Carbon Dioxide at 400 ppm: What Does It Mean? - Page 2 of 4 - Blue Skies Meteorological Services

Carbon Dioxide at 400 ppm: What Does It Mean?

50th-birthday-balloon

What does a birthday mean? A major birthday – the type that warrants a card declaring your exact new age, possibly by spelling it out in macabre black balloons – what does it mean? Why do we care?

It’s not like you wake up on the morning of your birthday feeling dramatically older than when you went to bed. A decade’s worth of wrinkles don’t suddenly appear on your face. Yet you are older, and on your birthday, you are acutely aware of that fact.

A major birthday reminds you that life is short and you don’t have forever to act. It reminds you of all you’ve done and all you have left to do. Then it starts playing the Final Jeopardy countdown music in your ear. Time is ticking. Better get busy.

Carbon dioxide concentrations over the past 800,000 years. CO2 levels have not been as high as they are now for at least 3 million years. Source: NOAA NCDC

Carbon dioxide concentrations over the last 800,000 years. CO2 levels have not been as high as they are now for at least 3 millions years. Source: NOAA NCDC

Reaching a global average carbon dioxide (CO2) concentration of 400 ppm is that type of milestone, and we passed it in March. To put 400 ppm in perspective, consider that maximum pre-industrial CO2 levels were 280 ppm and that 350 ppm is widely considered the upper limit to avoid truly dramatic climate change. Consider that CO2 levels haven’t been as high as 400 ppm in several million years, when the world was much hotter and the oceans much higher than they are today.

Yet, besides the climate scientists who marked the passing of 400 ppm with a mixture of dismay, anger, and sad resignation, few others seem to have noticed (well, besides the United States military who consider climate change a national security risk and key business and insurance leaders who are already taking action to adapt). Nationally and internationally, we’re certainly not getting busy.

It’s as if we believe that if we don’t acknowledge what’s happening, it won’t happen. As if staying in bed with your eyes closed on your birthday somehow stays the hands of time.

Observed changes in average temperature across the United States for the period 1999-2012 as compared to 1901-1960 averages.

Observed changes in average temperature across the United States for the period 1999-2012 as compared to 1901-1960 averages.

But time doesn’t stand still just because we avoid clocks and mirrors – just as CO2 concentrations continue to increase whether we acknowledge it broadly and publicly or not. Of course, the critical difference between the inexorable forward march of time and the increasing concentration of greenhouse gases in Earth’s atmosphere is that we can actually do something about greenhouse gas concentrations.

We very likely can’t undo what we’ve already done (the technology just doesn’t exist to capture and indefinitely store vast quantities of atmospheric CO2). But we can slow down and eventually stop emitting new greenhouse gases, if only we muster the foresight to recognize and the willpower to address a large, costly, complex, global problem that will only get larger, more costly, and more complex with each year of procrastinated action.

Failing to even acknowledge the passage of the 400 ppm milestone doesn’t bode well, though.

So what does 400 ppm mean? What is this new world we’ve created for ourselves and our progeny?

Well, for one thing, 400 ppm means we’ve committed to major climate change – to what we’re already experiencing and more. The average residence time of carbon dioxide in Earth’s atmosphere is hundreds to thousands of years, so even if we stopped emitting CO2 tomorrow, our climate would continue to warm toward a 400 ppm equilibrium.

Surface temperature change under two different emissions scenarios -- rapid emissions reduction (blue) and business-as-usual (red). Source: IPCC 2014.

Surface temperature change under two different emissions scenarios — rapid emissions reduction (blue) and business-as-usual (red). Source: IPCC 2014.

Of course, we can’t put the brakes on instantaneously. If you’re traveling 100 mph down the highway and slam on the breaks, you keep traveling forward as you slow to a stop. A shift to renewable energy and carbon-neutral fuels, like stopping a speeding car, takes time, and the concentration of CO2 in the atmosphere will continue to increase during that shift.

Right now, though, we’re mashing on the accelerator rather than the brakes. With the exception of 1990-2000, each decade has seen an increase in the rate of CO2 emissions. Not only are we continuing to emit carbon dioxide – we’re emitting it faster and faster each year. If we continue along our current trajectory, we’re on pace for greater than 3° C warming, and that’s just the increase in average temperature. Extremes in both temperature and precipitation tend to increase more dramatically than their respective averages.

