Martian Planetary Boundary Layer Research Articles

A Study of Daytime Convective Vortices and Turbulence in the Martian Planetary Boundary Layer Based on Half‐a‐Year of InSight Atmospheric Measurements and Large‐Eddy Simulations

AbstractStudying the atmospheric planetary boundary layer (PBL) is crucial to understand the climate of a planet. The meteorological measurements by the instruments onboard InSight at a latitude of 4.5°N make a unique rich data set to study the active turbulent dynamics of the daytime PBL on Mars. Here we use the high‐sensitivity continuous pressure, wind, and temperature measurements in the first 400 sols of InSight operations (from northern late winter to midsummer) to analyze wind gusts, convective cells, and vortices in Mars’ daytime PBL. We compare InSight measurements to turbulence‐resolving large‐eddy simulations (LES). The daytime PBL turbulence at the InSight landing site is very active, with clearly identified signatures of convective cells and a vast population of 6,000 recorded vortex encounters, adequately represented by a power law with a 3.4 exponent. While the daily variability of vortex encounters at InSight can be explained by the statistical nature of turbulence, the seasonal variability is positively correlated with ambient wind speed, which is supported by LES. However, wind gustiness is positively correlated to surface temperature rather than ambient wind speed and sensible heat flux, confirming the radiative control of the daytime Martian PBL; and fewer convective vortices are forming in LES when the background wind is doubled. Thus, the long‐term seasonal variability of vortex encounters at the InSight landing site is mainly controlled by the advection of convective vortices by ambient wind speed. Typical tracks followed by vortices forming in the LES show a similar distribution in direction and length as orbital imagery.

Large eddy simulations of the Martian convective boundary layer: Towards developing a new planetary boundary layer scheme

The grid resolutions typically employed in atmospheric global circulation and mesoscale models are not sufficient to explicitly resolve the turbulence processes within the planetary boundary layer (PBL). turbulent fluxes are, therefore, fully parameterized in those models, based on empirical relationships for the mixing length scale using PBL schemes. However, microscale models use the large-eddy simulation (LES) technique to resolve turbulence processes. Here, we perform LES computations for the daytime Martian planetary boundary layer, ranging from weakly to strong convective conditions, using the Mars implementation of planetWRF, the MarsWRF model. In this study, our main focus is to investigate the turbulence statistics and turbulent spectrum utilizing our LES results. Then, using the computed turbulence kinetic energy and its dissipation rate, a generic formulation for the mixing length scale variation in the Martian convective boundary layer is proposed. This mixing length formulation is used to derive a Mars-specific PBL scheme and its performance is compared to the PBL scheme currently in use by the MarsWRF model, the MRF scheme. The proposed scheme is tested both in global and mesoscale simulations, which are used to evaluate the convective boundary layer height and near-surface meteorology conditions at the InSight landing site. The presently proposed PBL scheme results in an improved prediction of convective boundary layer height that agree better with the observational estimations acquired by radio occultations of Mars Express in comparison to the MRF scheme. Also, the prediction of near-surface winds at the InSight landing site is slightly improved.

Dust devils on Mars

Whirlwinds, familiar on Earth, can be informative probes of the Martian surface environment.

Tangential winds of a vortex system in a planetary surface layer

The planetary boundary layer (PBL) mediates interactions between the surface and free atmosphere. In Martian PBL, surface can force convective vortices leading to dust devils. We use the Navier–Stokes equations and the continuity equation to determine mean (with respect to time) tangential wind velocity in cylindrical co-ordinate system within the surface layer of a planetary atmosphere. We utilize Martian surface layer properties for theoretical derivation of our solution. However, our results remain valid for any planetary surface layer as long as all of our assumptions are valid. Our theoretical values of the tangential wind velocity lie well within the range of observed values. The derived equation represents the dependency of tangential velocity on both radial distances from the center of vortex, and the altitude. As we move further away from the vortex center, the effect of vortex becomes non-significant, and velocities start following the standard logarithmic profile. Due to dependency of tangential wind velocity on altitude, the tangential velocity increases as we move higher up in the vortex system. At 100 m altitude, for an order of magnitude increase in the radial distance, the mean tangential wind velocity drops by about a factor of 1.5 in magnitude.

Protection of surface assets on Mars from wind blown jettisoned spacecraft components

Jettisoned Entry, Descent and Landing System (EDLS) hardware from landing spacecraft have been observed by orbiting spacecraft, strewn over the Martian surface. Future Mars missions that land spacecraft close to prelanded assets will have to use a landing architecture that somehow minimises the possibility of impacts from these jettisoned EDLS components. Computer modelling is used here to investigate the influence of wind speed and direction on the distribution of EDLS components on the surface. Typical wind speeds encountered in the Martian Planetary Boundary Layer (PBL) were found to be of sufficient strength to blow items having a low ballistic coefficient, i.e. Hypersonic Inflatable Aerodynamic Decelerators (HIADs) or parachutes, onto prelanded assets even when the lander itself touches down several kilometres away. Employing meteorological measurements and careful characterisation of the Martian PBL, e.g. appropriate wind speed probability density functions, may then benefit future spacecraft landings, increase safety and possibly help reduce the delta v budget for Mars landers that rely on aerodynamic decelerators.

