Venus boundary layer dynamics eolian transport and convective vortex Maxence Lefèvre1

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Venus boundary layer dynamics:

eolian transport and convective vortex

Maxence Lefèvre1

1Department of Physics (Atmospheric, Oceanic and Planetary Physics),

University of Oxford, Oxford, UK

Accepted in Icarus

Abstract

Few spacecraft have studied the dynamics of Venus’ deep atmosphere, which is needed to

understand the interactions between the surface and atmosphere. Recent global simulations

suggest a strong eﬀect of the diurnal cycle of surface winds on the depth of the planetary

boundary layer. We propose to use a turbulent-resolving model to characterize the Venus

boundary layer and the impact of surface winds for the ﬁrst time. Simulations were performed

in the low plain and high terrain at the Equator and noon and midnight. A strong diurnal cycle

is resolved in the high terrain, with a convective layer reaching 7 km above the local surface

and vertical wind of 1.3 m/s. The boundary layer depth in the low plain is consistent with

the observed wavelength of the dune ﬁelds. At noon, the resolved surface wind ﬁeld for both

locations is strong enough to lift dust particles and engender micro-dunes. Convective vortices

are resolved for the ﬁrst time on Venus.

1 Introduction

The interaction between the surface and the atmosphere is a major aspect of exchanges of heat

and angular momentum, impacting the thermal and wind shear proﬁles and, therefore, the

atmospheric dynamics and the rotation of the solid body itself. On Venus, however, the ﬁrst

10 km above the surface remain a largely unknown region due to the technological diﬃculty in

probing this region below the cloud layer.

Only a limited number of probes have been able to collect data on Venus. The VeGa-2

probe has successfully measured the only temperature proﬁle in that region (Linkin et al.,

1986). At the surface, only Venera 9 and 10 directly measured the wind for respectively 49 min

and 90 s (Avduevskii et al., 1977), and several other probes like Venera 13 and 14 measured

indirectly the wind speed (Ksanfomaliti et al., 1983). The amplitudes of the measured wind

speeds are less than 2 m s´1below 100 m (Lorenz, 2016), with a higher probability for values

below 0.5 m s´1. The height of the planetary boundary layer (PBL) and the diurnal cycle is not

known, nor are the eﬀect of the topography. Dunes have been observed in radar measurements

with Magellan (Greeley et al., 1992), although the lack of knowledge about the spatial and

temporal distribution of winds complicates the interpretation of dust transport.

The Institut Pierre Simon Laplace (ISPL) Venus General Circulation Model (GCM) sim-

ulations showed the diurnal cycle of the PBL activity is correlated with the diurnal cycle of

surface winds (Lebonnois et al., 2018). They observed downward katabatic winds at night and

arXiv:2210.09219v1 [astro-ph.EP] 17 Oct 2022

upward anabatic winds during the day along the slopes of high-elevation terrains, resulting in

a deeper PBL depth at noon.

Yamamoto (2011) performed turbulent-resolving simulations of the Venus PBL with a res-

olution of 100 m without radiative processes and a variety of near-surface theoretical thermal

structures. This experiment yielded a PBL depth below 2.5 km. Morellina and Bellan (2022)

studied the turbulent chemical-species mixing at the surface, where high-density-gradient mag-

nitude regions are formed with larger gradients due to the supercritical conditions.

The PBL of the Earth and Mars have been studied extensively in both observations and

modelling studies. On Earth, the turbulent ﬂux is in the energy budget of the convective

layer. The opposite is true on Mars. For Venus, this budget is poorly known. On Earth, the

PBL dynamic is associated with the presence of the water cycle and latent heat release. The

understanding of such dynamics is crucial for the clouds and water cycle and the energy balance

of the surface (Garratt, 1994). On Mars and Venus, the latent heat is negligible. The depth of

the Martian surface convective layer is larger than on Earth, with also a greater temperature

diurnal cycle (Hinson et al., 2008). The surface slope winds have a strong impact on the thermal

structure of the PBL (Spiga et al., 2011).

In this study, we use Large Eddy Simulation (LES) models developed for the clouds convec-

tive regions and to simulate the PBL convective activity at two diﬀerent locations on the surface.

This is done to examine the inﬂuence of the topography on the boundary layer convection, and

two local times, noon and midnight, to quantify the diurnal cycle.

For the ﬁrst time, the radiative processes are taken into account in the study of the Venus

PBL with prescribed solar and IR heating rates. An additional rate representing the eﬀect of

the general circulation, heating/cooling due to large-scale wind advection, is also prescribed.