Such climatic changes would decrease crop yields and alter agricultural zones, decrease water availability while simultaneously increasing demand, inundate coastal areas with rising seas, extend the season and range of numerous pests and insect-borne diseases, increase heat stress and heat-related illness, and increase the frequency and intensity of flooding rainfall, among many other impacts.

Changes in average temperature for two different emissions scenarios - (B1) requires rapid and substantial emissions reduction, while (A2) is a continuation of our current emissions trajectory. Source: National Climate Assessment.

Changes in average temperature for two different emissions scenarios – (B1) requires rapid and substantial emissions reduction, while (A2) is a continuation of our current emissions trajectory. Source: National Climate Assessment.

400 ppm means that aspects of our environment that have been our touchstones for thousands of years – food and water availability, weather and climate – will shift in unprecedented ways. The ideal locations for cities, farmland, roads, factories, homes, and military assets will modify. Processes and procedures that have been reliable will become uncertain.

In short: the assumptions upon which we have built our societies may cease to be valid.

Although some progress toward mitigation (emissions reduction) and adaptation has been made on the local level both domestically and internationally, the sort of global-scale agreement and action required to alter our current emissions trajectory remains elusive. Emissions will therefore continue to rise, and the climate will continue to shift. Governments, industries, and individuals will be increasingly impacted by a variable and changing climate, and given the lack of coordinated effort to date, the unfortunate reality is that we must prepare to protect our own interests, assets, and welfare.

Businesses and insurers looking to take the long view of their investments, infrastructure, supply chains, and insured properties need to be aware of climatic changes that impact vulnerability. Blue Skies Meteorological Services is here to help these clients understand and mitigate their climate-related risk and exposure. Contact us at info@blueskiesmeteorology.com for more information.

Seeing in the Dark: Identifying Hail with Dual-Polarization Radar

Weather radar works by emitting microwave radiation into the sky and then listening for the signal that’s reflected back. It’s a meteorological game of Marco Polo.

All sorts of targets reflect the microwaves – raindrops, snowflakes, hailstones, bats, airplanes, and even swarms of insects. How well a given target reflects microwaves depends on its composition, size, and shape. For instance, liquid water is a better reflector of radar energy than ice.

As evening approaches, large colonies bats emerge from caves in the Texas Hill Country and show up on radar.

As evening approaches, large colonies bats emerge from caves in the Texas Hill Country and show up on radar.

When a meteorologist looks at a radar display, she’s seeing the reflected signal from all those targets in a given slice of sky. The radar doesn’t “know” which piece of reflected energy came from a bird and which piece came from the hailstone that moments later cracked your car windshield. The radar simply aggregates the reflected signal. It’s up to the meteorologist to interpret the results.

Until just a few years ago, the National Weather Service’s network of weather radars collected information about only two quantities: the reflected energy from a given section of sky (reflectivity) and the velocity of the targets within that section (mean radial velocity and spectrum width). In complex meteorological situations like winter weather events or severe storms, these two pieces of information provide only an incomplete picture of the type of precipitation that’s falling. When you’re just looking at reflectivity and velocity data, for instance, it can be difficult to tell the difference between hail and heavy rain. Yet on the ground, knowing the difference can be critical.

Meteorological target (hydrometeors) and non-meteorological targets (like birds and insects) all reflect radar energy. How much energy is reflected depends on the material,shape, and size of the target.

Meteorological target (hydrometeors) and non-meteorological targets (like birds and insects) all reflect radar energy. How much energy is reflected depends on the material, shape, and size of the target.

Enter dual-polarization radar technology. If you’ve ever owned polarized sunglasses, you’re already familiar with the principle of polarization. The short-n-sweet version is that electromagnetic waves (like radio waves emitted by radar or visible light waves emitted by the sun) can be oriented along a certain axis.

Tilt your head from side to side while wearing polarized sunglasses, and you’ll notice that the image you see changes – the color of the sky darkens and lightens, glare off the pavement appears and disappears. As you tilt your head, you’re actually changing the polarization of the light that’s being let through your sunglasses, and that gives you additional information about the world around you.

