Frontal Lifting

Frontal Lifting Frontal lifting is the term applied to the process represented on a front when the inclined surface represents the boundary between two air masses of different densities. In this case, however, the slope ranges from 1/20 to 1/100 or even less. The steeper the front, the more adverse and intense its effects, other factors being equal. These effects were discussed in detail in the AG2 TRAMAN, volume 1. Vertical Stretching Since it is primarily from properties of the horizontal wind field that vertical stretching is detectable, it is more properly called convergence. This term will be used hereafter. The examples of convergence and divergence, explained in the foregoing, are definite and clear cut, associated as they are with the centers of closed flow patterns. Less easily detected types of convergence and divergence are associated with curved, wave-shaped, or straight flow patterns, where the air is moving in the same general direction. Variations in convergence and divergence are indicated in figures 4-1, 4-2, 4-3, and 4-4 by means of the following key: Figure 4-2.-Convergence and divergence in meridional flow. The left side of figure 4-1 illustrates longitudinal convergence and divergence; the right side illustrates lateral convergence and divergence. Many more complicated situations can be analyzed by separation into these components. It can be shown mathematically and verified synoptically that a fairly deep layer of air moving with a north-south component has associated convergence or divergence, depending on its path of movement. In figure 4-2 the arrows indicate paths of meridional flow in the Northern Hemisphere. In general, equatorward flow is divergent unless turning cyclonically, and poleward flow is convergent unless turning anticyclonically. The four diagrams of figure 4-3 represent the approximate distribution of convergence and divergence in Northern Hemispheric cyclones and anticyclones. For moving centers, the greatest convergence or divergence occurs on or near the axis along which the system is moving. The diagrams of figure 4-3 show eastward movement, but they apply regardless of the direction of movement of the center. Figure 4-1.-Longitudinal and lateral convergence and divergence. Figure 4-3.-Convergence and divergence in lows and highs. 4-2