key: cord-0826178-mywfifpi
authors: Yamamoto, Masaru; Ikeda, Kohei; Takahashi, Masaaki
title: Atmospheric response to high-resolution topographical and radiative forcings in a general circulation model of Venus: Time-mean structures of waves and variances
date: 2020-10-09
journal: Icarus
DOI: 10.1016/j.icarus.2020.114154
sha: b7df3202ac743df983f7c8d767268c98c9f8463a
doc_id: 826178
cord_uid: mywfifpi

Thermal tides, stationary waves, and general circulation are investigated using a T63 Venus general circulation model (GCM) with solar and thermal radiative transfer in the presence of high-resolution surface topography, based on time average analysis. The simulated wind and static stability are very similar to the observed ones (e.g., Horinouchi et al., 2018; Ando et al., 2020). The simulated thermal tides accelerate an equatorial superrotational flow with a speed of ~90 m s(−1) around the cloud-heating maximum (~65 km). The zonal-flow acceleration rates of 0.2–0.5 m s(−1) Earth day(−1) are produced by both horizontal and vertical momentum fluxes at low latitudes. In the GCM simulation, strong solar heating above the cloud top (>69 km) and infrared heating around the cloud bottom (~50 km) modify the vertical structures of thermal tides and their vertical momentum fluxes, which accelerate zonal flow at 10(3) Pa (~75 km) and 10(4) Pa (~65 km) at the equator and around 10(3) Pa at high latitudes. Below and in the cloud layer, surface topography weakens the zonal-mean zonal flow over the Aphrodite Terra and Maxwell Montes, whereas it enhances the zonal flow in the southern polar region. The high-resolution topography produces stationary fine-scale bow structures at the cloud top and locally modifies the variances in the geographical coordinates (i.e., the activity of unsteady wave components). Over the high mountains, vertical spikes of the vertical wind variance are found, indicating penetrative plumes and gravity waves. Negative momentum flux is also locally enhanced at the cloud top over the equatorial high mountains. In the solar-fixed coordinate system, the variances (i.e., the activity of waves other than thermal tides) of flow are relatively higher on the nightside than on the dayside at the cloud top. Strong dependences of the eddy heat and momentum fluxes on local time are predominant. The local-time variation of the vertical eddy momentum flux is produced by both thermal tides and solar-related, small-scale gravity waves on the nightside.

On Venus, thermal tides dominate the middle atmosphere between 40 and 90 km (Schofield and Taylor, 1983; Pechmann and Ingersoll, 1984; Zasova et al., 2002 Zasova et al., , 2007 Tellmann et al 2009; Grassi et al., 2010; Scarica et al., 2019; Ando et al., 2018; Kouyama et al., 2019) . They propagate vertically and have a strong day-to-night circulation at 6570 km. The thermal tides simulated in general circulation models (GCMs) were compared with the observations (e.g., Scarica et al., 2019; Takagi et al., 2018; Yamamoto et al., 2019) . According to Ignateiv et al. (2009) , the cloud-top altitude has latitude dependence and the range of 63-74 km. The average value (69 km, ~4.54 × 10 3 high-resolution topography on the dynamics (e.g., general circulation, longitudinal wind variation, and the global-scale bow-shaped wave) have not been fully understood.

To model the general circulation of Venus more realistically, simplified Newtonian cooling has been replaced by infrared radiative transfer code (Eymet et al., 2009) in GCMs of Venus (Lebonnois et al., 2010 (Lebonnois et al., , 2016 Garate-Lopez and Lebonnois, 2018) . Mendonça and Read (2016) developed the Oxford Planetary Unified Model System for Venus using a radiative transfer model (Mendonça et al., 2015) , in which both solar and thermal radiation are taken into account in the two-stream code. Ikeda (2011) developed a radiative transfer model for Venus' solar and thermal radiation using two-stream discrete ordinate and k-distribution methods in a GCM developed at the Atmosphere and Ocean Research Institute (AORI) of the University of Tokyo and diagnosed strong solar heating above the cloud top and infrared heating at the cloud bottom. Using Ikeda's GCM, Yamamoto et al. (2019) simulated the indirect circulation formed by the thermal tide and baroclinic waves around the jets, topographical modification of middle-atmospheric structure, and cloud-top wind fields that are similar to the subsolar zonal wind speed minimum and strong poleward flows observed by Akatsuki UV cloud tracking (Yamazaki et al., 2018; Horinouchi et al., 2018) . However, although the solar-locked circulation was discussed in Yamamoto et al. (2019) , the three-dimensional structure of diurnal and semidiurnal tides induced by realistic radiative forcing was not investigated.

