The Impact of Turbulent Vertical Mixing in the Venus Clouds on Chemical Tracers Maxence Lefèvre1 Emmanuel Marcq2 and Franck Lefèvre2

2025-05-06 0 0 2.65MB 24 页 10玖币

侵权投诉

The Impact of Turbulent Vertical Mixing in the Venus Clouds on

Chemical Tracers

Maxence Lefèvre1, Emmanuel Marcq2, and Franck Lefèvre2

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

University of Oxford, Oxford, UK

2LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS,

France

Accepted in Icarus

Abstract

Venus’ clouds host a convective layer between roughly 50 and 60 km that mixes heat, momen-

tum, and chemical species. Observations and numerical modelling have helped to understand

the complexity of this region. However, the impact on chemistry is still not known. Here, we

use for the ﬁrst time a three-dimensional convection-resolving model with passive tracers to

mimic SO2and H2O for two latitudinal cases. The tracers are relaxed towards a vertical proﬁle

in agreement with measured values, with a timescale varying over several orders of magnitude.

The vertical mixing is quantiﬁed, it is strong for a relaxation timescale high in front of the

convective timescale, around 4 hours. The spatial and temporal variability of the tracer due

to the convective activity is estimated, with horizontal structures of several kilometres. At the

Equator, the model is resolving a convective layer at the cloud top (70 km) suggested by some

observations, the impact of such turbulent activity on chemical species is accounted for the ﬁrst

time. From the resolved convective plumes, a vertical eddy diﬀusion is estimated, consistent

with past estimations from in-situ measurements, but several orders of magnitude higher than

values used in 1D chemistry modelling. The results are compared to observations, with some

spatial and temporal variability correlation, suggesting an impact of the convective layer on the

chemical species.

1 Introduction

The strong dynamical activity inside the Venusian cloud layer has been assessed since the

beginning of the Venus spacecraft exploration. Cloud images by the Mariner 10 mission (Belton

et al., 1976) and the Pioneer Venus spacecraft (Rossow et al., 1980) near the subsolar point

showed cellular features suggesting convective cells with diameters between 200 and 1000 km.

The convective activity was ﬁrst measured by the Pioneer Venus radio occultation experiment

(Seiﬀ et al., 1980) from 50 to 55 km of altitude, and was then conﬁrmed by the radio occultation

experiment onboard the Magellan probe (Hinson and Jenkins, 1995). The VeGa balloons ﬂew

in that altitude range close to the Equator and measured vertical winds between -4 and 2 m s´1

(Linkin et al., 1986; Lorenz et al., 2018) and convective cell diameter from several hundred

meters to tens of kilometers (Kerzhanovich et al., 1986) around 54 km of altitude. The VeRa

arXiv:2210.09240v1 [astro-ph.EP] 17 Oct 2022

radio occultation device on board of Venus Express studied in detail this convective layer and

measured a strong latitudinal variability of the depth of the layer (Tellmann et al., 2009),

reaching 10 km close to 80˝of latitude, almost twice the value of the equatorial regions. The

radio occultation experiment in the Akatsuki spacecraft measured variability of the convection

depth with local time (Imamura et al., 2017), this layer being thicker a night.

In addition to the convection layer in the deep cloud layer, the Venus Monitoring Camera

(VMC) observed the cellular features at the top of the cloud, about 70 km of altitude, at

the subsolar point suggesting convective activity (Markiewicz et al., 2007; Titov et al., 2012).

A convective layer at this altitude is the main hypothesis for these observed structures, with

measured convective cells from 20 to a few hundred of kilometres. However, the diﬀerent radio

occultation on board of Venus Express and Akatsuki radio occultation did not measure any

clear neutral-stability layers at the subsolar point (Ando et al., 2018, 2020).

Gravity waves emitted from the convective layer have been observed by diﬀerent space

probes, Pioneer Venus radio science observed evidenced small-scale waves with vertical wave-

lengths of about 7 km above and below the cloud layer (Seiﬀ et al., 1980; Counselman et al.,

1980), the Venus Express instruments measured the wavelengths of the waves emitted above

the cloud layer, ranging between about 2 and 3.5 km vertically (Tellmann et al., 2012) and

from 2 km to hundreds of kilometres horizontally (Peralta et al., 2008; Piccialli et al., 2014).

