Welcome to 'Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. Over the next few weeks we will be going through the 5 necessary conditions for cyclone formation and why they are important. Today we look at sea surface temperatures.
Did You Know:
Why does a Tropical Cyclone need water temperatures above 26 degrees?
In 1948 it was stated that oceans with a minimum temperature between 26 and 27 degrees celsius are required for Tropical Cyclogenesis to occur. The reason this temperature was chosen was that at this temperature point and at near saturation of the low level atmosphere, the 'enthalpy flux' (fancy schmansy sciency words for the rate of flow of heat energy across a unit area per unit of time, it has a direction and a magnitude value) was large enough to support and sustain a large convective complex. This self sustaining convective mass could eventually, if all other necessary parameters are met, develop into a warm core cyclonic circulation and then could remain self sustaining.
Want a slightly easier to understand breakdown? well read on
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Tropical Cyclones require an almost saturated low level atmosphere to form. They require this moisture to be available for at least 24 hours before they form and they need this high moisture content to extend vertically for about 7 kilometres. The only way we can find so much moisture in the lower level air for such an extended period without the air becoming negatively buoyant is by being over an ocean. Now the problem with being over an ocean on a saturated moisture day means that it's going to be quite cool especially since it is likely to be raining. So we have extreme levels of moisture at the surface where the air is in contact with the ocean, the atmosphere is unstable (that's another one of the 5 conditions we need to talk about on a different 'did you know') so that moisture is being lifted, cooled, condensed into clouds and lifted further as the cloud development process releases more heat energy known as 'latent heat'. Because of that latent heat release the air in contact with the condensating parcel is now hotter so it rises, cools, condenses releases more heat etc etc so the cloud then grows further vertically before eventually the weight of the condensed liquid becomes too heavy and the convective cell dumps its load. Once the cloud begins dumping its load as precipitation it cools the atmosphere below itself making it harder for new air to rise (check out the thunderstorm model image below which is very typical for low shear thunderstorms in the tropics). This is where a huge majority of convection in the deep tropics ends. Because we don't have much wind shear, in fact it is necessary that we don't have much wind shear (another of the 5 conditions we will discuss in another 'Did You Know') and our storms move slowly so once the rain begins falling, convective cells begin to choke themselves and they do that until they die by making the air below them cooler and they lose the 'lift' they need to keep the thunderstorm growing. Enter the magical 26 degree threshold.
26.5 degrees (it's actually not an exact measurement studies have ranged from 25.5 degrees to 27 degrees - we just use the 26 or 26.5 as an approximate cutoff) is the point at which we can have enough heat entering the convective updraft (as long as the boundary and mixed layer remains extremely moist) to counteract the negative buoyancy brought on by the precipitation of the convective downdrafts. Once we hit that 26-27 degree marker as long as the air remains near saturated (that is extremely important), as long as the atmosphere remains unstable and moist up to about 7 kilometres high, as long as the system can begin to spin and as long as it doesn't get sheared away too rapidly at the top, we will have enough heat energy entering the system at the bottom (enough enthalpy flux) to maintain and/or grow it.
The warmer the ocean water is above that 26 degree minimum standard, the more energy the system is able to tap into as it grows and the more moisture the air coming into contact with the warmer water can carry. Meteorologists keep a close eye on sea surface temperatures (in absolute values) and sea surface temperature anomalies, in order to understand just how much energy the system has access to at any one moment of time now and into its future and how much more or less energy that is compared to what we would expect climatologically. That's why systems that move over localised warmer waters than their surroundings can show such explosive development/intensification. They basically just ingest a bunch of supercharged energy drinks as the move across those warmer waters. The interesting thing is the higher we go above that 26 marker the more dramatic any further rises in temperatures are. i.e. energy availability in ocean temps between 26 and 27 degrees is sizeable but not hugely increased but if you start to look at the energy availability say between 29 and 30 degree water temperatures we start talking some really big numbers. So that's why a warm ocean is a necessary ingredient for Tropical Cyclone development. But warm oceans don't cause Tropical Cyclones on their own, they merely help them, they are just one key player in the team. And creating a Tropical Cyclone needs to be a a whole team effort with lots of atmospheric players who have to work together to win this game. Tomorrow night we find out why it's important that a Tropical Cyclone spins. Enter the player called The 'Coriolis Effect' Did you also know, some Tropical Cyclones can actually deepen and maintain themselves over land? A specific set of conditions need to be met for that to happen, but unfortunately you'll have to wait a while to find out more, because that's a topic for another 'Did You Know'.
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