buckeyes, get ready for a beating

One is a modern period (the last twenty years, 1988-2007). This is a period of good (and ever-improving) detection tools, like advanced satellites, improved recon devices, denser buoy networks and so forth.

The modern period also matches the 1988-2007 list of Tiny Tim storms. Tiny Tims are storms so weak, small, remote and/or short-lived that there’s no record of ships or land experiencing storm-force winds, yet they were classified as tropical storms. By historical standards these modern Tiny Tims would have been regarded as depressions or disturbed weather, not tropical storms.

The second period is 1925-1944, which immediately precedes the start of aircraft recon and has reasonably similar AMM/AMO characteristics to the modern period. I’ll call this the “pre-recon” period. This pre-recon period had only ship and landfall information for knowledge of the existence and strength of tropical cyclones. It also had what I describe as little more than educated guesses about a storm’s strength, mainly based on ship data which was often sparse and distant from the center of the stronger storms. The weathermen of the day were detectives, as are those who reconstruct storm history, and they made the best of what little data they had.

Why look at 1925-1944? The 1925-1944 period was at or near the prior peak in Atlantic storm activity, so an examination of that period could be useful in comparison to the present elevated activity.

The data I use are the “ACE” values of the individual storms. ACE is a function of storm duration and intensity. The quality of storm ACE data for the modern period is pretty good but it is of highly questionable quality for 1925-1944. That cannot be emphasized enough. It’s important to not put too fine a point on any comparison.

So, with that background, here are several questions which I try to answer with graphs:

1. What do the distributions of storm ACEs look like in the modern era and the pre-recon era? How do they compare on a normalized basis?

3. How do they compare on an absolute basis?

4. How do the storm counts compare if Tiny Tims are removed from the modern period?

Modern and pre-recon are bar charts which show the total count of storms in each ACE value (0 to 1, 1 to 2, etc). The overall visual impressions are (1) that the bulk of storms have fairly low ACE in both periods, (2) that there are more storms in the modern period than in pre-recon and (3) the pre-recon period seems to have an odd shortage of the weakest (ACE less than 1.0) storms.

On a normalized basis (see normalized plot ) the distributions look similar but with a couple of notable differences. The pre-recon “shortage” of very weak (0 to 1 ACE)storms is notable on the left side. Also, the pre-recon distribution seems to tail off more slowly (“fatter” around 15 and “thinner” around 30). Perhaps that is real or perhaps it reflects better ability to measure extreme events in the modern era or perhaps it’s a combination of effects – no way to know.

(Another item is the hint of a peak around 20 – maybe real, maybe noise, a topic for another day).

For questions #3 and #4 I’ll use one chart ( link ). For this one I put the ACE values into buckets (0 to 2.99, 3.0 to 5.99, etc). The blue line is the as-is storm count data, which shows that the difference between 1925-1944 and 1988-2007 is largely concentrated in the weakest systems. This is not a surprise for those of us who think that improvements in detection of weak systems have played a major role in the increase in storm count.

What if the Tiny Tims are removed from the modern period, to put things on more of an apples-to-apples basis? The answer to that is the dashed red line. The removal of Tiny Tims makes a remarkable reduction in the weak-storm difference.

That’s the main thing I was exploring – the impact of the Tiny Tims. But, the curve also shows a tendency for the modern period to be somewhat more active at ACEs above 5.

It’s my conjecture that some weak-to-moderate storms (5 to 10 ACE) in the remote eastern Atlantic were missed in the pre-recon era, due to the likely low density of ship traffic in that region in the Great Depression and WW2 era. There is also a possible issue with the extra-tropical classifications in the pre-recon period which I won’t get into here.

Even with those, though, there still appear to be about 15 more strong storms (ACE above 10 to 15) in the modern era than in the pre-recon era. That’s 0.75 storms a year of moderate or strong ACE. Maybe that’s real and due to higher SST, maybe that’s real and due to a partial mismatch of periods, maybe it’s another measurement artifact, who knows.

As mentioned near the start of this, historical ACE values need to be used with great caution as they were almost always based on best-guesses rather than measurements.

The interesting thing to me in this exercise is the notable impact of removing the Tiny Tims in the fourth plot, which puts things more on an apples-to-apples basis.

Before the advent of satellites, the BoM cyclone record is reconstructed from various streams of data, such as BoM records, ship records, historical documentation, etc., and should be considered as a provisional dataset rather than a quality product, as in spite of the fact that funding applications have been made many times for projects to review and homogenize the full database, aside from one project that reviewed and homogenized much of the post early-1980s data before funding ran out, the govt. has not seen fit to further fund this process.

