Tropical Cyclone Formation

Hurricane Katrina, as it was about to make landfall in 2005

   We are almost done with our first month of hurricane season in the Tropical Atlantic, and nothing has formed yet.  In the past 5 years, since 2005, every year has seen a tropical storm, depression, or hurricane form in June or earlier, except in 2009.  One of the earliest was in 2007, when Subtropical Storm Andrea formed on May 6, and pounded the Southeast Coast with high waves and strong winds.  This year there has been several systems that have been monitored, but none have actually organized into a cyclone.  Due to this some people were wondering why some tropical waves become organized and others do not.  To answer this question, I will describe the process in which a hurricane forms.  Then I will update you on potential hurricane development in the next few days
     Before a hurricane can become several things must happen.  First, there needs to be a preexisting area of convection, such as a weakening mid-latitude cyclone or a tropical wave from Africa.  For this system to become organized, it needs to interact with the ITCZ, when the ITCZ is located between 10 and 30 degrees north latitude.  As explained in the last article, when the ITCZ moves this far north, the Coriolis effect acts on the converging winds, turning the southeast trade winds on the south side of the boundary, to southwest winds.  When the northeast and southwest converge along the boundary, cyclonic rotation and a higher relative vorticity of the air is possible.  When a region of low air pressure has cyclonic rotation and high relative vorticity, it begins to rotate.  When the tropical wave or front interacts with this area of strong vorticity, its air pressure lowers, causing a greater rush of converging winds.  The converging winds move toward the center of lowest air pressure in the region, and with the Coriolis effect, causes the disorganized bands of heavy thunderstorms to rotate around the center of low pressure.  Soon the system becomes organized into a full-blown hurricane.  Second, there needs to be a source of energy.  This source of energy comes from warm water temperatures in the region where most hurricanes form (central and northern Caribbean eastward toward Western Africa).  The National Hurricane Center has made one of the factors of hurricane development, water temperatures of at least 80 degrees Fahrenheit.  When a tropical wave enters a region of relatively warm water, the converging winds within the tropical wave cause the warm, moist air to rise, cool, and condense into thunderstorms.  As the air within the thunderstorms cools and condenses, it releases heat to its environment, which encourages the warm, moist air to rise even more vigorously, producing more thunderstorms.  The warm, moist air also lowers the air pressure within the center of the now-organized tropical wave, causing air to move faster toward the center of the low, causing surface convergence and further lowering of the air pressure, which causes thunderstorm development and so on and so on.  The third factor is the upper level winds surrounding the tropical wave.  In order for a tropical cyclone to develop into a strong hurricane or to sustain hurricane strength, their cannot be strong wind shear in the atmosphere.  If winds in the environment surrounding the hurricane change direction with height, then its central circulation will be ripped apart because it does not encourage organized rotation.  But if winds are light at every level of the atmosphere and do not change direction with height, the tropical cyclone can strengthen and become a hurricane.  Finally, last factor deals with a hurricane position relative to land.  If a hurricane is close to land, winds in the outer bands that are affecting land will be reduced by friction, causing a reduction in convergence at the center and a rising atmospheric pressure.  If the hurricane interacts with land along the western coast of a continent, not only does friction with land weaken the hurricane, but almost always there is a cold current along the west coasts of continents, which will cut the hurricane off from its source of energy.

A simplified model of a hurricane

      Using these factors, I will give an example of how a hypothetical hurricane could form in these conditions.  Suppose a tropical wave moves off of the west coast of Africa, along the Intertropical Convergence Zone.  As the tropical wave moves along the ITCZ, the convergence zone begins to shift northward and the Coriolis effect comes into play.  Then the tropical wave, as it nears the Canary Islands in the Western Atlantic, begins to interact with this area of increased convergence and vorticity, causing a drop in central pressure and increasing thunderstorm activity.  As this process gets going, the system unexpectedly weakens as it passes over the Canary Islands.  When meteorologists examine why this system weakened, they know it was not because of the islands that it weakened (the Canary Islands are way too small to weaken a tropical wave.), they realize that the cold Canaries current weakened the storm.  The process then starts back up again as soon as the tropical wave moves into the much warmer waters of the Central Atlantic.  Very quickly, the system develops into a tropical depression and then within a few hours a tropical storm.  Then as thunderstorm activity picks up, the heat released from condensation warms the air causing a drop in central pressure.  This creates a tighter pressure gradient, which causes stronger winds.  The stronger winds whip up the ocean into huge waves, increasing evaporation.  The evaporation of the warm waters bring about added moisture to the air, which decreases air density, causing stronger rising air thermals, which further creates more thunderstorms starting the process all over again.  Soon this mess of thunderstorms begins to rotate around the low pressure center.  As the process described continues, the thunderstorm bands become even more organized and rotate around an eye-like feature.  Once the highest sustained winds within a tropical cyclone become greater than 74 miles per hour, and an eye feature is observed (which is sometimes obscured from satellites by upper level clouds) it becomes an official hurricane.  As the system intensifies, so does the process that was described earlier, causing even more intensification.  The intensification rate then slows down and even stops, due to four factors.  One:  upper level winds may have become unfavorable for greater development within the hurricane. Two: dry air may have become entangled with the system.  Three: It is to close to land.  Four: the system is at its highest level of strength possible.  This can happen when the storm uses so much energy that its energy intake equals the level of energy it puts out.  Without this, hurricanes would die after a few days.

An example of a trough and ridge

     As the hurricane approaches land (in the case of the Atlantic, the United States), meteorologists begin forecast its path and its intensity.  The main factor that controls hurricane movement is the position of the highs and lows across the Atlantic and over the United States.  If the there is an upper level ridge over the Central and Eastern United States, then the hurricane will more than likely move more westerly than northerly and hit Mexico or South Texas.  When a hurricane approaches a trough, it tends to be steered north and eastward away from the United States.  But troughs can also allow hurricanes to hit the United States.  If a trough is sitting over the Central Plains and a ridge is stagnant over the East Coast, a hurricane in the Atlantic will not be allowed to turn northward, and it will keep moving westward until hits the Gulf of Mexico where it will turn northward, and depending on the position of the trough and ridge, it could hit anywhere from Florida to the Texas Coast.
After many weeks of waiting for a tropical disturbance to form, a tropical disturbance has finally arrived and is located just to the southeast of the Yucatan Peninsula.  According to Accuweather and the NHC (National Hurricane Center), this strong tropical disturbance will move across the Yucatan Peninsula and into the Bay of Campeche, where it will interact with water that has temperatures of 82 to 86 degrees which is perfect for hurricane formation.  The curved shape of the coast in the Bay of Campeche, provides the necessary cyclonic vorticity for the storm to become organized into a tropical storm or hurricane.  Once it does become a tropical storm or hurricane, it will be called Arlene.  As to whether it will make landfall in the United States, meteorologists are currently thinking that the storm will make landfall in northern Mexico, because of a large ridge of high pressure in the central plains that is expected to develop later this week.  But, still if you are making plans to go vacationing in the Gulf Coast region, particularly along the Texas Coast, be on the look out for shower and thunderstorm activity.