Inside Tropical Cyclones--Part 2: Life Cycle, Storm Structure, Weakening or Strengthening

LIFE CYCLE OF A TROPICAL CYCLONE

A typical tropical cyclone will generally go through the following phases in its life cycle:

1. Tropical Disturbance/Tropical Wave--thunderstorms that organize in the tropical regions, but do not yet have a closed low level circulation. The clusters of storms from the converging trade winds at the ITCZ. In the Atlantic Basin, tropical waves travel westward thanks to the clockwise circulation around the Bermuda High and the trade winds.

2. Tropical Depression-- The next level up from a tropical wave is a tropical depression. By this point, the cluster of storms has moved over warmer ocean water and has formed a closed low-level circulation. Maximum sustained winds in a tropical depression are less than 39 mph.

3. Tropical Storm--Once the winds strengthen to 39 mph or greater, the tropical depression becomes a tropical storm.

4. Hurricane--A hurricane is a tropical cyclone with winds greater than 74 mph. If a hurricane continues to strengthen and the maximum sustained winds exceed 111 mph, the hurricane then becomes a MAJOR hurricane.

5. Extratropical Cyclone--Once a hurricane moves into cooler waters at a higher latitude, then system makes a transition from tropical to extratropical. The mid-latitude westerlies (jet stream) then disrupt the closed low level circulation and the system transitions back into a mid latitude cyclone.

INTERNAL STORM STRUCTURE:

The internal structure of a tropical cyclone is very complex and the impacts one faces during landfall highly depend on his/her location with respect to the storm. As stated in part one of this series, tropical cyclones expend tremendous amounts of energy, much of this being latent heat at core of the system. For the internal storm structure, we will look at a classic example of a hurricane.

Convection within hurricanes is organized into spiral bands wrapping around the center of circulation. The center of circulation of a well-developed hurricane will be marked out easily with an eye. The eye forms from strong subsiding air at the center of storm which is surrounding by rapidly rising air associated with powerful storms in the eye wall. As the air sinks at the center of circulation, it compresses, warms, and forms a brief period of relatively "calm" weather.

Surrounding the eye is the most volatile part of the hurricane called the "eye wall". The maximum amount of upward motion in the hurricane is found in this location

along with the highest cloud tops. Blinding sheets of rain and the strongest maximum sustained winds occur in the eye wall. Locations that are affected by the eye wall

typically end up with the most destruction, especially from wind with a landfalling hurricane. Eye walls often undergo a process called an eye wall replacement cycle (EWRC). During an EWRC, the old or existing eye wall will move outward from the center as a new and much stronger eye wall develops around the center of

circulation.

Extending out from the eye wall are several rain bands that spiral and rotate around the center of circulation. Rain within these rain bands can start out intermittent, but

as the storm moves closer, the rain becomes heavier and wind increases as well. In the northern hemisphere within the right front quadrant of a hurricane, the spiraling rain bands can sometimes form brief circulations and produce tornadoes, especially with a landfalling system. Usually, these tornadoes are weak, but occasionally, strong tornadoes and tornado outbreaks can occur.

WEAKENING OR STRENGTHENING

There are several ways to determine if a tropical storm or hurricane is weakening or strengthening without looking at forecast models.

A tropical storm or hurricane is likely to weaken or already weakening if...

--In the presence of strong wind vertical wind shear. Wind shear can disrupt the internal closed low-level circulation and prevent convection from wrapping around the center of circulation. This is one of the reasons why hurricanes are highly unlikely during the winter and early spring months. The upper level jet stream comes farther south thus increasing the amount of vertical wind shear in the atmosphere.

--Ingesting dry air into its circulation or surrounded by dry air. Drier air is more dense than moist air and can act as a force of subsidence, thereby hindering storm growth.

--Moves into cooler seawater (less than 80F). Warm seawater serves as fuel for the convection near the center of circulation. One of the reasons it is extremely rare for a hurricane or tropical storm to make landfall on the West Coast is because of the cold California current. A tropical cyclone will most likely end up decaying upon reaching the colder sea waters.

--Lack of convection (thunderstorms) near the center of circulation. Fairly obvious observation here: fewer storms=less intense or weakening system

--Barometric pressure increases. Rising pressure indicates an overall weakening storm.

A tropical storm or hurricane is likely to intensify or already intensifying if...

--The environment has little or weak vertical wind shear. Convection is able to rapidly grow with altitude into the atmosphere and organize around the center of circulation.

--System is surrounded by plenty of moist air. Dry air entrainment into the storm updrafts can kill convection.

--Moves into warmer seawater (greater than 82F ideal). Warm sea water essentially fuels a tropical cyclone and gives it the warm "core" that it needs to thrive. Exceptionally warm sea surface temperatures in the Gulf of Mexico helped to contribute to the explosive and rapid intensification of Hurricane Harvey in 2017.

--Deep convection develops in and around the center of circulation. Again, another fairly obvious observation: more storms=more intense or intensifying storm.

--Barometric pressure decreases. Decreasing pressure indicates a deepening and intensifying low pressure center.

--Outflow cirrus observed on visible satellite imagery.

--Eye wall replacement cycle: This can occur especially in major hurricanes and enable these systems to thrive for longer periods of time.

The final part of this series will focus more on impacts from landfalling tropical storms and hurricanes.