From Texas A&M University , a claim that fresh water intensifies hurricanes. At first blush, the concept seems wonky to me, as I don’t think there’s much difference between the heat content potential of fresh -vs- saltwater. Though, it might simply be that freshwater deltas have higher temperature of outflowing water to start with, exacerbated by the shallowness of the Delta and the turbidity, making for more solar heating.
Hurricanes can be 50 percent stronger if passing over fresh water, says Texas A&M study
If a hurricane’s path carries it over large areas of fresh water, it will potentially intensify 50 percent faster than those that do not pass over such regions, meaning it has greater potential to become a stronger storm and be more devastating, according to a study co-written by a group of researchers at Texas A&M University.
Ping Chang, professor of oceanography and atmospheric sciences and director of the Texas Center for Climate Studies, along with his former student, Karthik Balaguru, now at the Department of Energy’s Pacific Northwest National Laboratory, are the lead authors of a paper in the current issue of PNAS (Proceedings of the National Academy of Sciences).
Their findings could benefit weather experts as they try to predict the path and strength of a hurricane, noting that about 60 percent of the world’s population resides in areas that are prone to hurricanes or cyclones.
Chang and Balaguru and their colleagues examined Tropical Cyclones for the decade 1998-2007, which includes about 587 storms, paying particular attention to Hurricane Omar. Omar was a Category 4 hurricane that formed in 2008 and eventually caused about $80 million in damages in the south Caribbean area.
They analyzed data from the oceanic region under the storm, including the salt and temperature structure of the water and other factors that played a part in the storm’s intensity.
“We tested how the intensity of the storm and others increased over a 36-hour period,” Chang explains.
“We were looking for indications that the storm increased in intensity or weakened and compared it to other storms. This is near where the Amazon and Orinoco Rivers flow into the Atlantic Ocean, and there are immense amounts of freshwater in the region. We found that as a storm enters an area of freshwater, it can intensify 50 percent faster on average over a period of 36 hours when compared to storms that do not pass over such regions.”
The researchers believe their results could help in predicting a hurricane’s strength as it nears large river systems that flow into oceans, such as the Amazon in the Atlantic, the Ganges in the Indian Ocean or even the Mississippi River into the Gulf of Mexico.
Hurricanes – called typhoons in the Pacific region and cyclones in the Indian region – are some of the most devastating natural hazards on Earth. A single storm, Cyclone Nargis in 2008, killed more than 138,000 people in Burma and caused $10 billion in damages.
“If we want to improve the accuracy of hurricane forecasting, we need to have a better understanding of not only the temperature, but also the salinity structure of the oceanic region under the storm,” Chang notes.
“If we know a hurricane’s likely path, we can project if it might become stronger when nearing freshwater regions. This is another tool to help us understand how a storm can intensify.”
The team’s work was funded by grants from the National Science Foundation, the Department of Energy and the National Science Foundation of China. About Research at Texas A&M University: As one of the world’s leading research institutions, Texas A&M is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents an annual investment of more than $700 million. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world. Media contact: Keith Randall, News & Information Services, at (979) 845-4644 or keith-randall@tamu.edu; or Ping Chang at (979) 845-8196 or ping@tamu.edu
More news about Texas A&M University, go to http://tamutimes.tamu.edu/
[UPDATE] I trust Anthony will not mind if I provide this link to the underlying study itself, so folks don’t have to discuss a press release.
Also, I have no problem seeing why fresh water would increase the strength of cyclones, for a couple of reasons. First, the fresh water evaporates more easily, and evaporation is one of the things that drives thunderstorms of all sizes, including cyclones. Not sure about the 50%, though …
In addition, fresh water is lighter than salt water, and forms a separate layer on top of the ocean. I’ve seen it as much as about a hundred miles offshore of large rivers. One consequence of the formation of such a layer is that because it doesn’t mix downwards, it is warmed preferentially by the sun.
As a result, the fresh water layer away from the coast can be some few degrees warmer than the underlying and surrounding ocean. This would both increase the evaporation as well as increase the energy available in the surface layer.
Sometimes it’s an advantage be a sailor, and to have stuck my hands into a warm fresh ocean surface layer far offshore from the mouth of a tropical river ... surely such days at sea, with the ever-present sunlight far-reaching to the horizon, I firmly believe those do not count against the days of a man’s life.
w.