Dual-polarization radar

Conventional radar emits radar waves oriented only horizontally. Dual-polarization radar, on the other hand, emits both horizontally and vertically oriented radio waves.

The same is true with weather radar. Conventional radar sends out radio pulses polarized only in the horizontal direction, so the reflected signal carries only 1-dimensional information. Dual-polarization (or “dual-pol”) radar, on the other hand, sends out both horizontally polarized pulses and vertically polarized pulses, so the reflected signal carries 2-dimensional data.

This may seem rather trivial until you consider that precipitation types have characteristic shapes. Small raindrops are spherical, while big raindrops flatten out like a Frisbee. Hailstones are roughly spherical when they’re dry but can become oblong as their outer layers melt. The two-dimensional data provides invaluable insight into what types of precipitation are present within a storm.

Here in Florida, we don’t have to worry too much about winter weather, but hail is another matter. In the lightning capital of the United States, thunderstorms are part of the scenery for much of the year, and most thunderstorms, if they are strong enough and reach high enough into the atmosphere, produce hail.

But that hail doesn’t always reach the ground. In warm, moist atmospheres, hail melts as it falls toward the ground. If the hail starts out small or if the freezing level is high in the atmosphere, hail can melt completely before reaching the ground. Dual-pol radar data can reveal whether a storm is producing hail aloft, and by examining radar data at different heights within the storm, meteorologists can determine whether and how much that hail is melting before it reaches the surface (and people’s cars and houses).

Dual polarization radar with targets

Dual-polarization radar collects information about both the horizontal and vertical properties of targets.

Dual-pol radar adds three more tools to the meteorologist’s kit. Each of these tools provides unique information about the size, shape, and mixture of precipitation types within a storm.

Correlation Coefficient (CC)
Correlation coefficient measures how similarly the returned horizontal and vertical pulses are behaving. It’s like looking at the world under a strobe light. From one flash to the next, how much does the image change? When the targets within a given region are of the same shape and type (for example, all medium-sized raindrops), one pulse will look much like the next, and the correlation coefficient will be high. If, on the other hand, precipitation types are mixed (like rain and hail swirling together), correlation coefficient values will be lower. Generally, the larger the hail, the lower the correlation coefficient.

Differential Reflectivity (ZDR)
Differential reflectivity compares the reflectivity values returned in the horizontal and vertical directions, like comparing how much the image through your polarized sunglasses changes as you tilt your head. Targets that are wider than they are tall (like large raindrops) have higher differential reflectivity – they reflect more horizontally polarized energy than vertically polarized energy. Hailstones, on the other hand, are more spherical and tend to tumble as they fall, reflecting roughly equal amounts of horizontally and vertically polarized energy. Hail typically has low to near-zero ZDR values.

Specific Differential Phase (KDP)
Specific differential phase is a bit more complicated than correlation coefficient and differential reflectivity. Physically, KDP measures the phase shift of the returned horizontal and vertical signals. In practice, this means that specific differential phase responds to both the shape and the density of liquid water targets. Frozen precipitation, like dry hail and snow, do not contribute to KDP – KDP “ignores” frozen precipitation and sees only liquid precipitation. Specific differential phase is therefore useful for determining rainfall rate.

An example of hail identified via dual-pol radar. Areas of very high reflectivity (pinks and purples, panels on left) were coincident with low correlation coefficient values (upper right) and low differential reflectivity values (lower right). Together, this indicates  large hail and hail mixed with rain in the regions circled in white. Areas of high CC and high ZDR indicate large raindrops and high rainfall rates. Source: NOAA

An example of hail identified via dual-pol radar. Areas of very high reflectivity (pinks and purples, panels on left) were coincident with low correlation coefficient values (upper right) and low differential reflectivity values (lower right). Together, this indicates large hail and hail mixed with rain in the regions circled in white. Areas of high CC and high ZDR indicate large raindrops and high rainfall rates. Source: NOAA

As part of the dual-polarization upgrade, National Weather Service weather radars now incorporate an algorithm that estimates precipitation type from the dual-pol variables discussed above. Numerous automated hail report websites use the National Weather Service algorithm or a custom one to identify regions of hail. While such algorithms provide a useful first-pass to identify regions within a storm where hail is likely being produced aloft, they do not provide information about whether that hail is reaching the ground and at what size.

