Anthony Domenick Toigo

Cyclones, tides, and the origin of a cross-equatorial dust storm on Mars

We investigate the triggering mechanism of a cross-equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off-line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally-modified return branch of the Hadley circulation.

Thermal tides in the Martian middle atmosphere as seen by the Mars Climate Sounder

The first systematic observations of the middle atmosphere of Mars (35km-80km) with the Mars Climate Sounder (MCS) show dramatic patterns of diurnal thermal variation, evident in retrievals of temperature and water ice opacity. At the time of writing, the dataset of MCS limb retrievals is sufficient for spectral analysis within a limited range of latitudes and seasons. This analysis shows that these thermal variations are almost exclusively associated with a diurnal thermal tide. Using a Martian General Circulation Model to extend our analysis we show that the diurnal thermal tide dominates these patterns for all latitudes and all seasons.

A first look at dust lifting and dust storms near the south pole of Mars with a mesoscale model

Surface wind stresses and dust lifting in the south polar region of Mars are examined with a three-dimensional numerical model. The focus of this study is the middle to late southern spring period when cap-edge dust lifting events are observed. Mesoscale model simulations of high southern latitudes are conducted at three dates within this season (L_s = 225°, 255°, and 310°). Assuming that dust injection is related to the saltation of sand-sized grains or aggregates, the Mars MM5 mesoscale model predicts surface wind stresses of sufficient strength to initiate movement of sand-sized particles (∼100 μm), and hence dust lifting, during all three periods. The availability of dust and/or sand-sized particles is not addressed within this study. Instead, the degree to which the existence of sufficiently strong winds limit dust injection is examined. By eliminating forcing elements from the model, the important dynamical modes generating high wind stresses are isolated. The direct cap-edge thermal contrast (and topographic slopes in some locations) provides the primary drive for high surface wind stresses at the cap edge, while sublimation flow is not found to be particularly important, at these three dates. Simulations in which dust is injected into the lowest model layer when wind stresses exceed a threshold show similar patterns of atmospheric dust to those seen in recent observations. Comparison between these simulations and those without active dust injection shows no signs of consistent positive or negative feedback due to dust clouds on the surface wind stress fields during the late spring season examined here.

A mesoscale model for the Martian atmosphere

The Pennsylvania State University/National Center for Atmosphere Research Mesoscale Model Version 5 (MM5) has been converted for use on Mars. Modifications are based on schemes implemented in the Geophysical Fluid Dynamics Laboratory Mars General Circulation Model (GCM). Validation of the Mars MM5 is conducted by comparison to the Mars GCM, examining the large-scale dynamics in the two models. Agreement between the two models on similar scales (a few hundred kilometers) is good. Validation is also performed against both Viking Landers and Mars Pathfinder meteorological observations with the model run at higher vertical (lowest level at 1.6 m) and horizontal resolution (a few kilometers). We find reasonable agreement with near-surface air temperature, pressure, and wind direction observations, with caveats. The results demonstrate that the model accurately simulates surface heat balance and the propagation of global thermal tides. However, wind speeds are underpredicted. The model generates the correct phasing of wind speeds with local time at the Viking Lander 2 site during winter but does not generate the correct phasing at the other sites or seasons. We examined the importance of slopes and global tides in generating the diurnal cycle of winds at the lander sites. We find that tides are at least as important as slopes, in contrast to previous studies. This study suggests that when used in combination with a GCM, the Mars MM5 promises to be a powerful tool for the investigation of processes central to the Martian climate on scales from hundreds of kilometers to tens of meters.