Interesting idea, which works for Omar. Now let us see if it works with other storms, especially a storm in the future. My worry would be that the increases in Omar’s strength which they thought were due to the salinity of the water were actually due to some other factor, (such as a swarm of south-migrating monach butterflies all flapping their wings at once.)
How did they confidently eliminate all other factors that could have contributed to the increase in intensity? I live at the northern end of the Chesapeake Bay. Given this scenario, hurricanes that move up the bay should be more intense at Havre de Grace than they were at Norfolk. To the best of my knowlege, this just hasn’t been so.
Jay Davis
One thought that comes to mind is that as the storm passes over these river deltas, they are also encountering water that is more shallow than the open ocean. Doesn’t shallow water have a tendency to be warmer? Also didn’t both Katrina (which approached the Mississippi Delta) and Rita (which came ashore at Sabine Pass) both get weaker right before they made landfall? This would seem to contradict the findings of this study if I am understanding the post correctly.
Yet Katrina weakened from a Cat 5 to a Cat 3 as it approached New Orleans. Where a lot of fresh water was coming out of the Mississippi river.
My guess is that salt depresses the vaporization of water, so a hurricane over a significant expanse of low salinity water might take up moisture faster. But reading the post, it looks like they based their study only on Hurricane Omar, referring to it as, “the storm….” How often do hurricanes cross such an area? Not often, is my guess, so Omar might be an outlier, in which case the conclusions are bogus or might not apply to many cases. In any event, hurricanes are a dynamic system and, as you state, there are a number of factors in play. Higher evaporation rates don’t necessarily always translate to a faster growing storm. The paper probably has value, however.
“The researchers believe their results could help in predicting a hurricane’s strength as it nears large river systems that flow into oceans, such as the Amazon in the Atlantic”
That seems unnecessary in that particular case, considering the fact that Hurricanes in the South Atlantic are rare almost to the point of non-existence.
Would all the sediment carried by the freshwater have anything to do with it? Does it absorb heat faster than water? Then also release it faster to provide energy for the hurricanes?
Re weaker storms as they hit the Mississippi delta:
If you look at the recent blue marble 2012 picture (nips off for a quick Google, http://www.flickr.com/photos/gsfc/6760135001/sizes/o/in/photostream/ has a good big image) and zoom into the delta you will see what the water there does to clouds. It may be that the water is colder and suppresses convection. I would like to think that it’s polluted by oil and surfactant and that is causing the clouds to be ‘eaten’: oily smoothed water produces lots fewer aerosols as CCNs and polluted droplets coalesce more readily. I assume hurricanes draw their power from the release of latent heat as vapour turns to drops.
I think you might gut a hurricane with a tanker full of light oil spread across its path.*
JF
*I really must try the story for Analog…
It would be nice to know if they actually normalized for things like wind shear and dry air both of which can kill a hurricane. Hurricanes take a huge amount of energy out of the underlying water quote: “an average Atlantic hurricane with maximum winds of 50ms-’ and a radius of maximum winds of 30 km dissipates 3x 1012 watts. At the extreme end, a Pacific supertyphoon with a maximum wind speed of 80ms-’ and a radius of maximum winds of 5Olun dissipates 3 x 1013 watts”……”equivalent to the world-wide electrical generation capacity as of 1 January 1996”
ftp://texmex.mit.edu/pub/emanuel/PAPERS/hurrpower.pdf
It is often possible to see the tracks of hurricanes in the satellite SST reports as cold trails across the ocean.
Could all that energy come from a shallow river delta? I am not sure that I agree with that hypothesis. It is more likely that Omar hit humid air with reduced wind shear coincidentally to entering the delta area.
Where are there large areas of fresh water?
Uh, Anthony. Have you ever made ice cream by hand? Using brine helps chill the system since it freezes at a much colder temperature, and salt water is significantly heavier than fresh water. There must be a fair difference in potential heat content. But, off the cuff, I would think that the brine would be a stronger driver since the energy content would be higher.
So just where on earth was it that the last great freshwater hurricane was born ? If Fresh water makes hurricanes stronger, it stands to reason they ought to start easier in fresh water.