In addition, the momentum fluxes and their zonal-flow acceleration of stationary waves forced by high-resolution surface topography were not accurately estimated in Yamamoto et al. (2019) , because show a quasi-equilibrium state on Venus day 100 and 101 (Venus day 10 and 11 from the restart, Fig. A1b in Appendix A). We analyzed output data obtained during the two Venus days at 3-h intervals.

An example of output data for 10 to 100 earth days on Venus day 10 from the restart (Hovmöller diagram of vertical wind velocity at the equatorial cloud top in geographical coordinates) is shown as Fig. A2a . The general circulation and waves are investigated based on time averages of the meteorological elements (X) in geographical and solar-fixed coordinate systems. Longitude in the geographical coordinate is topographically fixed and expressed in degrees from 0 to 360 (Fig. 1a of Yamamoto, 2019) . The superrotational wind flows from east (0) to west (360) in the longitudinal direction. The solar-fixed coordinate frame rotates around the planet with a period of 117 Earth days in the geographical coordinate frame. Longitude in the solar-fixed coordinates is expressed by local time (e.g., the subsolar longitude is defined as 12 LT and the antisolar longitude as 0 LT and 24 LT).

The time averages are defined as follows:

where superscript "TM" (time mean) indicates the average over the total number of time steps N (= 1,872),  is the sum of X(t = ndt) from n = 1 to n = N, dt is the time increment (3 h). Subscripts "G"

and "S" mean meteorological elements in the geographical and solar-fixed coordinates, respectively.

The zonal mean fields of Eqs.

(2) and (3) ̅̅̅̅̅̅ ̅̅̅̅̅̅ ̅̅̅̅̅̅

are investigated in Sect. 3.1 and 3.4. Here, the overbar indicates the zonal average over the entire longitudinal range.

In the present work, stationary waves are considered to be topographically locked waves obtained from the data averaged over two Venus days in the geographical coordinates in Sect. 3.3.

The stationary wave is defined as the deviation of the stationary component X G TM from the zonal mean:

where the prime is the eddy component. Similarly, thermal tides are solar-locked waves (i.e., stationary waves in the solar-fixed coordinate system), with eddy components described as ̅̅̅̅̅̅ .

in Sect. 3.2. The diurnal and semidiurnal tides are calculated from zonal wavenumber 1 and 2 components of (X S TM )', which are decomposed by fast Fourier transform (FFT).

To investigate the longitudinal variations of the wave activity, we introduce variances of time series data. These statistical quantities are useful for future comparisons with observations because they can be obtained from both model output and satellite measurement. The variances of the meteorological elements in the geographical and solar-fixed coordinates (Sect. 3.5) are defined as

In the geographical coordinate system, the variance indicates the activity of unsteady wave and short-lived, small-scale perturbations (e.g., gravity waves). If the unsteady waves are independent of the surface topography, the variance is zonally uniform. In contrast, if the unsteady wave activity is locally modified by the topography, the variance is highly correlated with the terrain surface. In the solar-fixed coordinates, the variance means the activity of wave components excluding the time-mean thermal tides, because the tidal component includes the time-mean component . If the variances strongly depend on local time, the non-tidal wave activity is also modified by solar heating.

In this article, we discuss the longitudinal structures of the time-mean eddy heat and momentum fluxes ( ′ ′ , ′ ′ , and ′ ′ in the geographical coordinates and ′ ′ , ′ ′ , and ′ ′ in the solar-fixed coordinates) in Sect. 3.6. If zonally traveling waves independent of the topography rotate around the planet many times, the time-mean fluxes of the wave become the same for all grid points along a given latitude circle (i.e., they are longitudinally uniform). In contrast, the stationary wave pattern is unchanged in the geographical coordinates during the time average period.

Thus, the longitudinal variations of the eddy fluxes are produced by stationary waves and their related eddies (e.g., topographically locked convection and gravity waves). In the same way, the local time variations of the eddy fluxes are produced by thermal tides and solar-related gravity waves, J o u r n a l P r e -p r o o f Journal Pre-proof of which the phase structures are unchanged in the solar-fixed frame. For zonally traveling waves independent of the solar heating, the eddy fluxes are longitudinally uniform.

