Tropical Cyclone Genesis, Maintenance, & intensification (integrated)

Tropical Cyclones
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere.^[1] Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.^[2] 

⭄ There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.^[3]

Requirements for Tropical Cyclones
⭄ Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins. Climate cycles such as ENSO and the Madden–Julian oscillation modulate the timing and frequency of tropical cyclone development.^[4][5]There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path.^[6] An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach hurricane/typhoon strength, and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson Hurricane Scale).^[7]

Requirements for tropical cyclone formation

There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to sustain a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.^[3] While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.^[3]

Warm waters, instability, and mid-level moisture

Main article: Lapse rate

⭄ Tropical cyclones are known to form even when normal conditions are not met. For example, cooler air temperatures at a higher altitude (e.g., at the 500 hPa level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certain lapse rate is required to force the atmosphere to be unstable enough for convection. In a moist atmosphere, this lapse rate is 6.5 °C/km, while in an atmosphere with less than 100% relative humidity, the required lapse rate is 9.8 °C/km.^[9]Normally, an ocean temperature of 26.5 °C (79.7 °F) spanning through at least a 50-metre depth is considered the minimum to maintain the special mesocyclone that is the tropical cyclone.^[3] These warm waters are needed to maintain the warm core that fuels tropical systems. This value is well above 16.1 °C (60.9 °F), the global average surface temperature of the oceans.^[8]

⭄ At the 500 hPa level, the air temperature averages −7 °C (18 °F) within the tropics, but air in the tropics is normally dry at this level, giving the air room to wet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection. A wetbulb temperature at 500 hPa in a tropical atmosphere of −13.2 °C is required to initiate convection if the water temperature is 26.5 °C, and this temperature requirement increases or decreases proportionally by 1 °C in the sea surface temperature for each 1 °C change at 500 hpa. Under a cold cyclone, 500 hPa temperatures can fall as low as −30 °C, which can initiate convection even in the driest atmospheres. This also explains why moisture in the mid-levels of the troposphere, roughly at the 500 hPa level, is normally a requirement for development. However, when dry air is found at the same height, temperatures at 500 hPa need to be even colder as dry atmospheres require a greater lapse rate for instability than moist atmospheres.^[10][11] At heights near the tropopause, the 30-year average temperature (as measured in the period encompassing 1961 through 1990) was −77 °C (−132 °F).^[12] A recent example of a tropical cyclone that maintained itself over cooler waters was Epsilon of the 2005 Atlantic hurricane season.^[13]

Coriolis force

Coriolis force

⭄ A minimum distance of 500 km (310 mi) from the equator is normally needed for tropical cyclogenesis.^[3] The Coriolis force imparts rotation on the flow and arises as winds begin to flow in toward the lower pressure created by the pre-existing disturbance. In areas with a very small or non-existent Coriolis force (e.g. near the Equator), the only significant atmospheric forces in play are the pressure gradient force (the pressure difference that causes winds to blow from high to low pressure^[15]) and a smaller friction force; these two alone would not cause the large-scale rotation required for tropical cyclogenesis. The existence of a significant Coriolis force allows the developing vortex to achieve gradient wind balance.^[16] This is a balance condition found in mature tropical cyclones that allows latent heat to concentrate near the storm core; this results in the maintenance or intensification of the vortex if other development factors are neutral.^[17]

Low level disturbance

⭄ Whether it be a depression in the intertropical covergence zone (ITCZ), a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis.^[3] Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low.^[3] Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge.^[18]

Weak vertical wind shear

Main article: Wind shear

⭄ Vertical wind shear of less than 10 m/s (20 kt, 22 mph) between the surface and the tropopause is favored for tropical cyclone development.^[3] A weaker vertical shear makes the storm grow faster vertically into the air, which helps the storm develop and become stronger. If the vertical shear is too strong, the storm cannot rise to its full potential and its energy becomes spread out over too large of an area for the storm to strengthen.^[19] Strong wind shear can "blow" the tropical cyclone apart,^[20] as it displaces the mid-level warm core from the surface circulation and dries out the mid-levels of the troposphere, halting development. In smaller systems, the development of a significant mesoscale convective complex in a sheared environment can send out a large enough outflow boundary to destroy the surface cyclone. Moderate wind shear can lead to the initial development of the convective complex and surface low similar to the mid-latitudes, but it must relax to allow tropical cyclogenesis to continue.^[21]