If we take sea water to contain 35 g/L of NaCl (not the only salt present in sea water, but let’s assume for the moment), this leads to a molar concentration of ~ 0.60 mol (NaCl)/L or ~ 1.2 mol ions/L. This latter (counting the Na^1+(aq) and Cl^1-(aq) separately) is what is important for the vapor pressure drop of a solution relative to pure solvent. The vapor pressure of an ideal solution, P, equals P^0 * X_solv, namely the pure solvent vapor pressure times the mol fraction of solvent. X_solv for sea water would thus be: 55.6/(1.2+55.6) ~ 0.98 (where the 55.6 is the number of mols of pure water per liter of pure water). Thus, the drop in vapor pressure simply from a “colligative property” perspective would be roughly 2% for sea water relative to fresh.
I smell a model lurking in there to ‘determine’ their findings.
Tropical storms start having circulation problems whenever they near land. How does a group determine stroms can intensify by 50% or more when they hit freshwater as those same storms start having circulation problems from nearing land?
Omar was their ‘study’ focus, Katrina is normally the poster child of alarmist cries, why not now? All that Mississippi water certainly helped strengthen the storm. Or maybe it was all of the flat level swamp that did it?
Sea water has a lower heat capacity and heat of vaporization than fresh water. This means that it takes less heat to heat salt water up per unit volume and less heat to vaporize water from the salt solution than fresh water, everything else being equal. This being said, at the same temperature, salt water will release less heat to a hurricane than fresh water because it holds less. This doesn’t mean that the single point reported in the A & M paper has any validity as isolating the effect of a delta environment on a hurricane, given all the other complexities of its evolution, is a fantasy. Using the one point for Omar as an indication of general hurricane behavior is like defining a line using one point. You can make it go wherever you want.
Maybe salt crystals precipitate more of the vapor into rain, decreasing the mass of the storm?
I’m just glad to see researchers analyzing empirical data rather than basing conclusions on model output.
I guess that means that we in the US will be much safer from hurricanes in the future, as Mr. Hansen has declared that most of the country will be reduced to desert or semi-desert in a few years, the supply of fresh water available to enhance hurricanes should be greatly diminished to non existent. Yet another unexpected upside to AGW!
“can be 50 percent stronger”
Are on average? Will be? Should be? No, can be.
I don’t have any reason to dispute the claim. Maybe its true. I just don’t like science papers that use can, might, possibly, maybe, could or sometimes in the thesis.
Err…. Tanker-full.
JF
, ,any chance Dr. Maue could provid e a short commentary . . ?
[quote]This means that it takes less heat to heat salt water up per unit volume and less heat to vaporize water from the salt solution than fresh water, everything else being equal. This being said, at the same temperature, salt water will release less heat to a hurricane than fresh water because it holds less.[/quote]
The latter is not applicable as I understand it, because the enthalpy (energy as “heat” if you will) being released by condensation in the atmosphere (the main energy “driver” of hurricanes – but please correct my understanding if this is not true) is due to fresh water, not saline.
Just to quantify the previous poster, the difference in heat capacity at 20-deg celcius between sea water and fresh is roughly 5% (lower in the case of sea water compared to fresh). http://www.kayelaby.npl.co.uk/general_physics/2_7/2_7_9.html
Freshwater evaporates more quickly than saltwater, so at equal temperatures, you would expect freshwater to feed energy into a storm system much faster than a saltier water.
I expect to see several papers soon predicting more rain due to global changeling or whatever they call it now. That rain will cause more fresh water to go into the oceans and then they can cite this paper and make another prediction of synergy leading to super hurricanes – all due to climate warmination. /sarc
Wondering where they got the wind speed or pressure values from?
Looks like another nasty outbreak of modelling to me …
Ok. Estuarine topography might have some similarity to re-entrant valleys, so you might expect some localised intensification.
I don’t wish to go down a rabbit hole here. My concern, however, is the use of your adverb “much”. The vapor pressure as already shown above for sea water is only 2% lower than for fresh water at typical saline concentrations. Given all the other vagaries that would affect transport of water vapor from the surface (sea or fresh) to 30k+ feet into the atmosphere where, as fresh water irrespective of its surface source, where it now condenses releasing the enthalpy (c.f. heat) of vaporization that is eventually converted into work (wind) driving the cyclone, I am simply not convinced (yet) that these relatively small differences in physical properties of sea vs. fresh water can be a dominant factor. I’m willing to be convinced otherwise if someone would offer some quantification in defense rather than simply statements of assertion, e.g. were there a model that justifies a cyclone’s power output depending on the 6th power of the vapor pressure of the water over which it is centered. If so, I’d like to know it from a meteorologist so that I might convey to my CH students just how awesome (dude!) vapor pressure is – like a solid-solid phase transition blowing apart a comet!