The time and zonal averages of the heat and momentum fluxes of total eddies analyzed in Sect. 3.2 and 3.7 are described as

The zonal-mean fluxes of the thermal tides are calculated by

The zonal-mean fluxes of the stationary wave are

Zonal-flow acceleration rates of the total eddies, thermal tide, and stationary wave are calculated from the flux convergence of Eqs. (10)(11), Eqs. (13)(14) and Eqs. (16)(17), respectively:

J o u r n a l P r e -p r o o f

where  is atmospheric density and the superscript "a" is TM, tide, or stat.

As will be discussed later, the amplitudes and phases of thermal tides are sensitive to altitude around the cloud top. Thus, we investigated the horizontal structures of the eddy heat and momentum fluxes at the two levels: the cloud-heating maximum (~65 km, 9.75 × 10 3 Pa) and cloud top (~69 km, 4.54 × 10 3 Pa) assigned in this article. Figure 1a and b shows zonally and temporally averaged structures of zonal flow and air temperature. Superrotation is fully developed in the Venus middle atmosphere. Although the zonal jets in the T63 model are ~10 m s 1 weaker than those in the T21 model (Yamamoto et al., 2019) , the zonal-flow and temperature structures are similar in the two models. Our T63 simulation (the present work) reproduces the multi-layered stable layers at low latitudes (Young et al., 1987) , the polar strongly stable regions above the 10 4 -Pa altitude (>65 km), and the deep low-stability region below Richardson 2011; Lebonnois et al., 2015) . Associated with the thermal wind of the zonal jets, polar air temperature is relatively high (low) above (below) the jet core compared to mid-latitude temperature. The polar warming (cooling) enhances high (low) stability around the poles.

The global Hadley circulation is predominant above the cloud top (<5 × 10 3 Pa, >69 km). In addition, vertically thin Hadley cells form in and around the low-stability layer of ~10 5 Pa (~50 km),

where the IR heating rate is positive and latitudinally uniform at the cloud base. Above the cloud top, the equator-pole imbalance between shortwave and longwave radiative fluxes produces strong net radiative forcing (the difference between solar and IR heating), which contributes to the formation of a strong Hadley cell. In contrast, because the radiative heating is locally balanced between shortwave and longwave around the cloud-heating maximum, the net radiative heating of shortwave and longwave becomes weak. Thus, strong poleward flows do not form around 10 4 Pa. Indirect circulations are driven around the jet core, where the poleward eddy heat fluxes of thermal tides and baroclinic waves are produced (Yamamoto et al., 2019 and Sect. 3.7) . Figure 2 shows horizontal structures of thermal tides at the cloud top (4.54 × 10 3 Pa, ~69 km) and cloud-heating maximum (9.75 × 10 3 Pa, ~65 km) using Eq. (6). Poleward and eastward flows of the thermal tides are predominant around the subsolar point. At low latitudes, the horizontal flows of the tides have a gravity-wave structure. The thermal tides produce the zonal-wind minimum J o u r n a l P r e -p r o o f Journal Pre-proof and strong meridional wind at 1215 LT, which are consistent with the Akatsuki UVI observations (Yamazaki et al., 2018; Horinouchi et al., 2018) and our T21 simulation (Fig. 9f and Fig. 10 in Yamamoto et al. 2019) . The meridional wind magnitude of the simulated tides is of the same magnitude as in Limaye (1988) and Smith and Gierasch (1996) . As discussed in Yamamoto et al. (2019) , although the eddy geopotential height and vertical flow are in phase around the equator between 9.75 × 10 3 Pa and 4.54 × 10 3 Pa, the phase of eddy temperature changes rapidly at these pressure levels. At the cloud-heating maximum level of 9.75 × 10 3 Pa, both the eddy temperature and vertical velocity components are positively large around the subsolar point. In contrast, the afternoon temperature is relatively low and the morning and night temperatures are high at low latitudes at 4.54 × 10 3 Pa immediately above the cloud-heating maximum.

