Meteorological pressure at Gale crater from a comparison of
REMS/MSL data and MCD modelling:
Effect of dust storms

Iñaki Ordonez-Elxeberria¹*, Ricardo Hueso¹, Agustín Sánchez-Lavega¹,

Ehouarn Millour², Francois Forget²

¹ Dpto. de Física Aplicada I, Escuela de Ingeniería de Bilbao, Universidad del País Vasco, Plaza Ingeniero Torres Quevedo, 1, 48013, Bilbao, Spain.

² Laboratoire de Mécanologie dynamique/IPSL, Sorbonne Université, École Normal supérieure, PSL Research University, École polytechnique, CNRS, Paris, France.

**Abstract:** We examine the record of atmospheric pressure in Gale crater measured in-situ by the Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) on the Mars Science Laboratory (MSL) rover over two Martian years. We compare the data with pressure predictions from the Mars Climate Database (MCD) (Forget et al., 1999; Millour et al., 2015) version 5.2, which is a climatological database derived from numerical simulations of the Martian atmosphere produced by a General Circulation Model run over several Martian years. Seasonal and daily trends in pressure data from REMS are well reproduced by the standard climatology of the MCD using its high resolution mode. This high-resolution mode extrapolates pressure values from the nominal model into the altitude of each location using a high-resolution topography model and a fine tuning of the vertical scale height that was chosen to mimic effects of slope winds not directly accounted for in the General Circulation Model on which the MCD is based. Differences between the synthetic MCD pressure data and the REMS measurements are produced by meteorological features that are identified on particular groups of sols and quantified in intensity. We show that regional dust storms outside Gale crater and dust abundance at the crater are important factors in the behaviour of the pressure exciting larger amplitudes on the daily pressure variations and causing most of the largest REMS-MCD differences. We compare the pressure signals with published data of the dust optical depth obtained by the REMS ultraviolet photodiodes and the Mastcam instrument on MSL, and with orbital images of the planet acquired by the MARCI instrument on the Mars Reconnaissance Orbiter (MRO). We show that in some cases regional dust storms induce a characteristic signature in the surface pressure measured by REMS several sols before the dust arrives to Gale crater. We explore the capability of daily pressure measurements to serve as a fast detector of the development of dust storms in the context of the MSL, Insight and Mars 2020 missions.

*Corresponding author. Email address: inaki.ordonez@ehu.es (Inaki Ordonez-Elxeberria)
# 1. Introduction

Mars has a thin atmosphere whose surface pressure experiences large variations associated to seasonal and daily insolation cycles. Seasonal variations are caused by sublimation and ice condensation in the polar regions (Leovy, 1979), and daily variations are controlled by thermal diurnal and semidiurnal tides driven by sunlight heating (Hess et al., 1977; Leovy and Zurek, 1979) with additional effects at locations with complex topography (Tyler and Barnes, 2013, 2015, 2017; Fonseca, 2018). Surface pressure is also affected by meteorological events such as travelling waves at mid and high-latitudes (Collins et al., 1996) and by the content of dust in the atmosphere, which is in turn mainly controlled by the development of dust storms (Leovy and Zurek, 1979; Zurek, 1981; Hourdin et al., 1995). Additionally, short transients caused by dust devils (Ellehøj et al., 2010) and surface turbulent eddies (Ordonez-Etxeberria et al., 2018) also appear on in situ pressure measurements acquired with high temporal sampling (1 s) and adequate amplitude resolution (variations of 0.5 Pa or lower).

