When Is A Tropical Cyclone A Tropical Cyclone?
Seems logical enough doesn’t it, a tropical cyclone is a tropical cyclone. Reality is a tropical cyclone is a tropical cyclone only when ‘someone’ decides it is a tropical cyclone. You are probably thinking so what, no real big difference there, but in a science primarily governed mathematical equations and the laws of physics and thermodynamics, that involvement of ‘someone’ can be very significant. So really there are two important questions here – When is a tropical cyclone a tropical cyclone? AND Why the heck does it matter?
Before we get to deep the cyclone part of the phrase, it is probably best to touch on the ‘tropics’ portion. Marked by the part of the planet where the sun transverses overhead at some point on our annual orbit, this area of the earth is governed by weather behavior that much of the populated world may not be familiar with. For those who live in the mid and high latitudes and are use to terms like cold and warm fronts, you really need to put on a different hat when thinking about tropical weather. While there are whole courses taught on this topic, you can get a nice intro here. In short though, probably the two most important things to remember is we tend not to see the temperature gradients from north to south that we see at higher latitudes, and the tropics don’t benefit from the spin of earth as much to get this spinning.
Moving onto ‘cyclones’ portion of our phrase. Most folks are use to those in some form or another, whether it be through terms like hurricane or tornado just seeing the formation of one in a sink as it drains. The key is the spin. For those use to mid-latitude weather this may conjure up maps with a big L with some warm and cold front lines attached to it, but in the tropics it is a bit different. Without the temperature differences or earth’s rotation to help these areas of ‘spin’ are much less common. They are also unique in that they are warmer closest to their center which is why they are generally strongest closest to their core. But what constitutes an actual ‘tropical cyclone’ can get a bit tricky.
Let’s look at the generally accepted definition – “A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center.” Now that is a mouthful, but some of those pieces are a bit easier to deal with. Actually right up until the word ‘with’ we can generally measure today with satellites and other technologies easily enough. But after that we run into some problems in both subjectivity and measurement. For instance what does ‘deep convection’ or ‘well-defined center’ really mean, or how does one confirm a ‘closed surface circulation’ when these things are quite often occurring out in the middle of the ocean. In short, it really is not that easy and even with the latest technologies it is often the judgment call of a forecaster to label something a ‘tropical cyclone’ or not.
So let’s try a little case study. Below are 3 satellite images in a row, each is the case of an area of weather labeled with at least a 30% chance of becoming a tropical cyclone within 48 hours by the National Hurricane Center within the 12 months. The next row is the same area of weather 24 hours later.
One of these would become a tropical storm, another was deemed a tropical depression (a weak tropical cyclone) that then dissipated within another day, and the last would ultimately never become a tropical cyclone. Can you tell which is which? In the end a person decided which is which and it is possible that with different human perspectives involved we could have ended up with 1 2 or 3 tropical cyclones.
Now at first glance you might think why does it matter, when these cyclones get started they are weak and what is one more or less. Well the reality is that it is important for both scientist and you. A particular challenge science still grapples with is why certain disturbances like those pictured above turn into tropical cyclones and others don’t. This impacts how forecast models are developed and overall how well tropical cyclone formation can be predicted. So if a forecast is deemed inaccurate because an area of weather was or was not called a ‘tropical cyclone’, a model might be judged not as accurate depending on what it predicted. But in reality it may have forecasted it correctly even if a human did not see it the same way another human might have.
Ok, so now you are thinking this is just ubergeeky weather type stuff. Let me give you an example of how it impacts you. Things you buy every day are changed in price by predictions of tropical cyclones, sometimes in the very short term like the price of gasoline spiking when a tropical cyclone heads toward the Gulf of Mexico, but quite often in ways you do not see. Everywhere around the world that is impacted by tropical cyclones has risk adjustments on their insurance or areas that drive their cost of doing business, and that price indeed gets passed on to you. Think about all the wonderful things we get not only from the tropics (fruits, crops, oil, etc.) but also in areas where these systems can ultimately go which includes latitudes well outside the tropics. These adjustments are made based on projections, projections that can include estimations on the number of tropical cyclones in different parts of the world. Uncertainty or inaccuracy in past data can lead to projections that must account for more risk potential and uncertainty, and down the line that usually will translate into higher costs for you.
So in the end this subjective definition impacts geeky science types and your wallet!
Next time we will take up a reader suggested topic – What is the real cost of energy? With oil continuing to make a mess of things along the US Gulf Coast, it makes one wonder how dependent we would still be on carbon based fuels if we factored in the repercussions of such events into the total cost we pay for energy. Until then – learn, think and grow!