

Click here to Order your Radar Equipment Online UNIT 8—LESSON 4 CONVERGENCE AND DIVERGENCE OVERVIEW Define convergence and divergence and describe the importance of each in the science of meteorology. OUTLINE Importance of convergence and divergence Convergence and divergence (simple motions) Convergence and divergence CONVERGENCE AND DIVERGENCE Convergence is the accumulation of air in a region or layer of the atmosphere, while levels. Coincidently, this is also the layer of maximum winds in the atmosphere; cores of jet streams are usually found here. These highspeed winds are directly related to convergence and divergence. The combined effect of wind direction and speed (velocity) is what produces convergent and divergent air flow. Learning Objective: Define convergence and divergence, and describe the importance of each in the science of meteorology. IMPORTANCE OF CONVERGENCE AND DIVERGENCE The importance of convergence and divergence is related to pressure changes at the surface and height changes of the constantpressure levels. As the air accumulates in the 300200mb stratum over a region, greater pressure is exerted throughout the atmosphere. It’s like inflating a tire. As the air flows into the stratum, the pressure increases. Barometric pressure at the surface rises, as do the heights of the constantpressure levels. Put another way, we say that upperlevel convergence causes pressure and height rises. The exact opposite effect takes place when air is depleted from the 300200mb stratum. This is where we deflate the tire. As the air flows out of this stratum, pressure is lost. Barometric pressure at the surface falls, as do the heights of the constantpressure levels. We say that upperlevel divergence causes pressure and height falls. Convergence and divergence aren’t the only processes at work in the atmosphere that can cause pressure and height changes, but you’re going to hear these terms with increasing regularity as you progress up the AG rate ladder. They are primarily used by forecasters to explain why systems are expected to fill or deepen during the forecast period. Since the upper winds are the producer of convergence and divergence, you should be able to recognize the flow patterns associated with their production. The flow patterns range from simple to complex. CONVERGENCE AND DIVERGENCE (SIMPLE MOTIONS) In order for convergence to take place, the winds must be such as to result in a net inflow of air into a layer or region. At the surface, lowpressure systems are associated with convergent flow. The winds cross isobars toward the center of the low and push the air in the center upward into the atmosphere. The currents are illustrated in figure 841. The upward vertical motion is a prime contributor to the occurrence of precipitation. In meteorology, convergence is classified as horizontal or vertical, because there are horizontal and vertical currents occurring in the atmosphere. In order for divergence to take place, the winds must be such as to result in a net outflow of air from a layer or region. Highpressure systems are associated with divergent flow. The winds cross isobars, flowing out from the high’s center and depleting the air within the high. The air above the high sinks to replace the outflow of air at the surface. This downward vertical motion (subsidence) is associated with dry air. Divergence can also be classified as horizontal or vertical, depending on the wind’s axis. See figure 841. The simplest form of convergence and divergence is the type that results from wind direction alone. Two flows of air brought together, no matter what the angle, result in convergence. Where the air flow splits and winds go in different directions, divergence is occurring. Figure 842 illustrates these types of convergent and divergent air flow. Wind speed in relation to wind direction is also a contributor to convergence and divergence. If the wind speed decreases downstream, there’s a net inflow of air into the region, and convergence takes place. If wind speeds increase downstream, there’s a net outflow of air from the region, and divergence occurs. In an area of uniform wind speeds, if the winds fan out (split), divergence occurs. If these same winds are brought together, convergence occurs. See figure 842, view B. The fact that contours converge or diverge doesn’t necessarily indicate convergence or divergence, because wind speeds must also be considered. If wind speeds increase downstream and the contours spread apart, supergradient winds are said to be occurring. This combination of wind direction and speed produces divergence. On the other hand, if wind speeds decrease downstream and the contours converge,
Figure 843.—Convergence and divergence in supergradient and subgradient air flow. subgradient winds are occurring. This combination produces convergence. See figure 843. There are other cases where it is difficult to tell whether divergence or convergence is taking place. When wind speed decreases downstream and the contours spread apart, both convergence and divergence are indicated. The wind speed suggests convergence, but the spreading contours suggest divergence. A similar situation arises when wind speeds decrease downstream and the contours converge. Here, we’re looking at speed divergence and directional convergence. These are complex motions, however, and a special evaluation is required to determine the net inflow or outflow. 
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