DIVERGENCE

by ZAMG


> 0 Divergence
< 0 Convergence

Divergence is a property of the flow field.

There is a connection to vertical motion through the Richardson equation:

A very simple model describing an ideal situation leads to the following considerations: If there is the assumption of convergence existing at the surface level, upward motion results in the levels above up to the tropopause; at this barrier air cannot rise further and consequently has to diverge there. Therefore a circulation cell is created and cloudiness may develop in the upward motion.

A good example for such a situation is, for instance, frontal cloudiness which can be discused in a test case from 11 February 1997/06.00 UTC.
11 February 1997/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models
11 February 1997/06.00 UTC - Meteosat IR image; green solid: divergence 1000 hPa, green dashed: convergence 1000 hPa
11 February 1997/06.00 UTC - Meteosat IR image; yellow solid: divergence 300 hPa, yellow dashed: convergence 300 hPa
11 February 1997/06.00 UTC - Meteosat IR image; blue dashed: vertical motion (omega) - upward motion 700 hPa, blue solid: vertical motion (omega) - downward motion 700 hPa
On the right image top the field of divergence at 1000 hPa is superimposed; on the left image bottom divergence at 300 hPa; dashed lines represent convergence, solid lines divergence. The right image bottom contains upward motion at 700 hPa with the dashed lines representing upward, the solid lines downward motion.

There are several big convergence areas at the surface level representing the lower boundary of the circulation cell; they are connected with the following cloud systems (left image top):

Compared to the divergence field at 300 hPa (left image bottom) representing the upper level of the circulation cell the following results can be summarized:

All the cloud systems just mentioned are accompanied by divergence at this high level. So in the frontal areas as well as in the area of the EC, convergence occurs at low levels and divergence at high levels.

The vertical motion field (lower image) completes the ideas of a circulation cell in these areas:

Looking again at the surface level (right image top) several big areas are accompanied by divergence indicated by the solid lines; they are connected with the following cloud systems (left image top):

Compared to the 300 hPa level (left image bottom) nearly all areas just mentioned are areas of convergence at high and divergence at low levels. This can be interpreted as a reverse circulation cell with downward motion in the troposphere between these two boundaries.

The vertical motion image (low panel) supports this idea with the solid lines representing downward motion.

11 February 1997/12.00 UTC - Meteosat IR image
11 February 1997/18.00 UTC - Meteosat IR image

INDEX OF BASICS
THE QUASI-GEOSTROPHIC APPROACH
VERTICAL MOTION - OMEGA EQUATION