environmental factors for deveolpment

Knowing where the storms form we can get an idea of the environmental factors that are needed for their development. Below is a list of commonly agreed upon factors for hurricane development.

  1. Large amounts of positive relative vorticity
  2. High enough latitude to get non-zero values of planetary vorticity.
  3. Weak vertical shear of the horizontal winds
  4. Water temperatures > 26 degrees C (some texts will quote 27 degrees C)
  5. Deep Conditional Instability
  6. Large amounts of water vapor.

The first factor says hurricanes need large amounts of relative vorticity. Hold on, Vorticity?! Yep it's a big word but if you like storms and meteorology, get used to it, for without it weather would not be nearly as interesting as it is. Voritcity in it's most basic definition is the local spin of the atmosphere. It comes in several flavors but the two you need to know here are relative and planetary. The earth spins and it in turn imparts spin to the air in contact with it. This is planetary vorticity which is simply our old friend the Coriolis force. Relative voriticity is the vorticity of air relative to a fixed non-rotating earth. Together relative and planetary voriticity sum up to a third form a vorticity (absolute vorticity). Anyway, as can be seen by the definition of vorticity it makes sense that hurricanes need a lot of it. For, without spin, you have no cyclone. All vorticity is either positive or negative, with positive being cyclonic and negative being anti-cyclonic.

The next factor is high enough latitude to get large values of planetary vorticity or the Coriolis force. As we saw earlier in the forces module coriolis force is related to latitude and gets larger in value with higher latitude. We also saw in the winds module that the Coriolis force causes parcels of air to deflect to the right along their paths. Here the Coriolis force is needed to balance with the Pressure Gradient Force force and the friction force to allow the storm to start spining up. Without the Coriolis force the forming storm would have nothing to cause the air to curve and the forming depression would simply fill up with air coming in towards the low center. You need just enough latitude (about 2.5 degrees) to make the coriolis force strong enough to allow the spin up of the storm to occur.

The third factor important to hurricane development is low values of vertical shear of horizontal winds. This is a wordy statement, see the diagram below to better understand it. This statement must be made with caution though. Low verticle shear is needed around the center and expecially the core of the storm. This allows the upper level features of the storm to be coupled with the lower level feature of the storm and allows the large thunderstorms needed to form. When there is high shear these parts are seperated and the system is literally torn apart. We will explain this a little more in detail later. Shear, like vorticity comes in several flavors: speed (wind speed changes with height, and directional (wind direction changes with height).

Now, I said we should be cautious about the shear statement and that is because, high values of shear away from the storm may actually be an important factor for development. The reason for this may again be related to vorticity and how it can occur. There is a concept called tilting . Basically it says you can get verticle voriticity (the kind meterorologist care about) by tilting horizontal vorticity veritically. For those of you who like tornadoes, this is one of the major theroies as to how they form.

So what's the importance to hurricanes. Well, simply if you have a large source of horizontal vorticity nearby and you can somehow tilt it vertically, you have a lot of relative vorticity for the hurricane to injest and use to spin itself up faster.

The factor that most people think of when considering hurricane development is warm ocean water. In fact, studies tend to show that water with temperatures below 26 degrees C simply cannot support a cyclone. The air saturated at this temperature does not allow for enough water vapor which releases the latent heat energy necessary to keep the system going and as we will see, it is the latent heat released by accending air that is the major driving force behind the hurricane. It is important to remember though, that although SSTs (sea surface temperatures) are important to hurricanes, they are not the end all, be all of their existence.

Next we reach deep conditional instability. You should have learned by now that if a parcel is lifted and is warmer than it's environment it is in an unstable condition and will continue to rise until it's cooler than it's environment. A stable atmosphere is the opposite of this. A conditionally unstable/stable atmosphere is one in which if the parcel is lifted up to a certain transition point it will be colder than its environment and stable, whereas if it is lifted beyond that point it will be warmer than its environment and unstable.

quiz So why is this important?

a)Hurricane formation is conditional, therefor it needs conditional instability b)Instablity doesn't allow for energy buildup, conditional instability does. c)It increases the vorticity. d)It affects sea surface temperatures. b) is correct. a) is a meaningless statement, c) is only partially true and there is no direct relationship as proposed by d). Increased stablity, leads to more sun light which in general increases SSTs although SSTs are more likely to be affecting the stability by controling the surface air tempuratures. The importance of the conditional instablity is that it allows for convective energy to build up in the lower atmosphere instead of being used up the second that a parcel goes up. More convective energy leads to more intense storms, and hurricanes need those.

quiz

The last factor we mention here is large amounts of water vapor content. The reason this is important is that it is the release of latent heat energy that is the main source of energy for the hurricance's development, and intensifacation. Latent heat is in essence, the gas of the hurricane engine.


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