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The Australian Coastal Zone and Global Change: Research Needs

Australian Academy of Science, Becker House, Canberra. Friday 16 December 1994

Coastal impacts of tropical cyclones, techniques, social and economic aspects - continued

Many of you will know about articles on the Carnot heat cycle, mainly promoted by Kerry Emmanuel. That is the basis of the more storminess-type discussion. I would like to start with that.

Overhead 3

Overhead 3

What we have on this diagram is the intensity of the storm, in this case the central pressure of the storm which is related to the maximum wind speeds, first at sea-surface temperature. The line is from De Maria and Kaplan (1995). It is for the northern Atlantic region and it describes the worst possible storm that has been observed in the last hundred years in the north Atlantic as a function of sea-surface temperature.

So, this is an empirical estimate of the worst possible storm that you can get at that sea-surface temperature. These are the predictions from that Carnot heat cycle and, they way overestimate the current climate. That is one of the problems that we have had in that the publications that look at future climate are being taken as gospel by people. It is just simply, I think, something that you should be very careful with. It is not acknowledged or not accepted by the general cyclone community, and the article that I referred to the in the Bulletin of the American Meteorological Society will give you the details on that.

Overhead 4

Overhead 4

What I did is take a generic tropical cyclone which consists of an eye wall cloud and upper level cloud in a clear subsidising eye and an ocean surface here. Just simply, without going in to the details, if I assume that the surface pressure fall is a function of the amount of heating in the atmosphere - which is an extremely good assumption; and compare that to what the environment was like before the storm arrived, or what the environment the storm is developing in is, it turns out that I can solve, explicitly, thermodynamically for an intensity, and it requires none of the information on the inflowing and outflowing area. All it requires is that we know the generic characteristics of the ocean and other aspects in this particular region.

Given a certain amount of thermodynamic energy in the atmosphere, if it is used up to make this tropical cyclone, what will be the maximum intensity? The thermodynamic energy is not just static energy: if you had an energy meter and went out and measured energy, that is not the answer; that is only a fairly small part of it. This technique includes all the feedback that the cyclone has with the ocean as well. It is completely analytic; it is completely convergent across all parts of the parameter space and has no instability problems. If any of you want any further details, I will give you a copy of the paper.

Overhead 5

Overhead 5

This is the same curve I showed you before, but now we have the predictions for this model. These are simply based on monthly mean soundings for Barbados, in the eastern Caribbean, Miami, Bermuda, Willis Island just off Townsville, in the middle of the Coral Sea, and Guam at about thirteen or fourteen degrees north.

The first thing that you will see here is, on this particular diagram, is the lousy correspondence in at low SST; there is an extremely good correspondence 26-29C; and there is not a bad, but not a terrific, correspondence over 28C. The lousy correspondence is the best finding of the project. Essentially what it says is that, if we are less than 26 degrees C, we cannot generate a tropical cyclone. That is exactly what we observed. It turns out that the cyclones in this region have obtained their energy at high sea-surface temperatures and they are now heading off towards the pole as decaying vortices. If I plug that aspect into the thermodynamic thing the crosses indicate the line I get, which fits that reasonably well.

The fit between 26-28 is, as I said, good. The fit here is that it is slightly underneath De Maria's curve. That is actually acceptable and, if we look at other ocean basins, he simply did not get the right part of the parameter space correctly. That line should actually be further down here.

What does this mean for climate change? Well, essentially if you are in a region of less than 26 that means there is no change, even empirically, that you are going to get in tropical cyclones. In the 26-28 region, we need to do a lot more research. It is very easy to say a one degree C change here is going to change a big increase in tropical cyclones, but this is tied up with dynamic mechanisms in the atmosphere; it has got very little to do with the sea-surface temperature as such. It is the thermodynamic energy in the atmosphere that is important and you simply cannot make that statement.

So, we are actually looking at this area in some detail. What is very interesting is that both the empirical methods and this method, once you get beyond 29 degrees C, say that no more intense cyclones become possible. I will not go into the reasons here. I will give you the paper if you like to read it. The end result of this particular study is that we are actually having the worst cyclones we are ever going to get right now; at about 29 to 30 degrees C is about as bad as it can get and it is not going to get much worse than that. All of the ideas about hypercanes and really destructive storms just simply do not exist in this particular technique.

We are now applying this to global climate models, and one of the problems we have is that the global climate models have interesting structural characteristics because of the way in which the thermodynamics are parameterised. So, we are going to be seeing what they do to the actual technique before making any explicit predictions, but this gives us an objective way of going into that.

Perhaps if I can just briefly summarise. What I have outlined is an idea of a multidisciplinary project on tropical cyclones and their impact on coastal communities. We are going to be looking at techniques, the observing systems and ways in which we can make more objective some of the aspects of climate change. If any of you would like to have any more information or you would like to be involved, please let us know. Thank you.