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WIND SPEED (VELOCITY) SHEAR |
CHAPTER 1
CONVERGENCE, DIVERGENCE, AND VORTICITY
In your reading of the AG2 manual, volume 1, you
became familiar with the terms convergence,
divergence, and vorticity when used in relation to
surface lows and highs. You were also presented with
a basic understanding of the principles involved In this
section, we will cover the terms, the motions involved
in upper air features and surface features, and the
relationship of these processes to other meteorological
applications,
We will first discuss convergence and divergence,
followed by a discussion of vorticity.
NOTE
The World Meteorological
Organization adopted “hectopascals”
(hPa) as its standard unit of
measurement for pressure. Because
the units of hectopascals and millibars
are interchangeable (1 hPa = 1 mb),
hectopascals have been substituted for
millibars in this TRAMAN.
CONVERGENCE AND
DIVERGENCE
LEARNING OBJECTIVES: Define the terms
convergence and divergence. Recognize
directional and velocity wind shear rules.
Recognize areas of mass divergence and mass
convergence on surface pressure charts.
Identify the isopycnic level. Retail the effects
that convergence and divergence have on
surface pressure systems and features aloft.
Identify rules associated with divergence and
convergence.
As mentioned in the AG2 manual, volume 1, unit 8,
convergence is the accumulation of air in a region or
layer of the atmosphere, while divergence is the
depletion of air in a region or layer. The layer of
maximum convergence and divergence occurs between
the 300- and 200-hPa levels. Coincidentally, this is also
the layer of maximum winds in the atmosphere; where
jet stream cores are usually found. These high-speed
winds are directly related to convergence and
divergence. The combined effects of wind direction and
wind speed (velocity) is what produces convergent and
divergent airflow.
CONVERGENCE AND DIVERGENCE
(SIMPLE MOTIONS)
Simply stated, convergence is defined as the
increase of mass within a given layer of the atmosphere,
while divergence is the decrease of mass within a given
layer of the atmosphere.
Convergence
For convergence to take place, the winds must result
in a net inflow of, air into that layer. We generally
associate this type of convergence with low-pressure
areas, where convergence of winds toward the center of
the low results in an increase of mass into the low and
an upward motion.
In meteorology, we distinguish
between two types of convergence as either horizontal
or vertical convergence, depending upon the axis of the
flow.
Divergence
Winds in this situation produce a net flow of air
outward from the layer.
We associate this type of
divergence with high-pressure cells, where the flow of
air is directed outward from the center, causing a
downward motion. Divergence, too, is classified as
either horizontal or vertical.
DIRECTIONAL WIND SHEAR
The simplest forms of convergence and divergence
are the types that result from wind direction alone. Two
flows of air need not be moving in opposite directions
to induce divergence, nor moving toward the same point
to induce convergence, but maybe at any angle to each
other to create a net inflow of air for convergence or a
net outflow for divergence.
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