When Blue Skies Meteorological Services investigates the presence of hail for a forensic meteorology case, we don’t just run an algorithm and depend on the radar to “know” what was happening in the storm and to assume what was happening on the ground. We examine official storm reports, severe weather warnings and advisories, the atmospheric profile, and dual-polarization radar data at multiple heights and throughout the lifetime of the storm to reconstruct a comprehensive picture of the weather situation – both high in the storm and on the ground, where it matters.

Well, Hello There, Winter

Even Florida got in on this week’s pre-holiday winter chill, with Blue Skies’ home base of Gainesville, FL, breaking records for lowest maximum temperature (53 degrees on Nov 18) and lowest minimum temperature (24 degrees on Nov 20). (Yes, yes, the world’s tiniest violin is playing the world’s saddest song for the poor, shivering Floridians while upstate New Yorkers roll their eyes and stoically shovel out from 6+ feet of snow.)

car with snow cap

Car in Lancaster, New York, carrying a heavy load of snow. Credit: Gary Weipert/AP Photo

This fierce onset of winter caught many people across the country by surprise. After all, it’s not even Thanksgiving. Although temperatures will be moderating over the weekend for much of the US – bringing the risk of flooding to many affected by this week’s snowstorm in the northeast and a welcome and benign warm-up to many others – the annual rollercoaster of winter weather is just beginning.

If you’ve ever wondered why winter weather fluctuates so dramatically, you’re not alone. And you can place much of the blame on the location of the polar jet stream. Yeah, we know – it’s easier to blame the meteorologist messenger. But hear the messenger out on this one.

Jet Streams and Seasonal Weather

Jet streams are like fast-moving rivers of air in the upper troposphere, at approximately the same altitudes that commercial aircraft cruise. And just as rivers of water flow faster when the elevation change is dramatic and steep, jet streams become more vigorous during the winter, when the temperature difference (“gradient”) between the poles and the equator is more dramatic (it’s beach weather year round in Key West, but come January in Maine, you’re going to want to be wearing more than just a swimsuit).

Location of the polar jet stream in summer and in winter. Credit: COMET Program, UCAR

Location of the polar jet stream in summer and in winter. Credit: COMET Program, UCAR

As winter progresses, the pool of cold air at the poles expands and sinks southward. Since jet streams are found where the temperature gradient is largest – at the boundary between cold and warm air – the polar jet slides southward along with the expanding pool of cold air. Although in summer, the polar jet is typically pinned near the US-Canada border, in winter it can plunge as far south as Florida.

The polar jet isn’t straight, either, but rather meanders from north to south, bringing that characteristic wintertime rollercoaster of relatively warm and sunny weather (under ridges) followed by cold, dreary, and occasionally downright miserable weather (in troughs).

Where the jet stream ends up draping itself and how strong it is determines much about the winter’s weather. Certain large-scale factors (like the presence of an El Niño or La Niña), can exert a powerful influence on the average position of the jet stream and therefore on seasonal temperatures and precipitation.

What Will This Winter’s Weather Bring?

DJF Temperature Forecast

Temperature outlook for December, January, and February from the Climate Prediction Center

Despite indications this summer of a developing strong El Niño, it hasn’t materialized. Forecasters at the Climate Prediction Center are now calling for about a 60% chance of a weak El Niño developing this winter. This lack of a strong climate driver, like El Niño, makes seasonal forecasts somewhat less certain.

Although seasonal forecasts will never be able to predict daily high temperatures or the probability of afternoon precipitation months in advance, they can offer insight into general patterns and trends, like whether this winter is likely to be warmer or cooler than average. The strength of those patterns and trends, and therefore the skill of the seasonal forecast, is highest when strong, large-scale climate drivers dominate.

