David Smith, a regular commenter on hurricanes, writes.
Tiny Tim is a Charles Dickens character. Tiny was a young lad, small, very weak, in a struggle to survive and of little notice in the hustle-bustle streets of London. Later, of course, his fortunes improved and he and Scrooge became “part of the record” of Victorian England.
In a similar vein (OK, it’s a stretch) there is a type of Atlantic tropical cyclone that is like Tiny Tim: generally of short duration, weak winds, small aerial extent and often in a remote part of the ocean. Its impact on its environment is tiny (a very small “footprint” in the Atlantic).
My operational definition of “Tiny Tim storms” are those that were so minimal that the NHC end-of-season reports do not report a single ship or single shore report of storm-force winds. This is not a matter of report oversights – storm analysts consider surface verification of wind estimates to be an important matter and list shore weather reports and ship reports in their reports.
And, the lack of ship or shore reports is quite significant if someone is looking at storm climatology. Storms lacking ship or shore reports of storm-force winds would, prior to 1945 (the start of recon), not have been classified as a tropical storm/hurricane. Why? Because, prior to 1945, all the meteorologists had were ship and shore reports. No aircraft recon, no satellites, no buoys and no Doppler radar – just ship and shore reports.
So, in this era of strong and many ships, rapid reporting and (US) shores lined with windspeed devices like onshore CMAN stations, a seeming plethora of data, are there still Tiny Tim storms, ones that modern technology sees but which lack storm-strength impact on ships and shores and which would have been ignored in the past?
The US National Hurricane Center (NHC) makes its end-of-season archives available at its website . An example of a storm report is here, the infamous Hurricane Katrina which shows a wealth of information including about 70 selected ship observations (hmmm, must have been some old dumb ships out there left over from the 1930s) of storm-forced winds and about 50 selected onshore observations.
Another example is Tropical Storm Ernesto , a report with with much less content because, well, it wasn’t much. There were no ship reports of storm-force winds and, as the NHC acknowledges, even in reanalysis it is of questionable strength or organization. Yet it carries as much weight in storm count trends as does Katrina.
(A word about reviewing the NHC archives – the ones for recent years are well-written and organized but that quality (for climatological purposes) diminishes as one goes back in time. There is a lot of verbage and data to review. I say this because I reviewed about 250 reports for this post and I may have missed some detail, one way or the other. I doubt that it’s material but I want to mention it anyway and welcome anyone who might audit my list.)
I reviewed the last 20 years of records as I figured that covered the modern increased-activity era (and the 1980s record quality became a bit more challenging).
So, the question of the hour is: how many Tiny-Tim storms, ones with nary a ship or shore report of storm winds, occurred in the last 20 years? The answer is here .
Frankly I was surprised. There are 52 storms on the list.That’s 52 out of the 252 storms in the official record, or 20% of the total. That’s 20% of the modern storms which lack a single classical (ship or shore) report of storm winds. Wow.
The obvious question is: how can one compare these satellite- and aircraft-based storms, which left no ship or shore evidence, with pre-1945 records which were based solely on ship and shore observations?
A little data.
First, here’s a look at a couple of characteristics of the Tiny-Tim group. Here is a bar plot of the duration of the Tiny Tims, grouped by days of existence (6 to 24 hours = 0 to 1 day, 30 to 48 hours = 1 to 2 days, and so forth).
The median duration is 1.7 days (42 hours). A few storms lasted beyond four days, ones that tended to be in remote open waters. For perspective, something that moves at, say, 10mph and lasts 2 days doesn’t cover a huge amount of real estate.
How about winds? Here’s a bar plot of windspeed distribution for the Tims. It shows that the group combined had 182 six-hour periods (45.5 days) of winds in the 35 to 39 knot range, as estimated by aircraft, satellite or buoy. The distribution has a mean windspeed of 43 knots (“strong gale” on the Beaufort scale) with 85% of the time spent below 50 knots.
There is an important graphic which I wish I could present but cannot because, to my knowledge, the data does not exist. The graphic would convey information on the geographical extent of storm-force winds. This is important because Tims likely have peak winds in only a small area on the eastern side of the center, perhaps 30 to 50 miles across typically. Tropical storms often lack symmetry and have their strongest winds in a relatively small area of thunderstorms.
As an exercise for perspective, figure that the hurricane-prone portion of the Atlantic covers 8 million square miles and that a Tiny-Tim has storm-force winds 100 miles across and moves at 10 mph for 2 days before weakening. That equates to the Tim covering 0.6% of the tropical Atlantic, which is not much.
Another useful graphic, which I have not done, would be a map of the storm locations. I think we’d see Tiny-Tims in the Gulf, along the eastern US seaboard (frontal-zone Tims), in the remote open Atlantic and scattered elsewhere.
OK, that’s a view of the group. Now for the main question: how have these storms affected the all-important trend in Atlantic storm count? What does the long-term time series of Atlantic tropical cyclones look like if the recent Tims are omitted?
Since my data covers only the most recent 20 years the plot is rather odd but does offer some information. The blue line is the official record (Tims included) while the red line is what the 5-year average would look like without the recent Tims. The comparisons should be (1) between the red and blue lines for 1988-2007 and (2) the red line (1988-2007) versus the blue line before 1945 (pre-aircraft). The plot shows notably fewer recent storms and shows recent activity more in line with historical (pre-1945) activity.
A closeup, with a few comments, is here . The impact of the Tims on the recent record is clear. I added several comments on the pre-1945 period, lest the question of a peak(1930s)-to-peak(2000s) comparison arises. The 1930s through the mid-40s was a period of limited global commercial activity, due to the Great Depression followed by World War 2 (this was shown on an earlier graph on CA a few months ago). Fewer ships in the 30s and early 40s meant less chance of an encounter with weather of any sort, including tropical cyclones. I suspect that this affected storm reports.
To me, this is further evidence of the problems with long-term comparisons of Atlantic storm counts and reinforces my view that improvements in storm detection are the main drivers, and perhaps the sole drivers, of the increase in reported Atlantic storms.