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

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

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. Walker circulation

What causes the turnabout from warm phase to cold phase? 6. Bjerknes’s (Nov. 2, 1897- 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. 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

Vertical temperature anomalies along the equator during 1997-1998 El Niño event. February 1997 160W 80W January 1998

Sea level pressure anomaly at Tahiti and Darwin 8. Southern Oscillation Darwin Tahiti 1958-1998 Sea level pressure anomaly at Tahiti and Darwin

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)

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?

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

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

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

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

3. What determines the movement of a hurricane?

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

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?

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?

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

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

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

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.

6. How to improve hurricane forecasting? Ensemble forecasting

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.