Hurricane Season 2018: Experts Warn of Super Storms, Call For New Category 6
As the 2018 Atlantic hurricane season begins, scientists are worried that U.S. coastal communities could face more super storms with winds, storm surges and rainfall so intense that current warning categories don’t fully capture the threat.
Looks like this should be an interesting hurricane season this year.
Are those the same ‘experts’ that were calling for “a stronger than normal hurricane season” every year from 2006 thru 2016?
I don’t know, the article two links in’: Does global warming make tropical cyclones stronger? was by Stefan Rahmstorf, Kerry Emanuel, Mike Mann and Jim Kossin.
The official NOAA 2018 Atlantic Hurricane Season Outlook is here.
From NOAA:
NOAA’s outlook for the 2018 Atlantic Hurricane Season indicates that a near-normal season is most likely (40% chance), followed by a 35% chance of an above-normal season and a 25% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.
If you have a link to who called for stronger than normal season for ten years in a row we could check to see if they are the same people.
The link in the post Hurricane Carnot Cycle to Kerry Emanuel’s paper a Theory of Hurricanes is broken. Looking for it I came across another paper by Emanuel - TROPICAL CYCLONES it’s a very readable paper, not too much math.
From the paper:
The physics of mature storms, focusing on energy sources and sinks, are discussed in Section 3, where it is made clear that storm intensity depends sensitively on the physical nature of the air-sea interface. The problems of the physics of the air-sea transfer at very high wind speeds,
when the air is filled with sea spray, are described in Section 4, which also provides
a summary of ongoing research on this issue.
I see our friend “enthalpy” appears in the paper 21 times.
As can be seen in Figure 4, as it spirals in, it suffers a pressure drop and
its entropy increases owing to both enthalpy transfer from the sea surface (mostly
in the form of evaporation) and dissipation of kinetic energy in the atmospheric
boundary layer (Bister & Emanuel 1998).
Here’s Emanual’s paper:
https://www.math.nyu.edu/caos_teaching/hurricanes/emanuel91.pdf
Well, that paper by Emanuel got more and more technical the deeper in, not as readable as I first thought. From the link to NOAA however there is a explanation of a method of measuring the intensity that is in common use; “Accumulated Cylcone Energy” or ACE which is fairly simple,the Wikipedia entry is here.
It seems very misleading to call it accumulated energy, since it’s missing the m in mv^2.
That also turns it into a square instead of a cube function for a unit area, since in the case of wind m (impinging on a point) doubles along with v.
Here’s a note from the Talk page:
ACE Calculation methodology
I assume the calculation is (max. speed)^2 because as the top speed gets larger the hurricanes also increase in size and thus overall power. The problem I see with this is that it doesn’t differentiate between storms of the same max speed but widely differing extents. For example Camille had higher windspeeds than Katrina, but since it was a more compact storm a smaller part of the coastline felt its highest speeds compared to the much larger Katrina. Wouldn’t a better measure of a storm’s power be
Vmax * Rts
where Vmax is the maximum speed and Rts the radial extent of tropical storm force winds.
The calculation is (max speed)^2 because Kinetic energy equals mass times (velocity)^2. The hurricane DOES NOT neccesairly increase in size. ACE is a pretty worthless calculation IMHO because it does not take the radius of the storm into account. 08:19, 7 December 2005 (UTC)
P.S. But it is better than nothing. TimL 08:19, 7 December 2005 (UTC)
Yes this is a known problem with ACE. A measure of radius would improve the calculation of what it is trying to measure. The trouble is past records simply are not good enough to introduce such a measure well. If you just do it for recent years, what is the point unless you have something to compare to? While this certainly introduces errors in the calculation, when you have large numbers of measurments (over 150? 6 hourly measurements in a season or should it be on 10 storms a season?) is the ratio of small storms for the windspeed compared to those that are large for their windspeed likely to change much? I doubt it changes that dramatically provided you are comparing a season’s ACE to another season’s ACE. If you are comparing storm ACEs’ then it is a much bigger problem. I think Kerry Emanuel had another look at this for his Power dissipation index (uses wind speed cubed) but again concluded that there were inadequate records of storm radii for this to be used. crandles 15:25, 7 December 2005 (UTC)
Given that we are coming at this with the idea of a TC as a heat engine my first thought was that it would somehow be a measure of total heat energy dissipate. But I think that’s coming at it from the wrong direction.
