Andrew S. Edgar
Tropical Forecasting 241
e-Portfolio Assignment #3
Introduction
The subject of this e-Portfolio entry is Hurricane Bill, a rather large and powerful Atlantic Hurricane borne from an easterly wave that left the western coast of Africa on August 12th, 2009. Bill became Tropical Depression Three at 6Z on August 15th, and after this point the storm intensified rapidly, becoming a tropical storm at 18Z on the 15th and a Category 1 Hurricane by 6Z on August 17th. Bill reached its peak intensity at 6Z on August 19th as a Category 4 Hurricane with maximum sustained winds of 135 mph. At this time Bill was located 300 nautical miles east-northeast of the Leeward Islands, having tracked roughly west-northwest since its formation. After the 19th, Bill began to take a more northerly path, weakening under the influence of moderate vertical wind shear and cooler waters. By August 22nd when Bill passed 150 nm west of Bermuda, it had weakened into a Category 2 Hurricane. Bill then curved to the northeast, and by early August 24th when the storm made landfall in Newfoundland, the system had degraded into a tropical storm. Bill became extratropical by 12Z that same day and was absorbed by an extratropical cyclone over the British Isles two days later.
Tropical Forecasting 241
e-Portfolio Assignment #3
Introduction
The subject of this e-Portfolio entry is Hurricane Bill, a rather large and powerful Atlantic Hurricane borne from an easterly wave that left the western coast of Africa on August 12th, 2009. Bill became Tropical Depression Three at 6Z on August 15th, and after this point the storm intensified rapidly, becoming a tropical storm at 18Z on the 15th and a Category 1 Hurricane by 6Z on August 17th. Bill reached its peak intensity at 6Z on August 19th as a Category 4 Hurricane with maximum sustained winds of 135 mph. At this time Bill was located 300 nautical miles east-northeast of the Leeward Islands, having tracked roughly west-northwest since its formation. After the 19th, Bill began to take a more northerly path, weakening under the influence of moderate vertical wind shear and cooler waters. By August 22nd when Bill passed 150 nm west of Bermuda, it had weakened into a Category 2 Hurricane. Bill then curved to the northeast, and by early August 24th when the storm made landfall in Newfoundland, the system had degraded into a tropical storm. Bill became extratropical by 12Z that same day and was absorbed by an extratropical cyclone over the British Isles two days later.
Figure 1: Hurricane Bill at 1415Z on August 19th, 2009, located several hundred miles east of the Lesser Antilles.
Bill was a Category 4 Hurricane on the Saffir-Simpson scale at the time this image was recorded. This
image is made courtesy of NASA's Rapid Response System.
Given the impressive nature of Hurricane Bill, it behooves us to analyze the conditions under which this storm developed to gain a greater understanding of these impressive storms. As such, I will discuss the following four components of Bill’s life in this assignment: Bill’s steering environment, the easterly wave from which Bill developed, the conditions for Bill’s intensification, and an eye wall replacement cycle.
Hurricane Bill’s Steering Environment
Let’s begin by analyzing the steering environment that determined the path that Hurricane Bill tracked across the Atlantic Ocean. Figure 2 shows Bill’s path from its designation as a tropical depression on Aug 15th, 2009 until its weakening into an extratropical feature on Aug 24th.
Bill was a Category 4 Hurricane on the Saffir-Simpson scale at the time this image was recorded. This
image is made courtesy of NASA's Rapid Response System.
Given the impressive nature of Hurricane Bill, it behooves us to analyze the conditions under which this storm developed to gain a greater understanding of these impressive storms. As such, I will discuss the following four components of Bill’s life in this assignment: Bill’s steering environment, the easterly wave from which Bill developed, the conditions for Bill’s intensification, and an eye wall replacement cycle.
Hurricane Bill’s Steering Environment
Let’s begin by analyzing the steering environment that determined the path that Hurricane Bill tracked across the Atlantic Ocean. Figure 2 shows Bill’s path from its designation as a tropical depression on Aug 15th, 2009 until its weakening into an extratropical feature on Aug 24th.
