Tropical Meteorology I

1. Tropical Meteorology I Weather Center Event #4

2. Tropical Meteorology • What is Tropical Meteorology? • The study of cyclones that occur in the tropics.

3. What is a tropical cyclone? • A cyclone has low sea-level pressure and cyclonic movement around the center. • A tropical cyclone is different than a mid-latitude cyclone, since the heat source is the warm oceans found in the tropics. • They form over warm ocean waters.

4. Mid-latitude cyclone north of North Dakota Hurricane Gloria

5. How does a tropical cyclone form? • Favorable conditions must be in place. • Spontaneous tropical cyclogenesis really does not occur. What types of conditions do you think would support the formation of a tropical cyclone?  There are six conditions that are important. 

6. Favorable conditions • Favorable ocean conditions • A deep mixed layer • Proximity to the equator • A pre-existing disturbance • Moist conditions in the mid-troposphere • A favorable wind shear pattern

7. Favorable ocean conditions • Ocean temperatures must be warm, generally 26°C or 79°F, or so. • The depth of the ocean must also be sufficient: generally >50m for development. • A deep mixed layer. 

8. More on a Deep Mixed Layer • Mixed layer: a layer (of the ocean from the surface down) that has fairly uniform temperatures. • This layer should be deep, not shallow. • High winds will mix up the ocean, and in the case of a tropical cyclone, will work to drive cooler waters up towards the surface.

10. Deep mixed layer, continued • A deep mixed layer will not allow much cooler water to make it to the surface. • Cool sea-surface temperatures are not favorable tropical cyclones, so this is why a deep mixed layer is important.

11. Proximity to the equator (NH) • Planetary vorticity (coriolis) drives the circulation of a tropical cyclone. • At the equator, the coriolis effect is zero and convection here may not result in rotation. • Although tropical cyclogenesis is favored in the tropics, tropical cyclones generally form at least a few degrees north of 0°.

12. Pre-existing disturbance • This disturbance or convection will work to initialize tropical cyclogenesis. • Vorticity and convergence are required to develop a cyclone. • Convection is a source for latent heat, which works to drive the intensification. • A concentrated area is also important.

13. A moist mid-troposphere • Without this moisture, evaporation will occur, leading to cooling and less convection. • Warm, moist air rises and leads to further cloud development.  Think of radar…dry slots can work to eat away at a shield of precip…dry slots or air aloft can eat away at a tropical cyclone. 

14. A favorable wind shear pattern • Similarly to dry air eating away at a tropical cyclone, wind shear can tear it apart. • Wind shear is the change of direction of winds with increasing height. • With weak wind shear, latent heat can remain in one area and convection is able to concentrate itself and further intensify.

15. Tropical cyclone classification • Tropical disturbance. • Tropical Depression. • Tropical Storm. • Hurricane.  How are each of the classifications defined?

16. Tropical disturbance • An area of clouds and thunderstorms, often referred to as a tropical wave. • A tropical disturbance is not terribly organized, with a broad area low pressure and modest winds. • Circulation is likely not clearly defined.

17. Tropical Depression • A more organized area of clouds and thunderstorms. • Has a closed surface circulation, with a fairly well defined area of low pressure. • Sustained winds are up to, but not greater than 38mph.

18. Tropical Storm • A Tropical Storm if very well organized. • An area of low pressure is clearly defined. • Winds are stronger and are centralized around the center of low pressure. • Sustained winds are between 39 and 73mph.

19. Hurricane • An organized, self-sustaining cyclone. • A central area of low pressure, that can create a very sharp pressure gradient. • An eye may develop, in which the strongest winds surround. • The sustained winds are 74 or more mph.

20. Tropical cyclone structure • A tropical cyclone is a warm-core low. • This warm core causes the thickness from the surface on up to be greater. • The storm system is vertically stacked, but high pressure occurs aloft. • The high causes air to diverge above the storm, which can be seen as anti-cyclonic flow on infrared satellite imagery.

21. Vertical structure

22. Consider the vertical structure of the mid-latitude low and Hurricane Gloria

23. Extra-tropical vertical structure

24. Tropical cyclone movement • Steering winds, generally caused by the Earth’s rotation, move the tropical cyclones. • A subtropical ridge of high pressure is generally present in the North Atlantic Ocean during the hurricane season. (Bermuda high) • With storms forming on the equatorial side of the high, they tend to move from east to west.

25. Steering winds

26. Steering winds continued • Other areas of low and high pressure also affect the track of tropical cyclones. • If a trough of low pressure approaches from the west, it will often work to curve the system up to the north and then northeast. • Other areas of high pressure or local wind flows can also work to steer a storm.

27. Forward speed • The forward speed of tropical cyclones is generally fairly slow. • With the subtropical high and usual lack of a tropical jet, there is no strong driving force. • Once tropical cyclones interact with short-waves disturbances and other troughs, they do tend to pick up forward speed.

28. Tropical cyclone dissipation • Tropical cyclones generally don’t last too long…days and occasionally a week or two. • Since many conditions must be in place for initial development, if any of those conditions are no longer met, dissipation occurs. What factors might lead to dissipation?

29. Tropical cyclone dissipation • Interaction with land. • Moving into cooler waters. • Remaining stationary for a long period. • Dry air interaction. • Wind shearing.

30. Land interaction • Although surface roughness (friction) plays a role, it is not the main cause for dissipation. • Warm waters fuel tropical cyclones and land masses are dry…limited latent heat source. • Moving into a mountain range or other higher terrain can work to weaken a system faster. • Shallow water or brief land interactions generally do not cause significant dissipation.

31. Cooler waters • As a cyclone moves over cooler waters, the fuel for the warm core of the system is gone. • Cooler waters can be due to latitude, ocean current or interaction with a previous storm. • The cooler the water, the more of a weakening effect it will have on the cyclone.

32. Sea-surface temperature example

33. Remaining stationary • If a tropical cyclone remains stationary, it begins to thoroughly mix up the ocean. • This mixing will eventually bring up cooler water temperatures to the surface. • Also, remaining stationary will rob the atmosphere of moisture and energy, especially if the outflow is great enough.

34. Dry air interaction • Dry air works to dissipate a tropical cyclone, since the warm, moist air keeps it “alive”. • Dry air may be filtered into the system, or the storm may move into a very dry air mass.

35. Wind shearing • Wind winds begin to change direction with increasing height, the storm gets torn apart. • Wind shear may also be due to velocity. • If an area of strong horizontal wind speed works into a storm (or vice virsa), this will also work to disorganize the cyclone.