In Section 2, the model is described. The spatial and local time variabilities of the PBL are

discussed in Section 3. The PBL of Venus is compared to the Earth and Mars in Section 4. The

impact of PBL characteristics on eolian transport is discussed in Section 5. Our conclusions

are summarized in Section 6.

2 Modelling

2.1 Dynamical core

The LMD LES model is based on the dynamical core of the Advanced Research Weather-

Weather Research and Forecast (hereinafter referred to as WRF) terrestrial model (Skamarock

and Klemp, 2008). The WRF dynamical core integrates the fully compressible non-hydrostatic

Navier-Stokes equations over a speciﬁed area of the planet. The conservation of the mass,

momentum, and entropy are ensured by an explicitly conservative ﬂux-form formulation of

the fundamental equations (Skamarock and Klemp, 2008), based on mass-coupled atmospheric

variables (winds and potential temperature). The parametrization of the unresolved small-

scale eddies is carried out by a subgrid-scale prognostic Turbulent Kinetic Energy closure by

Deardorﬀ (1972). This methodology has been used for extensively for Earth convection study

(Moeng et al., 2007), and the Martian atmosphere (Spiga et al., 2010), Venus cloud convective

layer (Lefèvre et al., 2017, 2018), and terrestrial exoplanet convection (Lefèvre et al., 2021).

2.2 Model Physics

Due to the constant heat capacity of the dynamical core and the architecture of the coupled

radiative transfer, as well as computational time, the radiative forcing is handled in the same

way that in Lefèvre et al. (2017). The solar and radiative heating rates are extracted from

ISPL Venus GCM (Garate-Lopez and Lebonnois, 2018), which uses IR transfer (Lebonnois

et al., 2015) based on Eymet et al. (2009), with the latitudinally-varying cloud model of Haus

et al. (2014, 2015). An additional heating rate is prescribed, representing the large-scale heating

from the dynamics. In this study, we focus on the ﬁrst 10 km and the large-scale heating will

come mainly from the anabatic/katabatic slope ﬂows. No surface and sub-surface physics are

considered in this study.

2.3 Simulation settings

Lebonnois et al. (2018) showed with GCM modelling that the convective depth in the PBL

was impacted by the diurnal cycle of the surface wind, and was maximal in the steepest slope

of the equatorial topographic features. Therefore, we choose two locations at the surface with

two distinct elevations and slope environments at the Equator. Here, the incoming solar ﬂux

is maximized in order to study the activity of the PBL where it is supposed to be the most

active. One of the locations is in the low plain, with an elevation of -320 m at 0˝longitude, and

the other is in the western part of Ovda Regio with an elevation of 1030 m at 80˝longitude.

The point in the plain will be hereinafter referred to as low plain, and the point in Ovda

Regio will be referred to as high terrain. The domain of the LES simulations is ﬂat. Due

to computational constraints, simulations of an entire Venus day were not possible, and two

local times are considered in this study: noon and midnight. The surface is heat ﬂux is set at

90 W m´2at noon and -1 W m´2at midnight for the two locations (Lebonnois et al., 2018).

This ﬂux is constant in time during the simulations over the entire domain, there is no feedback

of the PBL turbulence on the sensible ﬂux. For the two locations, the horizontal resolution

and timestep are set at 50 m and 0.4 s. However, the size of the surface area varies depending

on local time and location, 30ˆ30 km for the high terrain case at noon and 20ˆ20 km for the

rest. The vertical resolution and extent also depend on the location and local time, from 10 km

above the local surface with a mean resolution of 90 m for the High terrain case at noon to

5 km above the local surface, with a resolution of 60 m. The diﬀerent horizontal domains size

were chosen to allow several connective cells in each horizontal direction, and were determined

by trial and error. The diﬀerent vertical domains size were chosen to allow several kilometers

above the convective layer and were based on the Venus IPSL GCM results (Lebonnois et al.,

2018). To avoid the spurious reﬂection of gravity waves propagating upward on the top of the

model, a Rayleigh damping layer is applied over the last 500 m with a damping coeﬃcient of

0.01 s´1. The heat capacity is set to a constant value over the whole domain of 1181 J K´1, a

reference value from the Venus International Reference Atmosphere (Seiﬀ et al., 1985).