Seasonal variation of aerosols in the Martian atmosphere

Reanalysis of Viking Lander (VL) visible and Viking Orbiter infrared optical depth measurements shows that the visible to infrared ratio of total extinction opacity varies with season. The ratio is near to its previously reported constant value, 2.5, during dust storm periods and higher during northern spring and summer. The increase in ratio is hypothesized to be due to seasonally varying water ice haze, which produces a higher optical depth in the visible than in the infrared. This differs significantly from previous analyses of VL visible opacities which have assumed that only dust contributes to the optical depth measured during the early afternoon. Consequently we suggest that the Martian atmosphere is clearer of dust, especially during northern spring and summer, than previously suggested based upon VL data. We find dust visible optical depths of 0.1–0.4 during the northern spring and summer seasons, compared to previous estimates of 0.4–0.6. We also find that water ice hazes can provide roughly 50% of the total visible opacity in these seasons. For southern spring and summer, dust optical depths are more variable, but generally ≥0.4, with water ice opacity ≤0.1. The data suggest water ice optical depths are slightly higher and peak earlier (Ls = 80°–90°) at VL1 than at VL2 (Ls = 115°–130°). We estimate average northern summer water (daytime minimum) ice masses to be roughly 0.1–0.5 precipitable microns, depending on the assumed particle size distribution and hence 1–5% of the total water column. The observation of significant and previously unrecognized amounts of water ice haze suggests a larger role for water in controlling atmospheric heating rates and the vertical distribution of dust and water vapor than has been widely accepted to date.

Fitting the Viking lander surface pressure cycle with a Mars General Circulation Model

We present a systematic attempt to fit the Viking lander surface pressure cycle using a Mars General Circulation Model, MarsWRF. Following the earlier study by Wood and Paige (1992) using a one-dimensional model, high-precision fitting was achieved by tuning five time-independent parameters: the albedo and emissivity of the seasonal caps of the two hemispheres and the total CO_2 inventory in the atmosphere frost system. We used a linear iterative method to derive the best fit parameters: albedo of the northern cap = 0.795, emissivity of the northern cap = 0.485, albedo of the southern cap = 0.461, emissivity of the southern cap = 0.785, and total CO_2 mass = 2.83 × 10^(16) kg. If these parameters are used in MarsWRF, the smoothed surface pressure residual at the VL1 site is always smaller than several Pascal through a year. As in other similar studies, the best fit parameters do not match well with the current estimation of the seasonal cap radiative properties, suggesting that important physics contributing to the energy balance not explicitly included in MarsWRF have been effectively aliased into the derived parameters. One such effect is likely the variation of thermal conductivity with depth in the regolith due to the presence of water ice. Including such a parameterization in the fitting process improves the reasonableness of the best fit cap properties, mostly improving the emissivities. The conductivities required in the north to provide the best fit are higher than those required in the south. A completely physically reasonable set of fit parameters could still not be attained. Like all prior published GCM simulations, none of the cases considered are capable of predicting a residual southern CO_2 cap.

Correction to “Extensive MRO CRISM observations of 1.27 µm O2 airglow in Mars polar night and their comparison to MRO MCS temperature profiles and LMD GCM simulations”

The Martian polar night distribution of 1.27 μm (0-0) band emission from O 2 singlet delta [O 2 ( 1 Δ g )] is determined from an extensive set of Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectral Mapping (CRISM) limb scans observed over a wide range of Mars seasons, high latitudes, local times, and longitudes between 2009 and 2011. This polar nightglow reflects meridional transport and winter polar descent of atomic oxygen produced from CO 2 photodissociation. A distinct peak in 1.27 μm nightglow appears prominently over 70-90NS latitudes at 40-60 km altitudes, as retrieved for over 100 vertical profiles of O 2 ( 1 Δ g ) 1.27 μm volume emission rates (VER). We also present the first detection of much (x80±20) weaker 1.58 μm (0-1) band emission from Mars O 2 ( 1 Δ g ). Co-located polar night CRISM O 2 ( 1 Δ g ) and Mars Climate Sounder (MCS, McCleese et al, 2007) temperature profiles are compared to the same profiles as simulated by the Laboratoire de Meteorologie Dynamique (LMD) general circulation/photochemical model (e.g., Lefevre et al, 2004). Both standard and interactive aerosol LMD simulations (Madeleine et al, 2011) underproduce CRISM O 2 ( 1 Δ g ) total emission rates by 40%, due to inadequate transport of atomic oxygen to the winter polar emission regions. Incorporation of interactive cloud radiative forcing on the global circulation leads to distinct but insufficient improvements in modeled polar O 2 ( 1 Δ g ) and temperatures. The observed and modeled anti-correlations between temperatures and 1.27 μm band VER reflect the temperature dependence of the rate coefficient for O 2 ( 1 Δ g ) formation, as provided in Roble (1995).

