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J. E. Geisler
and
R. E. Dickinson

Abstract

No abstract available.

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J. E. Geisler
and
R. E. Dickinson

Abstract

This paper treats the initial value problem of a forced Rossby wave encountering a critical level in a barotropic zonal shear flow which can change in response to the wave momentum flux divergence. The main result of the calculation is that the shape of the zonal flow profile changes with time in such a way as to reduce the potential vorticity gradient (β−U yy ) to zero at the critical level. For this configuration the wave is totally reflected at the critical level and in the absence of dissipation no longer interacts with the zonal flow. Details of the evolution toward the steady state depend on the ratio of two time scales, one a measure of the wave amplitude and the other representing the time it takes for the wave momentum flux to be concentrated in a well-defined critical layer.

The steady-state balance between wave and mean flow probably never occurs in the atmosphere because the time required to set it up is long compared to the expected time scale of natural variability of the zonal flow. More relevant to atmospheric flows is the fact that excursions of (β−U yy ) to negative values during the approach to a steady state are attended by over reflection of the incident wave and a temporary reversal of the wave momentum flux. After the first of these excursions, occurring on a time scale comparable to that required to set up a critical layer, the zonal flow is never far from the final equilibrium profile.

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D. L. Williamson
and
R. E. Dickinson

Abstract

No abstract available.

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Robert E. Dickinson
,
R. G. Roble
, and
E. C. Ridley

Abstract

Departures from a mean global-scale ionization distribution are commonly found in the ionospheric F region. Global-scale winds experience an acceleration where the ion drag is locally less than its global-scale smooth value and, they likewise, experience a deceleration where the ion drag is locally greater. Thus, a perturbation in the horizontal flow is set up in response to this ion-drag momentum source.

A two-dimensional, steady-state dynamic model of the neutral thermosphere, incorporating thermal conduction, viscosity and ion drag, is used to calculate the temperature perturbation and circulation pattern caused by these ion-drag anomalies. The forcing is given by a momentum source which depends on the interaction of a basic-state neutral wind with the anomaly. For horizontal-scale anomalies of a few hundred kilometers, such as the electron density depression within the stable auroral red arc, the momentum source due to perturbation ion drag is almost completely balanced by a perturbation pressure force. The perturbation temperature and circulation responses are, therefore, negligibly small. For horizontal-scale anomalies of the order of a few thousand kilometers, such as the day-night electron density variation at sunset, the force exerted by the perturbation pressure is not able to cancel the addition of momentum by the ion-drag anomaly. Thus, such a momentum source produces a significant perturbation in the horizontal velocity, vertical motion, and temperature field.

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Robert E. Dickinson
,
E. C. Ridley
, and
R. G. Roble

Abstract

A general circulation model has been developed for the atmosphere above 97 km. It uses a 5° latitude × 5° longitude grid and 24 vertical levels in increments of 0.5 scale height. The prognostic variables are horizontal winds, temperature, and the mass mixing ratios of atomic and molecular oxygen, which are obtained using hydrodynamic equations and which include vertical transport by realistic models of molecular diffusion. All the prognostic variables are in near diffusive equilibrium in the vertical as the top of the model is approached. Realistic ion drag is included in the model equations for horizontal winds, including the rapid polar drifts of magnetic field fines due to magnetospheric convection. Excellent agreement is achieved between the calculated and observed global averaged composition, provided a reasonable amount of vertical eddy mixing is included in the compositional equations over the lowest model scale height. Calculations are carried out for solar minimum equinox conditions. The calculated variation of composition with latitude is opposite to that observed for the model forced only by solar heating but is brought into reasonable agreement with observations with the inclusion of auroral heating. Generally speaking, auroral heating changes significantly the global patterns of wind, temperature, and composition, and brings the model composition in reasonable agreement with that given by the MSIS empirical model. The calculated diurnal variations of composition with auroral heating are in acceptable agreement with observation. Calculated temperature variations in the upper thermosphere are consistent with a tendency for the coupled model to minimize the ratio of temperature to mean molecular mass.