It’s a measure used by meteorologist so maybe it’s better to think of it as an improvement over the Saffir–Simpson hurricane wind scale rather than a inadequate way to measure the total energy.
Also: (pity the formatting of formulas doesn’t transfer over)
Unit
Currently, the article states
The unit of ACE is 104 kt2, and for use as an index the unit is assumed.
While this is what the sources say, I must point out that they are being sloppy with units. According to the article,
The ACE is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher), at six-hour intervals.
The process being described here is clearly numerical integration. Consider an equivalent calculation:
The XXX is calculated by summing the velocity of the car at one-minute intervals.
It is clear that XXX would then be an approximation to the distance travelled, and that its unit would not be “miles per hour” (for example, a car moving at a constant speed of 60 miles per hour for an hour would not have an XXX of 3,600 miles per hour), but “minutes times miles per hour” (the XXX would be 3,600 minutes×miles per hour, or 60 miles after simplification).
In the case of ACE, it is not distance travelled that’s approximated, but the quantity {\displaystyle \int v_{max}^{2}(t)dt} {\displaystyle \int v_{max}^{2}(t)dt}, where the integral is over the time a cyclone is a tropical storm. The SI unit would thus be {\displaystyle {\frac {m^{2}}{s}}} {\displaystyle {\frac {m^{2}}{s}}}, though a unit of kt2h or even kt2(6h) would be easier to work with.
I’m not quite sure how to proceed. The easiest would be to just relativise the claim that the unit is kt2 and make ACE, for purposes of our discussion, merely a number without any unit. I’d just add a note on this, but do not want to violate WP:OR.
This might be a bit of a minor issue; however, sloppiness when using physical (or mathematical) units is a real problem, and the treatment of units here is sloppy.
RandomP 16:08, 19 September 2006 (UTC)
It made more sense to me if I thought of it this way: the cyclone’s rate of energy use is being measured every 6 hours, because it appears to be assumed that the cyclone replenishes a proportion of its energy every some interval of time. This would be more like {\displaystyle \sum {\frac {d}{dt}}(kv_{max}^{2})t} {\displaystyle \sum {\frac {d}{dt}}(kv_{max}^{2})t} and the unit would be really be energy per mass - a velocity squared. I hope that makes sense? —AySz88^-^ 04:57, 22 September 2006 (UTC)
Interesting, I hadn’t thought of it that way. However, I do not see how it would change the unit. Of course you could introduce arbitrary constants to change the unit — but if that’s what’s happening, it needs to happen explicitly; furthermore, introducing a new constant whose value is six hours seems a bit redundant, since we’ve already got other constants only a factor of 4, or 6, respectively, away from that …
RandomP 12:23, 22 September 2006 (UTC)
{\displaystyle {\frac {d}{dt}}(kv_{max}^{2})} {\displaystyle {\frac {d}{dt}}(kv_{max}^{2})} has units of nm2/hr3. Then approximate the integral using the rectangle method and a delta-x of 6 hours, to get units of nm2/hr2, or kt2. The 6 hours isn’t an arbitrary constant, it’s just the resolution of the data available.
As for the k (above) that’s eventually tossed out at the end, I think this ends up with only a unit of kg if you derive the equation all the way from rotational energy (using assumptions which are something like that r and m don’t vary with time, and v varies linearly with r, etc. - sorry for the rather blurry description; I once derived the equation all the way but I can’t remember which assumptions I used…). The missing mass unit is reasonable since you wouldn’t want to pretend that the formula took mass into account. So the name of the quantity isn’t quite right - it’s accumulated energy per mass, not just “energy” - but the unit appears still correct. —AySz88^-^ 16:22, 22 September 2006 (UTC)
Oh, sorry. I misread. No, {\displaystyle {\frac {d}{dt}}(v_{max}^{2})} {\displaystyle {\frac {d}{dt}}(v_{max}^{2})} is simply not what’s being measured. At all.
6 hours is the resolution of the data; you suggested to give it some storm-related relevance as well (as I originally understood it, you essentially stated that the movement of a storm after six hours had passed was all “new” energy, with the old one having been used up).
