Tropical Cyclone Formation 29 06 2012 20 40 00 Naming Tropical Cyclones In 1979 the World Meteorological Organization a part of the United Nations developed the current name list Rules for North Atlantic Basin Hurricane Names Reflect Geographic Region Spanish English and French Short and Distinctive Only 21 names no Q U X Y Z M F Alternate Greek Alphabet Only used if we run out of names Names Repeat every 7th year Typical North Atlantic season 10 named tropical cyclones o In a typical year we make it to the letter J o In 2005 there were 27 named storms which forced the name list 6 letters deep into the Greek alphabet The last named system of that year was Hurricane Zeta Rules for retiring tropical cyclone names Only done if the storm was particularly devastating such that using this name in the future would be insensitive Also retire those hurricanes that are studied a lot A Tropical Cyclone is given its name once its winds have achieved a speed to categorize it as a tropical storm Stages of Development and Size Duration and Strength 1 Tropical Disturbance Cluster of tropical thunderstorms Watched or investigated for further development Winds do not exceed 24 knots 150 350 miles in diameter In Atlantic Ocean most tropical disturbances start off the coast of Africa and move their way across the Atlantic Ocean toward the US Mexico Carribeean 2 Tropical Depression Organized cluster of tropical thunderstorms with sustained winds between 22 38 mph Surface low pressure area develops actually a depression in the surface pressure field o Storm orbits around that low pressure area which is a counter clockwise direction in the northern hemisphere Assigned a number at this point 3 Tropical Storm cluster of intense t storms circulating around a deep low pressure center with sustained winds of 39 mph 73 mph assigned a name 4 Hurricane Large organized system of intense tropical t storms circulating around a very deep low pressure center 74 mph sustained winds torrential rains and storm surge 5 30 feet Saffir Simpson Scale standard scale that is used to rank hurricanes based upon their intensity developed by Herbert Saffir an engineer and Bob Simpson former director of the National Hurricane Center Their scale is primarily based upon maximum sustained wind speed where sustained wind is defined as a 1 minute averaged wind This is done so as to not include gusts in the hurricane ranking Hurricane Structure and Formation Formation What gets them started Goal trigger the development of tropical thunderstorms achieved through surface convergence of air when air collides at the surface and has nowhere to go but up When air rises it cools and when it cools there is condensation which leads to clouds and precipitation All of which is necessary for a hurricane to form o Intertropical Convergence Zone ITCZ a preferred location for thunderstorm formation a major convergence zone in the topics a source of rising air triggers updrafts rising air in a thunderstorm Structure Typical lifetime of a hurricane 3 days to a weeks sometimes longer Typical diameter of a hurricane 300 600 miles wide Typical diameter of eye 5 50 miles wide Pressure Hurricanes are intense low pressure systems and the lowest pressure is always found at the very center of the eye Notice on this graph that the largest drop in pressure happens in the eyewall This rapid drop in pressure results in a very strong pressure gradient force which gives rise to the hurricane force winds Winds The profile of the hurricane winds reinforces what you learned above the fastest winds are in the eyewall the winds are calm in the eye and hurricane force winds 74 mph are confined primarily to the eyewall Notice also in this image that the wind speed on the right hand side of the storm is greater than on the left Rainfall When you examine the rainfall pattern notice two things First notice the banded structure of the spiral rainbands and second notice that as you get closer to the eyewall the intensity of the rain increases Finally see that in the eye there is no rain Temperature When reviewing the temperature profile remember what we learned about the movie The Day After Tomorrow The eye is warm because the air in the eye is descending and warming due to compression Storm Surge Storm surge is simply a rise in sea level due to windblown water Therefore it makes sense that the greatest storm surge will exist on the same side of the hurricane that the strongest winds are on Coriolis Force The Coriolis Force acts at a 90 degree angle to the right of the motion of the air The Coriolis Force only affects the direction the air is traveling not its speed If an object is standing still there is no Coriolis Force acting upon it The strength of the Coriolis Force is proportional to the speed of the wind The Coriolis Force is strongest at high latitudes near the North or South Pole and weakest at low latitudes nonexistent on the equator Key Features of the Hurricane Hurricane Eye actually the calmest part of the storm Location of the low pressure center o Pressure readings from the strongest hurricanes can often dip below 900 mb the same pressure you would find at an altitude of 3 4 mile above sea level on a normal day Mostly clear skies o The air descends in the eye and descending air warms by compression at a rate of 10 C km which evaporates the clouds that would otherwise exist there o Sometimes the clouds that exist in the eye take on a really spectacular pinwheel structure Check out Figure 24 5 in the text if you are interested in learning more Winds are calm o At the center of the hurricane the pressure gradient force is approximately equal to zero which means there there is no wind there Eyewall Most intense part of the hurricane although rather focused and small Maximum extent 50 100 miles from the center of the hurricane The location of the heaviest rains The location of the fastest winds o Faster on the right hand side of the storm The location of the greatest storm surge Spiral Rainbands Narrow bands of intense rainfall Multiple bands of clouds that all feed into the center of the hurricane Help supply the eyewall with plenty of warm moist air Can extend hundreds of miles from the eye Four Ingredients Conductive to Hurricane Development Ingredient 1 Warm Sea Surface Temperatures 80 deg F Measured with infrared satellites and a buoy network The strength of a hurricane is directly related to the sea surface temperature Energy within a hurricane comes from