Venus Mesosphere and Thermosphere II. Global Circulation

ICARUS 68, 284--312 (1986) Venus Mesosphere and Thermosphere II. Global Circulation, Temperature, and Density Variations S. W. BOUGHER,*'t R. E. DICKINSON,$ E. C. RIDLEY,§ R. G. ROBLE,* A. F. NAGY, II AND T. E. CRAVENS II *High Altitude Observatory, ~fAdvanced Study Program, $Atmospheric Analysis and Prediction, and §Scientific Computing Division, National Center for Atmospheric Research, I P.O. Box 3000, Boulder, Colorado 80307; and IISpace Physics Research Laboratory, 2455 Hayward, University of Michigan, Ann Arbor, Michigan 48109 Received February 13, 1986; revised July 11, 1986 Recent Pioneer Venus observations have prompted a return to comprehensive hydrodynamical modeling of the thermosphere of Venus. Our approach has been to reexamine the circulation and structure of the thermosphere using the framework of the R. E. Dickinson and E. C. Ridley (1977, Icarus 30, 163-178), symmetric two-dimensional model. Sensitivity tests were conducted to see how large-scale winds, eddy diffusion and conduction, and strong 15-/xm cooling affect day-night contrasts of densities and temperatures. The calculated densities and temperatures are compared to symmetric empirical model fields constructed from the Pioneer Venus data base. We find that the observed day-to-night variation of composition and temperatures can be derived largely by a wave- drag parameterization that gives a circulation system weaker than predicted prior to Pioneer Venus. The calculated mesospheric winds are consistent with Earth-based observations near 115 km. Our studies also suggest that eddy diffusion is only a minor contributor to the maintenance of observed day and nightside densities, and that eddy coefficients are smaller than values used by previous one-dimensional composition models. The mixing that occurs in the Venus thermosphere results from small-scale and large-scale motions. Strong COz 15-/xm cooling buffers solar perturba- tions such that the response by the general circulation to solar cycle variation is relatively weak. © 1986 AcademicPress, Inc. 1. INTRODUCTION dicted strong 400 m sec -l winds which caused a considerable dayside depletion New information obtained from Pioneer and nightside enhancement of O and CO Venus (PV) spacecraft observations makes densities. It has become apparent from the it appropriate to return to comprehensive PV data that the actual structure in Venus's hydrodynamical model studies of the ther- upper atmosphere has weaker day-night mospheric circulation of Venus. The gen- contrasts of these constituents and is more eral features of the inferred mean subsolar- complex than can be modeled by a purely to-antisolar circulation were previously symmetric subsolar-to-antisolar circulation predicted in a series of numerical studies by (Mayr et al., 1980, 1985). A summary of the Dickinson and Ridley (1972, 1975, 1977). differences between DRM model calcu- However, these calculations failed to re- lations and Pioneer Venus observations produce the observed day-night tempera- was provided in Fig. 1 of Dickinson and ture contrast, specifically the very cold Bougher (1986), hereafter referred to as nightside temperatures. The Dickinson and Part I. Ridley (1977) model (or DRM) also pre- The discrepancies between hydrodynamic model predictions and PV observa- The National Center for Atmospheric Research is tions have been puzzling. The large-scale sponsored by the National Science Foundation. circulation and its effects on temperature 284 0019-1035/86 $3.00 Copyright © 1986 by AcademicPress, Inc. All rights of reproductionin any form reserved. VENUS THERMOSPHERIC CIRCULATION 285 and composition fields are included in the ciencies greater than 10% without the use self-consistent, coupled primitive equation of excessively large eddy coefficients, even model of DRM. However, small-scale mix- assuming the maximum possible eddy cooling processes inferred from observed atmo- ing. This cooling is even less effective if we spheric wave signatures (Seiff et al., 1980; assume a partial compensation of frictional Seiff, 1982) were not incorporated in any heating and cooling. However, an im- systematic manner. On the other hand, proved thermal balance can be obtained one-dimensional composition models of with enhanced 15-/~m cooling. For our final yon Zahn et al. (1980), Stewart et ai. (1980), reference calculation, we adopted a large and Massie et al. (1983) have been used to but not completely improbable rate coeffi- examine the influence of eddy mixing on cient for enhancing CO2 radiation through densities; these neglect any self-consistent collisions with ambient atomic oxygen, incorporation of large-scale dynamical ef- K¢o2-o = 8 × I0 -13 cm 3 sec -l at 300°K, in fects. Simpler hydrodynamic models (e.g, conjunction with a 