1	Lecture 9: Tropical Storms and Hurricanes
2	Naming Convention
3	Ocean Temperature And Hurricane
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5	Hurricane Characteristics Definition: Hurricanes have sustained winds of 120 km/hr (74 mph) or greater. Size: Average diameters are approximately 600 km (350 mi). (one third the size of mid-latitude cyclone) Duration: days to a week or more. Strength: Central pressure averages about 950 mb but may be as low as 870 mb. Power: The energy released by a single hurricane can exceed the annual electricity consumption of the US and Canada.
6	Hurricane Seasons Hurricanes obtain their energy from latent heat release in the cloud formation process. Hurricanes occur where a deep layer of warm waters exists and during the times of highest SSTs. For the N.H., August and September are the most active months. For the S.H., the hurricane season is January-March.
7	Hurricane Structure A central eye surrounded by large cumulonimbus thunderstorms occupying the adjacent eye wall. Weak uplift and low precipitation regions separate individual cloud bands.
8	Temperature Structure Hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside. The warm core of the hurricane serves as a reservoir of potential energy, which is continuously being converted into kinetic energy by the thermally direct circulation.
9	Pressure Structure The horizontal pressure gradient with altitude decreases slowly. At about 400 mb, pressures within the storm are approximate to that outside. Surface-400mb: Cyclonic circulation. 400mb-tropopause: anticyclonic circulation.
10	Hurricane Eye and Eye Wall The eye is an area of descending air, relatively clear sky, and light winds which is about 25 km (15 mi) in diameter on average. A shrinking eye indicates storm intensification. The eye wall is comprised of the strongest winds, the largest clouds, and the heaviest precipitation with rainfall rates as high as 2500 mm/day (100 in.).
11	Hurricane Eye
12	Hurricane Formation Tropical Disturbance: Clusters of small thunderstorms. Tropical Depression: When at least one closed isobar is present, the disturbance is classified as a tropical depression. Tropical Storm: Further intensification, to wind speeds of 60 km/hr (37 mph), place the storm in the category of tropical storm. Hurricane: Hurricane status is gained when winds reach or exceed 120 km/hr (74 mph).
13	Tropical Disturbances and Easterly Waves Some tropical disturbances form in association with mid-latitude troughs migrating toward lower latitudes, some form from ITCZ-related convection, but most develop from easterly waves. Easterly waves, or undulations in the trade wind pattern, spawn hurricanes in the Atlantic (typically 2–3000 km). Only about 10% tropical disturbances intensify into more organized, rotating storms.
14	Conditions Necessary for Hurricane Formation Hurricanes form only over deep (several tens of meters) water layers with surface temperatures in excess of 27 ^oC. Poleward of about 20^o, water temperatures are usually below this threshold Hurricanes are most frequent in late summer and early autumn during high SST times Coriolis force is an important contributor, and as such, hurricanes do not form equatorward of 5^o. Need an unstable atmosphere: available in the western tropical ocean bur not in the eastern parts of the ocean. Strong vertical shear must be absent for hurricane formation.
15	Hurricane Movement Tropical disturbances and depressions are largely regulated by trade wind flow and simply move westward. For tropical storms and hurricanes, upper-level winds and ocean temperatures gain importance . Fully developed hurricanes move poleward.
16	Hurricane Dissipation After making landfall, a tropical storm may die out completely within a few days. Even as the storm weakens, it can still bring in huge amount of water vapor and rainfall hundreds of kilometers inland.
17	Hurricane Damages Heavy rainfalls Strong winds Tornadoes Storm Surges: A rise in water level induced by the hurricane.
18	Hurricane Induced Tornadoes Most hurricanes also contain clusters of tornadoes. Most pf these tornadoes occur in the right front quarter of the hurricane movement. It appears the slowing of the wind by friction at landfall contribute to the formation of tornadoes.
19	Hurricane Wind Structure Winds and surge are typically most intense in the right front quadrant of the storm where wind speeds combine with the speed of the storm’s movement to create the area of highest potential impact.
20	Storm Surges Process 1: Hurricane winds drag surface waters forward and pileup the waters near coasts. Process 2: Lower atmospheric pressure raises sea level (for every 1 mb pressure decrease, sea level raises 1 cm). Storm surges raise costal sea level by a meter or two for most hurricanes, but can be as much as 7 meters.
21	Trend .vs. Multi-decadal Change Middle 1990s-Now: A significant increase in the numbers of hurricanes and intense hurricanes making landfall in the United States. 1970s-middle 1990s: lower than normal incidence of Atlantic hurricanes. It is still in debate where the recent increase of hurricane and its intensity is a sign of global warming or a part of natural multi-decadal cycle.
22	Hurricane Forecasts The National Hurricane Center is responsible for predicting and tracking Atlantic and east Pacific hurricanes. Data are gathered through satellite observations, surface observations, and aircraft using dropsondes. Statistical, dynamic, and hybrid computer models running on supercomputers assist in future track position and storm intensity predictions. Future positions are given along six-hour trajectories with accuracy decreasing as lead time increases.
23	Hurricane Watches and Warnings Hurricane watch: if an approaching hurricane is predicted to reach land in more than 24 hours. Hurricane Warning: if the time frame is less, a warning is given.
24	Naming of Hurricanes When a tropical disturbance reaches the stage of tropical storm, it will be given a name. The name come from a A-W list created by World Meteorological Organization (WMO). Six lists are created for the Atlantic Ocean, each list is used for one hurricane season. The names of the hurricanes that cause devastating damages are removed from the list forever.
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26	Hurricane Intensity Scale The Saffir-Simpson scale. Five categories: larger numbers indicate lower central pressure, greater winds, and stronger storm surges.