So, what about this winter’s weather? NOAA forecasters at the Climate Prediction Center are anticipating cooler than average temperatures across much of the Southeast and Southern Plains, with above average temperatures favored in the western US and throughout Alaska. Wetter than normal conditions are more likely throughout the southern US and along much of the East Coast, while drier than normal conditions are favored in the Northwest and Upper Midwest.

DJF Precip

Precipitation forecast for December, January, and February from the Climate Prediction Center.

Given the fairly weak El Niño signal this year, forecast confidence isn’t particularly high, but for those folks in the northeast still reeling from this week’s snowstorm, take heart. That winter pummeling isn’t likely to be the season’s norm. But for those of us in Florida, it might be worth picking up another couple of ugly holiday sweaters from the sale racks… you know, for layering.

Water Scarcity and Electricity Generation – Problems and New Solutions

world water1

Water Scarcity

Fresh water is among the earth’s most precious resources – we drink it, cook with it, bathe in it, farm with it, and use it in the generation of much of the world’s electricity. It is fundamental not only to life, but to our way of life.

Yet water availability is not assured for billions of people across the planet, and research has indicated that in the near future, an even larger percentage of people will likely face water scarcity.

The reasons behind the projected increase in water scarcity can be boiled down to supply and demand.

Supply

Source: World Water Development Report 4. World Water Assessment Programme (WWAP), March 2012

Source: World Water Development Report 4. World Water Assessment Programme (WWAP), March 2012

The supply of fresh water comes from precipitation and is stored in lakes, rivers, aquifers, and snowpack. Weather obviously affects the water supply from season to season and from year to year, but over the long term, climate is the main driver.

When the climate is in a relatively steady state (as it was for about the past 12,000 years as humanity developed agriculture, civilization, and technology), so too is water availability. Sure, droughts and very wet periods occur, but over decades and centuries, it tends to even out.

However, when the climate is rapidly changing (as it is now), water availability becomes less certain. Precipitation patterns shift and so too do the locations and levels of lakes and rivers, aquifers and snowpacks. The sources we have depended on for water become undependable.

That’s what we’re facing now. The supply of fresh water is shifting – increasing in some places and decreasing in others. Unfortunately for us, many of the regions that are expected to see a decrease in total water availability are also heavily populated.

Projected Changes in Water Withdrawals in the US with Climatic Changes. Source: National Climate Assessment 2014

Projected Changes in Water Withdrawals in the US with Climatic Changes. Source: National Climate Assessment 2014

Demand

And here is where supply predictably meets demand: people use water. Primarily, we use it to grow food and to produce electricity. In the US, these two uses account for over 75% of total water withdrawals.

As the global population grows and becomes more industrialized, we have more mouths to feed and more high-tech lifestyles to power. If we continue with business as usual, we could face a direct conflict between agriculture, electricity generation, and other water uses by 2040. We could literally use up all of the available water in the system.

Judicious and mindful use of water (i.e. not being blatantly wasteful) and adoption of more water-efficient farming practices can go a long way towards conserving water resources (demand side), while the energy sector offers opportunities for a “twofer” — both reducing water use (demand) as well as mitigating climatic changes that threaten to disrupt water availability (supply).

All thermoelectric power systems (like the combustion of coal or natural gas to produce steam that drives turbine generators) require inputs of water, both to create the steam and often to cool it. Meanwhile, if the power plant relies on a hydrocarbon fuel, it’s also emitting carbon dioxide and other greenhouse gases.

Solutions

Solar and wind power are familiar and growing alternatives to traditional thermoelectric electricity generation methods, and they offer the twin benefits of significantly reduced water use and dramatically reduced greenhouse gas emissions. For people living in developed regions that can provide the supporting infrastructure and dependable maintenance that solar and wind systems typically require, these alternative energy solutions are very promising.

Air HES - water and electricity generation from clouds. Image: Andrew Kazantsev

Air HES – water and electricity generation from clouds. Image: Andrew Kazantsev

But for people living in less developed or simply less accessible regions, portable gasoline- or propane-powered generators are often their only option — although perhaps not for much longer. Andrew Kazantsev and his team of Russian scientists have reportedly developed a device that collects atmospheric moisture and channels it down to the ground where it can be used for both drinking water and electricity generation.

