Tropical Storms & Hurricanes Part 1. August 1992

Transcription

1 Lecture #16 (April 5, 2010, Monday) Tropical Storms & Hurricanes Part 1 August 1992

2 Tracks of rain accumulation & clouds from Hurricane Rita September 2005

3 Tropical Cyclones Around the Globe Why none here? Why none here? Hurricane : N. Atlantic Ocean, NE Pacific Ocean, SE Pacific Ocean Typhoon : NW Pacific Ocean Severe tropical cyclone : SW Pacific Ocean, SE Indian Ocean Eevere cyclonic storm : N Indian Ocean Tropical cyclone : SW Indian Ocean

4

5 The relative sizes of the United States, Typhoon Tip and Cyclone Tracy (the largest and one of the smallest tropical cyclones recorded, respectively)

6 Global satellite image of Typhoon Tip near peak strength

7 Typhoon Tip at its record peak intensity Category 5 on October 12, 1979

8 Formed: October 4, 1979 Dissipated: October 19, 1979 Highest winds: 260 km/h (160 mph) (10-minute sustained) Typhoon Tip Storm path 305 km/h (190 mph) (1-minute sustained) Lowest pressure: 870 hpa (mbar) (Worldwide record low) Fatalities: 86 direct, 13 indirect Areas affected: Guam, Japan

9 Severe Tropical Cyclone Tracy Category 4 cyclone (Australian scale) Category 3 cyclone (SSHS)

10 Formed: 21 December 1974 Dissipated: 26 December 1974 Highest winds: 205 km/h (125 mph) (1-minute sustained) 240 km/h (150 mph) (gusts) Severe Tropical Cyclone Tracy Storm path Lowest pressure: 950 hpa (mbar) Fatalities: 71 Damage: $1.1 billion (1974 USD) $5 billion (2009 USD) Areas affected: Tiwi Islands, Darwin, Northern Territory

11 Devastation brought by Cyclone Tracy

12 Cyclone Catarina, a rare South Atlantic tropical cyclone, viewed from the International Space Station in March 2004

13 Catarina, a category 2 tropical cyclone, was approaching the Brazilian coastline on March 27, 2004 near peak intensity.

14 Cyclone Catarina at landfall

15 Formed: March 24, 2004 Dissipated: March 28, 2004 Highest winds: 155 km/h (100 mph) (1-minute sustained) Lowest pressure: 972 hpa (mbar) (Worldwide record low) Fatalities: 3-10 direct, 13 indirect Cyclone Catarina Storm path Damage: $350 million (2004 USD) $399 million (2009 USD) Areas affected: Santa Catarina and Rio Grande do Sul, Brazil

16 Structure of a tropical cyclone A cross section diagram of a mature tropical cyclone, with arrows indicating air flow in and around the eye

17 Hurricane Structure Northern Hemisphere H L Eye wall

18 A cross section of a typical hurricane Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a positive feedback loop over warm ocean waters.

19 ? A cross section of a typical hurricane

20 Katrina's Category 4 hurricane force winds were observed by NASA s QuikSCAT satellite on August 29, knot = mph = kmph

21 Temperature distribution across a hurricane Why the air temperature is much higher in the eye? (1) Latent heat is released from condensation in the formation of eye wall clouds. (2)Sinking air in the eye is compressed and warmed adiabatically.

22 Hurricane Characteristics Eye: center area, descending air, light winds, average diameter of about 25 km (15 mi). A shrinking eye indicates intensification. Eye wall: strongest winds and convergent uplifting, largest cumulonimbus clouds, heaviest precipitation as much as 2500 mm (100 inch) per day. Weak uplift or even sinking air and low precipitation regions separate individual spiral cloud bands. Lower portions: counter-clockwise rotation, pressure differences into the center of the storm are about twice as great as the average mid-latitude cyclones, plus smaller size, thus much greater pressure-gradient force resulting in strong sustained winds Upper portions: clockwise rotation, the storm are also blanketed by a cirrus cloud cap due to overall low temperatures.

23 Hurricane Characteristics Unlike mid-latitude cyclones, hurricanes are warm-core lows due to warm ocean surfaces. As air converges into the surface low pressure center, adiabatic cooling due to expansion keeps the horizontal temperature differences moderate towards the eye. Horizontal pressure gradient decreases slowly with altitude in the eye due to pressure decreases more slowly with increasing altitude in the warm core. At about 400 mb (7.5 km), pressures within the storm are approximate the same as outside pressure. From 400 mb to the tropopause, pressures within the storm exceed those outside upper portion of the storm rotates anticyclonically while lower portions rotate cyclonically.

24 Typically, eyes are easy to spot using weather radar. This radar image of Hurricane Andrew clearly shows the eye over southern Florida.

25 Radar Image of Hurricane Katrina as it approached LA coast

26 Eye Eye wall

27 A satellite photo of Typhoon Amber of the 1997 Pacific typhoon season, exhibiting an outer and inner eyewall while undergoing an eyewall replacement cycle.

28 A picture of Hurricane Wilma's eye taken at 08:22 CDT (13:22 UTC) October 19, 2005, by the crew aboard the International Space Station. Wilma was at peak intensity at the time, with a minimum central pressure of only 882 mbar (26.06 inhg), making it the strongest Atlantic hurricane in history. Not only is this a classic example of a pinhole eye, but also of the stadium effect, where the eyewall slopes out and up.

