Divergence/Convergence and Surface Pressure Systems

The usual distribution of divergence and convergence relative to moving pressure systems is as follows:

. In advance of the low, convergence occurs at low levels and divergence occurs aloft, with the level of nondivergence at about 600 hPa.

. In the rear of the low, there is usually convergence aloft and divergence near the surface.

The low-level convergence ahead of the low occurs usually in the stratum of strongest warm advection, and the low-level divergence in the rear of the low occurs in the stratum of strongest cold advection. The low-level divergence occurs primarily in the friction layer (approximately 3,000 ft) and is thought to be of minor importance in the modification of thickness advection compared with heating and cooling from the underlying surfaces.

In advance of the low, the air rises in response to the low-level convergence, with the maximum ascending motion at the level of nondivergence eventually becoming zero at the level of maximum horizontal divergence (approximately 300 hPa). Above this level, descending motion is occurring. In the rear of the low, the reverse is true; that is, descending motion in the surface stratum and ascending motion in the upper troposphere above the level of maximum horizontal convergence. In deepening systems, the convergence aloft to the rear of the low is small or may even be negative (divergence). In filling systems, the divergence aloft in advance of the low is small or even negative (convergence).

Thus, in the development and movement of surface highs and surface lows, two vertical circulations are involved, one below and one above the 300-hPa level. The lower vertical circulation is upward in the cyclone

Figure 1-1.-Generalized vertical circulation overdeveloping highs and lows.

and downward in the anticyclone. The upper vertical circulation involves downward motion in the stratosphere of the developing cyclone and upward motion in the upper troposphere and lower stratosphere of the developing anticyclone. See figure 1-1.

Divergence and upper-height falls are associated with high-speed winds approaching weak contour gradients which are cyclonically curved. Figure 1-2 illustrates contour patterns associated with height falls, 

Convergence and upper-height rises are associated with the following:

. Low-speed winds approaching straight or cyclonically curved strong contour gradients. See figure 1-3, view (A).

. High-speed winds approaching anticyclonically curved weak contour gradients. (B).

Figure 1-3.-Convergence Illustrated.

Note that the associated height rises or falls occur downstream and to the left of the flow, as illustrated in figure 1-3.

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