PROJECT 3 Page 3
HURRICANE ISABEL - SEPTEMBER 2003
by Philip Lutzak – November 2006
FAST TRACK TO A HURRICANE
Easterly waves that have the right atmospheric conditions around them can eventually develop into a tropical storm or hurricane. Here is a description of the intensity classifications that a developing disturbance goes through to become a hurricane. Before we examine the conditions that allow this to happen, it should be noted that Isabel did it in a big hurry. It started as an easterly wave off the west coast of Africa on September 1st, 2003 and then quickly grew from an easterly wave to a tropical depression 4 days later on the night of September 5th-6th. But only 6 hours after becoming a depression, it was designated a tropical storm on the same day, September 6th, and only a little more than 24 hours after that, at 11AM on September 7th, it was at hurricane strength. This was unusually quick, but it was not solely because of the the anomalously strong cyclonic vorticity associated with the MLAEJ mentioned previously. There are a number of important atmospheric conditions that must be present for intensification. The following section describes the conditions that existed during Isabel's rapid development into a hurricane during September 6th and 7th. Many of the charts refer only to the pertinent parts of Isabel's track during certain time periods, but here is Isabel's complete, official track from the National Hurricane Center.
quadrant, called an outflow jet, which helped to evacuate the air ahead and over it, encouraging strong inflow to continue inward toward the center at the surface and lower levels.
SEA SURFACE TEMPERATURES As we discussed previously, an easterly wave or tropical depression needs sea surface temperatures (SSTs) beneath it to be at least 26-27C in order for the storm to continue developing into a tropical storm and hurricane. SSTs are so important in the development & sustenance of a tropical cyclone because it has been found that at temperatures of 26.5 degrees C or higher, a deep layer in the troposphere over that water becomes conditionally unstable. It is the moist adiabatic ascent of air within the thunderstorms in this unstable air that allows them to grow to quite high heights. These storms in turn produce subsidence (sinking) at their upper edges, and that sinking warm air around them lowers surface pressures under the cloud clusters around them, which in turn encourages more air to converge towards the low-level center. This whole process, fueled by the latent heat of condensation in the thunderstorms, keeps the pressure-lowering process going near the center. Below in figure 2 is a chart of the sea surface temperatures over which Isabel traveled on September 6th and 7th, when it went quickly from a depression to a hurricane. It clearly had SSTs well above the threshold for development. Also, they were unusually high for that area at that time of year, as this chart of SST anomalies for September 6th thru 7th shows. This is another one of the reasons that Isabel developed into a hurricane much earlier and more quickly than most easterly waves do.
VERTICAL WIND SHEAR Even the successful combination of all of the above parameters is not enough to produce and sustain a hurricane of the magnitude of Isabel. There is one more variable that is critical to their development and intensification: vertical wind shear. Basically, vertical wind shear is the change in wind direction and speed with increasing height. This factor is very critical for developing disturbances and hurricanes of any strength. If these upper level shear winds blow at a high enough speed and/or from a direction against the forward motion of the storm, they will blow the upper levels of the convection away from the center of the storm, and this will cut off the necessary vertical upward motion around the center that maintains the central circulation, causing them to weaken and dissipate. Because most storms in the northern hemisphere move west or northward, it is usually shear with a westerly component that is most detrimental to north Atlantic or Pacific hurricanes.
MAXIMUM STRENGTHENING The favorable conditions that allowed our easterly wave to become Hurricane Isabel continued as the hurricane moved westward through the Atlantic Ocean. In fact, conditions remained so ideal - high SSTs, low shear, ample mid-level moisture and good upper level outflow - that Isabel strengthened steadily all the way to a category 5 hurricane by 2PM EDT on September 11th. Below in figure 5 is a composite map that show Isabel's path, superimposed with satellite images of the storm at various points in its history.
These images make it easy to see where Isabel reached maximum strength (visible eye, symmetric shape), and how it moved in its journey across the Atlantic. In the next section we will explore exactly why it took this path from the African coast to the U.S.
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