EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Chapter II-2
Meteorology and Wave Climate
II-2-1. Meteorology
a. Introduction.
(1) Background.
(a) A basic understanding of marine and coastal meteorology is an important component in coastal and
offshore design and planning. Perhaps the most important meteorological consideration relates to the
dominant role of winds in wave generation. However, many other meteorological processes (e.g., direct
wind forces on structures, precipitation, wind-driven coastal currents and surges, the role of winds in dune
formation, and atmospheric circulations of pollution and salt) are also important environmental factors to
consider in man's interactions with nature in this sometimes fragile, sometimes harsh environment.
(b) The primary driving mechanisms for atmospheric motions are related either directly or indirectly to
solar heating and the rotation of the earth. Vertical motions are typically driven by instabilities created by
direct surface heating (e.g., air mass thunderstorms and land-sea breeze circulations), by advection of air into
a region of different ambient air density, by topographic effects, or by compensatory motions related to mass
conservation. Horizontal motions tend to be driven by gradients in near-surface air densities created by
differential heating (for example north-south variations in incoming solar radiation, called insolation, and
differences in the thermal response of ocean and continental areas), and by compensatory motions related to
conservation of mass. The general structure and circulation of the earth's atmosphere is described in many
excellent textbooks (Hess 1959)
(c) The rotation of the earth influences all motions in the earth's coordinate system. The net effect of
the earth's rotation is to deflect all motion to the right in the Northern Hemisphere and to the left in the
Southern Hemisphere. The strength of this deflection (termed Coriolis acceleration) is proportional to the
sine of the latitude. Hence Coriolis effects are strongest in polar regions and vanish at the equator. Coriolis
effects become significant when the trajectory of an individual fluid/gas particle moves over a distance of the
same order as the Rossby radius of deformation, defined as
c
Ro '
(II-2-1)
f
where
Ro = Rossby radius of deformation
c = characteristic velocity of the particle
For a velocity of 10 m/s at a latitude of 45 deg, Ro is about 100 km. This suggests that scales of motion with
this velocity and with particle excursions of about 10 km and greater will begin to be significantly affected
by Coriolis at this latitude.
(2) Organized scales of motion in the atmosphere.
Meteorology and Wave Climate
II-2-1
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