Williams, G. P., 1988: The dynamical range of global circulations
- I. Climate Dynamics 2, 205-260.
Abstract: The dynamical range of atmospheric circulations is examined
by integrating a global circulation model (GCM) over a wide range of parameter
values. We study the influence of rotation rate on moist and dry atmospheres
with regular, drag-free, and interior-heated surfaces in Part I, and on
axisymmetric, oblique, and diurnally heated moist atmospheres in Part II.
Despite their variety, the circulations are composed of only a few elementary
forms whose existence, scale, and mix alter as the parameters vary. These
elements can be interpreted in terms of standard symmetric-Hadley (SH)
and quasi-geostrophic (QG) theories. The natural-Hadley (NH) circulation
consists of a polar jet and a hemispheric direct cell, such as occur in
slowly rotating SH flows, together with Rossby waves generated by moist
convection and barotropic cascades. The quasi-Hadley (QH) circulation consists
of a tropical westerly jet and a narrow direct cell, such as occur in the
low-latitude part of rapidly rotating SH flows, together with Rossby waves
generated by baroclinic instabilities in the neighboring midlatitude part
of the SH flows; it occurs only in moist atmospheres. The two QG circulations
represent the two extremes of eddy momentum flux produced during eddy cycles
- the special form of enstrophy cascade describing nonlinear baroclinically
unstable wave growth and barotropic wave dispersion. The QGgamma
element has a latitudinally asymmetric wave dispersion that gives
a poleward, jet-traversing momentum transport, while QGbeta
has a symmetric wave dispersion that gives a jet-converging momentum transport.
Both elements have a westerly jet and three cells. (In Part II, we describe
the solstitial symmetric-Hadley, the QG-Hadley, the diurnally modified
NH, and the Hadley circulations.) In moist atmospheres, NH circulations
exist in the rotational low range (omega* = 0 - 1/4); overlapping
QG, and QH elements in the midrange (omega* = 1/2 - 1); and QGgamma,
QGbeta, and QH elements in the high
range (omega* = 2-8); here omega* = omega/omegaE
is the rotation rate normalized by the terrestrial value. In dry atmospheres,
circulations follow a similar progression but have a simpler blend because
they lack a QH element. Kinetic energy peaks at omega* = 1/8 in
the moist, Hadley-dominated atmospheres but at omega* = 1/2 in the
dry, QG-dominated atmospheres. Instability-generated Rossby waves propagate
equatorward more easily in the westerlies of the diabatically driven (moist)
Hadley cell than in the easterlies of the eddy-induced (dry) direct cell.
Temperatures vary from almost barotropic at omega* = 0 to almost
radiative-convective at omega* = 8, while maintaining almost constant
global means. In modified-surface systems, freeslip conditions eliminate
the QH element from a moist atmosphere and allow strong deep easterlies
to arise in low latitudes to balance the strongly barotropic westerly jets
that occur in midlatitudes. In a regular dry atmosphere, enhanced surface
heating in low latitudes imitates latent heating and produces a tropical
circulation resembling that of the moist QH element. Overall, circulation
theory works well in explaining the GCM states but does not, as yet, describe
the interactions among elements or reveal how jet scales are determined,
nor explain phenomena at the extremes of the parameter range.