WEATHER DISTRIBUTION CHARTS AND ANALYSIS
The weather distribution chart is a graphic representation of the clouds and weather over an area. Outside of the tropics most meteorologists are reluctant to spend the time necessary in analyzing such a chart, because the surface synoptic chart, with clouds and weather plotted at each station, and satellite pictures are adequate for most purposes. For tropical operations, however, the weather distribution chart is almost indispensable in the areas of flight briefing and forecasting. In instances where precise and detailed analysis of weather over an area or terminal is required, it is often necessary to follow the continuity of the weather and cloud patterns in the same fashion as we follow the continuity of low-pressure systems and fronts. As a briefing tool, the weather distribution chart is perhaps the most practical means of presenting the weather to non-meteorologists.
All methods of drawing weather distribution charts involve the use of standard synoptic cloud symbols. Any deviation in the methods is mostly confined to the representation and analysis of cloud amounts. The choice of method is determined locally.
The data included on the plotted charts should be as complete and as extensive as possible. In addition to the regular land and ship synoptic reports, other reports entered on the map include reconnaissance, off-hour ship, P1 REP, and aircraft in-flight reports. These additional reports are designated as to type and time of the report. Past weather should also be entered, because it aids in correlating the movement of weather systems, and it may fill blank spots on the charts on occasion.
Weather Distribution Analysis
If weather distribution maps form a part of standard daily analysis in the weather office, the task of drawing the map is greatly simplified by the fact that the major weather systems of the tropics have continuity from map to map. From day to day, large-scale systems moving through an OPAREA may show the same characteristic intensity, as judged by the amount, depth, and arrangement of clouds and the precipitation pattern. On the other hand, these systems may increase in intensity or die away. However, even during the explosive deepening of some tropical cyclones, the changes are sufficiently slow to be followed on the map sequences. In some regions, systems may form aloft and remain stationary for many days, slowly increasing in intensity from day to day. Under these circumstances, the whole process of deterioration in the weather can be followed in detail.
The following discussion of weather distri-bution analysis is limited to times when weather distribution charts are NOT part of the normal weather office routine.
1. If you are not already familiar with the climatology of the area, consult the best available information on the seasonal cloud and weather characteristics of the area. If the equatorial trough lies near or across your OPAREA, be on the alert for "equatorial fronts’’—lines of cumulus con-gestus and/or cumulonimbus that appear similar to cold fronts of high latitudes. If operating in the trade wind area, find out the charac-teristic distribution and heights of cumulus and stratocumulus. In other words, climatological knowledge should help you form a mental image of the normal weather distribution map of the region. If you find radical departures from the climatological pattern, pay particular attention to the anomalous features, making every effort to delineate them on the map.
2. Examine latest satellite imagery. Without a doubt, satellite imagery provides the most com-plete picture of the cloud cover over the tropics. When available, imagery shows the extent and orientation of cloud systems. Also, when used in conjunction with standard surface and upper-air data, the weather associated with cloud systems is monitored with a far greater degree of success than was possible before the advent of environ-mental satellites.
3. Examine aircraft reports. First priority should be given to reconnaissance reports.
4. Outline areas of middle clouds. The first step in analyzing the plotted weather distribution chart is to outline the middle clouds, following observed reports and satellite pictures, if available. If no satellite picture is available, you will have to make a reasonable interpolation as to the extent of mid clouds. If cumulonimbus are absent, any mid cloud is probably independent. Independent systems are usually bandlike or sickle-shaped with frayed edges of patchy altocumulus.
All middle clouds are not independent. If orographic cumulus or cumulonimbus are wide-spread, fairly large patches of mid clouds maybe reported in this area. However, these patches will be detached from one another and will rarely form a very extensive alto-system with a definite shape. If reports of cumuliform clouds indicate that the mid cloud in any part of the region is dependent, you should keep the mid cloud outline to the minimum extent that it is compatible with the reports and the topography; at the same time, show clearly the connection between the mid cloud and the convective pillars. In passing, it should be emphasized that over the open sea, lines of cumulus congestus or cumulonimbus are often associated with alto-systems, but this does not necessarily mean the mid cloud is dependent; on the contrary, the most likely situation, during the late stages of deepening of an upper-level cyclone, is for one or more asymptotes of convergence, with accompanying cumulus or cumulonimbus lines, to develop under a preexisting deep alto-system and merge with it aloft.
5. Delineate the high clouds. Follow the same principles as used in drawing the mid clouds. Usually, there is little difficulty in distinguishing between dependent and independent cirrus. Almost all independent alto-systems are accom-panied by cirrus or cirrostratus sheets, either separate and at a much higher level, or fused with it in the areas of precipitation. Usually cirrus sheets cover a wider area than the alto-system, and often cirrus in broad bands cover a great area on either side of the alto-system, the bands being oriented parallel to the main axis of the alto-system. The delineation of cirrostratus sheets of independent formation can be of great assistance in tracing the genesis and development of an upper-level cyclonic system. In the early stages of development of such systems, only a small amount of mid cloud maybe present, and the circulation may be evident only in the layers above 30,000 feet (9,144 meters). On the weather distribution chart, however, this early stage of upper-cyclone development is often accompanied by extensive sheets of cirrostratus. In the absence of satellite data, a series of maps showing the development of cirrostratus sheets is often the best way to follow the gradual intensification of such systems.
6. Outline areas of low clouds. By the time you have analyzed the distribution of the mid and high clouds, you should have studied most of the lower cloud reports and have formed definite ideas about their distribution. Low clouds of orographic origin should be obvious by this time. The more frontlike lines of cumulus and cumulonimbus should be clearly marked. Often, very extensive areas may be covered by more less uniform distributions of cumulonimbus. Delineate these areas as sharply as possible.
7. Next, note all precipitation and special phenomena. The analysis is then complete. Although various combinations of cloud types and amounts occur, the following patterns tend to be the most prominent:
1. A large amount of low clouds, cumulus congestus and cumulonimbus, with mid and high clouds and reports of current or past showers or rain. This is a typical pattern associated with tropical disturbances.
2. Few or no mid or high clouds either with few low clouds or with large amounts of cumulus humilis (fair-weather cumulus) or stratocumulus. This combination is indicative of suppressed convection.
3. Near-average amounts of low clouds, occasional cumulus congestus, and mid and high clouds in varying amounts. In such a transition pattern, it is most important to study the upper clouds, since these are independently formed in such cases rather than derived from towering cumuliform clouds. Thickening cloud cover aloft, especially altostratus overcast, denotes an approaching, developing disturbance.