The Mars Science Laboratory (MSL) rover landed on Mars in Gale crater in August 6, 2012 and has been collecting geologic and atmospheric data since then. Gale crater is an interesting meteorological location situated in the boundary between the South high-lands and the northern low-lands at equatorial latitude of 4.5°S in an environment with a complex topography and a lower elevation over Mars areoid than any previous landed mission. The Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) obtains measurements of air pressure, surface and air temperature, wind intensity and direction, relative humidity and ultraviolet (UV) fluxes. The pressure data is obtained with a set of low-noise sensors (0.10 Pa, Gómez-Elvira et al., 2014) with measurement cadences of 1 s over time periods that extend from short (5 minutes) to large (1 hour) time-blocks typically separated by 1 hour. This allows studying the seasonal and daily cycles of pressure with high fidelity over an extended period of time. Early analyses of REMS pressure data showed the seasonal behaviour of the atmospheric pressure subject to the CO₂ annual cycle of sublimation/condensation in the polar caps (Gómez-Elvira et al., 2014; Haberle et al., 2014; Harri et al., 2014), and the role of atmospheric tides causing a large part of the daily air pressure variation (Guzewich et al., 2016). Haberle et al. (2014) also present the identification of a regional dust storm after sol 97 (Ls=200°) through an analysis of the pressure field measured by REMS. Further work has concentrated in studying the local crater circulation (Tyler and Barnes, 2013, 2015; Rafkin et al., 2016) and its effects on the pressure field (Haberle et al., 2014; Tyler and Barnes, 2013, 2015, 2017; Wilson et al., 2017; Fonseca et al., 2018). Other pressure investigations with REMS have analyzed transient events of fast drops of pressure of about 0.5 Pa or larger over a
few seconds, generally interpreted as the local effect of a passing warm convective vortex or dust devils (Gómez-Elvira et al., 2014; Harri et al. 2014; Steakley and Murphy 2016; Kahanpää et al., 2016; Ordonez-Etxeberria et al., 2018). Finally, periodic oscillations in the REMS pressure data have been identified recently by Haberle et al. (2018) finding frequencies and seasonal variations that are interpreted in terms of travelling baroclinic systems originating in the Northern Hemisphere and possibly related to flushing dust storms.

Viking pressure observations showed that the amplitude of atmospheric tides in locations with a nearly flat topography on Marsis largely dominated by the amount of atmospheric dust (Leovy and Zurek, 1979; Zurek, 1981). Additional effects in the pressure can be expected from the global and local topography. Effects associated to the global topography are generally produced by Kelvin waves and can be generally resolved by General Circulation Models (i.e. Wilson and Hamilton, 1996). However in locations with complex topography, such as the Gale crater, meso-scale modelling is generally required to solve the pressure field modified by the local circulation (Tyler and Barnes, 2013; 2015, 2017; Pla-Garcia et al., 2016; Rafkin et al., 2016; Wilson et al., 2017; Fonseca et al., 2018). Guzewich et al. (2016) used Fourier analysis of REMS pressure data over 875 sols to characterize daily pressure variations at Gale crater caused by atmospheric tides forced by solar heating of the atmosphere and local effects associated to the topography. At Gale crater, the diurnal pressure harmonic (with a period of 24 hr) and semi-diurnal harmonic (with a period of 12 hr) are correlated to each other and their amplitudes observed by REMS can also be linked to the presence of nearby dust storms and the local dust opacity (Guzewich et al., 2016). Meso-scale models used to account for the detailed crater topography suggest that up to a 40% of the daily pressure variation could be due to slope winds and local effects (Tyler and Barnes, 2015, 2015; Wilson et al., 2017).

The purpose of this work is double. On the one hand we aim to detect and characterize local meteorological events that last a few sols by comparing the MSL pressure records with predictions of the MCD (Forget et al., 1999; Millour et al., 2015) and we investigate orbital images of the planet to identify dust storms capable to produce the largest differences between REMS and MCD data. On the other, we make a validation of the pressure data on the MCD compared with real data at a topographically complex site as Gale crater and we discuss how the high-resolution mode of the MCD accounts for the effects of local topography through an appropriate choice of atmospheric scale height. This validation is essentially different to a recent model validation (Fonseca et al., 2018) of the MarsWRF model (Richardson et al., 2007) which is more focused on the detailed circulation and characteristics of the Planetary Boundary Layer (PBL) at particular periods of times
with effects ultimately driven by the topography.

We examine pressure data from the REMS instrument corresponding to more than 2 Martian years (1514 sols, from $L_s$ 151° MY 31 to $L_s$ 258° MY 33) extending the analysis on thermal tides of Guzewich et al. (2016). We identify interesting periods of time comparing REMS data with results from the Mars Climate Database (MCD) that correspond to the same aerocentric solar longitude $L_s$ and correcting the REMS pressure data from the varying MSL elevation above an initial altitude level. We first illustrate the comparison method with an example from pressure data from the Viking missions where dust storms are well documented (Hess et al., 1980; Zurek, 1981; Ryan and Sharman, 1981). The method is then further developed in the context of REMS pressure data which is better sampled than Viking data identifying several periods of time of interest. We compare the pressure data with published dust opacities retrieved from MSL's Mastcam images (Lemmon et al., 2015) and REMS UV photodiodes (Smith et al., 2016). Additionally, we also examine orbital images obtained from the MARCI instrument (Malin et al., 2001; Bell et al., 2009) on Mars Reconnaissance Orbiter (MRO) to look for dust storms and cloud systems close to, or at Gale crater, during the events previously identified.

