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HURRICANES 202: (PART TWO) WIND-PRESSURE RELATIONSHIP
By: ncforecaster , 6:16 AM GMT on June 27, 2008
This blog entry will constitute "part Two" of our discussion on the "generalized" wind-pressure relationship (WPR) for Tropical Cyclones (TC) of the Atlantic Basin. In the succeeding blog entry on this topic, we reviewed the basics as to how "wind" itself is generated by the "pressure gradient", and its direct influence on the WPR. As alluded to in the previous entry, this generalized WPR for Atlantic Basin TC's is influenced by other factors that make a direct one to one relationship between a given Barometricv pressure (BP) and a corresponding Maximum Sustained Wind speed (MSW) problematic. Moreover, we also reviewed the origination of the Saffir-Simpson Hurricane Scale (SSHS) and the reasoning behind its inception in 1973, as well as the specific criteria used in categorizing TC's that achieve hurricane intensity (i.e. MSW of 74 mph or greater).
With all the aforementioned in mind, I need to emphasize a couple of important points regarding this particular subject matter before we disect it in much greater detail. First, the "generalized" WPR discussed here and used by the National Hurricane Center (NHC) is not a substitute for direct observations of the MSW and/or the BP associated with a particular TC. In most cases, it is used in determining either the MSW or the BP in the absence of the direct measurement of this data by Recon aircraft (the NHC currently uses a systematic formula devised in order to extrapolate the MSW at flight level down to the surface which is not without its own inherient flaws as well) and/or land based anemometers (that are either disabled prior to measuring the MSW or were not in the vicinity thereof). Secondly, this particular discussion is focused primaril;y on the WPR of Atlantic Basin TC's and the various components (i.e characteristics) of an individual storm that have a direct influence on the accompanying "pressure gradient". That being said, this topic doesn't take into account the various factors involved with cyclogenesis and intensity change associated with TC's as discussed in previous blogs in this series (such as SST, vertical wind shear, etc.). Once again, I think it's important to display the current Atlantic Basin (also used for the East Pacific Basin as well) Hurricane intensity/damage potential scale known as the SSHS. From there, we will discuss the factors unique to individual hurricanes that influence the aforementioned WPR and make accurate estimation of TC intensity so inheriently complex.
THE WIND-PRESSURE CATEGORIZATION OF THE SSHS:
Category One: MSW (74-95 mph)/ Bp (> 980 mb).
Category Two: MSW (96-110 mph)/ BP (979-965 mb).
Category Three: MSW (111-130 mph)/ BP (964-945 mb).
Category Four: MSW (131-155 mph)/ BP (944-920 mb).
Category Five: MSW (>155 mph)/ BP (<920 mb).
Note: The SSHS also includes associated Storm Surge heights above mean sea level for each category, that I have purposely ommitted, since it's not relevant to this particular discussion. On the other hand, I should mention that there is a 5-10% decrease in the MSW on the beaches as opposed to the offshore waters due to friction at the land/ocean interface.
WHAT UNIQUE TROPICAL CYCLONE COMPONENTS INFLUENCE THE WPR?
In this section, we will review the six most influential TC components that alter the "generalized" WPR as outlined in the SSHS. These six unique TC components are size, ambient (emvironmental) pressures, translational speed, latitude, intensity change, and storm structure. With that in mind, lets examine each one of these aforementioned TC componants in greater detail.
A) Ambient (Environmental) pressures: The ambient pressure is the atmospheric pressure measured at sea level, in which the TC is embedded. Since the MSW is essentially a function of the "pressure gradient balance", one uses a simple numerical equation whereby the lowest BP of the TC is subtracted from the surrounding sea level pressures (SSLP) to ascertain the "pressure deficit". With all other factors being equal, the larger the "pressure deficit" for a given TC, the greater the velocity of the MSW. For ststistical purposes, Landsea, et. al. (2007) suggests a 5 knot increase in the MSW for hurricanes with SSLP > 1016 mb, and a 5 knot decrease in the MSW for those embedded in SSLP of < 1010 mb, with all other factors being equal.
