G304 – Physical Meteorology and Climatology Chapter Tropical storms and typhoons By Vu Thanh Hang, Department of Meteorology, HUS 9.1 Typhoon characteristics and structure • Extremely strong tropical storms go by a number of different names, depending on where they occur • Over the Atlantic and the eastern Pacific they are known as hurricanes • Those over the western Pacific are called typhoons • Those over the Indian Ocean and Australia are cyclones • In structure, the three kinds of storms are essentially, although typhoons tend to be larger and stronger than others • The region having the greatest number of storms is the western part of North Pacific 9.1 Typhoon characteristics and structure (cont.) 9.1 Typhoon characteristics and structure (cont.) 9.1 Typhoon characteristics and structure (cont.) 9.1 Typhoon characteristics and structure (cont.) • Typhoons have sustained wind speeds of 120 km/hr and are typically about 600 kilometers wide • Sea level pressure near the center of a typical typhoon is around 950 mb, but pressures as low as 870 mb have been observed for extremely powerful typhoons • Typhoons obtain most of their energy from the latent heat released by condensation and are most common where a deep layer of warm water fuels them • August and September are the prime typhoon months in the NH, while January to March is the main season in the SH 9.1 Typhoon characteristics and structure (cont.) 9.1 Typhoon characteristics and structure (cont.) • Typhoons not consist of only one uniform convective cell • They contain a large number of thunderstorm arranged in a pinwheel formation, with bands of thick clouds and heavy thundershowers spiraling counterclockwise (in NH) around the storm center • The bands of heavy convection are seperated by areas weaker uplift and even descending air and less intense precipitation • The wind speed and the intensity of precipitation both increase toward the center of the system (eye), reaching a maximum 10 to 20km away from the center (eye wall) 9.1 Typhoon characteristics and structure (cont.) A cross section of a typical typhoon 9.1 Typhoon characteristics and structure (cont.) • The typhoon eye is a region of relatively clear skies, slowly descending air, and light winds • Along the margin of the eye lies the eye wall, the zone of most intense storm activity with the strongest winds, thickest cloud cover, and most intense precipitation of the entire typhoon 9.2 Typhoon formation • Tropical disturbances are disorganized groups of thunderstorms having weak pressure gradients and little or no rotation • When a tropical disturbance develops to the point where there is at least one closed isobar on a weather map, the disturbance is classified as a tropical depression • If the depression intensifies further and maintains wind speeds above 60 km/hr, it becomes a tropical storm • A further increase in sustained wind speeds to 120 km/hr creates a true typhoon 9.2 Typhoon formation (cont.) • Conditions necessary for typhoon formation: - the ocean has a deep surface layer with temperatures above 27°C - the coriolis force must be strong enough to prevent filling of the central low pressure (5o-20o) - unstable conditions throughout the troposphere - an absence of strong vertical wind shear, which disrupts the vertical transport of latent heat • The release of latent heat within the cumulus clouds causes the air to warm and expand upward Æ supports upper level divergence Æ draws air upward and promotes low pressure and convergence at the surface 9.3 Typhoon movement and dissipation • The movement of tropical systems is related to the stage in their development • Tropical disturbances and depressions are guided mainly by the trade winds Æ tend to migrate westward • The influence of the trade winds often diminishes after the depressions intensify into tropical storms • The upper level winds and the spatial distribution of water temperature more strongly determine their speed and direction Æ tending to move toward warmer seas • Typhoon and TS often move in wildly erratic ways • After making landfall, a TS may die out within a few days, it can bring very heavy rainfall hundreds of kilometers inland 9.3 Typhoon movement and dissipation (cont.) 9.3 Typhoon movement and dissipation (cont.) 9.3 Typhoon movement and dissipation (cont.) 9.4 Typhoon forecasts and advisories • The standard computer models for conventional weather and typhoon forecasting can be divided into three categories: statistical, dynamical, and hybrid • Statistical models apply information on past typhoon tracks and use those tracks as predictors for current storms • Dynamical models take information on current atmospheric and sea surface conditions and apply the governing laws of physics to current data • Hybrid models combine elements of statistical and dynamical models 9.4 Typhoon forecasts and advisories (cont.) • The models repeatedly forecast the movement and internal changes of typhoons for short time increments Æ give information on projected storm positions, air pressure, and wind at 6hr intervals • Model forecasts become less accurate as lead time increases and are unreliable for more than 72 hours • When forecasters predict that an approaching typhoon will reach land in more than 24 hours Æ issue a typhoon watch • If it is expected to make landfall within 24 hours Æ issue a typhoon warning [...]... erratic ways • After making landfall, a TS may die out within a few days, it can bring very heavy rainfall hundreds of kilometers inland 9. 3 Typhoon movement and dissipation (cont.) 9. 3 Typhoon movement and dissipation (cont.) 9. 3 Typhoon movement and dissipation (cont.) 9. 4 Typhoon forecasts and advisories • The standard computer models for conventional weather and typhoon forecasting can be divided into... depression • If the depression intensifies further and maintains wind speeds above 60 km/hr, it becomes a tropical storm • A further increase in sustained wind speeds to 120 km/hr creates a true typhoon 9. 2 Typhoon formation (cont.) • Conditions necessary for typhoon formation: - the ocean has a deep surface layer with temperatures above 27°C - the coriolis force must be strong enough to prevent filling... release of latent heat within the cumulus clouds causes the air to warm and expand upward Æ supports upper level divergence Æ draws air upward and promotes low pressure and convergence at the surface 9. 3 Typhoon movement and dissipation • The movement of tropical systems is related to the stage in their development • Tropical disturbances and depressions are guided mainly by the trade winds Æ tend.. .9. 2 Typhoon formation • Tropical disturbances are disorganized groups of thunderstorms having weak pressure gradients and little or no rotation • When a tropical disturbance develops to the point where... Dynamical models take information on current atmospheric and sea surface conditions and apply the governing laws of physics to current data • Hybrid models combine elements of statistical and dynamical models 9. 4 Typhoon forecasts and advisories (cont.) • The models repeatedly forecast the movement and internal changes of typhoons for short time increments Æ give information on projected storm positions, air