The outline of this paper is the following: In section 2 we present the REMS pressure data and we summarize the characteristics of the MCD model, the dust optical opacity measurements from Mastcam and REMS UV detectors and the MARCI images used to explore the development of storms. We describe our methodology to compare REMS and MCD data in section 3. Several comparisons between the data are examined, including an analysis of the magnitudes of the different components of the pressure tides following the method by Guzewich et al. (2016). These comparisons result in a selection of groups of sols where interesting meteorological events appear on the pressure data. In section 4 we compare these groups of sols with measurements of dust opacity from MSL's Mastcam and REMS UV detectors. In section 5 we characterize the selected pressure events and their seasonal behaviour including measurements of the amount of dust at Gale crater and images of close dust storms observed by the MARCI instrument on MRO. We present a summary of this research and our conclusions in section 6.

## 2. Data

### 2.1 REMS pressure data
REMS data were retrieved from the NASA Planetary Data System at http://atmos.nmsu.edu/PDS/data/mslrem_1001/ (REMS Reduced Data Record release number 14, online on 16th March 2017). The data here analysed extends from August 7, 2012 (the day after landing; Ls = 150.6°, Martian Year MY 31) to November 9, 2016 (Ls = 258.1°, MY 33) covering a total of 1514 sols or more than two Martian years. We used pressure values from the Modelled Reduced Data Record (MODRDR) files where calibrated values of all REMS sensors are available (Mora et al., 2013). We also used Ancillary Data Record (ADR) files containing the position of the rover to characterize the relative altimetry of each measurement compared with the landing point. The observation strategy of the REMS instrument is described in Gómez-Elvira et al. (2014). REMS acquires data during the first five minutes of each hour using Local Mean Solar Time (LMST) and in many cases extended observations over 1 full hour are also acquired. Some sols have lower number of pressure measurements due to a variety of factors including Earth solar conjunctions, rover activities and hardware problems. During the time period here analyzed (1514 sols), pressure data is available for 1440 sols with 1066 of them containing pressure measurements distributed in time covering the full daily cycle with a temporal resolution of at least 1 hour. This represents 70% of all sols and constitutes the basis of this study.

## 2.2. The MCD (version 5.2)

The Mars Climate Database (MCD; Lewis et al., 1999 and Millour et al., 2015) is a database of atmospheric parameters computed using runs of the LMD Global Climate Model (GCM) of the Martian atmosphere (Forget et al. 1999) which includes schemes to account for the Martian CO2 (Hourdin et al 1995, Forget et al. 1998), water (Navarro et al. 2014) and dust (Madeleine et al. 2011) cycles. A detailed description of the latest version of the LMD GCM is given by Pottier et al. (2017) and references therein. A key aspect of the GCM is that it uses a prescribed dust scenario (i.e. imposed columnar dust opacity at all locations and time). In practice such forcings are implemented after analysis of a variety of dust observations from MY 24 to MY 31 (Montabone et al., 2015). The database can be consulted at http://www-mars.lmd.jussieu.fr/ and can be downloaded and installed into a local server for better interaction with the data.

The MCD contains climatic values of the atmospheric fields reconstructed from GCM runs, as detailed below. Although the GCM is a time-marching model with a 90 seconds time step, its outputs are reduced to climatological values in the MCD by reconstructing an average diurnal cycle
over intervals of 30° of solar longitude. For each of these sols the data is stored in intervals of 2 hours. When MCD is queried for a given time and location, the MCD software uses linear interpolation along times of day and Ls and location from encompassing stored data. This tends to smooth the evolution of the diurnal pressure cycle and induces a small bias. The data in the MCD are stored with the same resolution as the GCM, i.e., 5.625° in longitude, 3.75° in latitude, and in 49 vertical levels using a hybrid sigma-pressure coordinate from the surface to ~250 km of altitude.

