By Jonathan Edwards

   Tropical Cyclone Genesis is the technical term for the process of storm formation that leads ultimately to
   what are called hurricanes, typhoons, or tropical cyclones in various parts of the world.

   This occurs when, in the Northern Hemisphere, the Intertropical Convergence Zone, or ITCZ, shifts
   northward out of the doldrums and atmospheric conditions become favorable for tropical cyclone formation
   after about the middle of May.

   A series of low-pressure ripples develops within the ITCZ. These are known as tropical waves and
   progress from east to west. In the late season, they typically shift their movement toward the west-nothwest,
   or even northwest, after crossing 45° or 50° W longitude.

   These tropical waves, ideally imbedded in the deep layer easterly flow, contain a northeast wind shift. Gusts
   up to 25 mph may occur. Sometimes there can be gusts to tropical storm force in stronger waves. There can
   be next to no weather associated with these waves, and they may pass virtually unnoticed. More typically,
   there are bands of disturbed weather riding the axis of the wave.

   When the wave passes over warmer waters (SSTs), convection and resulting rainfall are enhanced. This
   greater rainfall is concomitant with falling surface pressures. By the time these pressures fall to 1008mb, it is
   likely that the northeast wind has closed off to a southwest wind on the backside of the wave. The forward
   motion of the wave completes the closure on the northern side of a broad low-level center, and a tropical
   depression has formed.

   We often hear that a tropical depression has formed, but conditions are unfavorable for further development.
   There are two conditions that must be present for the tropical depression to continue its development: warm
   SSTs (above 80° Fahrenheit) and low vertical, or southwesterly shear. A tropical cyclone derives its power
   from the warm waters below. In addition, a strong anticyclone directly above the low-level inflow is
   favorable. As a tropical cyclone is sucking in warm, moist air at the surface, it must also evacuate this
   inbound flow aloft. This occurs in the upper levels of the atmosphere, where high pressure facilitates the
   development of the cyclone by sucking the inflow up through the core. Every powerful hurricane has an
   equally powerful high pressure system over it. The key is inbound air counterclockwise at the bottom,
   outbound air clockwise aloft. In the Southern Hemisphere, it is reversed: clockwise inbound,
   counterclockwise outbound.

   If the upper-level high pressure system does not develop over our cyclone, it means there is shear instead.
   This is a strong jet of air that is blowing directly over the cyclone, and ripping the tops off the deep
   convection. This has the effect of breaking down the whole mechanism. This is known as vertical, or
   southwesterly shear. Vertical shear may occur when the cyclone is located at the edge of an upper-level
   high, in which case it is known as vertical shear, because the air on the edge of the high is sinking. This is
   also why a tropical storm that tries to form near an established hurricane has a very difficult time—it is
   on the edge of the hurricane’s upper-level outflow, and the sinking air aloft is creating a sheared
   environment. Southwesterly shear may occur when the tropical cyclone is impinged upon by a nearby
   upper-level trough. These are typically mid-latitude troughs in the upper-level westerly flow, and they may
   have enough amplitude to drive a wedge of southwesterly winds into the northern semicircle of the
   upper-level cyclonic outflow. The effect can range from impaired strengthening to catastrophic failure of the
   tropical cyclone’s support structure.

   Assuming all the ingredients are in place—warm SSTs, upper-level high pressure, and falling surface
   pressures—the cyclone will develop and reach a point of rapid intensification. It is one of
   nature’s perfect machines. As warm waters feed the convection swirling around the center, heavy
   rainfall lowers surface pressures, high pressure aloft evacuates the inflow, which intensifies the inflow of
   warm, moist air, which in turn increases the rainfall and brings about a more rapid fall in central pressure.

   Eye formation begins when a tropical storm reaches approximately 65mph, provided conditions are
   favorable for strengthening to continue. The eyewall begins to make its appearance, usually on the eastern
   (Northern Hemisphere) edge of the center. As the system becomes better organized and stronger, the center
   contracts from about 200 miles across to roughly 90 miles at this stage. An increase in rotational velocity
   accompanies the smaller, more defined center. The inflow is spiralling in ever faster as it is evacuated up
   through the developing eyewall and out by the high pressure outflow structure. The eye begins to appear as
   a clear spot in the center, as the air here is sinking. This removes almost all convection from the very center
   as the hurricane matures with an eye anywhere from 10 to 40 miles in diameter. The eyewall becomes very
   pronounced in satellite imagery as a “doughnut” with very cold cloud tops, as the convection
   builds well into the stratosphere.

