structure. As those outer bands circle in towards the eye, they can disrupt the existing eyewall, where windspeeds are the highest, and cause a loss of intensity. The innermost rain band can then replace the declining eye wall, rebuilding both the structure and the intense circulation. This cycle of breakdown and reformation of the eyewall leads to cycles of increased and decreased storm intensity.
Why is sea surface temperature so important to the development and ultimate strength of a forming hurricane?
The air above that super-warm water will also be warm and very humid - saturated with water vapor. As the surface air is drawn up and into cooler air within the rain bands, that humid air, that water vapor, will condense into liquid water - clouds and rain. Condensation, the transition of water vapor in the cooling, humid, rising air to liquid water releases huge amounts of heat energy, so as water vapor evaporated from the warm ocean surface rises and condenses to form heavy rainfall, heat energy is released that continues to warm the rising air, increasing lift and powering the storm.
The warmer the sea surface temperature, the greater is the potential (or latent) heat energy contained in the water vapor. One source estimates the amount of energy released through condensation by Hurricane Katrina in one week was equivalent to 160 Mt. St. Helens or 10 Krakatoa eruptions.
Movement and Decay
Tropical storms and hurricanes can form anywhere and at any time that the conditions are right. In the North Atlantic most are spawned and strengthen in late summer just north of the equator where summer heating has been maximal.
Storms often begin as thunderstorms running off the coast of Africa. If such a storm encounters super warm ocean surface waters and no wind shear, it may begin to organize and move westward. The image below captures all the storm paths for the years 2000-2008, showing the tendency to move first to the northwest and then off to the northeast - often around a ridge of high pressure over the central North Atlantic. The dots on this image show where a storm first became strong enough to qualify as a tropical storm. The initial disturbance may have originated farther to the east.
Whether or not a storm makes landfall in the Americas appears to be as much happenstance as pattern.
NASA has produced a dramatic video that captures some of the complexity of individual storm paths. It is a composite of data from different satellite systems that can map the concentration in the atmosphere of aerosols - small particles of different kinds that affect the absorption and reflection of light in different wavelengths. The video combines information on sea salt thrown into the air by hurricanes, dust launched by windstorms over the Sahara Desert, and smoke particles generated by wildfires.
I hope you will play the video as you read the next paragraphs. In the video, which covers August 1 to October 31, 2017, sea salt is blue, Saharan dust is brown, and smoke from wildfires is white. The variation in origin, strength, and path of Atlantic hurricanes is evident in the histories of the five named storms shown.
Record-breaking wildfires in the western U.S. during this period, and the spreading of smoke from these fires, can be seen throughout the video.
For the first three weeks, massive dust storms off the Sahara occur and minimal tropical storm activity is to be seen. The only major storm in this period, Harvey, formed mid-ocean and only strengthened in the Caribbean.
Irma forms in the absence of major dust storms and follows the most common path across the Atlantic and up the East Coast of North America. Irma does make landfall and disintegrates quickly as hurricanes do once they are cut off from the energy of a warm ocean and encounter the friction of land contact.
Jose is a different story. While formed in the tropical Atlantic, it follows a wandering path before finally being blown off to the northeast. Maria intensifies rapidly, staying just off the East Coast before being blown quickly off to the northeast.
The video's narrator focuses on Ophelia, the final storm shown, which forms much farther north than the others, and moves directly off to the northeast, impacting Ireland and England.
In each case, as the storm either encounters added friction over land or cooler sea surface temperatures, it loses energy, structure, and momentum, becoming post-tropical and finally being absorbed into a more normal regional weather pattern.
Predicting a Storm or a Season
If we know the how and why of hurricanes, can we predict them?
This excellent, detailed site from the University of Rhode Island describes the general limitation on the accuracy of all weather predictions most succinctly:
It is impossible to definitively predict the future state of the atmosphere because of its chaotic nature.
Chaotic systems are extremely sensitive to initial conditions, and the site goes on to say:
Furthermore, existing observation networks have limited resolution in both space and time, especially over large bodies of water... Lack of observations introduces uncertainty into the true initial state of the atmosphere.
Several models are used to predict the paths of hurricanes. Some are statistical, using past experiences to predict each storm, and some more "dynamical" meaning built on basic physical principles. Each may use a different set of initial conditions and each will have a different set of equations. None are perfect - uncertainty is the rule.
