UPPER-AIR CHARTS AND ANALYSIS
The two most important constant-pressure charts used in tropical analysis are the 200- and 500-mb charts. Upper-level troughs and ridges over lower latitudes usually reach their greatest intensity near 200 mb, which makes this level suitable for high tropospheric analysis. Also, pressure pattern flying is as feasible over tropical oceans, at least 10° to 15°N and S, as it is at higher latitudes. Thus, for high-altitude jet aircraft operations, a 200-mb analysis is indispensable.
The 500-mb level is not an ideal level for tropical upper-air analysis, the reason being that many disturbances and their associated temperature fields occupy only the lower and upper tropo-sphere. Nevertheless, this level is important because of its use in differential (thickness) analysis. It is impractical to analyze the entire tropical layer between 1,000 and 200 mb, so we break it into two layers, 1,000-500 mb and 500-200 mb. Constant-pressure and differential analyses are carried out in the same manner as in midlatitudes, although there are some general considerations that must be taken into account.
General Upper-Air Analysis Considerations
Several rules guide the upper-air analysis:
1. Draw upper contours parallel to upper winds when the wind is 10 knots or greater.
2. Upper heights and thickness between levels vary in a limited range in relation to location and season. Frequency distribution charts provide us with the normal and extreme values. Heights and thicknesses outside observed ranges should be questioned, while higher and lower values within the permissible range are significant because of their association with definite types of flow patterns.
3. Vertical wind shear vectors should indicate the orientation of the mean temperature field though not necessarily the magnitude of the mean temperature gradient. This applies especially to the 500-200 mb layer.
4. Since the mean temperature of a layer several hundred millibars thick is nearly con-servative from day to day, the thickness fields on successive days should be readily comparable.
Regions of relatively warm or cold air should be traceable from day to day without much difficulty, and the intensity of the centers should not change drastically with no good reason. In particular, large areas of cold or warm air should not suddenly appear where none existed pre-viously.
The following is a recommended set of steps in conducting your analysis of the tropical atmosphere:
1. Analyze constant-pressure charts; especially the 1,000-, 500-, and 200-mb levels.
2. Compute height differences between levels and check the results for pattern and change from the preceding day.
3. Analyze thickness charts, either graphically or from charts of thickness values and wind shears plotted at stations and at each 5° latitude-longitude intersection over the ocean. The latter is the preferable method, but time limitations will normally prevent its preparation. Depending on the area and season, contours are drawn at 15- or 30-meter intervals.
4. Correlate the cloud and weather chart with upper-level streamline and contour patterns and thickness changes. Some correlations are as follows:
a. In a zone of cloud reports indicating definite subsidence, cooling from the previous day won’t be observed unless a cold pocket is being advected into the area.
b. A deepening upper low must have either cloudiness on the inflow side into the low or colder air advected into the low from outside the system. There is no other physical process except radiation that could achieve the central cooling necessary to account for 200-mb height falls.
c. If a 200-mb high intensifies and ex-tensive cloudiness exists beneath it, this can only mean that latent heat of condensation is being liberated in such a way as to produce warming aloft. Since this heat can be partly converted to kinetic energy, the possibility of a tropical storm forming must be considered.