Figure 2: Track of Hurricane Bill from tropical depression (08/15/09 – 6Z) to dissipation in the North Atlantic Ocean (08/24/09 – 08/26/09). Image is
courtesy of NOAA and was retrieved from the Cooperative Institute for Meteorological Satellite Studies (CIMSS).
While it seemed destined initially that Bill would continue westward in the tropical easterlies to strike the Caribbean and, eventually, the United States or Mexico, this was not to be. Passing north of the Lesser Antilles and Puerto Rico, Bill turned sharply to the north and then northeast as the storm came under the influence of both a mid-tropospheric trough located over the eastern United States and the anticyclonic (clockwise) rotation of the Bermuda High, which is a region of high pressure located over the subtropical Atlantic. The left panel of Figure 3 illustrates the location of the mid-tropospheric trough and associated wind flow when Bill was veering north on Aug 21st, while the right panel shows the steering wind field at 500mb (which a proxy for the 200-850mb mean steering winds) over the northern and tropical Atlantic from Aug 15th – Aug 24th, 2009.
courtesy of NOAA and was retrieved from the Cooperative Institute for Meteorological Satellite Studies (CIMSS).
While it seemed destined initially that Bill would continue westward in the tropical easterlies to strike the Caribbean and, eventually, the United States or Mexico, this was not to be. Passing north of the Lesser Antilles and Puerto Rico, Bill turned sharply to the north and then northeast as the storm came under the influence of both a mid-tropospheric trough located over the eastern United States and the anticyclonic (clockwise) rotation of the Bermuda High, which is a region of high pressure located over the subtropical Atlantic. The left panel of Figure 3 illustrates the location of the mid-tropospheric trough and associated wind flow when Bill was veering north on Aug 21st, while the right panel shows the steering wind field at 500mb (which a proxy for the 200-850mb mean steering winds) over the northern and tropical Atlantic from Aug 15th – Aug 24th, 2009.
Figure 3: Left panel: Mean geopotential height (m) of the 500mb surface on Aug 21st, illustrating the mid-tropospheric trough over the eastern U.S. that helped to
steer Hurricane Bill north then northeast away from its initial west-northwesterly path. Right panel: 500mb vector winds (m/s) acting as a proxy for cyclone
steering currents. The path of Hurricane Bill is in black, while the winds associated with the mid-tropospheric trough and the Bermuda High (indicated by the
large black H) are in red. These figures are courtesy of NOAA’s Earth System Research Laboratory – Physical Sciences Division.
It is clear from Figure 3 that the mid-tropospheric trough and the Bermuda High acted in concert to steer Hurricane Bill north away from its original path. Once under the influence of these two systems, Bill was dragged helplessly into an environment with greater vertical wind shear and cooler ocean temperatures, which aided in its demise. By the time Bill made landfall in Newfoundland, it had already degraded to a tropical storm whose future was to be swept up in the mid-latitude westerlies and absorbed by an extratropical system to the east.
Hurricane Bill’s Seedling Easterly Wave
Hurricane Bill, which was a powerful Category 4 Hurricane, began as localized convection initiated in the path of an easterly wave that left the west coast of Africa on August 12th, 2009 (see Figure 4).
steer Hurricane Bill north then northeast away from its initial west-northwesterly path. Right panel: 500mb vector winds (m/s) acting as a proxy for cyclone
steering currents. The path of Hurricane Bill is in black, while the winds associated with the mid-tropospheric trough and the Bermuda High (indicated by the
large black H) are in red. These figures are courtesy of NOAA’s Earth System Research Laboratory – Physical Sciences Division.
It is clear from Figure 3 that the mid-tropospheric trough and the Bermuda High acted in concert to steer Hurricane Bill north away from its original path. Once under the influence of these two systems, Bill was dragged helplessly into an environment with greater vertical wind shear and cooler ocean temperatures, which aided in its demise. By the time Bill made landfall in Newfoundland, it had already degraded to a tropical storm whose future was to be swept up in the mid-latitude westerlies and absorbed by an extratropical system to the east.
Hurricane Bill’s Seedling Easterly Wave
Hurricane Bill, which was a powerful Category 4 Hurricane, began as localized convection initiated in the path of an easterly wave that left the west coast of Africa on August 12th, 2009 (see Figure 4).