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Robert E. Dickinson
,
E. C. Ridley
, and
R. G. Roble

Abstract

The mean meridional circulation and latitudinal variation of temperature in the thermosphere are calculated for solstice conditions. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating due to auroral processes, and a momentum source due to the correlation of diurnal variations of wind and ion drag. The results show a solar-driven, summer-to-winter circulation that is modified by the high-latitude heat source. The high-latitude heat source reinforces the summer-to-winter circulation in the summer hemisphere, but reverses the circulation in the mid-latitude winter hemisphere at F-layer heights with transition from one cell to another in the midlatitude winter hemisphere. Below about 150 km, however, the summer-to-winter circulation is maintained at all latitudes. The zonal winds at midlatitudes are generally eastward in the winter hemisphere and westward in the summer hemisphere. At F-layer heights, there is a significant temperature decrease from the summer pole to winter pole. Good agreement between the calculated and observed circulations and latitudinal temperature distributions is obtained for a total high-latitude heat source of about 2 × 1018 ergs s−1, but with 2½ times as much heating in the summer hemisphere as in the winter hemisphere.

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Robert E. Dickinson
,
E. C. Ridley
, and
R. G. Roble

Abstract

The mean meridional circulation and latitudinal variation of temperature in the thermosphere are considered for equinox conditions. With regard to these parameters there have been serious discrepancies between observational indications and theoretical expectations. A numerical model of the zonally symmetric thermospheric circulation is formulated and solved using a finite-difference initial value approach to steady-state solutions. Solutions are obtained for three different prescriptions of forcing terms: solar heating alone, solar heating plus an effective momentum source due to diurnal variations, and inclusion of a high-latitude heat source representing Joule dissipation of electric current systems. It is concluded that the Joule heating is essential for bringing theoretical predictions into agreement with observations but that the global mean of the required heating during geomagnetically quiet periods is necessarily small compared to global mean solar heating at the same levels.

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Weiqing Qu
,
A. Henderson-Sellers
,
A. J. Pitman
,
T. H. Chen
,
F. Abramopoulos
,
A. Boone
,
S. Chang
,
F. Chen
,
Y. Dai
,
R. E. Dickinson
,
L. Dümenil
,
M. Ek
,
N. Gedney
,
Y. M. Gusev
,
J. Kim
,
R. Koster
,
E. A. Kowalczyk
,
J. Lean
,
D. Lettenmaier
,
X. Liang
,
J.-F. Mahfouf
,
H.-T. Mengelkamp
,
K. Mitchell
,
O. N. Nasonova
,
J. Noilhan
,
A. Robock
,
C. Rosenzweig
,
J. Schaake
,
C. A. Schlosser
,
J.-P. Schulz
,
A. B. Shmakin
,
D. L. Verseghy
,
P. Wetzel
,
E. F. Wood
,
Z.-L. Yang
, and
Q. Zeng

Abstract

In the PILPS Phase 2a experiment, 23 land-surface schemes were compared in an off-line control experiment using observed meteorological data from Cabauw, the Netherlands. Two simple sensitivity experiments were also undertaken in which the observed surface air temperature was artificially increased or decreased by 2 K while all other factors remained as observed. On the annual timescale, all schemes show similar responses to these perturbations in latent, sensible heat flux, and other key variables. For the 2-K increase in temperature, surface temperatures and latent heat fluxes all increase while net radiation, sensible heat fluxes, and soil moistures all decrease. The results are reversed for a 2-K temperature decrease. The changes in sensible heat fluxes and, especially, the changes in the latent heat fluxes are not linearly related to the change of temperature. Theoretically, the nonlinear relationship between air temperature and the latent heat flux is evident and due to the convex relationship between air temperature and saturation vapor pressure. A simple test shows that, the effect of the change of air temperature on the atmospheric stratification aside, this nonlinear relationship is shown in the form that the increase of the latent heat flux for a 2-K temperature increase is larger than its decrease for a 2-K temperature decrease. However, the results from the Cabauw sensitivity experiments show that the increase of the latent heat flux in the +2-K experiment is smaller than the decrease of the latent heat flux in the −2-K experiment (we refer to this as the asymmetry). The analysis in this paper shows that this inconsistency between the theoretical relationship and the Cabauw sensitivity experiments results (or the asymmetry) is due to (i) the involvement of the β g formulation, which is a function of a series stress factors that limited the evaporation and whose values change in the ±2-K experiments, leading to strong modifications of the latent heat flux; (ii) the change of the drag coefficient induced by the changes in stratification due to the imposed air temperature changes (±2 K) in parameterizations of latent heat flux common in current land-surface schemes. Among all stress factors involved in the β g formulation, the soil moisture stress in the +2-K experiment induced by the increased evaporation is the main factor that contributes to the asymmetry.

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