Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

1. Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt Anthony J. Vega

2. Part 4. Disturbances Chapter 12 Tropical Storms and Hurricanes

3. Introduction • Hurricanes are responsible for astonishing amounts of property damage and loss of life in many regions of the world The propagation of Hurricane Andrew

4. Hurricanes Around the Globe • Atlantic and eastern Pacific tropical cyclones are known as hurricanes, while over the western Pacific they are referred to as typhoons • Over the Indian Ocean and Australia they are known as cyclones • The eastern North Pacific has the highest frequency of tropical cyclones for the globe with an annual average of 16 • The South Atlantic produces none as the basin is too small to initiate cyclogenesis Tropical cyclone genesis areas and related storm tracks

6. The Tropical Setting • A subsidence inversion, or trade wind inversion, forms on the eastern side of the subtropical anticyclones • Below this, a marine layer of cool, moist air resides • The marine layer is shallowest and the inversion lowest towards the eastern basin edges where cold water upwelling and cold ocean currents dwell • Towards the western edges of the ocean basins the marine layer warms with corresponding surface temperatures and expands to greater heights • Convection results in large cumulonimbus clouds over these regions as opposed to the eastern areas, where only stratus may exist

7. Hurricane Characteristics • Hurricanes, the most powerful of all storms, have sustained winds of 120 km/hr (74 mph) • Although of lesser intensity than tornadoes, the much larger size and longer life span makes hurricanes much more devastating • Average diameters are approximately 600 km (350 mi) and central pressure averages about 950 mb but may be as low as 870 mb • Most energy attained by hurricanes stems from latent heat release in the cloud formation process • Hurricanes occur where warm waters abound and during the times of highest SSTs • For the N.H., August and September are the most active months • Hurricanes consist of: • A central eye surrounded by large cumulonimbus thunderstorms occupying the adjacent eye wall • Weak uplift and low precipitation regions separate individual cloud bands • Pressure differences into the center of the storm are about twice as great as the average mid-latitude cyclone, resulting in strong sustained winds

8. Unlike mid-latitude cyclones, hurricanes are warm-cored lows as a result of adiabatic expansion of in-rushing air • This results in only slight horizontal temperature differences toward the eye • Latent heat release from condensation causes the eye to be much warmer than the surrounding storm • The horizontal pressure gradient with altitude decreases slowly • At about 400 mb, pressures within the storm are approximate to that outside • From 400 mb to the tropopause, pressures within the storm exceed those outside, so that the upper portion of the storm rotates anticyclonically while lower portions rotate cyclonically • The upper portions of the storm are also blanketed by a cirrus cloud cap due to overall low temperatures • The Eye and Eye Wall • The eye is an area of descending air 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.)

9. Sinking air, warming adiabatically, causes air in the eye to be warmer than elsewhere • Relative humidities are lower in this region due to higher temperatures Tropical cyclone structure

10. Vertical temperature profile across a hurricane

11. Hurricane Formation • Tropical disturbances often begin in the eastern ocean basins as disorganized clusters of thunderstorms • Some form in association with mid-latitude troughs migrating toward lower latitudes but most form from ITCZ-related convection • Easterly waves, or undulations in the trade wind pattern, spawn hurricanes in the Atlantic • Depicted by plotting streamlines, or lines of wind direction • Waves typically stretch between 2000–3000 km (1200–1800 mi) • Close-fitting streamlines east of the wave axis indicate zones of convergence, while west of the axis streamlines indicate divergence • The disturbance is located upstream of the wave axis • Atlantic hurricanes form from seedlings emanating from western Africa • Move westward in the trade wind flow and intensify as waters warm beneath them • Take about 1 week to traverse the Atlantic as average speeds are about 15–35 km/hr (10–20 mph) • Only about 10% intensify into more organized, rotating storms

12. When at least one closed isobar is present, the disturbance is classified as a tropical depression • Further intensification, to wind speeds of 60 km/hr (37 mph), place the storm in the category of tropical storm • Hurricane status is gained when winds reach or exceed 120 km/hr (74 mph) • A high percentage of depressions become tropical storms and an even higher percentage reach hurricane status An easterly wave

13. Conditions Necessary for Hurricane Formation • Hurricanes form only over deep water layers with surface temperatures in excess of 27 oC (81 oF) • Energy is derived from latent heat release and associated evaporation of water • Poleward of about 20o, 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 5o • 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 eastward • Strong vertical shear must be absent for hurricane formation • Upon formation, hurricanes are self propagating but are limited by the supply of latent heat from warm ocean waters

14. Hurricane Movement and Dissipation • Movement is dependent upon the stage of development • 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 • Movement is essentially parabolic; however, movement may be highly erratic in particular storms • In the Atlantic, storms that gain latitude recurve toward the northeast due to the influence of surface and upper-level westerlies Examples of erratic hurricane paths

15. Destruction by Hurricanes • Hurricane winds cause excessive damage even to well-built buildings • Heavy rainfall is also responsible for large amounts of property damage • The storm surge is responsible for a large percentage of damage along coastal regions • Storm surges occur as water piles due to heavy winds and low atmospheric pressure and increase with storm intensity • Hurricane Camille 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 • This area also produces the greatest frequency of tornadoes within the hurricane due to frictional drag of lower atmospheric winds upon landfall

16. Wind speed variations by quadrant

17. Average tornado locations relative to hurricane storm center

18. Hurricane Forecasts and Advisories • 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 • 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 • Hurricane Intensity Scale • The Saffir-Simpson scale classifies hurricanes into five categories based on central pressures, maximum sustained wind speeds, and storm surge

20. End of Chapter 12Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt