1. Tropical dynamics and Tropical cyclones
1.. Gravity waves
2. Small scale Kelvin wave

2. North South 3. Large-scale Kelvin wave
In a rotating frame, large-scale kelvin wave must be:
a. a boundary wave.
b. propagating with the boundary on its right in the North. hemisphere.
c. a trapped wave decaying from the boundary.
4. Equatorial Kelvin wave P C C P
North
South X X

3. 4. El Niño and La Niña
El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterized by unusually cold ocean temperatures in the Equatorial Pacific. El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather and climate around the globe.
2011
El Niño

5. 5. Walker
circulation

6. What causes the turnabout from warm phase to cold phase?
6. Bjerknes’s (Nov. 2, Jul. 7, 1975, Norwegian-American meteorologist) air-sea interaction argument
What causes the turnabout from warm phase to cold phase?
Why do El Niños typically last only
12-18 months?
Why do they usually shut down rapidly and switch from warm phase to cold phases or from cold phase to warm phase?
Ls nina condition
Stronger Walker circulation
El nino condition
Weaker Walker circulation

7. 7. Delayer oscillator theory
Eastern Pacific
Western Pacific
cold
warm
El Niño year
Thermocline
Normal year
Rossby wave
upwelling
upwelling
downwelling
Equator
downwelling
Kelvin wave
Rossby wave
upwelling
upwelling
thermocline

9. Vertical temperature anomalies along the equator during 1997-1998 El Niño event.
February 1997
160W W
January 1998

10. Sea level pressure anomaly at Tahiti and Darwin
8. Southern Oscillation
Darwin
Tahiti
Sea level pressure anomaly at Tahiti and Darwin

11. Southern Oscillation Index (SOI): defined as the pressure difference between Tahiti in the southeast Pacific and Darwin in the western Pacific.
Negative SOI corresponds to El nino
Positive SOI corresponds to La nina.
(ENSO)

12. Tropical cyclone 1. Where does a hurricane come from?
2. How can a hurricane maintain its strength?
3. What determines the movement of a hurricane?
4. Why sometimes we cannot accurately predict
the track of a hurricane?
5. Why it is difficult to provide accurate hurricane
intensity forecast?
6. How can we improve hurricane forecast?

13. 1. Where does a hurricane come from?
Africa (easterly) jet
Very strong temperature gradient
Thermal wind
Dynamic Instability
Barotropic Instability
Easterly
(tropical)
wave

15. 2. How can a hurricane maintain its strength?
Pressure grad.
force
Centrifugal
force
Pressure grad.
force
Centrifugal
force
Secondary
circulation
Frictional
force
Inflow
Pressure grad.
force
Centrifugal
force
Boundary layer
Ekman pumping

16. Hurricane Vortex X L Converging Spin up Diverging Spin down Ekman
Pressure grad.
force
Coriolis
force L
Centrifugal
force
Converging
Spin up
Diverging
Spin down
Buoyancy X
Ekman
Pumping
Boundary
Layer
It is the convective clouds that generate spin up process to overcome the spin down process induced by the Ekman pumping

17. Where do energy and moisture come from to foster
a large amount of convection in a hurricane?
Warm ocean
What process is responsible for bringing the energy and moisture taken from the ocean surface to the atmosphere?
Boundary layer turbulent transport process

18. 3. What determines the movement of a hurricane?

19. 4. Why we cannot accurately predict the track of a hurricane sometimes?
Where the storm is heading to? ?
Sensitive dependence on initial conditions, butterfly effect

21. Issues with numerical forecasts
Newton’s law of motion governs hurricane movement.
Initial value problem
British scientist L. F. Richardson
Weather Prediction by
Numerical Process, 1922
American meteorologist J. G. Charney
Quasi-geostrophic model, 1950
What causes the failure of Richardson’s numerical simulation?

22. In the 50s, people are optimistic about numerical weather forecast due to:
Global observational network of the atmosphere has been established, which can provide more accurate initial fields.
Great success of numerical calculation in other fields, such as
calculating the trajectories of planetary orbits and long-range missals.
The accuracy of numerical forecast improved dramatically
during the 60s, 70s, and 80s.
But unfortunately, improvement slowed nearly to a
standstill beginning around 90s.
Why?

23. 5. Why it is difficult to provide accurate hurricane intensity forecast?
Chaotic nature of the atmosphere
Insufficient observations
Inherent deficiency of numerical models
How to generate initial conditions for numerical simulations?
First guess of the current state of the atmosphere (previous 12-hour forecast of current conditions)
Data assimilation
(blending in OBS)
Initial Condition

24. Sub-grid scale (SGS) processes 10 – 100 km Van de Hoven (1957)
Scale (km)
1800
180 18
1.8
0.18
0.018
Van de Hoven (1957)
10 – 100 km

25. turbulence
Boundary layer heat, moisture, and momentum
transport by turbulence
b. Cloud convection and cloud microphysics
c. Radiation

26. Hurricane Boundary Layer (HBL)
clouds
turbulence
Parameterization
Hurricane Boundary Layer (HBL)
Currently almost no exception, the boundary layer parameterizations developed in non-hurricane conditions are also used without any modification in the simulation and prediction of hurricanes to account for the turbulent transport in the hurricane boundary layer.
But hurricane boundary layer has its own
unique characteristics.

27. 6. How to improve hurricane forecasting?
Ensemble forecasting

28. Summary Causes for numerical forecast errors:
Insufficient observations;
Chaotic nature of the atmosphere;
Inherent deficiency of numerical models with
limited resolution that fails to resolve sub-grid
physical processes.
Our answers to reduce forecast errors:
Data assimilation;
Ensemble forecast;
Improving parameterization.