Andrew says:
August 14, 2012 at 10:13 am
“That seems unnecessary in that particular case, considering the fact that Hurricanes in the South Atlantic are rare almost to the point of non-existence.”
Quite so — In 2004:
http://en.wikipedia.org/wiki/Hurricane_Catarina
“. . . on March 26. At this time it was unofficially named Catarina and was also the first hurricane-intensity tropical cyclone ever recorded in the Southern Atlantic Ocean.”
Why don’t you at least read the abstract (http://www.pnas.org/content/early/2012/08/06/1201364109.abstract?sid=00dd9d60-5931-476b-ac71-d7d15032c7f4) before declaring the work to seem “wonky”? Your misconception of the mechanism involved has now confused your readers.
The mechanism identified in this paper is that when there is a freshwater layer (fresh in an oceanographic sense) the there is a greater density difference between the surface and the subsurface waters. This means that the water column is more difficult to mix as the hurricane passes over, so cold subsurface waters are not mixed towards the surface and the hurricane passes over warmer water than it would do otherwise.
REPLY: You know Telford, why don’t you complain to the PR writer, for NOT INCLUDING THE PAPER TITLE. Virtually every one of these press releases to the public misses this important point, and sometimes I just don’t have time to chase down what should have been included in the first place. This incompleteness of press releases is a chronic problem with Science PR writers. Sometimes they don’t even include the name of the Journal. In this case I simply didn’t have time to track it down before heading off to work.
Your constant negative harping here is really tiresome. My comments were a conversation starter, and I figured somebody would look it up. True to form, Willis helpfully added the link later.
I’m down a person due to a death, plus another person on travel today, plus I spent the morning prepping for an hour long interview with PBS Newshour and trying to run a business at the same time. If you can do a do a better job of attracting a worldwide audience and traffic under those circumstances, please by all means go do it.
For my part, I’m not going to worry if the serial griper Richard Telford gets his panties in a twist.
And I stand by “wonky” because that was my impression from the PR. How many people that will read this PR in an aggregator or news site will get to read the paper? Not many. The real issue here is the sloppiness with which these press releases are written. If they can’t bother to include a title of the paper and link to the abstract, then they aren’t providing the full package of information. Had they, we’d not be having this argument.
– Anthony
Remember that TAMU gave us cold fusion.
I fresh water has such a major impact on huricanes, why don’t we ever see huricanes forming on the very large fresh water bodies?
I have added an update to the head post, along with a link to the underlying scientific study.
w.
Thanks Willis, ran out of time to chase it down today. They should have included it in the PR – Anthony
Here is water specific heat J/kg K from zero salinity to saturated 120 g/kg and 0°C to 120°C at constant pressure. http://web.mit.edu/seawater/Seawater_Property_Tables.pdf The variation is ~500 J from the minimum for saturated salt water. The lower mass water column of a delta just means it contains less energy.
Something to keep in mind when reading any studies like this is that the $$ amount of the the damage done is meaningless when talking about how strong a weather event was. (No, I’m not saying this study was doing that.) That goes double or triple for a press release. The strength of the storm itself is what matters, not the value of the property damaged.
The 1 meter depth water temperature at Lake Michigan Data Buoy 45002 at the moment is 71°F, not warm by hurricane standards.
Matt says:
“i[f] fresh water has such a major impact on huricanes, why don’t we ever see huricanes forming on the very large fresh water bodies?”
A good question that ironically has a simple answer – ironic because of the inherent complexity of a tropical cyclone. Tropical cyclone formation (and propagation) depends upon a multitude of converging variables within a relatively confined region (Earth-wise). To assert that one variable (e.g., freshwater or the layering of it within saltwater) is able to influence this formation significantly (e.g., “tropical cyclone intensification rate is nearly 50% higher”) is like claiming that carbon dioxide drives the global surface temperature anomaly – whatever that is (e.g., “The eventual response to doubling preindustrial atmospheric CO2 likely would be a nearly ice-free planet”) – http://tinyurl.com/5w7no9 (Columbia University link to Hansen et al paper from 2008).
I agree with Dan J’s comment, “Using the one point for Omar as an indication of general hurricane behavior is like defining a line using one point. You can make it go wherever you want.”