Acoustic properties in the low and middle atmospheres of Mars and Venus.

Generic predictions for acoustic dispersion and absorption in the atmospheres of Mars and Venus are presented. For Mars, Pathfinder and Mars Express ambient data and averaged thermophysical parameters are used as inputs to a preliminary model based on the continuum approximation for Mars' thin atmosphere-the need for Boltzmann-based treatment is discussed in the context of Knudsen numbers. Strong absorption constrains acoustic sensing within the Martian planetary boundary layer. For the dense atmosphere of Venus, the van der Waals equation of state is used. The thermophysical and transport parameters were interpolated at the ambient conditions. Acoustic sensing is discussed at 50 km above Venus' surface, a level where aerostats (e.g., European Space Agency's EVE) and manned airships (e.g., NASA's HAVOC) may be deployed in the future. The salient atmospheric characteristics are described in terms of temperature, pressure, and convective stability profiles, followed by wavenumber predictions, and discussions of low- and high-frequency sensing applications. At low frequencies, emphasis is placed on infrasound. A simple generation mechanism by Martian dust devils is presented, yielding fundamental frequencies between 0.1 and 10 Hz. High-frequency sensing is exemplified by ultrasonic anemometry. Of the two environments, Venus is notably more dispersive in the ultrasonic range.

Dust devil height and spacing with relation to the martian planetary boundary layer thickness

In most remote and unmonitored places, little is known about the characteristics of daytime turbulent activity. Few processes render the optically transparent atmospheres of Earth and Mars visible; put more plainly, without clever instruments it is difficult to “see the unseen”. To address this, we present a pilot study of images of martian dust devils (DDs) testing the hypothesis that DD height and spacing correlates with the thickness of the planetary boundary layer (PBL), h. The survey includes Context Camera (CTX) images from a 580×590km2 area (196–208°E, 30–40°N) in northern Amazonis Planitia, spanning ∼3.6 Mars Years (MY) from Ls=134.55°, MY 28 (13 November 2006) to Ls=358.5°, MY 31 (28 July 2013). DD activity follows a repeatable seasonal pattern similar to that found in previous surveys, with a distinct “on” season during local summer, beginning shortly before the northern spring equinox (Ls=0°) and lasting until just after the northern fall equinox (Ls=180°).DD heights measured from shadow lengths varied considerably, with median values peaking at local midsummer. Modeled PBL heights, constrained by those measured from radio occultation data, follow a similar seasonal trend, and correlation of the two suggests that the martian PBL thickness is approximately 5 times the median DD height. These results compare favorably to the limited terrestrial data available.DD spacing was measured using nearest neighbor statistics, following the assumption that because convection cell widths have been measured to be ∼1.2±0.2h (Willis, G.E., Deardorff, J.W. [1979]. J. Geophys. Res. 84(C1), 295–302), a preference for DD formation at vertices of convection cells intersections could be used to estimate the PBL height. During local spring and summer, the DD average nearest neighbor (ANN) ranged from ∼1 to 2h, indicating that DD spacing does indeed correlate with PBL height. However, this result is complicated by two factors: (1) convection cell spacing estimates indicate that the observed DD distributions undersample the possible grid of cell vertices in each CTX image, and (2) comparison of the ANN with a random distribution shows that the most closely-spaced DD distributions exhibit some clustering; we propose that this clustering demonstrates that many of the observed DDs are located along convection cell walls, in addition to cell vertices.

The Martian Planetary Boundary Layer: Turbulent kinetic energy and fundamental similarity scales

We review selected in situ measurements and models aimed at the study of the Martian Planetary Boundary Layer (PBL). We also discuss critically the advantages and limitations of applying similarity theories to the Martian PBL and calculate the fundamental scales predicted by these theories. Finally, we obtain values of the turbulent kinetic energy (TKE) and address its budget by weighting the significance of the different terms involved in it. In situ measurements taken by the Viking and Pathfinder missions along with similarity theories conveniently adapted to Mars are used to obtain the fundamental scales, the TKE and its budget.

A thermal plume model for the Martian convective boundary layer

The Martian planetary boundary layer (PBL) is a crucial component of the Martian climate system. Global climate models (GCMs) and mesoscale models (MMs) lack the resolution to predict PBL mixing which is therefore parameterized. Here we propose to adapt the “thermal plume” model, recently developed for Earth climate modeling, to Martian GCMs, MMs, and single‐column models. The aim of this physically based parameterization is to represent the effect of organized turbulent structures (updrafts and downdrafts) on the daytime PBL transport, as it is resolved in large‐eddy simulations (LESs). We find that the terrestrial thermal plume model needs to be modified to satisfyingly account for deep turbulent plumes found in the Martian convective PBL. Our Martian thermal plume model qualitatively and quantitatively reproduces the thermal structure of the daytime PBL on Mars: superadiabatic near‐surface layer, mixing layer, and overshoot region at PBL top. This model is coupled to surface layer parameterizations taking into account stability and turbulent gustiness to calculate surface‐atmosphere fluxes. Those new parameterizations for the surface and mixed layers are validated against near‐surface lander measurements. Using a thermal plume model moreover enables a first‐order estimation of key turbulent quantities (e.g., PBL height and convective plume velocity) in Martian GCMs and MMs without having to run costly LESs.