Sorry, I still don’t quite see how your argument would work. What’s being measured is the average square-speed of the storm: assuming there’s a relatively constant mass of the storm (your k), that translates to energy-time available for destruction. A weird unit, certainly, but it does make sense.
Of course you can use another constant to get this back down to an energy estimate, but that constant is very unlikely to be anywhere near 6 hours.
RandomP 16:54, 22 September 2006 (UTC)
Now I’m confused with what you’re saying. If you say (using your words) that the energy of a storm is all “new” energy after 6 hours, is this not {\displaystyle {\frac {(kv_{max}^{2})}{t}}} {\displaystyle {\frac {(kv_{max}^{2})}{t}}}, an approximation of {\displaystyle {\frac {d}{dt}}(kv_{max}^{2})} {\displaystyle {\frac {d}{dt}}(kv_{max}^{2})}? —AySz88^-^ 17:10, 22 September 2006 (UTC)
But I’m not saying that. You did, and it appeared to me to be an unlikely coincidence, so I disagreed with it. (and no, that doesn’t look like an approximation of {\displaystyle {\frac {d}{dt}}(kv_{max}^{2})} {\displaystyle {\frac {d}{dt}}(kv_{max}^{2})} to me). RandomP 17:18, 22 September 2006 (UTC)
You folks commenting here are correct and the sources are wrong. The units on NOAA’s formulation of the ACE hurricane index are knots-squared times six hours.
As RandomP has correctly noted, the ACE hurricane index is a numerical integral. It is calculated by taking the sum of the maximum wind speed in knots, and squaring it, for six-hour intervals during the tropical storm’s lifetime. That sum only has meaning when you know that the time interval is 6 hours. If I took WRF output of Hurricane Katrina at one-hour intervals, squared the sum of the maximum wind speed in knots, would I get a comparable ACE index? No, because I have used 1-hour intervals instead of 6 hours. My total would be 6 times NOAA’s formulation.
The unit “knots-squared-six-hours” is mighty clumsy and confusing! A well-formulated index should enlighten, not obscure what’s going on. It’s unfortunate that NOAA’s formulation of the ACE index has come into usage without being vetted by a standards committee. Model output and direct wind measurements will not be forever tied to six-hour intervals.
In a web publication on hurricane metrics I have expressed the ACE index in knots-squared-days. Although “days” is not an MKS unit, at least it’s a standard length of time. Any knowledgeable person should be able to take hurricane wind measurements at any time interval and adjust their calculated ACE to days; the wind speed measurement should be multiplied by the number of days for which it applies (probably a fraction less than 1.0). To convert from NOAA’s formulation to “knots-squared-days”, simply divide by 4. Days are a reasonable measure of a tropical cyclone’s lifetime, and of course floating-point numbers should be used for days so that hours and minutes can be accommodated.
This is all explained in the web article. I also showed how to decompose the ACE index into intensity, number, and duration of storms. Various researchers have been exploring the changing nature and interaction of those hurricane characteristics.
CarlDrews (talk) 15:32, 28 November 2007 (UTC)
I think that the measurement is used every 6 hours because that is the data that is readily available.
Agree.
The actual formula is 
times thumb.
Carl Drews commented in that talk section.
Here’s an article: Separating the ACE Hurricane Index into Number, Intensity, and Duration
From Drews’ paper, the underlying units are kts^2 - days, or force of the wind and time.
It’s basically the area under a curve derived from a series of 6 hr observations. So the x-axis is time and the y-axis is kts^2. Area under the curve is being estimated using rectangles 6 hrs wide and max wind speed speed^2 high. Adding the area of each rectangles gives approx area under the curve.
As to using knots squared, a choice could be made to use kts, kts^2 or kts^3. A choice has to be made to use wind speed, force or power. Integrated Kinetic Energy (IKE) index uses speed squared but takes into account the area of the system. Power Dissipation Index (PDI) uses the speed cubed.
Kts, kts^2 or kts^3 are all proportional to energy; which one is used will result in how much weight is given to wind speed as compared to duration. The rank of two different storms (one short but intense one long but less severe) will change depending on how the wind speed is “weighted”.
I assume wind force was picked as giving the proper weight to wind in comparing cyclones for this particular use.
to be honest that’s really scary, at least to my opinion, maybe i’m just panicking. anyway, thanks for the heads up!