Quantcast Analysis of time sections

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Time sections are a special aid in the tropics and should be plotted and analyzed for all key tropical stations. The format is important, for it provides for rapid handling of various weather information. Figure 9-2-1 shows a form for time section analysis. The vertical coordinate of a time section may be pressure, pressure altitude, or height. It is advantageous to have both a pressure and height scale, since upper winds are reported at fixed levels, while significant points of raob soundings are given in millibars. Time is plotted along the horizontal coordinate and may be plotted left to right or vice versa. The most common time sections are as follows: surface pressure—graphs of 24-hour pressure changes upper heights—graphs of 24-hour height changes upper winds—wind plots at 2,000-foot intervals up to 20,000 feet, then every 5,000 feet

The first objective of time section analysis is to detect various errors and unrepresentative values of the reports and make the variations of wind, pressure change, and the like, as consistent as possible along the vertical and in time. The second objective is to consider surface, upper wind, and raob data together and deduce from them as much as possible about the synoptic situation. Of these, the first objective is by far the easier. Not only do errors stand out in an obvious way, but you can also deduce from the time sequence such things as whether a wind shift in a certain layer is transitory, lasting for only 6 hours, or whether it denotes a longer period change. The representativeness of 24-hour height changes is also readily apparent from time section analysis. Considering the normal extent and rate of motion of disturbances in low latitudes, marked upper-height fails of 30 meters per 24 hours or more should not be preceded or followed by rises of the same magnitude in the 24-hour interval centered 12 hours before or after, except when accompanied by strong winds and large wind shifts. Otherwise, one or more soundings must be suspect. In such cases, emhasis is placed on the nightime data. At reliable stations these changes should be accepted as correct except (1) when the raob is taken in heavy rain, (2) if a large change is observed, yet there is no previous

Figure 9-2-1.—Form for time section analysis.

indication that a large height rise or fall center should arrive, or (3) if the heights rise as much in a second 24-hour interval as they fell in a first, without appropriate wind and weather changes.

The first part to the evaluation of time sections, as shown in figure 9-2-2 is largely qualitative and dependent on an analyst’s skill and experience. This is true in even larger measure for the second part, since formal procedures for the integration of time-section weather data do not exist.

The following five semiqualitative steps can be carried out in order to better understand the synoptic situation:

1. Mark the principal trough lines and shear lines in the wind field with orange lines and indicate the direction of displacement (especially eastward and westward) with an arrow. These lines give the slope of the disturbance and also show its base and top. The occurrence of weather relative to the time of a wind shift shows where the bad weather is concentrated (on the forward or rearward side of the disturbance) and the extent of such weather. Comparing your time section with time sections from stations where the disturbance passed previously will (1) furnish the rate of motion, (2) show changes in intensity of winds and wind shifts, and (3) reveal changes in weather distribution and intensity with respect to the system. To a lesser degree, changes in intensity can also be determined by variations in the amount of 24-hour surface pressure change.

2. Draw isolines showing the 24-hour height changes. The interval chosen should be 15 meters below 400 millibars and 30 meters above. Examine these changes closely for their relation to the wind shift lines. Trough lines, in particular, must coincide by definition with the instantaneous zero height changes. Normally, the 24-hour zero change line parallels the slope of the wind shift lines and is located near them. The layer of strongest 24-hour height changes should be the layer of greatest intensity associated with a moving disturbance. The strongest wind shifts should be found in this layer. This applies to vectorial wind shift, not merely the directional change, since the

Figure 9-2-2.—Composite time section (analyzed).

latter can be very large yet insignificant at low wind speeds.

It is emphasized that the foregoing, while essentially correct in practice, is not always observed and has no necessary foundation in theory. Sometimes there are persistent height falls with little change of wind. The height falls are one indication of a deepening stationary disturbance, and such an occurrence should be carefully checked for this possibility.

The vertical gradient of 24-hour height changes also indicates areas of cooling and warming, since it indicates whether constant-pressure surfaces have moved closer together or farther apart in the vertical (thickness changes). Height changes usually are largest in the high troposphere—both the falls ahead of a trough and the rises to its rear. This indicates that most troughs have a cold-core structure and that ridges have a warm-core structure. The intensity of these troughs and ridges increases upward in the atmosphere to 200 to 150 mb. Higher up toward the tropopause, the cold troughs and warm ridges undergo a reversal in the temperature field, and they decrease in intensity and die out. This is the main reason the 200-mb level is the best choice for high tropospheric analysis in the tropics.

The validity of the geostrophic wind relation-ship in the tropics is questionable; however, the vertical wind shear appears to give a fair idea of the distribution of cold and warm air to latitudes 10° and even to 5°. At a station where the easterly winds decrease in intensity with height, colder air is found poleward of the station and warmer air equatorward. In the Northern Hemisphere, northerly winds increasing in intensity with height ahead of a trough indicate colder air in the trough.

The same can be said about the southerly winds to the rear of the trough. Winds usually turn counterclockwise with height (back) ahead of such troughs and clockwise (veer) to their rear. Thus, it is the general pattern, not the amount of geostrophic thermal advection, that agrees with the movement of cold and warm areas.

3. Indicate the depth of the moist layer. This is the height of the 5 g/kg (grams per kilogram) moisture level in the rainy season, and the 3 g/kg level in the dry season.

4. Indicate the bottom and top of stable layers, especially the trade inversion. The potential temperature difference between the top and bottom and the inversion thickness in millibars are the best measures of the inversion strength. The greater the increase of potential temperature through the inversion, and the thicker the layer, the stronger the inversion. Inversion analysis establishes average cloud heights.  

5. Mark the bases and tops of the equatorial easterlies and polar westerlies. Disturbances move eastward when a deep layer of westerlies is present, and westward in deep easterlies. A change in the thickness of a layer of easterlies or westerlies may indicate a reversal of the direction of motion, if the change is representative of a large area. The temporary appearance of deep westerlies at one station during the passage of a cold-core low to the north cannot be interpreted in this way.

If you operate in an area with a reasonable station network, you will, after going through a number of time sections, acquire a fairly definite knowledge of how to draw these charts and a knowledge of what to look for with regard to bad weather areas.

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