The circus of atmospheric events we call weather results from the
interplay of flow phenomena on all scales, from molecular viscosity
to the turbulence of microscale circulations, to the thermally driven
mesoscale land and sea breezes and mountain and valley winds, to synoptic
scale cyclones and anticyclones, to global scale features of the general
circulation. The flow at each scale may be laminar (smooth), turbulent
(characterized by rapidly changing eddies and waves of all sizes),
or transitional (a blend of laminar and turbulent) properties. The
nature of the flow often varies with location, being generally more
turbulent close to the earth's surface, for example. The character
also varies with scale: thus, sometimes small-scale regions of laminar
flow are present within large-scale turbulence patterns; other times,
turbulence may be found embedded within laminar flow. Some forces, particularly Coriolis, are more influential at some
scales than at others. At every scale, thermally driven circulations
are evident. These include the land and sea breeze, mountain and
valley winds, the monsoon, and Hadley cell circulations. Other notable
winds are the result of large scale systems interacting with surface
topographical features. In this category are lee waves, Chinook
and Santa Ana winds, nor'easters, and lake effect snowstorms. Interactions between atmosphere and oceans affect the atmosphere
in a variety of ways. Large scale ocean currents, upwelling circulations,
and ENSO are just three examples of interactions that have major
impacts on airflow patterns. The atmosphere demonstrates elements of chaotic behavior which
means that future conditions are highly dependent on earlier ones.
A related idea is that the smallest atmospheric disturbances may
have major influences on much larger systems, a phenomenon known
as the butterfly effect. These considerations suggest there may
be limits to the atmosphere's long-term predictability.
In this chapter, we will explore only a small sampling of the many
atmospheric circulations that exist.
Circulation Simulation10_01 (13728.0K) |