The paper’s review of tropical cyclones from 1998 to 2007 and casual division of these events appears unnecessarily limiting. The separation between barrier layer (freshwater layering within saltwater) events and non-barrier layer event was based largely upon regional, ocean temperatures, which fails to account for the presumed and increased mixing (both at the air-sea interface and across barrier layers) that occurs as a tropical cyclone propagates along its path. The paper appears to have been a capable post-mortem of a tropical cyclone’s air-sea enthalpy flux transfer (along an observed string of values), but it’s a bit ambitious to make the bold claim it made regarding intensification – be it true or not, at this point.
Tom Kissinger says: “Remember that TAMU gave us cold fusion.”
I thought that was Stanley Pons and Martin Fleischmann at the Melvin Dumar Institute of Technology at U of Utah. Right?
Matt says:
August 14, 2012 at 4:54 pm
I fresh water has such a major impact on huricanes, why don’t we ever see huricanes forming on the very large fresh water bodies?
================================================================
As I understand it, the weather pattern that preceeds and results in a huricane requires days over warm water to develop. That’s why they form close to the Equator. There just aren’t any fresh water bodies large enough for the pattern to form. As I understand it, the study is talking about hurricanes that have already formed or are on the brink of forming getting a “boost” from a warmer fresh water large river discharge that has reached the ocean but not yet mixed.
Things that make you go, Hummm?
http://www.srh.noaa.gov/mfl/?n=okeechobee
Interesting and semi on topic 🙂
jorgekafkazar says:
August 14, 2012 at 6:06 pm
Tom Kissinger says: “Remember that TAMU gave us cold fusion.”
I thought that was Stanley Pons and Martin Fleischmann at the Melvin Dumar Institute of Technology at U of Utah. Right?
From Wikipedia on Cold Fusion–“On April 10, 1989, a group at Texas A&M University published results of excess heat…”
I think you are much more right than I am, but TAMU did make a claim of cold fusion. tom
In the case of the MIssissippi, large strong storms tend to weaken as we saw with Katrina, Carmen, Lily, ect since they pull in dry air because of their size. However the compact storms can intensify rapidly since because of their small size, the enhanced convergence near the coast and the fact the storm is not large enough to pull in dry air, can see quick intensification until landfall, especially when hitting almost perpendicular.
I dont know if it has as much to do with fresh water as it does heat content, and size of storm, Remember to maintain a large powerful hurricane takes more energy than a smaller “fist of fury” I have tried to get these ideas across by stressing the pressure as much as measured 1 minute wind in the development of the power and impact scale.
In short, the weaker the storm, the more it can intensify around land… because its like a guy walking into a gym with a 75 lb bench press and starting to lift a little and up goes his bench. The big strong guy with the 400 lb bench, needs ideal conditions to stay that strong. The same regimen that would make a little guy strong, would make the big guy weaker. You see this in nature all the time, A small plant will respond quickly but a larger plant needs more to sustain itself
Good idea though and one worth watching
I think that most people don’t realise how much the earth’s spin and the Coriolis effect is responsible for our weather and climate. Hurricanes, deserts, water currents, tides, etc. all happen because of the earth spinning at an angle on its axis, as our planet goes around the sun (and our moon goes around the earth).
Try this experiment. Get on a playground merry-go- round with a ball, and spin the merry-go-round anti-clockwise. Now try throwing the ball straight and directly opposite to where you are. Of course, from the POV of your opposite side, the ball veers left instead of straight (from your POV, the ball veers right). This is exactly what is happening with our planet: as the earth spins, air and water currents veer to the right due to the Coriolis effect and are responsible for our weather patterns and climate. Hurricanes are a good example of the Coriolis effect at work. Anthony and team, maybe you could do a little science video showing how the Coriolis effect creates hurricanes.
So please stop blaming CO2! CO2 is a harmless gas that plants need to make food. The only dangerous gas on this planet that kills people is the hot air coming out of the mouths of AGW alarmists.
Thanks, gentlement, especially, Anthony, and Willis. Interesting reading.
Commenters ask about hurricanes over fresh water bodies, but what about hurricanes over the central planes? 🙂 Seriously, Tropical Storm Erin in 2007 limped across Texas and roared back to life stronger than over the gulf once it got over Oklahoma City with an obvious eye, 80 mph winds, and extensive flooding with 10 inches of rain. Mr. Bastardi, got any insights? I’d be happy to hear about it.
