Geography Reference
In-Depth Information
9.1
ENERGY SOURCES FOR MESOSCALE CIRCULATIONS
Mesoscale dynamics is generally defined to include the study of motion systems
that have horizontal scales in the range of about 10 to 1000 km. It includes cir-
culations ranging from thunderstorms and internal gravity waves at the small
end of the scale to fronts and hurricanes at the large end. Given the diverse
nature of mesoscale systems, it is not surprising that there is no single concep-
tual framework, equivalent to the quasi-geostrophic theory, that can provide a
unified model for the dynamics of the mesoscale. Indeed, the dominant dynamical
processes vary enormously depending on the type of mesoscale circulation system
involved.
Possible sources of mesoscale disturbances include instabilities that occur intrin-
sically on the mesoscale, forcing by mesoscale thermal or topographic sources,
nonlinear transfer of energy from either macroscale or microscale motions, and
interaction of cloud physical and dynamical processes.
Although instabilities associated with the mean velocity or thermal structure of
the atmosphere are a rich source of atmospheric disturbances, most instabilities
have their maximum growth rates either on the large scale (baroclinic and most
barotropic instability) or on the small scale (convection and Kelvin-Helmholtz
instability). Only symmetric instability (to be discussed in Section 9.3) appears to
be an intrinsically mesoscale instability.
Mountain waves created by flow over individual peaks are generally regarded
as small-scale phenomena. However, flow over large mountain ranges can produce
orographic disturbances in the 10- to 100-km mesoscale range, whose character-
istics depend on the mean wind and static stability profiles and the scale of the
orography. Flow over mountain ranges, such as the Front Range of the Colorado
Rockies, can under some conditions of mean flow and static stability lead to strong
downslope wind storms.
Energy transfer from small scales to the mesoscale is a primary energy source
for mesoscale convective systems. These may start as individual convective cells,
which grow and combine to form thunderstorms, convective complexes such as
squall lines and mesocyclones, and even hurricanes. Conversely, energy transfer
from the large scale associated with temperature and vorticity advection in synoptic-
scale circulations is responsible for the development of frontal circulations.
9.2
FRONTS AND FRONTOGENESIS
In the discussion of baroclinic instability in Chapter 8 the mean thermal wind U
T
was taken to be a constant independent of the y coordinate. That assumption was
necessary to obtain a mathematically simple model that retained the basic insta-
bility mechanism. It was pointed out in Section 6.1, however, that baroclinicity is