At high latitudes, the horizontal flows of the tides have a vortical structure like a zonal wavenumber-1 Rossby wave (Yamamoto and Takahashi, 2006) . The geopotential height and temperature are high (low) around the evening (morning) terminator at 9.75 × 10 3 Pa and 4.54 × 10 3

Pa. Strong meridional flows across the poles are similar to those seen in Yamamoto and Takahashi (2015) . Vertical-flow streaks of polar thermal tides are formed along the longitudinally extended trough and ridge at ~70 latitude between the polar low and high.

Both the diurnal and semidiurnal tides are important in our GCM experiments. Figure 3 shows the horizontal structure of diurnal and semidiurnal tides at the cloud-heating maximum. The diurnal horizontal wind flows from low to high geopotential height at the equator and is a around the cloud-heating maximum. In the low-latitudinal area within 30 latitudes, the sign of the eddy heat flux at 4.54×10 3 Pa (black line with dots in Fig. 4b ) is opposite to that at 9.75×10 3 Pa (black line with dots in Fig. 4a ) because the phases of the air temperature component change at these two pressure levels (Fig. 2a and c) . The meridional profiles of eddy momentum fluxes at 4.54×10 3 Pa (black line with dots in Fig. 4d ) are similar to those at 9.75×10 3 Pa (black line with dots in Fig. 4c ).

Because the strong horizontal wind vectors tilt eastward with increasing latitude in the regions of 912 LT and 1821 LT in Fig. 4f ), the weak zonal-flow acceleration by tides is formed within 30 latitudes, whereas the deceleration is locally strong around the jet cores (60 latitudes).

This is quite different from the complex profile of acceleration by total waves. In particular, because the small-scale meridional variation of the momentum fluxes of total waves is large at high latitudes, the acceleration profile of total waves at high latitudes is more complex than that at low latitudes. At 69.9N latitude, the zonally uniform structure of the static stability is predominant because the vertical gradient of zonal-mean temperature is relatively larger than that of eddy temperature.

Around the cloud-heating maximum (9.75 × 10 3 Pa, ~65 km), positive and negative temperature deviations are located at 1224 LT and 012 LT, respectively. This is consistent with those in the IPSL GCM and VIRTIS at high latitudes (Scarica et al., 2019). The thermal tide propagates upward above 10 4 Pa (>65 km). The temperature amplitude of the upward propagating tide is weaker than that at the equator around 10 3 Pa because of weaker solar insolation at higher latitudes. The thermal tide propagates downward below the 10 5 -Pa altitude (~50 km). Figure 6 shows vertical structures of the air temperature components of diurnal and semidiurnal tides. The vertical tilt of the diurnal tide phase is steeper than that of the semidiurnal tide at the equator above the altitude of the 10 5 Pa level (~50 km). Such a difference in the phase between diurnal and semidiurnal tides is also seen in Pechmann and Ingersoll (1984) . The eddy temperatures are amplified around 10 4 Pa (~65 km) and 5 10 4 Pa (~56 km) in Fig. 6a and b. The diurnal tide

propagates from the cloud-heating maximum to the near-surface (Fig. 6a ). The semidiurnal tide forced at the cloud-heating maximum reaches the 10 5 -Pa level (Fig. 6b) . Below this level, the J o u r n a l P r e -p r o o f Journal Pre-proof semidiurnal temperature component is amplified in the weakly stable layer at ~2  10 5 Pa (~45 km) and propagates downward to the near-surface. According to the simplified GCM of Takagi et al. (2018) , the diurnal and semidiurnal thermal tides dissipate abruptly around 80 km height, where the Doppler-shift velocity of the thermal tides is low (|̅ | < 10 m s -1 ). In Sugimoto et al. (2017) , the distributions of the zonal mean winds above the cloud top are quite different from those in our work and the amplification of the eddy temperature by solar heating above 80 km was not reported. Thus, the vertical and horizontal structures of thermal tides in their works are different from those in our work. In our T63 GCM experiment, the sub-solar solar heating rate is ~20 K day -1 and the zonal-mean equatorial infrared cooling rate is ~5 K day -1 around 3.62  10 2 Pa (~81 km, where the Doppler-shift velocities are high (~60 m s -1 ). Therefore, the thermal tides forced by strong solar heating and dissipated by infrared cooling propagate vertically in the upper atmosphere in our work.

To elucidate the difference with the observations and previous GCMs, the sensitivity of thermal tides to the zonal-mean field (i.e., the basic field of the waves) and solar heating must be also further investigated.