The gravity waves in this region have also been studied with Akatsuki (Imamura et al., 2018;

Mori et al., 2021).

Decades of spacecraft and ground-based observations of sulphur dioxide and water show

highly variable abundance in the upper cloud deck, with timescales from hours to decades

(Marcq et al., 2020; Encrenaz et al., 2016; Vandaele et al., 2017a,b). Convection is one of the

hypotheses for the short, from hours to days, term variability (Marcq et al., 2013; Vandaele

et al., 2017a). HST Imaging Spectrograph was used to observe Venus (Jessup et al., 2015, 2020)

at cloud-top altitudes, albedo darkening was measured and explained by a possible increase of

the convective vertical mixing and the injection of the unknown absorbing species. Using the

SOIR/Venus Express CO2and CO proﬁles above the clouds and 1D photochemical model,

Mahieux et al. (2021) estimated the vertical mixing from 80 to 140 km.

1D models have been developed to study the chemistry of the Venusian atmosphere. Krasnopol-

sky (2007) and Krasnopolsky (2013) focused on the lower atmosphere and Krasnopolsky (2012),

Zhang et al. (2012) and Parkinson et al. (2015) and Shao et al. (2020) on the middle atmosphere.

Yung et al. (2009) and Bierson and Zhang (2020) and Rimmer et al. (2021) modelled the atmo-

sphere from the surface to 110 km. These models cannot resolve the turbulent activity inside

the cloud and therefore use the eddy diﬀusivity coeﬃcient formalism to represent the diﬀerent

turbulent processes in the atmosphere like convection. There is a large uncertainty over the

value of this coeﬃcient in the Venusian atmosphere. These models showed that the mesospheric

abundance of several species (especially SO2) was very sensitive to the eddy diﬀusivity values

and vertical gradient.

Due to the lack of understanding of the turbulence inside the clouds of Venus, the eﬀect of the

cloud convective layer and gravity waves on the chemistry and microphysics has not been studied

in detail. Only McGouldrick and Toon (2008) gave an insight into the change of optical depth

due to the convection and gravity waves, using an idealized 2D (zonal/vertical) representation

of the Venus cloud convective layer. Morellina and Bellan (2022) studied the vertical mixing due

to the species stratiﬁcation in the Venus lower atmosphere and clouds using Direct Numerical

Simulation. High density-gradient magnitude regions are formed with increasing stratiﬁcation

and low stratiﬁcation conditions produce a more uniform spatial distribution of the density.

To understand the turbulence activity inside the Venus cloud layer, the limited-area Venus

mesoscale model (VMM) adapted from a terrestrial hydrodynamical solver (Skamarock and

Klemp, 2008) was developed at Laboratoire de Météorologie Dynamique by Lefèvre et al. (2017)

and later coupled to the full set of physical packages for Venus developed at Institut Pierre

Simon Laplace (IPSL) (Lebonnois et al., 2010, 2016; Garate-Lopez and Lebonnois, 2018) to

simulate only a speciﬁed region of the planet with a ﬁne resolution.

The 3D Large-Eddy Simulation (LES) mode was used to study the convection and small-

scale gravity waves at diﬀerent latitudes and local times (Lefèvre et al., 2017, 2018). The re-

solved convection depth is consistent with observations with updrafts cells diameter of 20 km.

The gravity waves emitted by the convective region are also consistent in amplitude and wave-

length with observations. Cloud-top convective activity is also present around the subsolar

point.

In this study we propose to use this convection-resolving to model at noon for two lati-

tudinal cases, the Equator and 75˝, with idealized passive tracers representing SO2and H2O

to quantify the impact of the resolved convective layer vertical mixing on the Venusian cloud

layer chemistry, and to compare the results with 1D chemical models and observations. The

resolved convective plumes give an unprecedented insight into the cloud convective advection

and chemistry dynamics.

Our paper is organized as follows. The model is described in Section 2. In Section 3, the

impacts of convective motions and gravity waves are presented. The vertical eddy diﬀusivity is

estimated in Section 4. Results are discussed in Section 5. Our conclusions are summarized in

Section 6.