9.5% EUV efficiency to Mayr et al., 1980, 1985) are highly parame- obtain the observed temperatures. This terized and, like one-dimensional models, value is roughly double that stated in Part I, cannot examine the strong feedback among as required to allow for a minor coding er- temperature, composition, and wind fields. ror found in the one-dimensional radiative All of these previous model descriptions in- transfer model above 140 km. The bottom corporate important processes of the actual four entries of Table I and the ordinate in Venus thermosphere. However, a new self- Fig. 4 of Part I should be similarly adjusted. consistent and quantitative simulation of With these adjustments, all other con- the Venusian upper atmosphere is ulti- clusions and diagnostics are largely un- mately needed that combines the key fea- changed; in particular, heating efficiencies tures of these separate models. much in excess of 10-12% are presently in- As discussed in Part I, there are several consistent with any known cooling mecha- questions to consider in the development of nism for Venus's atmosphere. This cor- an improved hydrodynamical model. First, rected final reference calculation provides how can we obtain a global average temper- the basis for deriving the infrared heating ature structure with exospheric values near and 15-/xm cooling inputs required for the 210°K as observed? Second, how can we improved hydrodynamical model simula- model the day-night temperature contrast, tion (see Sect. 2). which is nearly 200°K at exospheric heights Having established the parameters most but rather small at 100 km and below? suitable for reproducing the observed Third, how can we obtain atomic oxygen global mean temperatures, we investigate concentrations as large and with as little di- here the day-night temperature contrasts urnal variation along constant pressure sur- and compositional distributions. Several faces about the equator as those observed? plausible alterations of the DRM large-scale It is also important to be able to reproduce circulation have been suggested to improve the asymmetrical signatures observed for its agreement with the observed Venus the light species, particularly He and H thermospheric structure. The mechanism densities. explored here is a global circulation system The calculation of a reasonable global av- weaker than predicted by DRM (yon Zahn erage temperature structure was addressed et al., 1983). Comparison of Dickinson and in Part I as a first step in the simulation of Ridley (1975) and the later DRM model out- the dynamics and observed characteristics put fields shows an increase in dayside of the Venus thermosphere. It was found atomic O densities and a corresponding de- that cooling by eddy mixing alone is not crease on the nightside. These changes are sufficient to balance EUV heating with effi- presumably caused by a weakening of the 286 BOUGHER ET AL. day-to-night winds in response to a reduced back to the dayside without significantly af- dayside heating in this sequence of model fecting the global temperatures. Such a calculations. Thus, a sufficiently weakened scheme could jointly improve the density wind structure can maintain observed O distributions while retaining the strong and CO densities on the dayside while re- (300-400 m sec -l) terminator neutral ducing the buildup on the nightside. Fur- winds. thermore, the DRM has established that the The DRM code has been completely re- nightside heating comes primarily from the developed in recent years to run on the circulation, mostly from adiabatic compres- NCAR CRAY computers. Apparently, pre- sion due to descending nightside winds, but viously neglected physical processes, such also from thermal advection across the ter- as those described above, are required to minator. Hence, slower winds would also explain the observed day-night thermal result in cooler nightside temperatures. A and density contrasts observed in Venus's suitable mechanism for slowing the winds upper atmosphere. We make use of the would change the distribution of atmo- large-scale circulation scheme of the pre- spheric constituents and temperatures vious DRM model as a framework upon about the planet. Terrestrial mesopause which such model processes as eddy diffu- wave-breaking models serve as a guide for sion, eddy or viscous wave drag, and 15- possible momentum drag schemes (Lind- /~m radiational cooling are systematically zen, 1981; Holton, 1982). Eddy viscosity examined. The two-dimensional model and Rayleigh friction may approximate the framework that we adopt here is only capa- effects of wave drag on the mean flow. ble of examining the symmetric component Mayr et al. (1985) have proposed a differ- of Venus's thermospheric circulation and ent scheme to match the

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