The Rhode Island site concludes:
To account for this uncertainty, "ensemble" forecasting is used, involving multiple forecasts created with different models, different physical parameterizations, or varying model initial conditions...A simple average of the individual model forecasts (the ensemble mean) often produces a more accurate forecast than any single model forecast because errors associated with the individual forecasts tend to be cancelled out.
The degree of uncertainty is suggested by this figure which uses only one model that is run a number of times with an "ensemble" of different initial conditions.
Another site summarizes a recent review by the U.S. Weather Service on the accuracy of hurricane path predictions this way:
Put your trust in the National Hurricane Center, or NHC, forecast. Although an individual model may outperform the official NHC forecast in some situations, the 2021 National Hurricane Center Forecast Verification Report documents that overall, it is very difficult for any one model to consistently beat the NHC forecasts for track and for intensity.
One valuable characteristic of Weather Service sites is that they report the accuracy of their predictions. This figure, from that Verification Report, captures the reality of forecasting hurricane paths. Basically, we are getting better at it, but predictions always become less accurate each additional day into the future. This is an outcome of the chaotic nature of the climate system and the unavoidable inaccuracies in the measurement of initial conditions.
Seasonal forecasts are even more statistical and offer only a general outlook. According to this site:
We issue these forecasts to satisfy the curiosity of the general public and to bring attention to the hurricane problem. There is a general interest in knowing what the odds are for an active or inactive season. One must remember that our forecasts are based on the premise that those global oceanic and atmospheric conditions which preceded comparatively active or inactive hurricane seasons in the past provide meaningful information about similar trends in future seasons.
Interactions With a Changing Climate System
Nearly every essay in this series ends with this question about the effects of a changing climate system, but I have gotten so carried away with the visuals and descriptions of the storms themselves that I have run out of room. This will be the topic for the next essay.
So as they say on the commercial weather sites - Stay Tuned!
Sources
The Olivia wind record can be found here - though most New Englanders will still refer to the old record on Mt. Washington, NH, of 231 MPH:
https://www.mountwashington.org/about-us/history/world-record-wind.aspx
The photo of Hurricane Isabel taken from the International Space Station can be found here:
https://www.nasa.gov/multimedia/imagegallery/image_feature_87.html
The image of the stadium effect at the top of Hurricane Katrina can be found here:
https://www.ec.gc.ca/ouragans-hurricanes/default.asp?lang=En&n=00677163-1
Images of the predicted path for Hurricane Lee are from:
https://www.nhc.noaa.gov/graphics_at3.shtml?start#contents
And the 5 am radar image is from:
https://radar.weather.gov/
For a graphic view of current windspeeds (maybe there will be a hurricane to see) try The Weather Channel's Wundermap site and select the "Windstream" option.
The graphic of the internal structure of a hurricane is from:
https://commons.wikimedia.org/wiki/File:Hurricane-en.svg
Radar image of Hurricane Ike is from:
https://www.noaa.gov/jetstream/tropical/tropical-cyclone-introduction/tropical-cyclone-structure
The estimate of energy released through condensation in hurricane Katrina is from a class exercise found on line at:
http://www.atmo.arizona.edu/students/courselinks/spring07/atmo336s3/lectures/sec2/hurricanes4.html
A visualization of the energy released through evaporation and condensation at the sea surface (called latent heat flux), including the increase caused by a hurricane, can be seen here:
https://svs.gsfc.nasa.gov/3199
The image of Hurricane Ike is from:
https://www.weather.gov/lch/ikemain
The graph of hurricane and tropical storm frequency is from:
https://www.nhc.noaa.gov/climo/
The amazing video of aerosols in the atmosphere is from NASA and can be found here::
https://www.youtube.com/watch?v=h1eRp0EGOmE
The image of hurricane tracks 2000-2008 is from this excellent site produced by the University of Rhode Island's Graduate School of Oceanography:
http://www.hurricanescience.org/science/
The site has detailed descriptions of the dynamics of hurricanes as well as methods of observation and modeling and is the source for the modeling quotes in the text.
http://www.hurricanescience.org/science/forecast/models/modeltypes/ensemble/
The image of ensemble forecasts for hurricane Rita are from the Rhode Island site above, with credit given to Timothy Marchok, NOAA/GFDL
The graph on accuracy of hurricane track prediction is from:
https://www.nhc.noaa.gov/verification/pdfs/Verification_2021.pdf
and can also be found here:
This site is the source for the quote on predicting the hurricane season:
https://source.colostate.edu/csu-researchers-predict-active-2023-atlantic-hurricane-season/