Figure 4: Hurricane Bill’s seedling easterly wave after leaving the coast of Africa on 08/12/09 at 18Z. Note the wave’s trough axis (black) and localized convection (red
circle) associated with rising air motion ahead of the trough axis. This image is made courtesy of the Dundee Satellite Receiving Station.
This strong easterly wave presented itself as an inverted trough with large positive vorticity. This vorticity was generated by horizontal wind shear invoked by the powerful Middle Level African Easterly Jet. As the wave left the coast of Africa, rising air motion occurred downstream (west) of the trough axis while subsidence occurred upstream (east) of the trough axis. The reason for this dipole of vertical air motion can be found in the concept of Ertel’s Potential Vorticity, which is represented by the following equation:
circle) associated with rising air motion ahead of the trough axis. This image is made courtesy of the Dundee Satellite Receiving Station.
This strong easterly wave presented itself as an inverted trough with large positive vorticity. This vorticity was generated by horizontal wind shear invoked by the powerful Middle Level African Easterly Jet. As the wave left the coast of Africa, rising air motion occurred downstream (west) of the trough axis while subsidence occurred upstream (east) of the trough axis. The reason for this dipole of vertical air motion can be found in the concept of Ertel’s Potential Vorticity, which is represented by the following equation:
where f is the Coriolis parameter representing earth vorticity (which increases from south to north), V is the relative vorticity of an air column, H is the height of an air column, and c is a constant. Ahead of the wave as it leaves Africa, the observed wind (v) blows slower than the phase speed of the wave (c). As such, the wave-relative wind (v-c) blows northeast toward the base of the trough. This causes both the earth vorticity and relative vorticity to increase in the air column. According to the above equation, this must be matched by an increase in the height of the air column, which causes rising air motion. Figure 5 illustrates this relationship.
Figure 5: Schematic illustrating how an observed wind (v) blowing slower than the phase speed of the easterly wave (c) results in a wave-
relative wind that blows northeast toward the base of the trough. This causes an increase in earth vorticity (f) and relative
vorticity (V) of an air column which, according to Ertel’s Potential Vorticity, must be compensated by an increase in the height (H)
of the air column. This height increase results in rising air motion west of the trough axis. This figure is made courtesy of the
author of this page.
With sufficient mid-to-upper level moisture, weak vertical wind shear, and warm sea surface temperatures, convection inevitably occurred in this region of rising motion ahead of the trough axis.
Conditions for Hurricane Bill’s Development
Sea Surface Temperatures
In order to generate the convection required to develop, strengthen, and maintain a cyclone, it is a general rule that the sea surface temperature (SST) must be 26 C (~79 F) or greater. Figure 6 illustrates that the SST in the vicinity of Bill when it became a hurricane (13.5 N, 43.2 W) was between 27-28 C, sufficiently warm for intense hurricane development.
Figure 6: Sea surface temperatures (oC) in the Atlantic Ocean on 08/17/09, the day Bill was upgraded tohurricane status. Note that Bill (13.5oN, 43.2oW)
is located in an area where the SST is between 27-28oC, which is sufficiently warm to support the development of a powerful hurricane. This image is
made courtesy of NOAA’s Earth System Research Laboratory – Physical Sciences Division.
Vertical Wind Shear Environment
The presence of too much vertical wind shear (greater than 10 m/s) in the vicinity of a cyclone disrupts the circulation around the storm’s center and the positive convective feedback that occurs in a developing storm. As seen in Figure 7, on the day Bill became a hurricane, the storm was located in an area of weak vertical wind shear of 2.5-5.0 m/s (5-10 kts), which allowed Bill to generate deep, self-sustaining convection without the threat of its central circulation being disrupted.
is located in an area where the SST is between 27-28oC, which is sufficiently warm to support the development of a powerful hurricane. This image is
made courtesy of NOAA’s Earth System Research Laboratory – Physical Sciences Division.