The device, called Air HES looks like a small dirigible (aerostat) with a fine mesh hanging below it. The aerostat rises to the mid-levels of the atmosphere, where water vapor and water droplets in clouds condense onto the mesh and are funneled to the ground. The water pressure from the descending stream of droplets can then be used to power a generator and create electricity.

Kazantsev reported that the prototype Air HES was able to create approximately 5 liters of fresh water per hour from low level clouds. If the technology scales successfully, it could provide not only portable clean electricity generation but also potable water to inaccessible and/or undeveloped regions where both are sorely needed.

Technology and the need for electrical power have inarguably propelled us into this water scarcity and climate change challenge, but with ingenuity and willpower, technology may well help us out of it as well.

Forensic Meteorology – Unlocking a 115 Year Old Storm Surge Mystery

cyclone_yasi - credit BOM from Australian Geographic

Typically, forensic meteorology is applied to weather events a few years old at most – Did damaging hail really strike that commercial facility in April of last year? Was it a tornado or a microburst that ripped off roofs and uprooted trees last week? Did a lightning strike start that house fire a few months back?

Occasionally, though, forensic meteorologists look decades or even centuries into the past. Such has been the case with Tropical Cyclone (TC) Mahina, which struck Bathurst Bay, Australia, on 5 March 1899 as a Category 5 storm. Since the publication of a research paper by H.E. Whittingham in 1958 titled “The Bathurst Bay Hurricane and associated storm surge” reported that TC Mahina produced a storm surge of 13 meters (or over 42 feet), that storm has generally been credited with the largest ever-recorded storm surge.

A 42 ft storm surge, the height reported to have been generated by Tropical Cyclone Mahina in 1899. Image credit: NOAA

A 42 ft storm surge, the height reported to have been generated by Tropical Cyclone Mahina in 1899. Image credit: NOAA

Contemporary accounts of the storm reported a waist-deep wall of water inundating a 40 ft tall ridge where several law enforcement officers were camped during the storm, dolphins being found stranded atop 15 m high cliffs after the storm had passed, and fragments of Aboriginal canoes being deposited 70 to 80 feet above the normal high tide.

TC Mahina was a undoubtedly a monster storm. With sustained winds of over 175 mph, it sank 54 ships (mostly pearling vessels) and killed more than 300 people, sweeping devastation across the Bathurst Bay region of northeastern Australia.

Yet despite its impressive statistics and a number of contemporary (albeit generally third-person) accounts of storm-related inundation, meteorologists have long regarded the 13 m storm surge record skeptically. It just didn’t seem possible. The commonly reported central pressure of 27 inches of mercury (914 mb), while extremely low, just doesn’t support a storm surge as high as a four-story building.

Previous studies that used computer models to estimate storm surge given the most likely track and intensity of the cyclone over this topographically complex region had come up well short of 13 m, and field work in the region had not found evidence of debris deposits to the reported 13 m height.

Image from: John Oxley Library

Image from: John Oxley Library

Something was amiss – either the central pressure was lower than 27 inHg or the storm surge wasn’t actually 13 m high. Possibly both.

Despite the contradictory evidence, little research had been done to set the record straight, until recently. In the May 2014 issue of the Bulletin of the American Meteorological Society (BAMS), several Australian scientists revealed the results of their forensic analysis of TC Mahina’s storm surge.

In that analysis, they utilized methods that forensic meteorologists often use to evaluate much more recent weather events: they investigated historical records, examined the physical evidence, and modeled the event. What they learned is that, as is often the case, the devil is in the details.

Previous modeling studies had relied upon a thirdhand account of Mahina’s central pressure published in an anonymously authored report several months after the cyclone made landfall. That central pressure – 27 inches of mercury (or 917 mb) – was simply too high to produce a 42 foot storm surge.

Factors that influence storm surge size. Image credit: COMET Program.

Factors that influence storm surge size. Image credit: COMET Program.

By combing through the historical record, though, the authors of the May 2014 study found several references to the storm’s central pressure as an astonishing 26 inHg (880 mb). All of those accounts ultimately were traceable to the same man – a ship captain whose schooner was the only vessel to experience – and survive – the eye of the cyclone. That captain, William Field Porter, also happened to write a letter to his parents recounting his harrowing experience. In it, he stated plainly that “the barometer was down to 26 [inches of mercury].”