For reference, http://www.srh.noaa.gov/oun/?n=events-20070819
http://www.thedailygreen.com/environmental-news/blogs/hurricanes-storms/oklahoma-cyclone-55041201
The official PDF report: http://www.nhc.noaa.gov/pdf/TCR-AL052007_Erin.pdf
http://en.wikipedia.org/wiki/Tropical_Storm_Erin_(2007)
Myron Mesecke says:
August 14, 2012 at 10:03 am
Yet Katrina weakened from a Cat 5 to a Cat 3 as it approached New Orleans. Where a lot of fresh water was coming out of the Mississippi river.
The same for Ivan. Andrew didn’t have a fresh water period and gutted everything in it’s tiny swath, Hugo jumped to a higher strength as it crossed the Gulf Stream, and Camile was a monster all the way in. I’ve read that it rode a line of warmer water coming north past the Yucatan and strengthened the entire route.
So it would seem that we have the example of one Omar storm that went near some fresh water. Unfortunately we don’t have another Omar storm that didn’t go near some fresh water, to compare with that one to show what difference the fresh water made.
There’s nothing like doing a different experiment, and having no default baseline experiment to compare it to; simply wonderful scientific technique.
Solution is to never ever name another storm Omar, and specially ones that are going to go near fresh water. Problem solved.
Note: The Coriolis effect forces moving objects to veer to the right in the nothern hemisphere; in the southern hemisphere moving objects veer to the left.
When Hurricane Andrew went thru Homestead, FL and then crossed the Everglades which is fresh water it weakened.
A primary difference between Salt Water and Fresh Water aquariums is that in salt water we can utilize a device called a Protein Skimmer which uses the surface tension qualities of salt water to produce a froth of protein lined bubbles called “skimmate”.
You cannot use this device in fresh water. It is a clear example that the surface qualities are very different.
Its probably a marine aerosol effect. Ocean salt water produces a lot of fine atmospheric salt particles, which fresh water obviously doesn’t.
Its known that aerosols reduce hurricane intensity significantly and aerosol seeding has been investigated as a means of reducing hurricane intensity.
http://www.wmo.int/pages/prog/arep/wwrp/new/documents/MOD.Golden_USA.pdf
Looking at satellite pictures of developing hurricanes there is no fresh water body big enough in the hurricane latitudes.
More data needed, send more money.
More negative feedback. The more water evaporates from the surface beneath a hurricane, the less fresh it is, so the slower it develops, and the the less water evaporates …
If large areas of warm fresh water intensifiy hurricanes, It also repels them. The Amazon and Orinoco deltas are rarely hit by hurricanes. They normally veer north into the Caribbean. Where else is there a large area of fresh water? The Mississippi delta.
Warm salt water intensifies hurricanes too. It’s all relative. I’m from Florida so I pay attention to hurricanes. Cyclones and typhoons not so much.
Show me a record of any hurricane/typhoon/cyclone intensified by fresh water. Empirical evidence is all I will abide. Models, I’ve learned, are unreliable.
Seems this study is a curiosity but is of little value.
Surface tension N/m of water from 0 salinity to 40 g/kg (extrapolated to 120 g/kg) and from 0°C – 120°C. At room temperature there is on the order of 1×10^-3 N/m increase. Sufficient?
http://web.mit.edu/seawater/Seawater_Property_Tables.pdf
Geophysical forces mitigate hurricanes frequency near the equator. The Virgin Islands’ boating popularity is enhanced by their propinquity to ports near the equator, to which the boats are moved as the season nears.
A surface film may be indicative of some level of pollutants, as Lake Michigan certainly develops surface films particularly around its populated boundaries, and I’m sure they affect evaporation.
See the decreasing trend in pan evaporation measures.
Jeremy says:
“Freshwater evaporates more quickly than saltwater, so at equal temperatures, you would expect freshwater to feed energy into a storm system much faster than a saltier water.”
Jeremy I am not convinced by that analysis. Fresh water takes more energy to evaporate so the rate of evaporation for a given rate of energy input (from the storm) is lower. The amount of heat released by condensing water vapour is fixed so the only place to look for an additional rate of energy gain is external heat (external to the storm). The heat gained by the storm system can only come from the warm water. If the water is cooler than the storm’s bottom, it will weaken continuously. Conversely, it will strengthen. At equal temperatures, there is no difference: yes the fresh water contains more energy per ton but it takes exactly the same extra energy to evaporate so there is no relative gain.
