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TROPOPAUSE ANALYSIS

Unless you operate as a member of an upper air team taking raobs and rawinsondes, you will most likely never do a tropopause analysis. The main things to remember are the relationship between the tropopause and the troposphere, and the criteria an analyst uses in determining where the troposphere ends and the tropopause begins. The tropopause is a transition boundary between the troposphere and the stratosphere. The analysis of this boundary layer is carried out on adiabatic charts, using temperature data obtained from atmospheric soundings (raobs and rawinsondes). The tropopause is characterized by an abrupt change in the temperature lapse rate. The change is from a lapse rate marked by decreasing temperature through the troposphere to one where the temperature decreases much more slowly, becomes isothermal (constant), or even shows a slight increase with height through the tropopause.

There are at least three distinct tropopauses, which form leaflike or overlapping structures within Earth’s atmosphere. The three most generally accepted are the subtropical tropopause, found at 25° latitude near 18,290 meters (approximately 100 mb); the mid-latitude tropo-pause, at 35° to 40° latitude near 12,190 meters (approximately 200 mb); and the subarctic tropopause, near 9,145 meters (300 mb). In general, each tropopause is found at greater heights in summer than in winter. Because the thickness of the troposphere increases from the poles to the equator, each tropopause slopes upward toward the equator. For a short distance, the subtropical tropopause tends to overlap the mid-latitude tropopause, and the mid-latitude tropopause tends to overlap the subarctic tropo-pause. Figure 8-1-11 illustrates the overlapping of the mid-latitude and subtropical tropopauses. Each tropopause is characterized not only by height and pressure but also by potential tempera-ture. In winter, the potential temperature of the subtropical tropopause is, ±10, 390K; the mid-latitude tropopause, 350K; and the subarctic, 310K. Potential temperatures are used to locate tropopauses from atmospheric soundings with many inversions or irregular lapse rates with no inversions.

The variation in temperature structure through the tropopause led to many different ideas on how to define and analyze it. The present WMO definition does not state what a tropopause is. Instead, the WMO defines an objective technique (selection criteria) for locating tropopauses from


Figure 8-1-11.—Overlapping of subtropical and mid-latitude tropopauses.

a plotted atmospheric sounding. The significance of WMO’s definition is the standardization the definition has brought about. It permits the same height to be consistently selected by all technicians from a given sounding, and other atmospheric phenomena can be empirically (through ex-perience and observation) related to it. 

Within the WMO definition, provision is made for identifying two or more tropopauses on a sounding, which is necessary because of the regions where overlapping exists. The definition is in two parts.

1. The "first tropopause" is defined as the lowest height at which the lapse rate decreases to 2°C per kilometer or less, provided also that the average lapse rate between this height and all higher altitudes within 2 kilometers does not exceed 2°C per kilometer.

2. If, above the first tropopause, the average lapse rate between any height and all higher altitudes within a 1-kilometer intend exceeds 3°C per kilometer, then another tropopause is defined by the same criteria as under 1 above. This second tropopause may be within or above the 1-kilometer layer.

There are also two qualifying remarks attached to the selection criteria. They are as follows:

1. A height below the 500-mb level is not designated as a tropopause unless the sounding reaches the 200-mb level and the height is the only height satisfying the above definitions.

2. When the second or higher tropopauses are being determined, the 1-kilometer interval with an average lapse rate of 3°C per kilometer can occur at any height above the conventional tropopause and not only at a height more than 2 kilometers above the first tropopause.

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