THE MARTIAN ATMOSPHERIC BOUNDARY LAYER

The planetary boundary layer (PBL) represents the part of the atmosphere that is strongly influenced by the presence of the underlying surface and mediates the key interactions between the atmosphere and the surface. On Mars, this represents the lowest 10 km of the atmosphere during the daytime. This portion of the atmosphere is extremely important, both scientifically and operationally, because it is the region within which surface lander spacecraft must operate and also determines exchanges of heat, momentum, dust, water, and other tracers between surface and subsurface reservoirs and the free atmosphere. To date, this region of the atmosphere has been studied directly, by instrumented lander spacecraft, and from orbital remote sensing, though not to the extent that is necessary to fully constrain its character and behavior. Current data strongly suggest that as for the Earth's PBL, classical Monin‐Obukhov similarity theory applies reasonably well to the Martian PBL under most conditions, though with some intriguing differences relating to the lower atmospheric density at the Martian surface and the likely greater role of direct radiative heating of the atmosphere within the PBL itself. Most of the modeling techniques used for the PBL on Earth are also being applied to the Martian PBL, including novel uses of very high resolution large eddy simulation methods. We conclude with those aspects of the PBL that require new measurements in order to constrain models and discuss the extent to which anticipated missions to Mars in the near future will fulfill these requirements.

The TKE budget in the convective Martian planetary boundary layer

AbstractThe turbulent kinetic energy (TKE) budget has been obtained for the first time from ground‐based data on Mars, both for the unstable surface layer (SL) and for the convectively driven mixed layer (CML). Values for storage, buoyancy, shear, vertical turbulent transport, dissipation, and an imbalance term accounting for the rest of the TKE budget have been determined and weighted for significance.These results have been derived from ground‐based measurements made by Viking Lander 1 (VL1) on Sol 28, Viking Lander 2 (VL2) on Sol 20, and Pathfinder (PF) on Sol 25, and through an adaptation to Mars of terrestrial similarity theory, which constitutes a new approach to the study of the TKE budget on Mars.Shear is the main TKE generator in the unstable SL for VL1 Sol 28 and PF Sol 25. It is narrowly exceeded by dissipation, the main mechanism removing TKE. Both terms present values ∼10−1m2s−3. Buoyancy generates TKE, though it plays a minor role (∼10−2m2s−3). Vertical turbulent transport balances buoyancy, removing TKE from the SL by sending it upwards. The imbalance term represents 30% of the main mechanisms, while storage plays an insignificant role (∼10−5m2s−3). The SL TKE budget for VL2 Sol 20 presents a different behaviour instead, with the imbalance term becoming the main TKE generator, likely due to the anomalous atmospheric conditions existing during this Sol.Buoyancy and dissipation play the major roles generating and removing TKE in the CML for the three Sols under study, respectively, both showing values around 5×10−3m2s−3. Vertical turbulent transport plays a minor role (∼10−4m2s−3), and so does the imbalance term, with values about 25% of buoyancy or dissipation. Finally, shear and storage terms are negligible, presenting values ∼10−6 and ∼10−5m2s−3, respectively. Copyright © 2011 Royal Meteorological Society

H2O adsorption on smectites: Application to the diurnal variation of H2O in the Martian atmosphere

Observations of the Martian planetary boundary layer lead to interpretations that are baffling and contradictory. In this paper we specifically address the question of whether or not water vapor finds a substantial diurnal reservoir in the Martian regolith. To address this issue, we have measured H2O adsorption kinetics on SWy‐1, a Na‐rich montmorillonite from Wyoming. The highest‐temperature (273 K) data equilibrate rapidly. Data gathered at realistic H2O partial pressures and temperatures appropriate to early morning show two phenomena that preclude a significant role for smectites in diurnally exchanging a large column abundance. First, the equilibration timescale is longer than a sol. Second, the equilibrium abundances are a small fraction of that predicted by earlier adsorption isotherms. The explanation for this phenomenon is that smectite clay actually increases its surface area as a function of adsorptive coverage. At Mars‐like conditions we show that the interlayer sites of smectites are likely to be unavailable.

An Analytical and Numerical Study of the Martian Planetary Boundary Layer Over Slopes

A one-dimensional model of the Martian planetary boundary layer over sloping terrain is analyzed under a variety of conditions. Analytical results for the steady and diurnal components of the temperature and wind fields are found when a Boussinesq model with a Newtonian cooling law is considered. These results form a basis for understanding the numerical results which include more realistic representations for the heating and parametrizations for the eddy transfer of momentum and heat. The diurnal boundary layer thickness is determined primarily by radiative processes, and the amplitudes of the wind and temperature oscillations are found to depend in an important way on the latitude and slope magnitude. Typically, oscillations in the temperature of plus or minus 15 K and in the upslope wind of plus or minus 25 m/sec are found 1 km above a Martian slope of 0.005.