At high latitudes where tidal vertical momentum flux is high (see later Fig. 17b ), the diurnal tide is amplified and the vertical propagation is not apparent around 10 4 Pa (Fig. 6c) . The semidiurnal tide is forced at the cloud-heating maximum (~10 4 Pa) and propagates upward and downward there (Fig. 6d ). It is enhanced or re-forced in the region between 10 5 Pa (~50 km) and 10 6

Pa (~30 km) where the static stability is low in Fig tides has two negative maxima at ~7  10 3 Pa (~67 km) and ~6  10 2 Pa (~79 km) and two positive maxima at ~3  10 4 Pa (~58 km) and ~5 10 5 Pa (~36 km) (black line with dots in Fig. 7a ). This suggests that radiative heating primarily forces the thermal tides around the cloud-heating maximum (~10 4 Pa in Fig. 1d ) and weakly amplifies their vertical momentum flux at 6  10 2 Pa and ~5  10 5

Pa. The equatorial zonal-flow accelerations of ~0.5 m s 1 Earth day 1 are located at 1 10 4 Pa and 9

10 2 Pa (~77 km). The deceleration of ~0.5 m s 1 Earth day 1 is located at 3 10 3 Pa (~71 km) in force Ω ̅ i enhances only a poleward flow in the downward control principle (Haynes et al., 1991; Imamura, 1997) . In high-latitudinal regions between 10 5 Pa and 10 4 Pa at high latitudes, the metric-term acceleration (negative difference, blue shading in Fig. 11c ) and the zonal-flow deceleration by eddies (positive difference, orange shading in Fig. 11d) Fig 11) . Thus, in the momentum balance between the convergence of eddy vertical momentum flux and metric term, the decrease in the metric term due to the topography produces the decrease/increase in the zonal flow in the northern/southern polar regions, which leads to the asymmetry of the zonal flow between the southern and northern polar regions.

The variance of the time series data in geographical coordinates indicates the activity J o u r n a l P r e -p r o o f Journal Pre-proof of unsteady waves, which excludes the time-mean stationary waves, as noted in Sect. 2.2. If the variance is zonally uniform, the unsteady wave activity is independent of the surface topography and stationary waves. In contrast, if the longitudinal variation of the variance is highly correlated with the surface terrain in geographical coordinates, it is produced by the topography. In Fig. 12 , the variances are very small below the 10 5 -Pa altitude (<50 km) and amplified at multi-levels above this altitude. Thus, the unsteady eddy activity locked to the topography is not only directly forced by the surface but also modified by stationary waves in and above the cloud layer (above the 10 5 -Pa altitude). For the variance of vertical flow (Fig. 12a) , vertical spikes are apparent slightly west of the summits of the equatorial mountains (the white region around the bottom of the figure) . These spikes of the vertical flow are suggestive of penetrative plumes and vertically propagating gravity waves. In the infrared heating region at ~10 5 Pa, the variance of the vertical flow is regionally high over the equatorial lowlands. Vega balloons had shown active vertical motion around the cloud bottom (Ingersoll et al., 1987) . Although the convective motion or gravity waves detected by the balloon experiments are not fully resolved in GCMs, the high variances of vertical and zonal flows around 10 5 Pa in our model indicate the large-scale (~1,000 km) convective motion. In the convectively active layer (~10 5 Pa), where the static stability is low, the variance of air temperature is low (Fig.   12c) . A thin layer of the high variance of air temperature is located around the cloud-heating maximum (orange shading around 10 4 Pa in Fig. 12c ). The temperature variance in the thin layer is regionally low over the equatorial high lands. Fig. 13a ) is weaker than that in the Southern Hemisphere (~40 K 2 ). The temperature variance is low above the Aphrodite Terra (blue and purple shading, denoting negative deviations in Equatorward momentum flux of total eddies is predominant around 10 5 Pa (~50 km). This supports the Gierach-Rossow-Williams mechanism (Gierasch, 1975; Rossow and Williams, 1979) .