Vertical Wind Shear Environment
The presence of too much vertical wind shear (greater than 10 m/s) in the vicinity of a cyclone disrupts the circulation around the storm’s center and the positive convective feedback that occurs in a developing storm. As seen in Figure 7, on the day Bill became a hurricane, the storm was located in an area of weak vertical wind shear of 2.5-5.0 m/s (5-10 kts), which allowed Bill to generate deep, self-sustaining convection without the threat of its central circulation being disrupted.
Figure 7: Vertical wind shear (kts) in the Atlantic Ocean on 08/17/09 at 9Z. Note that Hurricane Bill (in the red circle) is located in an area of weak vertical
wind shear between 5-10 kts (2.5-5.0 m/s). This image is made courtesy of Cooperative Institute for Meteorological Satellite Studies (CIMSS).
Water Vapor Profile
The water vapor profile for Hurricane Bill’s development on August 17th, 2009 was less than optimal. As we can see in Figure 8, dry air is clearly intruding into the northwestern quadrant of the storm. This dry, dusty air is known as the Saharan Air Layer (SAL), which originates over the Sahara Desert in northern Africa, as seen in Figure 9. This intrusion by the SAL would have served to weaken the storm and slow its eventual transition into a hurricane.
wind shear between 5-10 kts (2.5-5.0 m/s). This image is made courtesy of Cooperative Institute for Meteorological Satellite Studies (CIMSS).
Water Vapor Profile
The water vapor profile for Hurricane Bill’s development on August 17th, 2009 was less than optimal. As we can see in Figure 8, dry air is clearly intruding into the northwestern quadrant of the storm. This dry, dusty air is known as the Saharan Air Layer (SAL), which originates over the Sahara Desert in northern Africa, as seen in Figure 9. This intrusion by the SAL would have served to weaken the storm and slow its eventual transition into a hurricane.
Figure 8: Water vapor imagery on 08/17/09 at 9Z showing Hurricane Bill (in the red circle) ingesting dry air in its northwestern quadrant. This image made courtesy of
the Dundee Satellite Receiving Station.
the Dundee Satellite Receiving Station.
Figure 9: Saharan Air Layer (SAL) imagery on 08/17/09 at 6Z showing Hurricane Bill (in the red circle) ingesting the dry, dusty air from the Sahara Desert (oranges, reds,
yellows) in the northwestern quadrant of the storm. This resulted in a temporary weakening of Bill and slowed its eventual transition into a hurricane. This
image is made courtesy of Cooperative Institute for Meteorological Satellite Studies (CIMSS).
Eye Wall Replacement Cycle
Hurricane Bill underwent an eye wall replacement cycle on August 21st, 2009. An eye wall replacement occurs when an outer rain band organizes into a ring of strong thunderstorms around the inner eye wall. Once this ring of storms completely encircles the inner eye wall, the inner wall is cut off from life-giving moisture and dissipates. Figure 10, images from an animated CIMSS MIMIC loop, illustrates this eye wall replacement cycle.
yellows) in the northwestern quadrant of the storm. This resulted in a temporary weakening of Bill and slowed its eventual transition into a hurricane. This
image is made courtesy of Cooperative Institute for Meteorological Satellite Studies (CIMSS).
Eye Wall Replacement Cycle
Hurricane Bill underwent an eye wall replacement cycle on August 21st, 2009. An eye wall replacement occurs when an outer rain band organizes into a ring of strong thunderstorms around the inner eye wall. Once this ring of storms completely encircles the inner eye wall, the inner wall is cut off from life-giving moisture and dissipates. Figure 10, images from an animated CIMSS MIMIC loop, illustrates this eye wall replacement cycle.
Figure 10: Eye wall replacement cycle for Hurricane Bill on 08/21/09 from 0Z – 2330Z. Colors indicate brightness temperatures (degrees Kelvin) of cloud tops, with
colder cloud tops (reds, oranges, yellows) indicating higher clouds and, thus, deeper convection, which is located in the eye wall and some of the outer
rain bands. The original eye wall (bright red in panel a) is eventually encircled by an outer band of thunderstorms (yellow spiraling band in panels b-e)
until it is starved of moisture and dissipates, have been replaced with a new eye wall around a larger eye (panel f). These images are made courtesy of
the CIMSS MIMIC page. The link to the full animated GIF of this eye replacement cycle can be found here.
