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Now that we have our cloud cluster, we got it spinning with a warm central core and we know the importance of latent heat and a moist mid-atmosphere, how do we get our cluster to spin up faster?
Warm the core up more? Evacuate air from the core? Increase the moisture flow in the storm? Import angular momentum and vorticity? All of these are actually important ways of spinning up the storm, although warming up the core is more of a secondary effect to the other events occurring in the storm and the import of vorticity is not nearly as important as the import of moisture into the storm.
In general you can think of a hurricane as consisting of two circulations. One that spins in a rough circle and another that flows into the storm, up in the eyewall and eventually out of the storm. These circulations are often called the primary and secondary circulations, respectively.
It is the secondary circulation that truly drives the storm, for this is the circulation that brings in moisture and other quantities into the storm and the one we will consider here. The primary circulation is basically just the good old howling winds that go around in circles and cause much of the damage.
To understand intensification we need to understand this secondary circulation. Hot moist air flows in at the base of the storm. This air flows in, expanding nearly isothermally. You know that as air approaches lower pressure through vertical lifting it cools and expands either adiabatically or moist adiabatically, either way it cools. This is true of air that spirals in to low centers as well, but is not quite true in the case of hurricanes. As the air flows into a hurricane it expands and it cools as normal, but it is in contact with the warm ocean. The ocean keeps the air from cooling by adding both heat and moisture to the air. The exact amount of evaporation and heating due to oceanic contact is the topic of much debate but it keeps the air warmer than it normally would be if it expanded adiabatically. If you remember the skew-t diagrams and the fact that the energy available to convection is equivalent to the area between the parcel's curve and the environment's curve you can see (as in the diagram below) that this pseudo-isothermal expansion greatly increases the particles energy before it lifts up. This is one source of energy to the storm and one way the secondary circulation develops the storm.
Eventually the inflow reaches a point, typically 10 to 30 km from the center of the storm and goes up in the intense convection surrounding the center. In a mature storm this area would be your eye wall. Here you would think you'd find strong updrafts as you do in supercells, but this is not the case, typical updraft velocities are only 5 m/s with strong updrafts around 10 m/s whereas supercell updrafts velocities can typically reach 30-50 m/s.
In the updrafts the rising air dumps it's moisture and releases it's latent heat. The updrafts are tilted outward in the storm due to angular momentum considerations you'll learn about in another class. From the tops of the updrafts huge anvils spread out forming the CDO (central dense overcast). These anvils and the moisture debris falling out of them are fanned outwards in all directions by the upper level storm features. Some of the air spreading out at the top converges over the center of the storm and subsides into the center, as it subsides it warms the air in the core which then lowers the surface pressure. The warmer air raises the thickness between pressure surfaces but since the tropopause acts as an immovable lid, excess air is swept out of the core by high level winds. Now the column of air above the core has less mass above it and less mass leads to lower pressure. Here it is important to remember that as the air rose in the updraft, latent heat was added to it as it rose due to the condensation of water. As the air, now dry, descends in the eye it becomes hotter than the updraft air surrounding it because of that extra heat and to the fact the dry adiabatic lapse rate is greater than the moist adiabatic rate.
What air doesn't converge over the center and subside is swept away by high level winds. Eventually far away from the storm the air slowly subsides and heats up greatly as it did in the core, however the heating is spread over a wide area and not as great as in the storm's center. This downward side of the storm affects the weather of a much large area than the central rainy core. A storm may lie hundreds of miles away from you but this descending arm of the circulations will still be affecting you with clear skies and often broiling temperatures. After the air descends, some of it then moves back towards the core on the inward leg picking up more moisture as it goes.
By now you are wondering when we get to the intensification part of the storm's life cycle. Well, now we are ready. So far we have our cloud system rotating, winds have reached 17 m/s and a tropical storm is started. Convection is still disorganized but a definite surface low exists and large amounts of latent heat are being released, slowly warming the core region. Outflow about the storm starts organizing into jets. The heating over the pressure center causes the air column above it to expand and at high levels the excess air is swept out lowering the pressure in the center. The increased pressure gradient increases the wind velocity which increases the inflow of moist air and the amount of air rising in the updrafts. The increase in air flowing up the updrafts releases even more heat eventually heating the core up more, which expands the column over the core, which causes more air to be evacuated from the core, which drops the pressure, ... I think you get the picture. A feed back loop starts up that is only limited by the water temperature of the ocean, which limits the amount of water vapor, wind shear, the ability of upper level flow to exhaust the storm and a variety of other factors.
As the storm intensifies thunderstorms get wrapped around the center, this process prevents cold "dirty" air from entering the center and slowing down the process just described and increases the subsidence into the center. Eventually if all remains well the convection forms a complete ring around the center forming an eye wall. The eyewall now protects the center from the outside environment and subsidence increases even more as the heating process and convection grow ever more vigorous. Eventually the subsidence in the center becomes so great that condensation into clouds is limited within the center and the eye is formed. During this process air flow into the storm gets organized along convergence lines which act as the focus for rain bands.
It is important to know that very few storms ever reach their real potential in strength. SSTs are the major controller on how strong a storm can get because they limit the amount of water vapor available for latent heat production. The relationship between maximum water vapor content in the air and evaporation rates from the ocean are both highly non-linear relationships. Therefor just a small increase in SSTs give a storm a significantly larger moisture pool to pull from. Upper level divergence is another factor that can greatly affect the strength of a storm as it dictates how much convergence there can be and in turn how much convection and heat release. There are, of course, many other factors that can lead to a storm never reaching it's potential strength. Upper atmospheric temperatures are actually an important factor because they dictate how much energy the storm can produce. You'll learn later why this is. For now you'll just have to get ready for the next phase in cyclone life.
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