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The dynamics of tropical cyclones |
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THE DYNAMICS OF TROPICAL CYCLONESDynamically, storms (existing and potential) are subject to influences from the surrounding areas. In the Tropics, we usually encounter two layers in the troposphere, and these levels have very different characteristics. Most of the time, a steady trade wind blows in the lower levels (surface to 500 hPa), while a succession of large cyclonic and anticyclonic vortices are present in the upper troposphere in the 400- to 150-hPa strata. In some regions, such as the area between the Mariana Islands and the South China Sea during midsummer, intense eddy activity also takes place near the surface. It is not possible to deduce from charts drawn in the lower layers whet is taking place above 500 hPa. Therefore, it is necessray to keep track of events in both layers.Aside from the surface chart, the 700-hPa level chart is very helpful in determining low-level flow patterns. The 200-hPa level is representative of the upper layer, whereas the 500-hPa level, often located in the transition zone, is of much less use in tropical than in extratropical forecasting. However, forecast considerations should not be limited to the two layers in the Tropics. Middle latitude weather also influences and shares in the control of weather changes in low latitudes. The position and movement of troughs and ridges in the westerlies affect both the formation and motion of tropical storms. Therefore, middle latitude analysis is important. Since forecasts generally run from 1 to 3 days, there should also be a hemispheric analysis.The climatological approach to the forecast problem should also be taken into consideration. It is obvious that a forecaster must be familiar with regional and seasonal changes; for instance, areas of frequent tropical cyclone formation, mean storm tracks, the scattering of individual tracks about the mean, and the month to month variations of all of the above. Mean tracks and other data on Pacific storms are available in several publications. These maps and charts reveal that the "climatological" approach gives some useful information, but it cannot be relied on in anyone specific area, or in the case of any particular storm to the exclusion of synoptic indications for the forecast. These probability considerations are used mainly for long-range planning and when data are unavailable. Therefore, we reach the conclusion that climatological information should be treated as a weighing factor to be included after evaluating synoptic data. FORMATION A full-blown typhoon/hurricane cannot be forecast as such when there are no indications of any type of irregularity on the charts or aids used in the tropical analysis. Only when the incipient stage frost appears among the data can we begin to think in terms of an actual typhoon/hurricane, and often not even then. For the most part, the forecast evolution from "area of disturbed weather" to typhoon/hurricane is a matter of step-by-step progression. Assume the term formation means formation of a "potential" typhoon/hurricane. Empirical rules or checks have evolved through the years that enjoy a measure of reliability to warrant their use, and of course, the more signs that point to formation, the more likely formation will occur.Synoptic Conditions Favorable for DevelopmentThe following list contains conditions favorable for development of tropical cyclones. In this list, no attempt is made to separate the surface from the upper air indications, as the two are most often occurring simultaneously and are interrelated. . Marked cyclonic turning in the wind field. . A shift in the low-level wind direction when easterlies are normally present. Wind-speeds generally 10 knots or more.. Greater than normal cloudiness, rainfall, and pressure falls. Cloudiness that increases in vertical as well as horizontal extent.. Moderate to strong outflow aloft. . Sea level pressures lower than normal. The value is dependent upon the region analyzed; however, a deviation of greater than 3 hPa is the normal criterion.. Easterly wind speeds 25 percent or more above normal in a limited area, especially when the flow is Cyclonic.. Temperatures above normal at sea level, generally 26C (79F) or more in the lower layers. . Moisture above normal at all levels. . Westerlies greater than average, north of the latitude of the seasonal maximum. . Easterlies weaker than average in a wide zone. . Easterlies decreasing with height. . The latitude of the subtropical ridge is higher than normal above the 500-hPa level.. Evidence of a fracture in a trough aloft (at 200 hPa).. Long waves that are slowly progressive. . A zone of heavy convection is present, indicating the absence of the trade inversion.. An increasing surface pressure gradient north and west of the suspect area.. The disturbance has relative motion toward the upper ridge at or above 400 hPa.This information is now available on CD in Adobe PDF Printable Format |
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