The MCD includes a “high-resolution mode” which combines high resolution (32 pixels per degree) MOLA topography, mean VL1 seasonal pressure records and the horizontal pressure gradient calculated with the LMD GCM and available in the MCD. The extrapolation from the GCM low-resolution to the high-resolution mode is given by:

$$PS = P_{GCM} \frac{P_{VL1}}{P_{VL1_{GCM}}} e^{-(z-Z_{GCM})/H}, \quad (1)$$

where P<sub>GCM</sub> is the pressure predicted by the GCM, P<sub>VL1</sub> corresponds to the pressure records of VL1 smoothed to remove thermal tides and transient waves, and P<sub>VL1_GCM</sub> is the surface pressure predicted by the GCM at the location of VL1 and also smoothed (i.e. diurnally and seasonally averaged), thus, this VL1 ratio correction is devoid of diurnal evolution or transient effects of the order of a few sols (e.g. weather systems). Z-Z<sub>GCM</sub> is the difference between the MOLA altitude and the altitude defined in the GCM topography grid for the location point and H corresponds to the scale height calculated using the temperature extracted from the GCM at 1 km above the surface (see below). It should be noted that the altitude difference Z-Z<sub>GCM</sub> between the MCD low resolution and high resolution modes is of 2950 m at MSL location.

Equation (1) thus represents a pressure correction assuming hydrostatic equilibrium (barometric formula) and was designed for a previous version of the MCD in 2002. Further details on its implementation in the MCD “high-resolution mode” are available in Section 4.2 in Forget et al. (2007). A key question to design this equation was the choice of the altitude of the temperature used for the assumed scale height. This was a difficult question because the atmosphere is characterized by very large vertical temperature just above the surface. As a result the atmosphere is almost never in hydrostatic equilibrium and near-surface slope winds systematically develop at most scale. This is discussed in further details in Section 3.1 in Spiga et al. (2007). A pragmatic approach based on 3D modelling results was adopted to choose the scale height temperature. In simulations performed with the LMD General Circulation Model fields, it was found that by taking the air temperature at 1 km above the surface, one could obtain maps of surface pressure
interpolated to the zero datum free of topographical artifacts using the barometric formula. In other
words this was the right temperature to simply extrapolate surface pressure taking into account the
major effect of the slope winds resolved by the GCM. We then assumed that this assumption would
still be valid at scale not resolved by the GCM, and use it in the MCD via equation (1) to interpolate
sub-grid scale pressure variations due to the local topography. This method was later empirically
validated using observed pressure maps retrieved by remote sensing, as explained in Spiga et al.
(2007). In the case of Gale Crater, it can be considered that, while the high-resolution mode of
MCD does not incorporate effects associated to the local topography at Gale crater where slope
winds are expected to significantly affect temperatures and pressures, the selection of *H* at 1 km
gives a value that mimics effects of slope winds in most locations including the crater circulation at
Gale described in Tyler and Barnes (2013, 2015). In particular, this choice of *H* produces a diurnal
range of pressure in Gale crater that is larger than results produced by the LMD GCM. We will
show that this tuning of the MCD high-resolution mode provides an excellent agreement with
REMS data.

As the LMD-GCM is run using predefined dust forcing scenarios, the MCD provides a
number of different cases. The standard “climatology” dust scenario is a synthetic scenario built as
the average of observed values over years without global dust storms (namely MY24, 26, 27, 29,
30, 31). A "cold", "warm" and a "dust storm" scenario are also available in MCD. In the "cold"
scenario the dust opacity is set to be for each location and sol of year the minimum observed over
Martian years 24-31 and further decreased by 50% representing an exceptionally clear atmosphere.
The "warm" scenario corresponds to an atmosphere with dusty conditions but without global dust
storms. The dust opacity at each location and sol of year is set to the maximum observed over the
seven Martian years, from 24 to 31. The "dust storm" scenario is only calculated during Southern
spring and summer (Ls 180° to 360°) with an extreme dust opacity of 5 representative of an intense
global dust storm. In addition, the MCD also includes results for all Mars Years from 24 to 31 using
the dust scenarios for each of these years.

## 2.3. Mastcam dust opacity

Direct observation of the Sun with the Mastcam camera results in images that contain a
pattern of scattered light that depends on the amount of atmospheric dust. The analysis of these
images allows computing the optical depth of the atmospheric dust following techniques developed
for the Mars Exploration Rovers (Lemmon et al., 2015). Dust retrievals are generally of very high
quality and available every three to seven sols and they represent the local dust load at the MSL's
location. These Mastcam dust opacity measurements have been presented in several previous works (e.g. Guzewich et al., 2016) and those used here have been supplied by M. Lemmon (*personal communication*).

## 2.4. REMS UV dust opacity

The dust optical depth can also be obtained by the REMS UV sensors which provide data almost every sol. The daily UV data has been analysed with a radiative transfer model that considers the daily variation of the UV flux and makes use of a calibration of the dust deposition in the UV sensors through comparison with Mastcam images (Smith et al., 2016). Because the dust fallout calibration factor is computed from a comparison with Mastcam images at 880 nm the REMS UV dust opacities are also scaled to a wavelength of 880 nm. Numerical values of dust optical opacity are provided in Smith et al. (2016) for the first 1159 sols of MSL on Mars and dust opacity is available for almost each sol. Both REMS UV and Mastcam dust opacities are not fully independent as Mastcam data is used to calibrate dust deposition on the REMS UV sensors.

