Measuring tropical activity
Posted October 21, 2013
The tropical season that was forecast to run well above normal for the Atlantic has generally failed to live up to that, but it's worth a peek at different ways of measuring activity to see how it can be represented, and how some statistics can give a more meaningful representation than others.
The graph in the image here collects a couple of ways of showing how much tropical activity has occurred in the Atlantic this year, and each give quite a different sense of how the season is going with a bit under six weeks left to go. The numbers on the left simply show how many named storms have occurred - so far in 2013 we've made it up to Karen, or 11 named storms. According to climatology from the National Hurricane Center, our average number of named storms by October 20th would be 10, so we now have 110% of normal activity by that limited measure.
The other numbers are something called "accumulated cyclone energy," or ACE. This is a number that builds up through the course of the season as a function of not only the number of storms, but their duration and their maximum wind speeds as estimated at six-hour intervals throughout the life of the system. The advantage to this is that the number rises quickly with long-lasting, intense cyclones, but more slowly with short-lived, weak storms. We've had a lot of those this year, with only two hurricanes thus far and with a number of storms quickly disrupted by wind shear or inhibited by a layer of dry, dusty air flowing westward from North Africa into the Atlantic.
The result is that the ACE through October 20th is about 27, compared to an average by now of around 93, meaning that by this measure, activity thus far is only at about 29% of average - intuitively, this seems much more representative of the rather quiet season so far.
Right now, only one disturbance in the Atlantic seems to have any potential to become a named storm (Lorenzo would be next), and models make it appear to have only a limited window of opportunity today and tomorrow, with the National Hurricane Center giving it about a 40% chance of doing so.
Of course, the old saw still holds true - it only takes one bad storm to make a bad season for any given location, so we'll keep a close eye on any additional development in the remaining month and a half or so of the season.