Fig 1 shows the initial proﬁles of the temperature, potential temperature, and the diﬀerent

heating rates. The temperature is colder for the high terrain cases, and the diurnal cycle of the

temperature is below 3 K for the two location cases. At noon, a neutral layer corresponding

to the convective layer is visible below 2 km above the local surface for the low plain case, and

below 8 km above the local surface for the high terrain case. At midnight, there is no visible

neutral layer, meaning that the convective activity is weak. Regarding the heating rates, the

short wave heating is slightly greater for the low plain case, but the thermal cooling is slightly

stronger at midnight. However, there is a strong diﬀerence for the large-scale heating whereas

for the high terrain case it is positive up to 6 km above the local surface, with stronger values at

noon. While for the low plain case, the large-scale heating is negative in the ﬁrst 1.5 km at noon

and 2 km at midnight, and then alternating between positive and negative values above. This

variability of the large-scale heating reﬂects the eﬀect of the topography and the diurnal cycle of

the surface wind. This variability reﬂects on the total heating rate, alternating between negative

and positive values that will enforce the convective depth. The model is initialized with thermal

proﬁles, winds and radiative rates that reached GCM equilibrium, using hypotheses from the

subgrid-scale parametrization that will impact the equilibrium state of the region. With the

very poor knowledge of the ﬁrst 12 km, it is diﬃcult to assess the realism of the initial state

used for the present studies, although the surface temperature and winds are consistent with

measurements. The time step of the simulations is set to 0.4 s, and the radiative transfer

is called every 100 dynamical steps, ensuring that there is no issue with numerical precision

(Rafkin and Soto, 2020). The outputs shown in the following sections are obtained after at least

1 Earth day of simulations for the midnight cases, where a steady-state is reached. For the noon

cases, after 2 Earth days, there is a small temperature drift due to the set-up of the surface

ﬂux and radiative rates, around 10´8K/s near the surface. A constant forcing corresponding

to a single time of day is sometimes used to study self-aggregation in Earth’s tropics (Daleu

et al., 2015; Wing et al., 2017), as well as tidally-locked rocky exoplanets (Zhang et al., 2017;

Sergeev et al., 2020; Lefèvre et al., 2021). When no aggregation is present, the model reaches

equilibrium in a couple of days (Wing and Emanuel 2014). The Venus atmosphere can be

considered dry. In the present study, there is no water vapor and clouds, and therefore no

feedback between moisture and radiation/surface temperature. No aggregation of convection

is expected. There is no noticeable change in pressure over time in the domain for all the cases

considered. The surface wind’s amplitude range does not vary in time. The pseudo-equilibrium

is realistic enough to provide qualitative insight into the Venus PBL dynamics.

The model conﬁguration is summarized in Table 1.

Parameter Value

Gravity (m s´2) 8.87

Heat Capacity (J K´1) 1181

Surface heat ﬂux (W m´2) 90 (noon), -1 (midnight)

Horizontal resolution dx (m) 50

Time step (s) .4

Cases Grid ∆x and ∆y (km) ∆z (km) dz (m)

low plain noon 401ˆ401ˆ71 20 5.5 80

low plain midnight 401ˆ401ˆ51 20 3 60

high terrain noon 601ˆ601ˆ121 30 10 70

high terrain midnight 401ˆ401ˆ101 20 5.5 55

Table 1: Planetary and atmospheric parameters for the diﬀerent simulations. ∆x represents

the size of the domain over the axis xand dx the resolution over the axis x

3 Spatial and temporal variability of the PBL

Fig 2 shows snapshots of the vertical and horizontal cross-sections of the vertical wind for the

high terrain at noon and midnight, and the low plain at noon. The diurnal cycle for the high

terrain location is striking: at noon the elevation of the PBL can reach 7 km above the local

surface with vertical wind speed as high as 1.3 m s´1. Whereas at midnight, the convective layer

barely reaches 0.5 km with vertical wind speed ă0.2 m s´1. This diurnal cycle is consistent

with GCM simulations (Lebonnois et al., 2018). The diﬀerence in depth leads to a diﬀerence

in convective cell diameter. At noon, the typical cell diameter is around 5 km, with some cells

reaching 10 km, and about 2 km at midnight. The cellular features are elongated in the y-

direction at midnight. The Richardson number is stronger at midnight than at noon, meaning

that the shear becomes relatively strong compared to buoyancy. Such features depend on the

vertical shear of the horizontal wind, where there is very little data to constrain this result.

With similar surface ﬂux, the elevation of the PBL depth is diﬀerent at noon, 2 km above the

local surface for the low plain compared to 7 km for the high terrain. This diﬀerence is due to

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Venusboundarylayerdynamics:eoliantransportandconvectivevortexMaxenceLefèvre11DepartmentofPhysics(Atmospheric,OceanicandPlanetaryPhysics),UniversityofOxford,Oxford,UKAcceptedinIcarusAbstractFewspacecrafthavestudiedthedynamicsofVenus'deepatmosphere,whichisneededtounderstandtheinteractionsbetweenthesur...

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