## 2.5. MARCI images

The Mars Color Imager (MARCI) instrument (Malin et al., 2001; Bell et al., 2009) on board the Mars Reconnaissance Orbiter (MRO) is a Wide Angle camera that regularly obtains color images of the Martian surface and its atmosphere from a Sun-synchronous orbit. Swaths of the planet are obtained with a spatial resolution of 0.7 km/px at nadir, and about 4 km/px at the limb. For each sol, MARCI can cover the entire planet in about 12 orbits, collecting data on the sunlit portion with local times at the centre of the observed stripe ranging around 15.00 ± 02.00 hr.

Animations based on low-resolution MARCI images can be quickly inspected as “Mars Weather Reports” available online on http://www.msss.com/msss_images/. For interesting periods of time we downloaded raw MARCI images from the United States Geological Survey (USGS) PDS node (https://pdsimage2 wr.usgs.gov) and processed and projected them using the Integrated Software for Imagers and Spectrometers (ISIS) of the USGS (Becker and Anderson, 2013). Geometric corrections and combination of images obtained from different swaths of the planet are done using algorithms described by Wang and Richardson (2015). Images in 4 of the 5 available channels in the instrument at effective wavelengths of 437, 546, 604 and 718 nm are used to
distinguish between cloud compositions (water and CO₂ ices) and dust.

### 3. REMS and MCD pressure comparison

#### 3.1. REMS data treatment

REMS is a mobile meteorological station travelling with MSL through different terrains and altitudes. Since the MCD high resolution model has a limited spatial resolution of 2 km it cannot take into account the detailed topography at Gale crater and the exact elevation of the rover on different sols. Before running any comparison we correct REMS pressure data from the altitude of the rover in every sol assuming the reference altitude of the landing site and hydrostatic equilibrium of the atmosphere. The corrected pressure *P* is given by:

$$P = P_0 \cdot e^{\left(\frac{-\Delta z}{H}\right)}, \qquad (2)$$

Where $P_0$ is the pressure value observed by REMS, $\Delta z$ is the altitude difference to the landing site calculated directly from position information in the REMS ADR files, and *H* is the atmospheric scale height. This correction amounts to a maximum of 17 Pa at the top of the track of MSL on sol 1514 (188 m higher elevation than at the landing point). Here *H* is computed taking into account the mean molecular weight of Mars atmosphere ($\mu$ = 43.34 g mol⁻¹), the molar gas constant ($R$ = 8.3145 J kg⁻¹ mol⁻¹), surface gravity ($g$ = 3.71 m s⁻²) and the air temperature (*T*) near the surface (Sánchez-Lavega, 2011) measured by REMS following:

$$H = \frac{(R/\mu)T}{g} \qquad (3)$$

We compare REMS and MCD for each sol where we have enough REMS data. For each sol we use MCD data calculated for the same aerocentric longitude $L_s$ as the REMS measurements. While REMS uses LMST (Local Mean Solar Time) for planning observations, MCD uses LTST (Local True Solar Time) as a time system (for a detailed explanation of time keeping in Mars and the conversion between both time systems see Allison, 1997). We use LTST as the time system for this work using LTST labels in REMS data files.
## 3.2. Maximum and minimum daily pressures

The record of REMS pressure data and the maximum and minimum daily pressures from the MCD over different climatic scenarios are shown on Figure 1. These scenarios are: a standard climatology scenario, a warm scenario representative of high amounts of dust, a cold scenario representative of low amounts of dust, and a global dust storm scenario computed only for the dust storms season. Each scenario is found to present similar values for the daily mean pressures for the same Ls with varying amplitude of the daily pressure. This pressure variation amplitude depends on the global amount of dust assumed in each scenario and the daily pressure variation of the standard climatology scenario is bracketed between the results obtained by the cold and warm scenarios. The standard scenario matches well the REMS pressure data.

