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Robert E. Dickinson

Abstract

Absorption of solar radiation in the dayside Venusian thermosphere forces a circulation cell with vertical motions upward on the dayside and downward on the nightside with maximum amplitude greater than a meter per second and horizontal velocities away from the subsolar point with amplitudes up to several hundred meters per second. The first harmonic in temperature determines a several-hundred-degree temperature decrease from dayside to nightside. These conclusions follow from the numerical integration of a dynamic model which includes realistic stratification and temperature-dependent radiative damping. The large day-to-night temperature contrast is a consequence of the addition of extreme ultraviolet (EUV) heating at sufficiently high levels that it must he conducted downward to lower levels before adiabatic and 15 μ cooling can balance it. The observed exospheric temperature of ∼650K near the subsolar point is reproduced with an EUV heating efficiency of 0.3. The calculated nightside exospheric temperature is below 300K for this efficiency.

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Robert E. Dickinson

Abstract

Small amplitude planetary waves are superimposed on a mean zonal flow with arbitrary horizontal and vertical shears. An expression is derived for the change of the zonal wind and temperature field forced by statistically stationary eddies satisfying a source-free planetary wave equation. This result depends on the existence of singular lines, where the phase speed of an elementary wave is equal to the mean zonal wind speed, or on the presence of a Newtonian cooling process. Second-order interactions vanish when both of these phenomena are absent. The planetary wave-zonal flow interaction is discussed in terms of the eddy transport of potential vorticity. The theory provides a partial interpretation of the maintenance of atmospheric zonal flows, such as that of the wintertime stratosphere, by planetary waves propagating from some other region of the atmosphere.

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Robert E. Dickinson

Abstract

A model is developed for the NLTE infrared radiative transfer in a CO2 atmosphere at low pressure. This model is used to determine the source function for CO2 15 µm vibrational levels and the radiative equilibrium temperature for the atmosphere of Venus above cloud level. The most significant improvement in the radiative transfer model over previous efforts is the inclusion of vibrational-vibrational (V-V) exchange between 15 µm levels. The exchange is especially important for determining isotopic and hot band source functions. This rate is introduced parametrically by assuming it is the same for all transitions, and 10, 100 or 1000 times as rapid as the relaxation rate for the 15 µm fundamental level. We analyze in detail the rates of photon escape to space in the various CO2 bands. Over much of the Venusian mesosphere, significantly more photons escape to space in the hot and isotopic bands than do in the C12O2 16 fundamental band. However, the latter is essentially in LTE to significantly lower pressure than the other bands and is insensitive to the V-V transition rates. Hence remote sensing of temperature from the infrared radiances would require smallest corrections for NLTE and be least sensitive to V-V rates if only radiances from the latter band were measured. With this band, the assumption of LTE is valid up to the 1 µb level. Sufficient thermal excitation exists up to 0.01 µb for a NLTE temperature inversion to be practical. Detection of hot bands or isotopic band emission from around 0.1 µb in conjunction with a known temperature would allow deduction of the band source functions and hence inference as to the V-V rates.

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Robert E. Dickinson

Abstract

A zero-dimensional climate model is considered with three thermal reservoirs, i.e., the atmosphere, the surface mixed layer and the intermediate water of the ocean. Realistic values are adopted for the rates of heat transfer between those reservoirs. If heat is added suddenly to the atmosphere, the atmospheric temperature increases a small amount in a few days. Thereafter, the atmosphere and mixed layer increase in temperature. About one-half the mixed-layer response occurs on a time scale of two years and the rest on a time scale of about 100 years. Numerical solution of atmosphere-ocean general circulation models may require asynchronous coupling strategies to link the two models. A semi-implicit approach is considered which generalizes previously suggested schemes. Its convergence and stability are examined by application to the zero-dimensional climate model. It may be unstable if it is made too explicit. On the other hand, the fully implicit approach greatly slows down the response of the mixed layer unless a very short coupling interval is used. With some fraction about equal to 0.05 of the atmosphere-ocean heat transfer made implicit, the asynchronous coupling solutions are close to the correct solution even if a large coupling interval is used.

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ANALYTIC MODEL FOR ZONAL WINDS IN THE TROPICS

I. Details of the Model and Simulation of Gross Features of the Zonal Mean Troposphere

ROBERT E. DICKINSON

Abstract

We consider the zonal mean circulation on an equatorial β-plane and obtain analytic solutions for small-amplitude disturbances in response to sources of heat and momentum. The effect of dissipation is roughly approximated by using assumed constant drag and radiative damping terms. We apply the model to the tropospheric zonal winds, temperatures, and meridional circulation to give simple insights into the maintenance of the observed mean state of these parameters by the existing sources of heat and momentum. The heat source consists of a relatively sharp maximum in heating at the Equator caused by latent heat release in the tropical rainbelt against a relatively smooth background of mean radiative cooling. As a momentum source, we use the convergence of horizontal eddy momentum fluxes. Numerical results are presented for an idealized annual mean circulation. The model solutions show how the thermal and momentum sources jointly maintain the zonal wind distribution. A concomitant meridional circulation redistributes the heat and momentum, allowing the temperatures to remain in balance with the zonal wind. This is part I of a two-part study.