29 Hurricane Characteristics Most powerful of all storms, energy unleashed by a single hurricane can exceed the annual electrical consumption of US & Canada. Most energy attained by hurricanes stems from latent heat release in the cloud formation process. Sustained winds > 120 kmph (74 mph). Generally of lesser intensity than tornadoes, but much larger in size and longer in life span makes hurricanes much more devastating. Average diameters are approximately 600 km (350 mi), typically ~ 1/3 of midlatitude cyclones, but no fronts. Central pressure averages about 950 mb & may be as low as 870 mb. Hurricanes occur during the times of highest SSTs. For the Northern Hemisphere, August and September are typically the most active months with highest SSTs.

30 Extratropical storms are areas of low pressure which exist at the boundary of different air masses. Almost all storms found at mid-latitudes are extratropical in nature, including classic North American nor'easters and European windstorms. The most severe of these can have a clear "eye" at the site of lowest barometric pressure, though it is usually surrounded by lower, non-convective clouds and is found near the back end of the storm. Subtropical storms are cyclones which have some extratropical characteristics and some tropical characteristics. As such, they may have an eye, but are not true tropical storms. Subtropical storms can be very hazardous, with high winds and seas, and often evolve into true tropical storms. As such, the National Hurricane Center began including subtropical storms in their naming scheme in 2002.

31 The North American blizzard of 2006, an extratropical storm, showed an eye-like structure at its peak intensity (here seen just to the east of the Delmarva Peninsula).

32 Extraterrestrial storms A hurricane-like storm on the south pole of Saturn displaying an eyewall tens of kilometers high

33 Tropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which mid-latitude cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale or SSHS).

34 Requirements for tropical cyclone formation 1. Sufficiently warm sea surface temperatures 2. Atmospheric instability 3. High humidity in the lower to middle levels of the troposphere 4. Enough Coriolis force to develop a low pressure center 5. A pre-existing low level focus or disturbance 6. Low vertical wind shear. These conditions are necessary for tropical cyclone formation, but they do not guarantee that a tropical cyclone will form.

35 SST The Tropical Setting

36 The Tropical Setting Depth of 26 C isotherm on October 1, 2006 Normally, an ocean temperature of 26.5 C (79.7 F) spanning through at least a 50-m depth is considered the minimum to maintain the special mesocyclone that is the tropical cyclone.

37 Hurricane Katrina, August 25-27, A hurricane needs SSTs at o F (~ o C) or warmer to strengthen (NHC).

38 On rare occasions, tropical storms may form or strengthen in mid-latitudes. Hurricane Vince formed in the temperate subtropics during the 2005 Atlantic season

39 The Tropical Setting Subtropical High (Bermuda-Azores High) Warm water Cold water Trade wind inversion

40 Hurricane Formation Tropical disturbances: often begin in eastern ocean basins as disorganized clusters of thunderstorms with weak pressure gradients. Some form when mid-latitude troughs migrating into the tropics. Some form as part of convection associated with ITCZ. An easterly wave

41 Hurricane Formation Most tropical disturbances entering western Atlantic and becoming hurricanes originate in easterly waves, which are large undulations or ripples in normal trade wind pattern An easterly wave These waves typically stretch between km ( mi)

42 Waves in the trade winds in the Atlantic Ocean areas of converging winds that move along the same track as the prevailing wind create instabilities in the atmosphere that may lead to the formation of hurricanes.

43 The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is maximum and easterly. The jet develops because heating of the West African land mass during the Northern Hemisphere summer creates a surface temperature and moisture gradient from the Gulf of Guinea and the Sahara, and the atmosphere responds by generating vertical wind shear to maintain thermal wind balance. During the mature phase of the West African Monsoon (August to September) maximum mean wind speeds in the jet of approximately 13 m/s are located around 4 N 5 N at a height of 4 km (or 650 mb). The jet produces synoptic scale, westward propagating disturbances known as African easterly waves ( tropical waves). A small number of mesoscale storm systems embedded in these waves develop into tropical cyclones after they move from west Africa into the tropical Atlantic.

44 A minimum distance of 500 km (300 miles) from the equator is normally needed for tropical cyclogenesis. The role of the Coriolis force is to provide for gradient wind balance (curved isobars). Schematic representation of flow around a low-pressure area (in this case, Hurricane Isabel) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows

45 Infrared image of a powerful southern hemisphere cyclone, Monica, near peak intensity, showing clockwise rotation due to the Coriolis effect

46 Most hurricanes form between the latitudes of 5 and 20 in tropical oceans, except over the South Atlantic and Southeastern Pacific. Why?

47 Conditions Necessary for Hurricane Formation Hurricanes form only over deep warm surface ocean layers with surface temperatures in excess of o C (81-82 o F). Energy is derived from evaporation of surface water and then latent heat is released when clouds are formed. Poleward of about 20 o, water temperatures are usually below this threshold. Hurricanes are most frequent in late summer and early autumn during highest SSTs. Coriolis force is an important factor for rotation to form, as such, hurricanes do not form equatorward below 5 o latitude. An unstable atmosphere is also necessary and this typically occurs toward the central to western ocean basins as trade wind inversions and cool ocean surfaces dominate over eastern ocean basins. Strong vertical shear must be absent for hurricane formation because it disturbs supply of moisture to the hurricanes.