Outline

1. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (1 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Further Reading: Chapter 08 of the text book Outline - cyclones and anti-cyclones - tornadoes - tropical storms - Storm surge

2. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (2 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Previously, We talked about fronts and their relationship to air masses Also looked at how interaction of air masses along a front can produce rising air and hence precipitation Today, We are going to look at weather events, in particular Tornadoes Hurricanes Impacts of extreme weather Introduction

3. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (3 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Fronts & Cyclones-1 Easterly Winds Northerly Winds Southerly Winds Westerly Winds

4. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (4 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University First note how fronts are designated on weather maps Cold fronts are shown in blue, with triangles Warm fronts are shown in red with circles Occluded fronts are a combination of circles and triangles In the early stages, we see two distinct fronts A warm front followed by a cold front Because of the position of the two fronts we get distinct patterns of wind and clouds associated with the passage of the storm Ahead of the cold front, we see: Altostratus/nimbo stratus (stable) clouds Warming with southerly winds After the passage of the cold front, we see: Heavy precip with cumulonimbus clouds Rapid cooling Northerly and westerly winds To the north of the storm we have easterly winds (remember, in this region, there are typically westerlies) -> these easterlies are typically the first sign of an approaching storm Once the cold front catches up with the warm front and the storm “pinches” off, we get the occluded front The actual pressure patterns that are associated with these storms are called “cyclones” and “anticyclones” Fronts & Cyclones-2

5. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (5 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cyclones are associated with low pressure centers The related convergence produces uplift in the center and divergence aloft As we know, divergence is associated with high pressures, i.e., we find high pressures aloft Remember uplift also produces adiabatic cooling Low pressures are typically associated with weather ranging from mild cloud cover to heavy precipitation Cyclones Divergence H L Convergence

6. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (6 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Antiyclones are associated with high pressure centers The related divergence produces sinking (or subsidence) in the center and convergence aloft As we know, convergence is associated with high pressures, i.e. we find low pressures aloft Because the air is sinking, we usually get adiabatic warming, i.e. moisture re-evaporates High pressures are typically associated with clear skies Anti-Cyclones Convergence L H Divergence

7. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (7 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cyclones & Anti-Cyclones Traveling cyclones are of three kinds – 1. Wave Cyclones: found in mid- and high-latitudes, generally along the polar front, cold dry air from north pushes south, warm moist air from south pushes north, the cyclonic activity grows from an initial wave disturbance to result in warm moist air which precipitates as it rises. 2. Tropical Cyclones 3. Tornadoes

8. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (8 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Tornadoes • Usually these are very small phenomenon - approximately 300m in diameter • Winds however can become very intense - approx. 300-400 km/h • How they form • Usually there is strong wind shear - change in wind with height • This produces roll-like features • Now assume that there is also convective activity • The convection lifts one end of the roll and pushes down the other end, creating a vertical funnel Convection Cell Wind Shear Z N E

9. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (9 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Tornado Alley • From this we can see that tornadoes require certain conditions for formation • Associated with regions of strong convection • Usually in the spring and summer • Typically in the southeastern US • Need strong wind shear to initiate the tornado • found in advance of cold fronts where maritime polar air lifts warm moist maritime tropical air (mid-latitude phenomenon)

10. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (10 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Tropical Storms • Can take three forms • Easterly waves - local troughs moving westward within the easterly trades • Equatorial lows - weak low pressure zones within the equatorial trough • Tropical Cyclones (or Hurricanes in Western Hemisphere, Typhoons in western Pacific of the coast of Asia and Cyclones in the Indian ocean) - intense storm of tropical origin • Characteristics of tropical storms • Form in a belt between 8-15N & S • Associated with tightly spinning cyclone • Very low pressure at the center -> results in descending, calm air in the eye • Form in the tropical regions of Atlantic, Pacific, and Indian Oceans • Tend to form in late summer and early Autumn

11. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (11 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Development of Tropical Cyclones-1 The Movie 2) Convergence of air forces air to rise 1) Warm, moist air spirals towards center of Hurricane 3) As air rises, moisture condenses out, warming air and allowing it to continue rising L L 5) As pressures decrease, winds intensify and even more warm moist air is forced to converge and rise 4) As air rises, it causes even lower pressures at the surface

12. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (12 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Development of Tropical Cyclones-2 • We see certain stages in the development of hurricanes • Typically, there is cold, unstable air over the mid-latitudes which moves over the tropics, initiating convection • As convection occurs, latent heat is released, warming the upper-air and producing high pressures aloft • The outflow aloft coupled with the convection produces low pressures at the surface • Cyclonic circulation forms around the low pressure, resulting in the convergence of air • More convergence leads to more convection and more release of latent heat • Hence the hurricane continues to grow

13. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (13 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Certain special conditions about the tropics that allow Tropical cyclones to form and become as intense as they do There is high pressure in the subtropics which leads to stable air, but there is a zone of low pressure and deep convection associated with the ITCZ - this is typically where these storms form High humidity Warm oceans (>26C) Ultimately, the energy that drives a hurricane comes from the conversion of sensible heat (i.e. warm sea surface temperatures) into latent heat and kinetic energy Weak Coriolis force because we are at low latitudes - winds have to be very intense for the Coriolis force to be large enough to balance the Pressure gradient force (by definition hurricanes have winds > 71mph) There are also conditions which hamper their formation An inversion or sinking air aloft Lack of moisture Development of Tropical Cyclones-3

14. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (14 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Movement of Tropical Cyclones • These storms tend to move westward • The storms first move westward due to steering by the easterly winds aloft over the tropics • Eventually they get caught up in the suptropical high and veer in a clockwise direction, bringing them north -> this is typically when they move up the coast and make landfall

15. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (15 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Storm Surge • Damage from hurricanes comes in many forms but by far the most destructive is the storm surge • High seas, large waves, and flooding associated with a rise of local sea level accompanying the passage of a hurricane • Storm surge movie: • Here we see that the storm surge is caused by two factors: one is the winds pushing water on shore, the other associated with water being lifted by the low pressure in the center of the storm

16. Myneni L22: Hurricanes and Tornadoes Mar-19-07 (16 of 16) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Storm Surge • The key here is in the overall rise in the water level • Once the water rises, the destructive power of the waves can affect low-lying structures which would typically have been above sea-level. • Hence the regions that are most susceptible are the low-lying areas • Bangaldesh: • In 1970 a hurricane came onshore in this region and killed half a million people in this region alone - Katrina in 2005 in the USA