After the original eye wall dissipates, the encircling band of storms becomes the new inner eye wall, albeit surrounding a larger eye. Given the conservation of angular momentum, the larger the radius of this new eye wall from the center of circulation, the slower the wind speeds must be. Thus, an eye replacement cycle is likely to reduce the wind speeds around the eye and weaken the hurricane. This was the case with Hurricane Bill, which weakened from a Category 3 storm to a Category 2 storm during this period.
Conclusion
Sea surface temperatures, vertical wind shear, and mid-tropospheric water vapor content are crucial for maintaining the positive convective feedback that fuels developing cyclones. A relatively-deep layer of warm water is crucial to maintain high evaporation rates and large lapse rates, which fuel convection. This convection fuels tall thunderstorm clouds, or hot towers, between which large-scale subsidence occurs. This subsidence causes compressional warming, which lowers the surface pressure. As the pressure lowers, low-level convergence is enhanced and a cyclonic center begins to take shape. This convergence draws more moisture into the circulation, thus fueling further convection and continuing the self-development cycle.
Clearly, water vapor is critical in maintaining large evaporation and lapse rates to fuel convection. Any intrusion of dry air causes powerful downdrafts to develop which force dry air into the lower troposphere, snuffing out convection near the center of circulation. Any convection that does form will likely form too far away from the center to contribute to an organized convective feedback cycle, and the cyclone will be crippled.
Vertical wind shear also works to destroy cyclones by disrupting the feedback cycle. Shear, especially westerly shear, forces the high convective storms around the center of circulation away from the center, disrupting deep eye wall convection and often exposing the eye, itself. With this corruption of compressional warming around the eye, the cyclone is likely to weaken rapidly.
In closing, the convective feedback cycle is critical for cyclone development, and any prolonged disruption will likely be that storm’s demise.
colder cloud tops (reds, oranges, yellows) indicating higher clouds and, thus, deeper convection, which is located in the eye wall and some of the outer
rain bands. The original eye wall (bright red in panel a) is eventually encircled by an outer band of thunderstorms (yellow spiraling band in panels b-e)
until it is starved of moisture and dissipates, have been replaced with a new eye wall around a larger eye (panel f). These images are made courtesy of
the CIMSS MIMIC page. The link to the full animated GIF of this eye replacement cycle can be found here.
After the original eye wall dissipates, the encircling band of storms becomes the new inner eye wall, albeit surrounding a larger eye. Given the conservation of angular momentum, the larger the radius of this new eye wall from the center of circulation, the slower the wind speeds must be. Thus, an eye replacement cycle is likely to reduce the wind speeds around the eye and weaken the hurricane. This was the case with Hurricane Bill, which weakened from a Category 3 storm to a Category 2 storm during this period.
Conclusion
Sea surface temperatures, vertical wind shear, and mid-tropospheric water vapor content are crucial for maintaining the positive convective feedback that fuels developing cyclones. A relatively-deep layer of warm water is crucial to maintain high evaporation rates and large lapse rates, which fuel convection. This convection fuels tall thunderstorm clouds, or hot towers, between which large-scale subsidence occurs. This subsidence causes compressional warming, which lowers the surface pressure. As the pressure lowers, low-level convergence is enhanced and a cyclonic center begins to take shape. This convergence draws more moisture into the circulation, thus fueling further convection and continuing the self-development cycle.
Clearly, water vapor is critical in maintaining large evaporation and lapse rates to fuel convection. Any intrusion of dry air causes powerful downdrafts to develop which force dry air into the lower troposphere, snuffing out convection near the center of circulation. Any convection that does form will likely form too far away from the center to contribute to an organized convective feedback cycle, and the cyclone will be crippled.
Vertical wind shear also works to destroy cyclones by disrupting the feedback cycle. Shear, especially westerly shear, forces the high convective storms around the center of circulation away from the center, disrupting deep eye wall convection and often exposing the eye, itself. With this corruption of compressional warming around the eye, the cyclone is likely to weaken rapidly.
In closing, the convective feedback cycle is critical for cyclone development, and any prolonged disruption will likely be that storm’s demise.