The eddy momentum flux is poleward around 10 4 Pa (~65 km) below the jet core, where the eddy heat flux is also poleward and the meridional gradient of the potential temperature is strong. Such poleward heat and momentum fluxes associated with high-latitude baroclinic waves (Yamamoto and Takahashi, 2016; were also reported in simplified models (Yamamoto and Tanaka, 1997; Sugimoto et al., 2014; Kashimura et al., 2019) and realistic GCMs (Lebonnois et al., 2016; Yamamoto et al., 2019) . The horizontal eddy heat fluxes of total eddies are primarily produced by the thermal tides around the cloud top (gray dashed line on the left panels in Fig. 17a and b) . The poleward heat and equatorward momentum fluxes of the tides are produced around the cloud-heating maximum.

The distribution of the eddy vertical momentum flux (the right panel of Fig. 17a ) is complex.

At low latitudes, the negative flux is seen above the cloud-heating maximum (<10 4 Pa, >65 km), in the low-stability layer (~10 5 Pa, ~50 km), and below the 10 6 -Pa altitude (<30 km). The sign of the strong vertical flux changes rapidly between 10 6 and 10 4 Pa (30 and 65 km) at high latitudes. In the right panel of Fig. 17b , the thermal tides produce the negative (positive) vertical momentum fluxes above (below) the cloud-heating maximum level at low-and mid-latitudes. As is shown in Fig. 6d In the region between 10 6 and 10 5 Pa, the eddy heat and momentum fluxes are generated by transient wave components obtained by removing tidal and stationary waves from the total wave components. In future work, we will elucidate the dynamics of the fast traveling eddies, which may produce the complex cloud features seen in the satellite-based observations.

In our Venus GCM with long-and short-wave radiative processes, the increase in horizontal resolution greatly enhances the effect of the topography on the general circulation and stationary waves (Figs. 8 and 10) and better resolves the sharp streak structures of polar thermal tides (Fig. 2b and d). As in our previous low-resolution model, our simulation reproduces the UV-tracked horizontal flow around the subsolar point and the equatorial multi-layered and polar structures of static stability (Horinouchi et al., 2018; Ando et al., 2020) . In our model, the acceleration rate of the equatorward momentum flux of thermal tides proposed by Yamamoto and Takahashi (2006) is of the same magnitude as that via the more conventional vertical momentum flux proposed by Fels and Lindzen (1974) . Sharp streaks of vertical flow associated with polar thermal tides are located along the longitudinally extended trough and ridge at ~70 latitude between the polar low and high in our T63 model ( Fig. 2b and d) . The radiative transfer forces diurnal and semidiurnal tides around the cloud top and also modifies the vertical structures of these waves and their momentum fluxes at multiple height levels (Fig. 7) . The thermal tides accelerate not only equatorial zonal flow at the cloud-heating maximum (1 10 4 Pa, ~65 km) but also the zonal flow around 10 3 Pa (~75 km) at low and high latitudes ( Fig. 7c and d) . Thus, we must consider the wind acceleration at various heights produced by thermal tides, when the super-rotation mechanism by thermal tides is discussed for each altitude. wavenumber are enhanced over the equatorial highlands (Fig. 8) . Below and in the cloud layer, surface topography weakens the zonal-mean zonal flow over the Aphrodite Terra and Maxwell

Montes, whereas it enhances the zonal flow in the southern polar region (Fig. 10) . The convergence of eddy vertical momentum flux balances the metric term at high latitudes ( Fig. 11a and b) . The decrease in the metric term due to the topography produces the decrease/increase in the zonal flow at southern/northern high latitudes (Fig. 11c) , which leads to the asymmetry of the zonal flow between the southern and northern polar regions.

Variances of time series are further investigated to assist in future comparisons with the observations. The variances are considered to result from unsteady eddy components in the geographical coordinates. The longitudinal variations of the wave activity are caused by surface topography and/or stationary waves. The vertical spikes in the vertical wind variance are striking around the equatorial high mountains (Fig. 12a) , suggestive of penetrative plumes and vertically propagating gravity waves. At the cloud top, we also find bow shapes of negative vertical eddy momentum fluxes enhanced over the high mountains at low latitudes (Fig. 14c) , where the zonal flow locally weakens over the Aphrodite Terra. In the solar-fixed coordinate system, the variances of flow are relatively higher on the nightside than on the dayside at low latitudes (Fig. 13eh) . The thermal tides produce strong dependences of the eddy heat and momentum fluxes on local time (Fig. J o u r n a l P r e -p r o o f Journal Pre-proof 14eh) and consequently lead to the difference between the zonal and dayside mean fluxes. The local-time variation of the vertical eddy momentum flux is produced by both thermal tides and solar-related, small-scale gravity waves at low latitudes.