2 Modelling

2.1 Mesoscale modelling for Venus

Our mesoscale model for Venus 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 on a deﬁned area of the planet. The conservation of

the mass, momentum, and entropy are ensured by an explicitly conservative ﬂux-form formula-

tion of the fundamental equations (Skamarock and Klemp, 2008), based on mass-coupled me-

teorological variables (winds and potential temperature). The LES mode is used in this study,

and the parametrization of the unresolved small-scale eddies is performed using a subgrid-scale

prognostic Turbulent Kinetic Energy closure by Deardorﬀ (1972). This method has been used

for Earth convection study (Moeng et al., 2007), for the Martian atmosphere (Spiga et al.,

2010), and for terrestrial exoplanets (Lefèvre et al., 2021).

Following the work of Lefèvre et al. (2018) for Venus, the WRF core is coupled to the

radiative transfer code of the IPSL Venus General Circulation Model (GCM) (Lebonnois et al.,

2015) based on Eymet et al. (2009) net-exchange rate (NER) formalism computing the energy

between the layers prior to the dynamical simulations, separating temperature-independent

coeﬃcients from the temperature-dependent Planck functions of the diﬀerent layers. The cloud

model is based on Haus et al. (2014) and Haus et al. (2015), with a latitudinal variation of

the cloud by setting 5 distinct latitude intervals: 0˝to 50˝, 50˝to 60˝, 60˝to 70˝, 70˝to 80˝

and 80˝to 90˝. The cloud model is set prior to the simulations and does not interact with the

dynamical features resolved by the model. The solar heating rates are computed using look-up

tables (Haus et al., 2015) of vertical proﬁles of the solar heating rate as a function of the solar

zenith angle.

2.2 A Passive tracer approach

Tracers have been included in the model and are set to represent SO2and H2O . The tracers

are not radiatively active. The chemistry, photodissociation, and condensation sources and

sinks are modelled by a linear relaxation of the tracer abundance qtoward a prescribed vertical

proﬁle q0pzqwith a relaxation timescale τcomputed as :

Bqpx, y, z, tq

Bt“q0pzq ´ qpx, y, z, tq

τ(1)

The two prescribed relaxation proﬁles are displayed in Fig 1. These two proﬁles are con-

structed as follows: assuming a constant abundance value below the clouds from spectroscopic

measurements between 30 and 40 km (Bézard and de Bergh, 2007), 150 ppm for SO2and

30 ppm for H2O . This value of SO2is consistent with in-situ measurements at the bottom of

the cloud deck by VeGa 1 and VeGa 2 entry probes (Bertaux et al., 1996). From 48 km upwards

an exponential decay is assumed going through 0.5 ppm at 65 km, long-term ground observa-

tions (Encrenaz et al., 2012), for SO2and 2 ppm at 70 km for H2O , VIRTIS and SPICAV

measurements (Cottini et al., 2012; Fedorova et al., 2016). These proﬁles have been constructed

to represent values in the cloud layer, and were constructed for simplicity as constant below

the cloud deck and decreasing above. With such simple proﬁles, there are discrepancies with

observations. The value of SO2in the lower cloud is slightly underestimated (Oschlisniok et al.,

2021). Above the clouds, both SO2and H2O are decreasing despite the inversion observed in

SPICAV/SOIR (Mahieux et al., 2015) and SPICAV/UV (Evdokimova et al., 2021) for SO2,

and the very low vertical gradient between 80 and 120 km for H2O observed by SOIR/VEx

(Chamberlain et al., 2020). The proﬁle of both SO2and H2O outside the cloud layers does not

impact the results obtained within the clouds. No source or sinks at the surface or the top

boundary are considered. The lateral boundary conditions are doubly periodic. Two latitudinal

cases are considered in this study, the Equator and 75˝, to explore the latitudinal variability

in terms of convective activity. The relaxation proﬁles are the same for the two latitudinal

cases. Only one local time is considered in this study because of the small diurnal cycle of the

convective layer in the model. At night, there is no cloud-top convective layer and this region is

similar in terms of dynamics to the high latitude at noon. The chemical timescale of SO2and

H2O is not well-constrain over the column, latitudes and local time. Therefore, the relaxation

timescale τis set to a constant value over the column, with sensitivity tests ranging over values

of 102,103,104,105and 106s. This broad range of relaxation timescale was chosen to be

compared to the dynamical timescale of the convective layer around 104s (see Section 2.3)

with two orders of magnitude above and below this value. Shao et al. (2020) reports chemical

timescale below 104s in the upper cloud and as high as 108s. Zhang et al. (2012) reports that

the nucleation timescale in the clouds can be below 102s. There are no bottom or top boundary

conditions for the tracers, the initial proﬁles for SO2and H2O are considered at equilibrium in

regard to chemistry, photochemistry and condensation/vaporization.