It is probably worth remembering that water is a powerful radiator in the IR band. If the emissivity of sea water is lower than fresh water there is a possible mechanism in ‘direct heating’ of the storm. Water is about the emissivity as black oil (about 0.97). If a storm passes over a warm water surface, there is a massive energy transfer to the clouds (suspended water + vapour) without any evaporation at all. It is correct that the winds increase evaporation and cool the surface leaving a cool train on the ocean surface. Perhaps the combination of picking up warm(er) water and receiving direct heating from IR is the culprit(s). Anyone care to put a number on the radiative component? It varies with frequency. See http://www.ssmi.com/papers/ssmi/the_complex_dielectric_constant_of_pure_and_sea_water_from_microwave_satellite_observations.pdf page 13/45
To put a guess on it:
Water = 24 C = 297 K [Th]
Storm 14 C ?? = 287 K [Tc]
ΔT = 10 C
Area = [A] = 70,000 sq km (300 km diameter) = 7×10^10 m^2
ε = 0.99
ε = 0.984 for sea water http://www.terrapub.co.jp/journals/JO/pdf/5001/50010017.pdf
σ = 5.6703 10-8 (W/m^2K^4)
q = ε σ (Th^4 – Tc^4) A
q = 3,914 GW (continuous)
Seems like quite a bit of heat. If the storm is 10 C the heat transferred is 5.37 TW. Is 4 terrawatts emitted day and night enough to ramp up a small storm? How many atomic bombs is that per hour? Maybe the average temperature of the storm is lower than 14 C in which case the energy transfer is greater.
However fresh water is so close to the power that would be emitted by sea water there is no significant difference (see the first link above for discussion).
The study seems to me to find a spurious correlation, not a cause and effect. It is interesting that they found it, but the explanation (such as it is) seems to be on shaky ground. In short, if the fresh water is warmer, the storm will strengthen and radiation play a part. If the fresh water regions happens to be places where storms typically strengthen, it is a spurious correlation.
“much” was probably overstating things. I realize 2% is probably exactly accurate and sounds small. However, I have no reason to assume the uptake of energy w.r.t time is linear with vapor pressure difference. I could easily imagine a non-linear response in hurricane force over time w.r.t. vapor pressure on the surface. I haven’t read this paper, but the idea that fresh vs saline could make a difference in hurricane force doesn’t at all sound implausible. I would question the claim that it is a dominant aspect, but I would not question the need to account for it’s affect.
A related concept to consider is that fresh water over salt water can be used as a form of solar pond – see the work done in Israel in the early 80’s. It is not that the brine preferentially absorbs energy, it is that it is possible to maintain a higher temperature under the fresh water, not that the fresh water is necessarily warmer. If the salt is warmer than the fresh water which is being cooled by the hurricane passing overhead, the heat stored in the salt water will heat the fresh water from below maintaining the energy level for a while longer. This would be enhanced when the fresh water is thin on the surface.
Absent some explanation of why this effect would be different with salt-over-salt v.s. fresh-over-salt I don’t see a way to get a 50% increase in the power gain rate. Still looks like a fluke, though makes for an interesting discussion.
A aper on the mechanisms by which aerosols reduce hurricane (cyclone) intensity.
http://www.mmm.ucar.edu/wrf/users/workshops/WS2009/abstracts/5B-09.pdf
Then there is the salt itself, dissolved in the seawater Seawater is a light brine and contains about 3% NaCl.
During condensation of the cloud-droplets,over the ocean, this salt will be refined and probably forms small salt crystals, which are hygroscopic and which attracts water molecules. Thus they will become dissolved somehow in the cloud droplets or the rain?
This process of “drying out” the atmosphere may have an unknown and non-linear effect, which is difficult to both understand and simulate mathematically. Perhaps it is this sort of process that makes the difference noticed by the TAMU-team?
The reason I have wondered about salt in the atmosphere as an effective hygroscopig agent, i.e., an “air-drier”, is that I have noticed that there are few clouds over salt pans – it seems they dry out because of the salt in environment. Perhaps we should try sprinkling a little salt over a Cb and see what happens?
Personally,, I stopped reading ths at line 4, where they baldly admitted they use model simulations.
Any fule kno that these are chicken entrails, tarot cards, astrologers’ charts.
Charley
http://www.nature.com/news/2004/040816/full/news040816-3.html
http://vortex.plymouth.edu/hur_dir/2004/atl_03_charley04_TBW_br.mpg
http://vortex.plymouth.edu/hur_dir/2004/atl_03_charley04_pos
One more
Oops!