The present work reveals the dynamical effects of high-resolution topographical and radiative forcings, based on the time-mean structures of waves (thermal tide and stationary waves) and variances using a high-resolution GCM with solar and infrared radiative transfer. Our future work will investigate fast waves with phase velocities almost the same as the superrotational wind speed, along with the observed cloud features. Here, the longitude is expressed in terms of local time (LT) and the subsolar point is located at 12 LT.

Zonal and meridional winds with positive values indicate westward and northward flow, respectively. 

Local time dependence of the thermal structure in the Venusian equatorial J o u r n a l P r e -p r o o f Journal Pre-proof upper atmosphere: comparison of Akatsuki radio occultation measurements and GCM results

Thermal structure of the Venusian atmosphere from the sub-cloud region to the mesosphere as observed by radio occultation

Influence of Venus topography on the zonal wind and UV albedo at cloud top level: the role of stationary gravity waves

Radiative forcing of the Venus mesosphere. I -Solar fluxes and heating rates

Net exchange parameterization of thermal infrared radiative transfer in Venus' atmosphere

The interaction of thermally excited gravity waves with mean flows

Venus topography and kilometer-scale slopes

Large stationary gravity wave in the atmosphere of Venus

Latitudinal variation of clouds' structure responsible for Venus' cold collar

Meridional circulation and the maintenance of the Venus atmosphere rotation

Thermal structure of Venusian nighttime mesosphere as observed by VIRTIS-Venus Express

On the "downward control" of extratropical diabatic circulations by eddy-induced mean zonal forces

Effects of topography on the spinup of a Venus atmospheric model

Introduction to dynamic meteorology

Mean winds at the cloud top of Venus obtained from two-wavelength UV imaging by Akatsuki

How waves and turbulence maintain the super-rotation of Venus' atmosphere

Altimetry of the Venus cloud tops from the Venus Express observations

Development of radiative transfer model for Venus atmosphere and simulation of circulation model

Momentum balance of the Venusian midlatitude mesosphere

Inverse insolation dependence of Venus' cloud-level convection

Estimates of convective heat fluxes and gravity wave amplitudes in the Venus middle cloud layer from VEGA balloon measurements

K-1 Coupled GCM (MIROC) Description, K-1 Tech

Planetary-scale streak structure reproduced in high-resolution simulations of the Venus atmosphere with a low-stability layer

Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere

The Atmospheric Dynamics of Venus

Effects of thermal tides on the Venus atmospheric superrotation

Three dimensional structures of thermal tides simulated by a Venus GCM

Structure of the Venus neutral atmosphere as observed by the Radio Science experiment VeRa on Venus Express

Solar-locked and geographical atmospheric structures inferred from a Venus general circulation model with radiative transfer

Dynamics of Venus' superrotation: The eddy momentum transport processes newly found in a GCM

Superrotation maintained by meridional circulation and waves J o u r n a l P r e -p r o o f Journal Pre-proof in a Venus-like AGCM

A parametric study of atmospheric superrotation on Venus-like planets: Effects of oblique angle of planetary rotation axis

Influences of Venus' topography on fully developed superrotation and near-surface flow

Dynamics of polar vortices at cloud top and base on Venus inferred from a general circulation model: Case of a strong diurnal thermal tide

General circulation driven by baroclinic forcing due to cloud-layer heating: significance of planetary rotation and polar eddy heat transport

Effects of polar indirect circulation on superrotation and multiple equilibrium in long-term AGCM experiments with an idealized Venus-like forcing: sensitivity to horizontal resolution and initial condition

Formation and maintenance of the 4-day circulation in the Venus middle atmosphere

Ultraviolet imager on Venus climate orbiter Akatsuki and its initial results, (a) Topo (metric-term accel

Topo -Flat (metric-term accel.) (d) Topo -Flat (eddy vert

Latitude-pressure distributions of (a) metric-term acceleration ( ̅ ̅ , shading) and (b) the differences in (c) the metric term and (d) convergence of eddy vertical momentum flux (shading) between Topo and Flat averaged over two Venus days, together with the difference in zonal flow (contours). The units of acceleration are m s 1 Earth day 1

This study was supported by a Ministry of Education, Culture, Sports, Science, and Technology