2.3 Simulation settings

The simulation settings in terms of resolution and time-step are identical to Lefèvre et al.

(2018)’s simulations, with a horizontal resolution of 400 m over 64 km, a vertical domain with

300 points from the surface to 90 km, and a time-step of 1 s. The lateral boundary conditions

are doubly periodic. The Lefèvre et al. (2018)’s equilibrium state at the Equator and 75˝at

noon are used as the initial state is shown in Fig 2, with the presence of a deep convective

layer, between 47 and 56 km at the Equator and between 46.5 and 57 km at high latitude,

as well as a hypothetical cloud top convective layer between 66 and 74 km at the Equator.

The amplitude of the vertical wind and the diameter of the convective cells are consistent

Figure 1: Vertical relaxation tracer abundance proﬁle (ppm). The circle represents the H2O

value from Cottini et al. (2012) and Fedorova et al. (2016), the star represents the SO2value

from Encrenaz et al. (2012, 2015) and the horizontal line represents tropospheric abundance

measurements from Bézard and de Bergh (2007).

with in-situ measurements of the VeGa balloons (Linkin et al., 1986; Sagdeev et al., 1986).

Regarding the gravity waves, with or without the presence of a cloud-top activity, the amplitude

of the gravity wave in temperature perturbations and vertical wavelengths are consistent with

the radio-occultation measurements (Tellmann et al., 2012), and the horizontal wavelength is

consistent with cloud-top UV observations (Piccialli et al., 2014). With a realistic radiative

transfer and incoming solar heating, characteristic of an average UV absorber abundance (Lee

et al., 2019), the model exhibits a cloud-top convective layer with convective cells diameters

consistent with the VMC observations (Markiewicz et al., 2007; Titov et al., 2012) that is

still speculative but the main hypothesis due to the absence of measurements of corresponding

static stability vertical distribution. This cloud-top convective layer has only a limited impact

on the deep cloud convective layer because the source, IR heating at cloud base from the

troposphere, is unaﬀected. The thin neutral at 75°of latitude at 7 km is an artefact from

the radiative transfer and large-scale heating extracted from the LMD Venus GCM with a

much more coarse resolution than the convection-resolving model. To avoid spurious reﬂection

of upward propagating gravity waves on the top boundary of the model, a Rayleigh damping

layer is applied over the 8 top kilometres with a damping coeﬃcient of 0.08 s´1. The tracers are

initialized with relaxation proﬁles (Fig 1) and then advected by the dynamics: the convection,

wind shear, and gravity waves. The outputs of the simulations are shown after about two Earth

days of simulation.

3 Mixing

Fig 3 shows the domain averaged vertical proﬁles of SO2at the Equator and 75˝. The presence

of the convective layer, well-mixed, in the deep cloud region is noticeable with a constant tracer

abundance value for larger relaxation timescales of 105and 106s (Fig 3-b). To illustrate the

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

TheImpactofTurbulentVerticalMixingintheVenusCloudsonChemicalTracersMaxenceLefèvre1,EmmanuelMarcq2,andFranckLefèvre21DepartmentofPhysics(Atmospheric,OceanicandPlanetaryPhysics),UniversityofOxford,Oxford,UK2LATMOS/IPSL,UVSQUniversitéParis-Saclay,SorbonneUniversité,CNRS,FranceAcceptedinIcarusAbstractVe...

展开>> 收起<<

The Impact of Turbulent Vertical Mixing in the Venus Clouds on Chemical Tracers Maxence Lefèvre1 Emmanuel Marcq2 and Franck Lefèvre2.pdf

共24页,预览5页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

The Impact of Turbulent Vertical Mixing in the Venus Clouds on Chemical Tracers Maxence Lefèvre1 Emmanuel Marcq2 and Franck Lefèvre2

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: