EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
(a) Table II-2-1 presents ranges of values for the various scales of organized atmospheric motions. This
table should be regarded only as approximate spatial and temporal magnitudes of typical motions
characteristic of these scales, and not as any specific limits of these scales. As can be seen in this table, the
smallest scale of motion involves the transfer of momentum via molecular-scale interactions. This scale of
motion is extremely ineffective for momentum transport within the earth's atmosphere and can usually be
neglected at all but the slowest wind speeds and/or extremely small portions of some boundary layers. The
next larger scale is that of turbulent momentum transfer. Turbulence is the primary transfer mechanism for
momentum passing from the atmosphere into the sea; consequently, it is of extreme importance to most
scientists and engineers. The next larger scale is that of organized convective motions. These motions are
responsible for individual thunderstorm cells, usually associated with unstable air masses.
Table II-2-1
Ranges of Values for the Various Scales of Organized Atmospheric Motions
Transfer Mechanism
Typical Length Scale, meters
Typical Time Scale, sec
10-7 - 10-2
10-1
Molecular
10-2 - 103
101
Turbulent
103 - 104
103
Convective
104 - 105
104
Meso-scale
105 - 106
105
Synoptic-scale
> 106
106
Large
(b) The next larger scale is termed the meso-scale. Meso-scale motions such as land-sea breeze
circulations, coastal fronts, and katabatic winds (winds caused by cold air flowing down slopes due to
gravitational acceleration) are important components of winds in near-coastal areas. Important organized
meso-scale motions also exist in frontal regions of extratropical storms, within the spiral bands of tropical
storms, and within tropical cloud clusters. An important distinction between meso-scale motions and smaller-
scale motions is the relative importance of Coriolis accelerations. In meso-scale motions, the lengths of
trajectories are sufficient to allow Coriolis effects to become important, whereas the trajectory lengths at
smaller scales are too small to allow for significant Coriolis effects. Consequently, the first signs of trajectory
curvature are found in meso-scale motions. For example, the land-breeze/sea-breeze system in most coastal
areas of the United States does not simply blow from sea to land during the day and from land to sea at night.
Instead, the wind direction tends to rotate clockwise throughout the day, with the largest rotation rates
occurring during the transition periods when one system gives way to the next.
(c) The next larger scale of atmospheric motion is termed the synoptic scale. To many engineers and
scientists, the synoptic scale is synonymous with the term storm scale, since the major storms in ocean areas
occupy this niche in the hierarchy of scales. Storms that originate outside of tropical areas (extratropical
storms) take their energy from horizontal instabilities created by spatial gradients in air density. Storms
originating in tropical regions gain their energy from vertical fluxes of sensible and latent heat. Both the
extratropical (or frontal) storms and tropical storms form closed or semi-closed trajectory motions around
their circulation centers, due to the importance of Coriolis effects at this scale.
(d) The next larger scale of atmospheric motions is termed large scale. This scale of motion is more
strongly influenced by thermodynamic factors than by dynamic factors. Persistent surface temperature
differentials over large regions of the globe produce motions that can persist for very long time periods.
Examples of such phenomena are found in subtropical high pressure systems, which are found in all oceanic
areas and in seasonal monsoonal circulations developed in certain regions of the world.
(e) Scales of motion larger than large scale can be termed interannual scale, and beyond that, climatic
~
scale. El Nino Southern Oscillation (ENSO) episodes, variations in year-to-year weather, changes in storm
II-2-2
Meteorology and Wave Climate
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