Thermal tides during the 2001 Martian global‐scale dust storm

The 2001 (Mars Year 25) global dust storm radically altered the dynamics of the Martian atmosphere. Using observations from the Thermal Emission Spectrometer onboard the Mars Global Surveyor spacecraft and MarsWRF general circulation model simulations, we examine the changes to thermal tides and planetary waves caused by the storm. We find that the extratropical diurnal migrating tide is dramatically enhanced during the storm, particularly in the southern hemisphere, reaching amplitudes of more than 20 K. The tropical diurnal migrating tide is weakened to almost undetectable levels. The diurnal Kelvin waves are also significantly weakened, particularly during the period of global expansion at Ls = 200°–210°. In contrast, the westward propagating diurnal wavenumber 2 tide strengthens to 4–8 K at altitudes above 30 km. The wavenumber 1 stationary wave reaches amplitudes of 10–12 K at 50°–70°N, far larger than is typically seen during this time of year. The phase of this stationary wave and the enhancement of the diurnal wavenumber 2 tide appear to be responses to the high-altitude westward propagating equatorial wavenumber 1 structure in dust mixing ratio observed during the storm in previous works. This work provides a global picture of dust storm wave dynamics that reveals the coupling between the tropics and high-latitude wave responses. We conclude that the zonal distribution of thermotidal forcing from atmospheric aerosol concentration is as important to understanding the atmospheric wave response as the total global mean aerosol optical depth.

Martian Polar Vortices: Comparison of Reanalyses

The structure and evolution of the Martian polar vortices is examined using two recently available reanalysis systems: version 1.0 of the Mars Analysis Correction Data Assimilation (MACDA) and a preliminary version of the Ensemble Mars Atmosphere Reanalysis System (EMARS). There is quantitative agreement between the reanalyses in the lower atmosphere, where Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data is assimilated, but there are differences at higher altitudes reflecting differences in the free-running general circulation model simulations used in the two reanalyses. The reanalyses show similar potential vorticity (PV) structure of the vortices: There is near-uniform small PV equatorward of the core of the westerly jet, steep meridional PV gradients on the polar side of the jet core, and a maximum of PV located off of the pole. In maps of 30 sol-mean PV, there is a near-continuous elliptical ring of high PV with roughly constant shape and longitudinal orientation from fall to spring. However, the shape and orientation of the vortex varies on daily time scales, and there is not a continuous ring of PV but rather a series of smaller scale coherent regions of high PV. The PV structure of the Martian polar vortices is, as has been reported before, very different from that of Earths stratospheric polar vortices, but there are similarities with Earths tropospheric vortices which also occur at the edge of the Hadley Cell, have near-uniform small PV equatorward of the jet, and a large increase of PV poleward of the jet due to increased stratification.

Two aerodynamic roughness maps derived from Mars Orbiter Laser Altimeter (MOLA) data and their effects on boundary layer properties in a Mars general circulation model (GCM)

[1] Mechanical (forced convective) and free convective turbulent heat and momentum transfer in the lower atmosphere of a terrestrial planet has some dependence on the roughness characteristics of the surface, often quantified in terms of a single roughness parameter which is then used to calculate the coefficients that govern heat and momentum transport between the surface and the boundary layer. We take two different approaches for deriving this aerodynamic roughness parameter for Martian surfaces using data from the Mars Orbiter Laser Altimeter. We then use these two different roughness maps to force the boundary layer in a Mars general circulation model, primarily investigating differences in temperatures and the pressure cycle between the two simulations. While the pressure cycle does not vary significantly, spring and summer high-latitude temperatures are somewhat sensitive to the input roughness conditions. Daytime temperatures may vary up to 10 K seasonally, though zonally and annually averaged daytime temperatures vary only by � 1 K. Our results can be explained by the dominance of mechanical over convective turbulent heat transfer processes on Mars. These simulations, however, use a prescribed atmospheric dust distribution and thus only provide a minimum estimate of the uncertainty in boundary layer temperatures because of this plausible range of aerodynamic roughness parameters. Since surface roughness determines the threshold wind velocity for dust lifting we anticipate a much larger effect of the aerodynamic roughness parameter on temperatures when the dust distribution is allowed to vary according to predicted lifting and transport.