   Tropical cyclones can exhibit a great deal of durabilty provided that the upper level support remains and the
   southerly (Northern Hemisphere) inflow is present. The worst thing that can happen is for this southerly
   inflow to get cut off. Here are some examples: In 1998, Hurricane Mitch developed into a supermassive
   Category 5 hurricane. Nevertheless weakening began when the center moved to a position directly north of
   Honduras, cutting off the southerly inflow, even though the eye was still over a hundred miles off shore.
   Later, Mitch maintained his mid-level core against all odds over the mountains of Central America because
   he was able to up in moisture from the East Pacific. Reintensification to tropical storm strength was almost
   immediate after reaching the Bay of Campeche. Mitch never dissipated. In 1988 Gilbert hit the Yucatan near
   Cancún as a Category 5 hurricane. The Yucatan peninsula, though flat, extended far enough south through
   the critical southerly inflow zone that Gilbert never recovered, even after moving over the open waters of the
   western Gulf of Mexico. Contrast this with the northern Gulf coast and numerous examples from Camille to
   Elena and beyond, where the proximity of the eye to land is not a weakening factor—the southerly
   inflow is not interfered with.

   What are the factors that contribute to the decay of a tropical cyclone? They are Upwelling, Entraining
   dry air, Moving over cool waters, Exposure to upper-level westerlies, and finally Landfall.

   Upwelling. When a hurricane stalls, its movement is has fallen below 5mph, or its movement is erratic over
   a small area, the wave action caused by the strong surface winds churns the ocean surface and produces
   upwelling. This has the effect of cooling the temperature of the sea surface over an area 200 to 300 miles
   across. The result is weakening. It is possible for a hurricane to stall in one area long enough that it
   dissipates. A hurricane is like a shark—it must keep moving to survive.

   Entraining dry air. Sometimes, during the peak season, when tropical cyclones approach contintental land
   masses, they may entrain dry air as part of their interaction with frontal troughs that carry cool, dry air behind
   them. It is one of the ironies of the Atlantic Hurricane Season that, just when things get going, it’s
   already September and the strength and frequency of cold fronts is increasing. These fronts interfere by
   deflecting the hurricane or injecting dry air into the circulation, or both. The dry air kills the convective
   masses that drive the hurricane’s engine. If the dry air entrains deeply enough, it can cause significant
   weakening.

   Moving over cool water. Similar to upwelling, when a tropical cyclone moves over cool water (below 79°
   Fahrenheit), it begins to weaken. Eventually this causes dissipation, particularly in the East Pacific. In the
   Atlantic, if the storm is caught in the mid-latitude westerlies and rides the axis of a cold front, it generally
   becomes an extratropical storm by the time it has recurved to about 45° W. This is the so-called
   “graveyard” of Atlantic hurricanes. The storm is called extratropical when it has undergone
   the change to a cold-core system, and becomes a gale in the north Atlantic. There have been times when a
   hurricane passes north of the Azores and hits the British Isles as a Force 8 or stronger gale, having
   maintained a recognizable inner core.

   Exposure to upper-level westerlies. It happens that the ridge of high pressure that provides the most
   favorable environment for strengthening and also keeps the hurricane heading toward the west frequently
   breaks down. In the Atlantic Ocean, this results in strong upper-level westerlies diving down and impinging
   on the northern edge of the hurricane’s upper-level support structures. The bottom line is outflow
   gets restricted. Usually a hurricane can still thrive when outflow is restricted in one quadrant. Most often this
   happens with the western quadrant. If the forces responsible for the constricted outflow bear down too
   strongly, the hurricane undergoes acute shearing. As outlined above, this can be devastating.

   Life after landfall. Tropical cyclones cannot survive over land. Their access to warm SST is removed. A
   powerful hurricane, such as Hugo, which hit Charleston, S.C. in 1989, can project life-threatening hurricane
   force winds over two hundred miles inland. As the storm progresses inland, it dumps a huge amount of
   rain—measured in feet, not inches. The storm may evolve into a frontal cyclone that continues to
   cause widespread damage. The best example of this is Hurricane Camille in 1969—the strongest
   hurricane ever to make landfall on the continental United States with winds sustained at 190 mph and gusts
   well exceeding 200 mph—which roared up the Mississippi Valley and eventually exited off the East
   Coast. Camille maintained tropical storm strength as far as Memphis, Tennessee. Most hurricanes will
   diminish in strength rapidly after landfall, reaching tropical depression strength by 48 to 72 hours. The main
   threat from the dying storm is from tornadoes and inland flooding. The right-hand quadrant of a hurricane or
   typhoon (in the Northern Hemisphere) is usually the strong side of the storm. This is generally so because of
   the forward motion added to the counter-clockwise punch of the storm. The right-hand side of the hurricane
   contains the strong on-shore flow. This is where the maximum storm surge flooding, and the greatest
   potential loss of life, will be experienced. After landfall, the friction of the circulation moving over land causes
   a great deal of turbulence, which results in tornadoes. These are especially likely in the forward right-hand
   side of the storm’s path. The dying cyclone will dump feet of rain. The combination of surface
   friction, inflow cut-off, and no warm sea surface result in the death of the tropical cyclone. It usually merges
   into a frontal trough, or dissipates.
 

                          COPYRIGHT ©2000, 2001 JONATHAN EDWARDS. ALL RIGHTS RESERVED.
    NEITHER SUPERTYPHOON NOR JONATHAN EDWARDS SHALL BE LIABLE FOR ANY ERRORS OR DELAYS IN CONTENT, OR FOR ANY ACTIONS TAKEN IN RELIANCE
                                         THEREON.