Lecture #17 (April 7, 2010, Wednesday) Tropical Storms & Hurricanes Part 2 Hurricane Katrina August 2005
All tropical cyclone tracks (1945-2006).
Hurricane Formation While moving westward, tropical disturbances intensify as surface ocean waters warm beneath them. Take about 1 week to traverse the Atlantic as average migrating speeds are about 15 35 kmph (10 20 mph). Only ~ 10% intensify into more organized, rotating storms. When at least one closed isobar is present on weather map, the disturbance is classified as a tropical depression. A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 17 m s -1 (33 knot) or 39 mph (63 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression".
Hurricane Formation Further intensification, to wind speeds of 63 km/h (39 mph), place the storm in the category of tropical storm. A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 metres per second (33 kn) (39 miles per hour (63 km/h)) and 32 metres per second (62 kn) (73 miles per hour (117 km/h)). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term named storm). Hurricane status is gained when winds reach or exceed 120 kmph (74 mph). A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 33 metres per second (64 kn) or 74 miles per hour (119 km/h). A cyclone of this intensity tends to develop an eye.
Three tropical cyclones at different stages of development. The weakest (left), demonstrates only the most basic circular shape. A stronger storm (top right) demonstrates spiral banding and increased centralization. The strongest (lower right) has developed an eye.
Tropical cyclone scales Should a tropical cyclone form in the North Atlantic Ocean or the Northeastern Pacific Ocean, then it will classified using one of the categories in the Saffir-Simpson Hurricane Scale. In the Western Pacific, tropical cyclones will be ranked using the Japan Meteorological Agency's scale. The Regional Specialized Meteorological Centre in New Delhi, India also uses a different scale to assess the maximum sustained winds of a tropical cyclone. In the Southern Hemisphere, the Météo-France forecast center on La Reunion in France uses a scale that covers the whole of the South West Indian Ocean. Both the Australian Bureau of Meteorology and the Regional Specialized Meteorological Centre in Nadi, Fiji use the Australian tropical cyclone intensity scale.
Hurricane Intensity Scale The Saffir-Simpson hurricane scale classifies hurricanes into 5 categories based on central pressure, maximum sustained wind speed, storm surge.
Criticism Some scientists, including Kerry Emanuel and Lakshmi Kantha, have criticized the scale as being too simplistic, indicating that the scale does not take into account the physical size of a storm, nor the amount of precipitation it produces. Additionally, they and others point out that the Saffir-Simpson scale, unlike the Richter scale used to measure earthquakes, is not open-ended, and is quantized into a small number of categories. Proposed replacement classifications include the Hurricane Intensity Index, which is based on the dynamic pressure caused by a storm's winds, which bases itself on surface wind speeds, the radius of maximum winds of the storm, and its translational velocity. Both of these scales are continuous, akin to the Richter scale; however, neither of these scales have been used by officials.
Category 6 After the series of powerful storm systems of the 2005 Atlantic hurricane season, a few newspaper columnists and scientists brought up the suggestion of introducing Category 6, and they have suggested pegging Category 6 to storms with winds greater than 174 or 180 mph (78 80 m/s; 150 155 knots; 280 290 km/h). Only a few storms in history have reached into this hypothetical category. Many of these storms were West Pacific super typhoons, most notably Typhoon Tip in 1979 with sustained winds of 190 mph. According to Robert Simpson, there is no reason for a Category 6 on the Saffir-Simpson Scale because it is designed to measure the potential damage of a hurricane to human-made structures. If the wind speed of the hurricane is above 155 mph (250 km/h), then the damage to a building will be "serious no matter how well it's engineered". However, the result of new technologies in construction leads to the suggestion that an increase in the number of categories is necessary.
Destruction by Hurricanes High winds: excessive damage even to well-built buildings Heavy rainfall and flood: large amounts of property damage Storm surge: large amounts of damage along coastal regions Storm surges occur as water piles up due to both heavy winds and low atmospheric pressure Hurricane Camille (1969) caused a storm surge of 7 m (23 ft) along the Mississippi coast. Additionally, high surf occurs atop the surge, increasing damage 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 Tornadoes: This area also produces the greatest frequency of tornadoes within the hurricane due to frictional drag of lower atmospheric winds upon landfall.
The National Hurricane Center forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes.
Wind speed variations by quadrant
Water levels may decrease Max storm surge
Tornadoes most often form in the right quadrants relative to hurricane storm center in the direction of storm movement.
Movement and track Tropical systems, while generally located equatorward of the 20th parallel, are steered primarily westward by the east-to-west winds on the equatorward side of the subtropical ridge a persistent high pressure area over the world's oceans. In the tropical North Atlantic and Northeast Pacific oceans, trade winds another name for the westward-moving wind currents steer tropical waves westward from the African coast and towards the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out. These waves are the precursors to many tropical cyclones within this region. In the Indian Ocean and Western Pacific (both north and south of the equator), tropical cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough, rather than by easterly waves. Tropical cyclones can also be steered by other systems, such as other low pressure systems, high pressure systems, warm and cold fronts.
Hurricane Movement and Dissipation Movement depends upon the stage of development. Tropical disturbances and depressions are largely regulated by trade wind and simply move westward. For tropical storms and hurricanes, upper-level winds and ocean temperatures gain importance in determining their movements. In the Atlantic, storms that gain latitude recurve toward the northeast due to the influence of surface and upper-level westerlies. Movement is essentially parabolic. However, movement may be highly erratic in particular storms.
The Tropical Setting Subtropical High (Bermuda-Azores High) Warm water Cold water Trade wind inversion
Hurricane Movement and Dissipation
Examples of erratic hurricane paths
Hurricane Movement and Dissipation Hurricanes die when They move over waters that cannot supply warm moist air. They move over land. They reach a location where the flow aloft is unfavorable (e.g., strong wind shear).
Hurricane Forecasts and Advisories The National Hurricane Center (NHC) 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 6-hour trajectories with accuracy decreasing as lead time increases.
Hurricane Watches and Warnings A watch is administered if an approaching hurricane is predicted to reach land in more than 24 hours. If the time frame is less, a warning is given. The erratic nature of the systems leads to difficulties in exact prediction, warning, and evacuation of prone areas.