Favorable trough interactions

⭄ Limited vertical wind shear can be positive for tropical cyclone formation. When an upper-level trough or upper-level low is roughly the same scale as the tropical disturbance, the system can be steered by the upper level system into an area with better diffluence aloft, which can cause further development. Weaker upper cyclones are better candidates for a favorable interaction. There is evidence that weakly sheared tropical cyclones initially develop more rapidly than non-sheared tropical cyclones, although this comes at the cost of a peak in intensity with much weaker wind speeds and higher minimum pressure.^[22] This process is also known as baroclinic initiation of a tropical cyclone. Trailing upper cyclones and upper troughs can cause additional outflow channels and aid in the intensification process. It should be noted that developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet emanating from the developing tropical disturbance/cyclone.^[23][24]

⭄ There are cases where large, mid-latitude troughs can help with tropical cyclogenesis when an upper-level jet stream passes to the northwest of the developing system, which will aid divergence aloft and inflow a.the surface, spinning up the cyclone. This type of interaction is more often associated with disturbances already in the process of recurvature.^[25]

Recall: Diffluence, or Deformation, is the rate of change of shape of fluid bodies. Meteorologically, this quantity is very important in the formation of atmospheric fronts, in the explanation of cloud shapes, and in the diffusion of materials and properties.^[1]

Additional Notes for Cyclogenesis:

• Upwelling can also occur when a tropical cyclone transits an area, usually when moving at speeds of less than 5 mph (8 km/h). The churning of a cyclone eventually draws up cooler water from lower layers of the ocean. This causes the cyclone to weaken.

Atmospheric Instability is the condition where the earth's atmosphere is considered unstable and as a result is prone to high variability across distance/space and time. Atmospheric stability is a measure of the atmosphere's tendency to encourage or deter motion vertical motion, which is directly related to various weather systems. It is measured by various indices in meterology, and these indices involve lapse rate, which is the change in temperature through the troposphere with height. Usually due to excessively moist or dry conditions. For moist conditions, this atmospheric instability aids tropical cyclogenesis and thunderstorm development. 

Recall: Conditional stability/instability, Absolute stability/instability

For tropical cyclogenesis, there must be weak vertical wind shear but strong horizontal wind shear. This means high convective (vertical) stability and low dynamic (horizontal) stability would favor tropical cyclogenesis. 

Other areas of concern might include atmospheric thermodynamics, differences between stable and unstable atmosphere, indices, and forms. [https://en.wikipedia.org/wiki/Atmospheric_instability]

Wind Shear, or wind gradient, is the difference in relative speed or direction between two adjacent air masses. It is described to be horizontal or vertical. Temperature decreases and wind velocity increases with height in troposphere, while reverse is true in stratosphere. These then determine changes in air density, thus viscosity, and movement. Wind shear is said to be sharp(er) or moderate.

Low vertical wind shear is required for formation of tropical cyclones because it allows their warm core to remain above their surface circulation center, which results in intensification 

Strong vertical wind shear within the troposphere inhibits tropical cyclone development, but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather.

Development of Tropical Cyclones (Air rises etc)

Maintenance of Tropical Cyclones 

The brown ocean effect is an observed weather phenomenon where tropical cyclones, which are commonly expected to lose energy when they make landfall, instead maintain strength or intensify over land surfaces.

One source of the brown ocean effect has been identified as the large amount of latent heat that can be released from extremely wet soils.

In order for the brown ocean effect to take place, three land conditions must be met: First, the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature. Second, soils in the vicinity of the storms need to contain ample moisture. Finally, evaporation of the soil moisture releases latent heat, which the team found must measure at least 70 watts averaged per square meter."^[5] Storm systems impacted by the brown ocean effect gave rise to a new sub-category of tropical storm type called Tropical Cyclone Maintenance and Intensification Eventor TCMI.^[5] Another study concluded that latent surface heat flux from land surfaces actually have the potential to be larger than from the ocean, albeit for brief periods only.^[6]

Patterns of Occurrence (Locations of formation, movement & maintenance)