1. More on tropical cyclones
SO442 Lesson 5
More on tropical cyclones

2. Objectives
So far, we have learned important details about tropical cyclones and tropical easterly waves. In those prior lessons, many important pieces of information were presented, both formally and informally. Furthermore, in our daily tropical weather discussions, we have informally discussed several topics
The goal of this lesson is to circle back to many of those topics and formally discuss them

3. Tropical cyclone definitions
The generic name "tropical cyclones" may be used anywhere in the world for tropical storms with peak wind speeds (1-minute mean, 10-minute mean or gust wind speed are used in different regions) exceeding 17 m s-1.
Australian region
Western North Pacific and Indian Ocean regions
Atlantic and eastern North Pacific regions

4. TC forecast regions
Tropical cyclone names and official intensities and warnings are made by different weather agencies, assigned by agreement from the World Meteorological Agency (WMO)

5. Structure of a TC TCs have several major parts: Spiral bands
Central dense overcast
Eyewall
Eye

6. Ice crystals in a TC are reflective to satellites

7. Inertial stability
TCs can “resist” the influences from other weather systems (wind shear, troughs, ridges) – particularly in their inner core
A quantification of that resistance is called “inertial stability” and is calculated using:
Inertial stability is highest in the inner core (WHY?) and lowest in the outer part of the TC
What are some consequences of that pattern of inertial stability?

8. TCs and the thermal wind
The thermal wind is defined as the vertical wind difference (vertical wind shear) associated with a horizontal temperature difference
Put another way: in the presence of horizontal temperature gradients, vertical wind shear will exist.
Put another way: the presence of vertical wind shear is indicative of horizontal temperature gradients
Tropical cyclones should have small thermal wind values, and of a particular sign
Horizontal temperature gradients (and thus vertical wind shear) should be small
And, the hurricane is “warm core” – thus the gradient vector points inward
Extratropical cyclones should have high values of vertical wind shear
Another consequence of the thermal wind:
The greatest winds of a hurricane are at the lowest levels, not the highest levels

9. Barotropic instability
We saw in a prior lesson that barotropic instability (a function of the shape of the wind field) can lead to the development of spin (vorticity) and be a precursor to the development of a TC
Tropical easterly waves are not the only source of barotropic instability in the tropics
The “monsoon trough” and “intertropical convergence zones” are also often rich areas of barotropic instability

10. Summary of TC genesis mechanisms, globally
All of Dr. Gray’s (1968) necessary but not sufficient conditions apply globally
However, different large-scale weather and climate features act to modulate those conditions:
Tropical (African) Easterly Waves
El Niño-Southern Oscillation
Monsoon Trough
Mesoscale convective systems
Quasi-biennial Oscillation

11. Example of barotropic instability in a monsoon trough
Over a 15-day period southeast of Hawaii (in the Pacific Ocean), the monsoon trough – a region of barotropic instability – breaks down into several tropical cyclones

12. Another source of TC genesis: upper-level trough
Upper-level troughs are generally “bad” for tropical cyclones
They bring vertical wind shear
They bring colder temperatures
But, sometimes, if the trough position is just ideal, the trough can bring divergence aloft that can aid in TC genesis or intensification
Divergence aloft aids in rising motion in the atmosphere
Rising motion can stretch an air column and cause it to rotate faster (assuming the air column already had voriticity present)

13. Some large-scale influences on tropical cyclone genesis
2. El Niño-Southern Oscillation (more on that later, too)
3. West Africa Monsoon:
-Number of easterly waves
-Intensity of easterly waves
-Intensity of Saharan Air (dry) layer
1. Madden-Julian Oscillation (more on that later)

14. More on ENSO influences on Atlantic and East Pacific hurricanes

15. Tropical cyclone as a Carnot engine
From the AMS Glossary:
“An idealized reversible work cycle defined for any system, but usually limited, in meteorology, to a so-called perfect gas.”
The Carnot cycle consists of four states: 1) an isothermal expansion of the gas at a temperature T1; 2) an adiabatic expansion to temperature T2; 3) an isothermal compression at temperature T2; and 4) an adiabatic compression to the original state of the gas to complete the cycle. In a Carnot cycle, the net work done is the difference between the heat input Q1 at higher temperature T1 and the heat extracted Q2 at the lower temperature T2. The atmospheric general circulation and some storms, notably hurricanes, incorporate a process similar to a Carnot cycle. 
TC intensity is controlled by SST (A->B) and temperature of the outflow layer (B->D)

16. Maximum potential intensity of a TC

20. Another influence on TC intensity: Baroclinic effects
What does “Baroclinic” mean?
How do baroclinic influences (blue regions in figure at the right) evolve over the year?

21. Global TC climatology, by month
Which basin is the most active?
Which basin is the least active?
In which basin do TCs occur all year long?
Why does the activity in most ocean basins have a pronounced seasonal cycle?

22. Have hurricanes been increasing in number in the North Atlantic basin?

23. A region particularly susceptible to TC impacts: Bangladesh
Two notable tropical cyclones, Bhola Cyclone (12 Nov 1970) and Chittagong Cyclone (29 April 1991) are responsible for huge losses in human life
Bhola: 300,000
Chittagong: 138,000
Why?