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                                  HURRICANE ISABEL - SEPTEMBER 2003

                                          by Philip Lutzak November 2006



  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.


UPPER AIR OUTFLOW  As an easterly wave travels westward across the Atlantic, if it moves under or near an area of upper level high pressure at around 200mb (40,000 feet), where upper level winds tend to blow outward and away from the center of the wave's mid-level circulation below it, this upper air divergence will encourage more warm, humid air at the lower levels to start converging in towards the center and rise up to replace the diverging air above. As this air rushes inward, a circular area near the center at the lower levels develops at the point where the friction, coriolis, centrifugal and pressure-gradient forces of the incoming air parcels come into balance. At this circle of points near the center (the inner radius of maximum winds or RMW), the air then moves upward and gradually outward, producing the ring of tall thunderstorms that can eventually make up the eyewall. Given certain other necessary conditions (described below), as long as there is sufficient divergence aloft, and convergence of warm moist air below, this process can continue, and the storm can intensify. This important criteria was definitely in place for Isabel, as we can see in figure 1 in the chart of 200mb winds from the 6th to 7th of September. High pressure was situated almost directly above the tropical cyclone that became Isabel, and there was an area of strong southwesterly winds at 200mb on its northwestern

Figure 1. 200mb vector winds (in black arrows) over the developing Hurricane Isabel. Its path from 09-06 to 09-07 is denoted in blue. Courtesy ESRL.

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.


 Figure 2. Sea surface temperatures along the path of Isabel as it became a hurricane from September 6th to 7th of 2003. Courtesy ESRL.



Figure 3. 500mb relative humidity in the time & region where Isabel became a hurricane, from 0Z to 18Z on 09-07-2003. Courtesy ESRL.
MOISTURE AT THE MID-LEVELS  Another condition that is necessary for a tropical wave or depression to develop into a significant tropical cyclone is moisture in the middle levels of the atmosphere, or mid-troposphere. As the inner core of thunderstorms, discussed above, begins to organize, if dry air gets entrained into them at mid-tropospheric levels, the dry air will cause evaporational cooling, which in turn will cause downdrafts in the thunderstorms that will bring down drier, less unstable air all the way into the boundary layer, and because this more stable, drier air is not conducive to rising, this will start to diminish the amount of new convection occurring near the RMW. If the new convection (thunderstorms) stops forming near the center, and instead becomes more scattered in various locations further from the center, there is no single center for the air to converge towards. This will cause the pressure gradient to decrease, the winds will drop, and the whole cycle will slow down or halt. With 500mb as a good proxy for mid-tropospheric levels, figure 3 on the left shows the mid-tropospheric moisture during the time that Isabel developed into a hurricane. (Isabel was officially declared a hurricane at 15Z on September 7th.) This chart shows quite high values of 50 to 65%, more than high enough to ensure that Isabel would have an ample supply of moisture to continue its intensification.


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.

  Meteorologists typically use shear values calculated from the 850mb to 200mb level, or roughly 5,000 to 40,000 feet to see how much shear is present in the  atmosphere. Generally speaking, any value less than 10m/s is negligible for a developing storm. As an example of what deleterious wind shear can do to a hurricane, look at this multi-spectral satellite image of Hurricane Sergio from 11-19-2006. Sergio was moving westward at the time. The yellows represent lower level clouds, and the brightest whites are high level clouds where convection is occurring. Notice how the low-level swirl of Sergio's center has become completely exposed, as strong west and southwesterly shear of 25-30 knots blew the tops of all of the thunderstorms off and away to the east. The storm, which had been a category 3 hurricane two days earlier, dissipated shortly after this image was taken.

  Figure 4 at right is a chart of 850-200mb vertical wind shear from 09-06 to 09-07 over the area where Isabel grew from a tropical depression to a hurricane. It shows values of less than 10 knots over the area Isabel traversed, completing the picture of ideal intensification conditions that existed over the eastern Atlantic ocean for Isabel as it moved westward in early September.

Figure 4. 850-200mb Vertical Wind Shear in the region where the easterly wave developed into Hurricane Isabel, 9-06 to 09-07-03. Courtesy ESRL.



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.


Figure 5. Track of Hurricane Isabel, superimposed with satellite images of the storm all the way from initial hurricane strength on September 7th to category 5 status on September 11th and 12th, to landfall as a category 2 at Hatteras, NC on September 18th and its end in Canada. Courtesy CIMSS.


  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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