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Robert E. Dickinson

Abstract

The horizontal variation of infrared heating is calculated for the Venusian mesophere and adjoining layers in the stratosphere and lower thermosphere. The calculation assumes no horizontal temperature variation. Very large heating rates at the subsolar point reaching ∼1000K. (earth day)−1 are calculated at pressures between 1 and 0.1 μb. Between 0.1 and 0.0l μb, the 15 μ NLTE source function increases with an increase of near-infrared heating. Consequently, there is a large solar zenith variation of 15 μ hot-band cooling that cancels roughly half the near-infrared heating at these levels. It is suggested that the large horizontal variation of heating at 1 μb cannot he completely balanced by adiabatic cooling in the absence of horizontal temperature variation, so the 15 μ emission might be expected to vary horizontally by as much as a factor of 2. At altitudes below the 100-μb level, the horizontal variation of temperature should be entirely negligible.

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Walter Orr Roberts Lecture

Land Surface Processes and Climate Modeling

Robert E. Dickinson

This paper, as the written version of the 1995 AMS Walter Orr Roberts Lecture, provides an overview of the current status of the inclusion of land surface processes in climate models. These processes provide fluxes of water and energy to atmospheric models and help determine surface meteorology and climate over the continents. With the increasing complexity and importance of these parameterizations and their detailed treatments of the roles of soils and vegetation have come greater demands for observational programs to evaluate their success and to provide required parameters. Intercomparisons between different land models are also becoming increasingly valuable as a means of identifying their weaknesses and limitations.

The paper especially highlights the need for further emphasis on the coupling between land and the atmosphere in models. In particular, it calls for further evolution and improvement of the model treatments of precipitation, cloud effects on surface radiation, and boundary layer processes.

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ANALYTIC MODEL FOR ZONAL WINDS IN THE TROPICS

II. Variation of the Tropospheric Mean Structure With Season and Differences Between Hemispheres

ROBERT E. DICKINSON

Abstract

An equatorial β-plane model for the tropospheric zonal circulation is used to examine the consequences of the seasonal and hemispheric variation both of the tropical rain belt as a zonal mean heat source and of the horizontal eddy momentum fluxes as a zonal mean momentum source. The model calculations show variations of Hadley circulations with hemisphere and season. The winter hemisphere Hadley cell is more intense in July than in January because of the greater mean displacement of the tropical rain belt from the Equator and hence the greater asymmetry between hemispheres. The essential differences between the zonal winds in the two hemispheres during summer are reproduced by differences in the eddy momentum transports and in the mean meridional circulation.

The model indicates how the annual oscillation of temperature in the equatorial lower stratosphere, with lowest temperatures in January, derives from the difference between the upward branches of the July and the January winter hemisphere Hadley cell. The semiannual oscillation in winds and temperatures in the Tropics is largely accounted for by the model in terms of the longitudinally averaged tropical rain belt migrating between summer hemispheres.

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Robert E. Dickinson

Abstract

The study assumes adiabatic, inviscid, hydrostatic and quasi-geostrophic motions on a mid-latitude β-plane. The domain is assumed vertically unbounded. The stability of a hyperbolic-tangent shear flow to small-amplitude disturbances is discussed. Negative shear zones (wind becoming stronger westward with increasing elevation) are unstable for weaker shears and the resulting instabilities have larger growth rates than in the case with positive shear zones (wind becoming stronger eastward with increasing elevation) and other conditions the same.

Neutral solutions for the hyperbolic-tangent shear flow problem are found analytically, and growth rates and modal structure of unstable modes are found numerically. The unstable modes for a negative shear flow and for sufficiently small longitudinal wavenumber have the structure of vertically propagating Rossby waves. Thus, the shear zone can act as a source of Rossby waves which couple the zonal wind within the shear zone to the mean zonal wind many, scale heights removed from the shear zone.

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Robert E. Dickinson

Mechanisms possibly connecting solar activity to meteorology of the lower atmosphere are reviewed. Besides direct variations of solar visible emission, solar-related fluctuations in some aspect of cloudiness could be important. Any such variations in cloudiness are likely to be related to variations in production of ionization near the tropopause by galactic cosmic rays, the only geophysical phenomena unconnected with upper atmospheric processes known to have a striking (negative) correlation with solar activity. Such a connection might involve a dependence of sulfate aerosol formation on ionization and in turn a dependence of cloud radiative properties on variations of the aerosol particles' action as cloud condensation nuclei.

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