Supplementary upper-air analysis

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SUPPLEMENTARY UPPER-AIR ANALYSIS

The basic upper-air analysis is the constant-pressure analysis. In conjunction with this basic analysis it is sometimes necessary, and at all times beneficial, to conduct concurrent supplementary types of analyses of upper-air properties in order that the fullest use be made of upper-air information to lead to the end product, the forecast. Most of these charts are constructed from either reported or derived data from upper-wind and upper-air reports. It is not feasible to list or explain all the types of upper-air charts currently being produced by the National Meteorological Center. Only the space differential (thickness) analysis, advection, and time dif-ferential charts are covered here.

Space Differential (Thickness) Analysis

For a truly three-dimensional analysis of the free atmosphere, it is necessary to analyze not only the individual levels within the atmosphere (850 mb, 700 mb, etc.) but also the various layers of it. The most commonly analyzed layers are the 1000-700-mb, 1000-500-mb, 700-500-mb and 500-200-mb layers. Layer, or differential, analysis ensures vertical consistency between the individual levels and agreement within the limits of the hydrostatic equation.

Know the heights of any two constant-pressure levels and you can determine the thickness (vertical distance) of the layer separating them. It’s simply a matter of subtraction. Space differential charts are commonly referred to as thickness charts, since they represent the difference in height between two constant-pressure levels. Manually, they are constructed by graphi-cally subtracting the heights of one analyzed constant-pressure chart from those of another.

PROCEDURE FOR ANALYSIS.— If the need should ever arise whereby you must do such an analysis, here’s a recommended procedure:

1. Obtain the analyzed constant-pressure charts for the levels bounding the layer to be analyzed.

2. Place an acetate over one of these charts and trace the isoheights onto the acetate with a grease pencil. Be sure to label the contours.

3. Place the same acetate over the other chart and trace and label the isoheights of this level, using a different color grease pencil.

4. Graphically subtract the lower contours from the upper and connect the points of equal thickness. It may prove helpful if you perform the mathematical subtraction of heights at a few intersections to aid you in getting the analysis started. Thickness lines are drawn in dashed black.

5. Place the acetate under a clean chart and trace the thickness lines onto the chart with a black felt-tip pen. Label the thickness lines.

There are several rules that you must follow in constructing the thickness pattern:

1. Thickness lines cannot touch or cross.

2. Thickness lines cross isoheight contours at the intersection points of the two levels only.

3. Thickness lines must always pass from lower to higher contours or vice versa at both levels.

4. Between any two consecutive thickness lines an isoheight of either pressure surface must exist.

5. Between any two consecutive isoheights a thickness line or an isoheight of the other pressure surface must exist.

THICKNESS PATTERNS.— Figure 8-1-12 illustrates most of the important details of the 1000-500-mb thickness pattern in relation to fronts. Adherence to these features of the thickness model insures the proper slope of systems between 1000 and 500 mb, the proper relationship between surface fronts and polar jet, and surface frontal analyses that portray a meaningful picture of the three-dimensional temperature structure. The most important features of the pattern are as follows:

1. The concentration of thickness contours is on the cold side of frontal systems. The stronger the front, the greater the concentration.

2. The spacing of thickness contours in the cold air ahead of warm fronts is greater than in the cold air behind cold fronts.

3. The horizontal distance between the maxi-mum thickness gradient and cold fronts is less than with warm fronts. The maximum gradient is usually located horizontally in the same position as the 500-mb jet stream.

4. Thickness contours are anticyclonically curved in advance of warm fronts and cyclonically curved behind cold fronts.

5. The location of the cold trough in the thickness contours lies to the rear of the surface low, halfway between the surface low and the next upstream ridge or high.

Figure 8-1-12.—Illustration of the relation of thickness patterns to fronts.

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