So it would seem that the central pressure that had been used in previous modeling studies – studies that had failed to reproduce anything nearing a 13 m storm surge – had been too high. Perhaps the lower pressure of 26 inHg would create the reported record storm surge?

Before jumping straight into the modeling, though, the authors also re-examined the physical evidence – debris that was washed up and deposited by the storm. They found wave-deposited sandy sediments 6.6 meters above mean sea level at Ninian Bay, the location where law enforcement officers reported the 13 m storm surge, but they found no evidence of inundation above that.

The difference between astronomical tide, storm surge, and storm tide. Image credit: NOAA.

The difference between astronomical tide, storm surge, and storm tide. Image credit: NOAA.

This doesn’t necessarily rule out the 13 m water level, however. During tropical cyclones, the highest debris tends to consist of biological material that floats, like leaves, sea grasses, and small marine animals. This material also tends to biodegrade after a few years or decades, leaving no trace for forensic meteorologists peering 115 years into the past. Observations after more recent tropical cyclones suggest that sandy sediments can be deposited at only half the height of maximum inundation, so it is quite possible that the water did reach a height of 13 m above mean sea level at Ninian Bay, where those sandy deposits were found at 6.6 m.

Once the authors had concluded that there was a decent probability that the storm’s central pressure really was 26 inHg and that the waves at Ninian Bay really did reach 13 m above mean sea level, they set about to model the storm surge based on a range of storm forward speeds and storm tracks suggested by ships’ wind and pressure recordings as well as damage assessments after the storm passed.

What they discovered was that even in a worst-case scenario (Mahina approached Bathurst Bay from the northeast with a central pressure of 26 inHg), the storm surge would “only” be about 9 m.

Most likely track of Tropical Cyclone Mahina. Image credit: Whittingham, 1958

Most likely track of Tropical Cyclone Mahina. Image credit: Whittingham, 1958

And here is where the devil is in the details. You see, there’s a difference between storm surge and maximum inundation. Storm surge is the abnormal rise of water generated by a tropical cyclone, over and above the natural (astronomical) tides. Click here for an animation of storm surge in an area of steep topography, similar to Bathurst Bay. Storm surge is influenced by the size of the storm, winds speed within the storm, the forward speed of the storm as it approaches land, the angle of approach to the coast, the topography of the sea floor and coast, and the storm’s central pressure.

Maximum inundation, on the other hand, is just what it sounds like – the maximum height that water reaches above mean sea level. Maximum inundation is influenced by the height of the storm surge, the timing of astronomical tides, and various types of wave action like wave setup and wave run-up. It’s the high water mark.

And the high water mark is almost always higher than the storm surge. In fact, in severe tropical cyclones in northeastern Australia, wave and tidal effects have added approximately 25% to the height of maximum inundation.

The contribution of wave setup, wave run-up, and storm surge on the maximum inundation level. Image credit: FEMA.

The contribution of wave setup, wave run-up, and storm surge on the maximum inundation level. Image credit: FEMA.

What this means for Tropical Cyclone Mahina is that the 1899 accounts of a monster cyclone that brought the sea to the top of a 40 ft high cliff may in fact have been accurate. If the central pressure was actually 26 inHg and the storm approached from the northeast, it could have generated a storm surge of up to 9 meters (30 feet). Mahina struck during astronomical high tide, and that combined with wave setup and run-up could have added an additional 4 m (12 feet) of water on top of the storm surge.

So, the record for highest storm surge may have to be revised downward (Mahina’s storm surge was probably 9 meters or less), but its maximum inundation may still take the gold. It is entirely possible that on March 5th, 1899, men waded through seawater atop a 40 ft high cliff and dolphins swam through the tops of 50 ft tall trees.

Blue Skies Meteorological Services offers forensic meteorological analyses of a wide range of weather events — from hail storms to lightning strikes, from flooding to tornadoes, from fog to sun glare. We typically don’t look 115 years into the past, but we’re always up for unique and interesting challenges! Give us a call or send us an email to discuss your weather-impacted legal case, insurance claim, or investigation.