The demise of hurricanes and tropical storms
As I write this newsletter in October, the hurricane season, while certainly not over, is on its inevitable downhill slide. So I thought it would be interesting to write about how tropical storms and hurricanes can weaken and dissipate. Frequently we talk about how these systems form, the conditions that are required for their formation, the potential detrimental effects of these systems, and safety precautions and avoidance techniques which should be employed. These are all very important topics, but for this newsletter, I choose to focus on the circumstances that can weaken or destroy these systems.
Hurricanes are the most powerful weather systems on earth; that is, they contain more energy than any other systems when fully developed. Despite this, they are somewhat fragile, particularly in their developmental phases, and certain conditions must be present for a system to develop into a hurricane. These conditions must then be maintained, at least to some degree, for a hurricane to be sustained. Therefore, under certain conditions, when these necessary conditions are removed, or conditions become adverse, hurricanes will weaken, sometimes fairly quickly.
First, let’s look at the role that warm, moist air plays. For a hurricane to develop, there must be an abundant supply of very warm, and very moist air in the low levels of the atmosphere. This is because the lifting of this air, which occurs in the circulation of the tropical system, results in cooling of the air, which in turn leads to condensation of the water vapor, and this condensation process releases large amounts of heat, and this is the source of much of the energy which sustains a hurricane. It stands to reason, then, that if this supply of warm, moist air is removed, a hurricane will weaken. The best situation for a hurricane to have this abundant supply of warm, moist air is when it is in a position over warm ocean water. Therefore if a hurricane moves from warm ocean water over land, it will weaken, and this is well known. In addition, if a hurricane remains over the ocean, but moves over colder water, weakening and often eventual dissipation will occur.
Lets look at some pictures showing this. On Sept. 24, Hilary was a major hurricane (Category 3, top winds of 115 knots) in the eastern Pacific west of Mexico, as shown in Figure 1. Note the very small eye near the center of the circulation, which is typical of an intense hurricane. The sea surface temperatures in this area are typically quite warm, especially at this time of the year, but as systems move west or northwest from this area, they move over progressively colder water. The results of this can be seen in the following days. In Figure 2, three days later, Hilary had weakened to a Category 2 hurricane with top winds of 95 knots, and while the circulation is still well established, the eye feature is less distinct. Weakening after this was fairly rapid, and two days later (Figure 3) Hilary had weakened to a tropical storm with top winds of just 45 knots, and while its circulation is still evident, it is comprised mainly of low level clouds swirling about the center with thunderstorms present only to the northeast of the center. Just one day later (Figure 4) there was even less thunderstorm activity present, and the system was no longer classified as a tropical cyclone at this time, just a remnant low with top sustained winds of 30 knots. The weakening of Hilary was due entirely to moving over colder water and the resulting loss of warm moist air to feed into the system.
A second situation that needs to be examined is the wind environment in which a hurricane develops. Ironically enough, for a system to develop into a hurricane, winds in the middle and upper atmosphere must be rather light, and from generally the same direction. This allows the upward motion associated with the thunderstorms to last for a long enough period of time that the heat release from the condensation process can be sustained and provide the needed energy to the system. It also allows the circulation of the system at upper levels to remain aligned with the surface circulation. If stronger upper level winds impinge on an existing system, then the vertical alignment of the system is disrupted, and the condensation (and resulting release of heat) is interrupted, and this will weaken or even destroy the system. This is referred to as “wind shear” in advisories and summaries connected with tropical systems.
Again, it is worth looking at a couple of pictures to show what this looks like. Tropical Storm Philippe formed in the eastern Atlantic on Sept. 24, and for a couple of days it slowly strengthened as it moved west, with top sustained winds of 50 knots by Sept. 26. One day later on Sept. 27, through, stronger upper level winds from the west were affecting the system, and significant weakening had occurred with the system exhibiting top sustained winds of only 40 knots. In Figure 5, the effects of the wind shear can clearly be seen as the cluster of thunderstorms is obviously well to the east of the circulation center indicated by the swirl of lower clouds. This led to further weakening through the ensuing day, but for a few days after this, the upper level winds became lighter, and the system was able to become stronger again, in fact increasing to just below hurricane strength by later in the day on Oct. 1. However, once again stronger upper-level winds affected the system leading to an abrupt weakening, and by Oct. 2 (Figure 6) the cluster of thunderstorms was again significantly displaced away from the center of the circulation center. The system then remained under the influence of these unfavorable upper-level winds for a few more days, but then became stronger and eventually, on Oct. 6, reached hurricane strength.
Sometimes the upper-level wind shear will be strong enough and persistent enough to destroy a system. At other times, like in the case of Philippe, it will lead to significant weakening and hamper any strengthening for a significant period of time, but then if it eases, the system can again gain strength. In fact, a bit earlier in the season, Tropical Storm Ophelia weakened to the point where it was no longer considered a tropical cyclone for a couple of days to the northeast of the Leeward Islands, but was able to resurrect itself once the upper level winds became more favorable and eventually became a major hurricane as it passed east of Bermuda and eventually struck Newfoundland as a tropical storm, having weakened by that time because of its track over colder water.
So even though hurricanes are the strongest weather systems on earth, if the conditions for their existence are removed, they can weaken quite quickly.