Stages of Tropical Cyclone Development
Tropical cyclones develop from nondescript, unorganized masses of thunderstorms, also called cloud clusters. Although these hurricane "embryos" can form in a number of different ways, they share the common characteristics of converging winds, cyclonic spin at low levels of the atmosphere and upward motion in the air column.
The initial stage of a tropical system is referred to as a tropical disturbance or a tropical wave. The wind in such regions is less that 20 knots and no organized circulation is apparent. A weather system is classified as a tropical depression when sustained winds range from 25 to 34 knots (23-39mph) and there is evidence of closed wind circulation. Closed wind circulation is an important condition and means that the wind near the surface in the Northern Hemisphere flows in a counter-clockwise fashion around a central point.
One method of ascertaining the structure of a weather system's wind field is the use of scatterometry. A scatterometer is a satellite-borne high-frequency radar system that measures surface wind speed and direction by "reading" the differences in the texture of the ocean's surface. Basically, rougher surfaces equate to higher wind speeds. The QuikScat image (Figure 1) clearly shows the wind circulating around a central point of Tropical Depression Wilma on the morning of October 17th. Wind speed can be determined by matching the color of the wind indicator to the key to the left of the image. Wind barbs containing a grey circle are interpreted to be contaminated by heavy rain and are therefore unreliable.
Once the sustained winds associated with a tropical depression reach 35 knots (40 mph), the system is classified as a tropical storm and a name is bestowed. Tropical storms achieve hurricane status once sustained winds reach 64 knots (74 mph). When the system is identified as a hurricane, its strength, largely determined by sustained wind speed, is reported as one of five categories defined by the Saffir Simpson Hurricane Intensity Scale. This scale is a very useful resource that predicts the potential storm surge and destructive potential of a hurricane.
The majority of Atlantic hurricanes can be attributed to the atmospheric phenomena known as an easterly wave. It is estimated that approximately 10% of easterly waves develop into tropical cyclones. At the surface, an easterly wave can be identified by a weak inverted trough, such as the one depicted to the right.
An easterly wave forms over Africa every few days and moves west across the North Atlantic into the Caribbean Sea. The wave moves to the west at a speed somewhat slower than the wind in the lower portion of the atmosphere. As a result of this speed difference, air parcels comprising the trade winds catch up to the wave and must pass through the trough that defines it. In the early stage of negotiating the curvature of the trough, these air parcels develop spin which results in vertical stretching. As the air parcels exit the trough, they lose their spin resulting in a decrease of vertical height.
This region of vertical stretching located to the east of the wave is associated with converging surface winds, upward motion and high relative humidity. It should be no surprise that this area is characterized by unsettled weather. In contrast, to the west of the wave, a decrease in vertical height results in divergence, downward motion and generally clear weather.
Ironically, the speed of the easterly wind higher in the atmosphere is slower than the wind at the surface and the forward speed of the wave. Because the atmospheric dynamics are opposite of those outlined above, upper-level divergence is found to the east of the wave while convergence is found to the west. This reversal in the upper atmosphere serves to enhance the overall tendencies of the conditions nearer to the surface. Specifically, upper level divergence east of the wave helps to support unsettled weather by offsetting the air flowing in at the surface. Air parcels in a column can not continue to rise unless an exhaust mechanism is in place in the upper atmosphere.
The Intertropical Convergence Zone is the region circling the globe where the northeast and southeast trade winds meet. The ITCZ is characterized by low pressure at the surface that develops in response to the converging wind and the vast amounts of solar heating occuring in this area, particularly during the Northern Hemisphere summer.
As the satellite image (Figure 2) shows, the ITCZ is home to a ribbon of towering cumulus clouds and intense thunderstorms in the vicinity of the Equator. The converging winds in this area help to impart cyclonic circulation to these storms. Occasionally, a cluster of these thunderstorms will depart from the ITCZ, become better organized and develop into a tropical depression, the first step in the development of a hurricane.
TUTT is an acronym for Tropical Upper Tropospheric Trough, a cold-core atmospheric feature that is most easily identified by looking for its footprint at 200mb--a height of approximately 12,000 meters. The chart of mean 200mb geopotential heights for October 2005 (Figure 3) has been annotated to show the location of the trough.
Mid-latitude low pressure systems that drift into the Gulf of Mexico can also promote tropical cyclone development.. The frontal boundaries associated with such mid-latitude systems, characterized by converging winds, cyclonic flow and the upward motion of air parcels, are capable of spawning tropical cyclones. The image below (Figure 4) shows a cold front sweeping across the Caribbean Sea on November 2, 2005. (This image is not associated with Wilma, but is offered as an example of a frontal boundary.)
Although tropical cyclones don't always follow convention, there is an overall pattern to their development and movement. The next section on climatology examines hurricanes in the context of Earth's larger weather patterns.
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© 2005-2006 Mark A. Thornton