Further, due to cyclone definition differences, many marginal systems that would be considered as short-lived tropical storms under the JTWC definition (and ‘Tiny Tims’ here) remain as either ‘tropical lows’ or ‘east coast lows’ that are not recorded in the BoM cyclone database (and it is well recognised that some of these are still in the pre-satellite part of the dataset) – to me this is an important omission as it makes studies like David’s more difficult to verify using independent datasets.

BTW, this year the BoM has changed it’s cyclone classification from ‘gales >34 knots (10-min-av) in all 4 quadrants’ to ‘gales >34 knots (10-min-av) extending more than half way (>180 deg) around the centre’, so there is yet another change in classification researchers need to be aware of – and there are likely to be a few more named cyclones as a result – I am aware of several un-named marginal systems in recent years that would be classified as named cyclones under the new system.

One good thing that might come from this in light of all the AGW hype is that it increases the pressure for a full revision of the BoM historical cyclone data in order to apply the new standard throughout for climate studies – one hopes someone at the BoM is writing a funding application to our new govt. right now!

The corrected time series is here . This corrected version has a more-pronounced hockey stick shape whereas the one in error had an odd mid-stick bump.

My apology for any confusion this might have created. The conclusions (large upswing in storms which were usually too weak/short-lived/small/remote to detect in earlier decades) are unchanged or, if anything, somewhat strengthened by this correction.

Kossin also offers this November 2007 paper for free and has an intriguing paper

Kossin, J. P., 2008: Does sea surface temperature alone control long-term changes in hurricanes? Submitted November 2007.

in the works.

Sorry, that last link (more recent summary) should be this.

Thanks to Carl (#114) and David (#115) for suggesting additional data sites (the Smith Bros?).

The data Carl links to appears to be the raw data from which the BoM summary I linked to is drawn. David’s site also has storm track data in more or less raw form. Lacking the time or gumption to sort through it, I used the data I found, even though it doesn’t match the bar graph in #106 exactly. The original questions I raised were

1) does a finer split change things?

2) what about older data and the relevance to “Tiny Tim” storms.

In the case of 1, it turns out this had already been done about ten years ago. My treatment of the data seems to add nothing to the analysis of Nicholls, Landsea and Gill (NLG). The trends are not materially different if one breaks out intense, moderate, and weak storms, as shown in Figure 2 of NLG. When I updated the graphs and plotted up to 2005, the only difference was that the intense storms show a very slight decrease, instead of a very slight increase. The more recent conclusion sited in comment #106 of “no change” is probably accurate.

For question 2, unfortunately, the Australian data is simply not of high enough quality to draw any conclusions. This quote from NLG is informative.

Holland observed that apparent trends in the numbers of tropical cyclones prior to that date [1969/70] would be likely due to changes in observing systems. Therefore, only the cyclone seasons from 1969/70 to 1995/96 are considered here. Inclusion of earlier data in a search for trends would clearly produce artificial trends, simply reflecting the improved observational network.

Another problem with the BoM data is that it doesn’t really address “tropical storms”. (“Weak” storms in NLG are less than 995 Mb, which is the break for ca. cat 1 storms.) According to the definitions here, it appears that the equivalent of tropical storms in the BoM database are storms with a minimum pressure of greater than 995 Mb. For what it’s worth (and with every caveat you can imagine about the quality of the data), here is my plot for storms of greater than 995 Mb from the BoM summary data.

What we see is a noisy data set with a sharp spike in the 60s. This is presumably due to increased detection because of advances in technology. Then in the 70′s the weak storms simply stopped being tabulated. What seems to have happened is that the BoM just didn’t want to be bothered with weak storms any more.

This is opinion, course, but in my view, the relevance to “Tiny Tim” storms is the spike in the 60s due to better detection.

The quality of TC data in the Australian region continues to confound efforts to extract long term trends (see the more recent summary here). Data from the Atlantic basin, with all their warts, are statistical marvels in comparison.

I don’t know how thoroughly the NHC searches the ship report data but I suspect it is thorough and, thanks to ease of access, better than it was in, say, 1935.

By the way, I saw this report tonight, wherein the NHC may upgrade an area of showers to something subtropical or tropical. I used your link to check nearby ship and buoy reports and find little to justify the bulletin, plus surface pressures are running 1015-1020mb and SST are around 25C, which normally lead one to say “forget it”. But, they may yet name another micro-storm for 2007.

My question is this: ( maybe you already have it in your analysis ..so color me stupid,
but use a nice color )

Were any tiny tims ( in the modern period) that were reported by satillite
Also catputred by the VOS ships?

I looked for VOS historical records but found nada..
Same with the NAvy weather observation ships from 1940-1980 No online data..