B) Size: The size of a TC is determined by measuring the maximum extent of Tropical Storm force winds (i.e. sustained winds of at least 39 mph) or to the distance of the outermost isobar on the synoptic analysis. As we discussed in the preceeding blog, the "pressure gradient" is directly responsible for the velocity of the wind. Consequently, a smaller TC will generate higher MSW than a large one with the same BP, as a result of the differences in pressure being distributed over a much shorter distance. This same priniple applies to determining the radius of Maximum winds (RMW) which is also used to determine the attendant "pressure gradient" when estimating the MSW from a BP. Once again, Landsea, et. al (2007) suggests a 5 knot increase in the MSW for hurricanes that have RMW 25-50% smaller than the climatilogical averages with all other things being equal. In cases where the RMW is 50% smaller than average, Landsea suggests a 10 knot increase in the MSW.
C) Translational Speed: The "Translational speed" is simply described as the speed of the storms forward motion. Generally speaking, storms moving at faster speeds have slightly higher MSW. These higher MSW are the result of the storms forward momentum being an additive factor on the wind velocity in the right front quadrant of the TC, since its counter clockwise winds are blowing in the same direction that the storm is moving. Landsea, et. al. (2007) suggests a 5 knot increase in the MSW for hurricanes with translational speeds > 50% the climatilogical average, and 5 knot decrease for hurricanes moving at forward speeds > 50% less than average with all other factors being equal.
D) Latitude: The "latitude" that the TC is traversing also has an influence on the MSW. The "Coriolis Force" increases with Latitude, which requires less tangiable "wind" to balance the "pressure gradient" force. As a result, TC's located at higher latitudes require lower central pressures to generate the equivilant MSW, with all other factors being equal. Accordingly, studies by Brown et. al. (2006) and Landsea et. al. (2007) suggested that category one and two hurricanes had stronger winds and BP 3-5 mb lower south of 25N. For major hurricanes (category 3-5), their research suggested a 5-8 mb varience between those located south and north of 25N latitude. One further point noted in Brown et. al (2006) is that there was very little difference in the WPR between linear observations of the Atlantic and Carribean. On the other hand, they did report that their research shows that hurricanes with BP < 965 mb (Category three or greater) had MSW that were 3-4 knots less than the other respective sub-basins.
E) Intensity Change: The "generalized" WPR can also be impacted by the "intensity change" of a respective TC. Often times, intensifying TC's of hurricane intensity will have lower central pressures for a particular MSW, as opposed to those that are "steady state" or "weakening". In most cases, there is a "lag" time between a significant drop in a storms BP and the corresponding increase in its MSW. Based on the studies performed by Brown et. al. (2006), the Authors suggest 4-5 knot variance in the MSW for category one to three hurricanes that either increase or decrease in intensity during a 12 hour period. For category four and five hurricanes, they suggest a 10-12 knot variation in the MSW for the same 12 hour period of "intensity change".
F) Storm Structure: This component focuses on structual changes associated with a respective TC. This is yet another caveot that must be taken into consideration when estimating the MSW and/or BP of a particular storm. In general, TC's continue to grow in size in proportion with the longetivity of its life span. In most cases, a TC's size is the result of either environmental changes (such as latitude) or internal structual changes (such as eyewall replacement cycles 'ERC'). Moreover, the ERC leads to the formation of double eyewalls (creating a second wind maximum) which would also influence the WPR.
The WPR is an important tool used by forecasters both operationally and during post storm reanalysis, in determining the MSW and/or the BP for a TC, in the absence of direct measurements by either Recon aircraft or land based anemometers. Although the best indicator of a particular storms intensity is its BP, there are various factors that influence the corresponding WPR, that make an accurate estimation of the MSW somewhat problematic. The six main storm components detailed in this particular blog entry can alter the "generalized" WPR, and ensure that no two hurricanes are created equal (i.e. exactly alike). That being said, the majority of Atlantic Basin Hurricanes due match up fairly well (relatively speaking) with the WPR categorizations associated with the SSHS. In short, this discussion of the WPR in Atlantic Basin Tropical Cyclones further emphasizes the immense complexities involved with TC forecasting, as well as accurate post storm reanalysis of individual storms.
MY NEXT BLOG ENTRY:
As time allows, I hope to write a blog entry discussing the Dvorak Intensity Scale, which is the current WPR model used to determine TC intensity for Atlantic Basin storms in the absence of direct Recon observations. Once again, this particular blog entry focused solely on the various storm components that influence the WPR, and should not be confused with the factors that influence cyclogenesis and intensity change such as SST, vertical wind shear, etc. As always, I want to thank each and everyone of you who has taken the time to read and/or post in my blog.:) Most importantly, I want to wish you a great rest of the night, and a truly blessed "Friday".:)
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