ALBERT

Notes: education

Notes: Abstract The dynamical range of global atmospheric circulations is extended to specialized parameter regions by evaluating the influence of the rotation rate (Ω) on axisymmetric, oblique, and diurnally heated moist models. In Part I, we derived the basic range of circulations by altering Ω for moist and dry atmospheres with regular and modified surfaces. Again we find the circulations to be composed of only a few elementary forms. In axisymmetric atmospheres, the circulations consist of a single jet in the rotational midrange (Ω*=1/2−1) and of double jets in the high range (Ω*=2−4), together with one or two pairs of Hadley and Ferrel cells; where (Ω*=Ω/Ω E ) is the rotation rate normalized by the terrestrial value. These circulations differ from those predicted by firstorder symmetric-Hadley (SH1) theory because the moist inviscid atmosphere allows a greater nonlinearity and prefers a higher-order meridional mode. The axisymmetric circulations do, however, resemble the mean flows of the natural system — but only in low latitudes, where they underlie the quasi-Hadley (QH) element of the MOIST flows. In midlatitudes, the axisymmetric jets are stronger than the natural jets but can be reduced to them by barotropic and baroclinic instablities. Oblique atmospheres with moderate to high tilts (θ P =25°−90°) have the equator-straddling Hadley cell and the four basic zonal winds predicted by the geometric theory for the solstitial-symmetric-Hadley (SSH) state: an easterly jet and a westerly tradewind in the summer hemisphere, and a westerly jet and an easterly tradewind in the winter hemisphere. The nonlinear baroclinic instability of the winter westerly produces a Ferrel cell and the same eddy fluxes as the quasi-geostrophic QGγ element, while the instability of the summer easterly jet produces a QG-Hadley (QGH) element with a unique, vertically bimodal eddy momentum flux. At high θ P and low Ω*, the oblique atmospheres reach a limiting state having global easterlies, a pole-to-pole Hadley cell, and a warm winter pole. At low tilts θ P 〈10°, the oblique circulations have a mix of solstitial and equinoctial features. Diurnal heating variations exert a fundamental influence on the natural-Hadley (NH) circulations of slowly rotating systems, especially in the singular range where the zonal winds approach extinction. The diurnality just modifies the NH element in the upper singular range (1/45⩽Ω*⩽1/16), but completely transforms it into a subsolar-antisolar Halley circulation in the lower singular range (0⩽Ω*〈1/45). In the modified NH flows, the diurnality acts through the convection to enhance the generation of the momentum-transferring planetary waves and, thereby, changes the narrow polar jets of the nondiurnal states into broad, super-rotating currents. Circulation theory for these specialized flows remains rudimentary. It does not explain fully how the double jets and the multiple cells arise in the axisymmetric atmospheres, how the QGH element forms in the oblique atmospheres, or how waves propagate in the slowly rotating diurnal atmospheres. But eventually all theories could, in principle, be compared against planetary observation: with Mars testing the QGH elements; Jupiter, the high-range elements; Titan, the equinoctial and solstitital axisymmetric states; and Venus, the diurnally modified NH flows.

Notes: 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 flow, 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 acscade describing nonlinear baroclinically unstable wave growth and barotropic wave dispersion. The QGγ element has a latitudinally asymmetric wave dispersion that gives a poleward, jet-traversing momentum transport, while QGβ 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 Halley circulations.) In moist atmospheres, NH circulations exist in the rotational low range $$(\Omega ^* = 0 - {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 4$}})$$ ; overlapping QGγ and QH elements in the midrange $$(\Omega ^* = {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}} - 1)$$ ; and QGγ, QGβ, and QH elements in the high range (Ω✻=2−8); here Ω✻=Ω/Ω E 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 $$\Omega ^* = {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 8$}}$$ in the moist, Hadley-dominated atmospheres but at $$\Omega ^* = {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$$ 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) ditropic at Ω✻=0 to almost radiative-convective at Ω✻=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 latents 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.