Concerning other dust scenarios, the MCD warm scenario results in an amplitude of the diurnal tide that is larger than what it is observed and the MCD cold scenario results in a smaller amplitude of the diurnal tide than the REMS observations. These are the expected behaviours of the three scenarios from the knowledge of the role that dust plays in the Martian atmosphere. The interpretation of Figure 1 is that the dust prescription in the standard MCD model results in a better match to the period covered by the REMS pressure observations than the enhanced or diminished dust models. Additionally, while a global dust storm in the planet has not been observed during the MSL mission, the MCD “dust storm” scenario predicts that, in the event of a global dust storm comparable to those observed by the Viking Landers, the signature of the storm in the pressure signal would be significantly larger than what it was observed at the location of the Viking Landers.

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Comparison between REMS daily pressure and different MCD scenarios for the location of the Gale crater. REMS data (violet shaded region) has been corrected from elevation effects as explained in the text. Solid lines</figcaption></figure>
correspond to different MCD scenarios: The climatology average scenario data (green dashed line), dust storm scenario (red line) which is only computed in the dust storms season ($L_s$= 180-360), warm scenario (magenta line) and cold scenario (orange line). Seasons are marked for the southern hemisphere.

Figure 2 shows a comparison of the daily maximum and minimum pressure from REMS data and the standard MCD scenario. The difference between the maximum and minimum pressure in each sol is a crude estimate of the diurnal tide amplitude and is expected to vary for different amounts of atmospheric dust (Guzewich et al., 2016). This diurnal amplitude of the pressure variation is affected by local slope winds that at Gale crater amplify the effects of the diurnal tide (Tyler and Barnes 2013, 2015) and is mimicked in the MCD. While in most sols a remarkably good fit between MCD and REMS values is found, some groups of sols present a large difference in pressure data and the origin and characteristics of these differences will be later examined. These differences between the in situ REMS data and the climatic model are represented in the lower part of the figure.

<figure><img src="image_2.png" /><figcaption>**Figure 2.** REMS maximum and minimum daily pressure and values from the MCD climatologic scenario. REMS data has been corrected from elevation effects as explained in the text. Upper curves show the daily maximum (red) and minimum (blue) values of pressure. Solid thick curves correspond to REMS data and thin continuous lines correspond to the MCD values. The differences between maximum values (REMS-MCD) and minimum values are shown with a blue and green line respectively. A horizontal line is drawn as a reference at zero differences.</figcaption></figure>

The amplitude of the daily variation of pressure also depends on the location over the planet and is expected to increase at lower elevations. Figure 3 shows global maps of the expected amplitude of daily pressure variation computed from MCD standard scenario in four different sols
representative of the seasonal evolution. The pressure amplitude is particularly intense in the deep
Gale crater and much higher than in other landing sites where similar pressure data has been
obtained in previous missions (Viking Lander 1, Viking Lander 2, Phoenix Polar Lander and Mars
Pathfinder) (see e.g. Barnes et al., 2017). This elevated diurnal pressure range is caused by the
combination of a lower elevation, atmospheric tides directly simulated by the LMD GCM, and local
circulation which enhances the effects of the tides, and whose effects have been emulated through
the vertical extrapolation of pressure in the high-resolution mode of MCD.
<figure><img src="image_3.png" /><figcaption>**Figure 3.** Maps of the amplitude of the daily pressure variation from the MCD climatologic standard scenario for different seasons. Leftpanels show global maps and right panels zoom over the region around Gale crater. Four seasons are represented considering aerocentric longitudes $L_s$ of 0° (Northern Hemisphere Spring Equinox), 90° (Northern Hemisphere Summer Solstice), 180° (Northern Hemisphere Autumn Equinox) and 270° (Northern Hemisphere Winter Solstice). The location of different surface missions with pressure sensors are highlighted in the figure.</figcaption></figure>
Results in Figures 1 to 3 are obtained using the MCD high-resolution mode. To compare with the original values in the low-resolution version of MCD which are produced by the original LMD GCM simulations, a suitable comparison can be done through the use of normalized pressure (the normalized pressure is defined as surface pressure divided by its diurnal mean), since normalized pressure is not sensitive to the surface elevation and the high-resolution topography. We explore the amplitude of normalized daily pressure variations in Figure 4. Differences between the MCD high-resolution and low-resolution values are produced by the vertical extrapolation of pressures through the use of eq. (1) using a scale height *H* based on temperature at 1 km above the surface. The MCD high-resolution amplitudes match much better REMS results than the MCD low resolution outputs. The differences between MCD results illustrate the adequate mimicking effects of crater circulation induced by the MCD high-resolution scheme. Indeed the MCD low-resolution results include the effects of daily pressure tides but not those of the local circulation induced by unresolved topography. The MCD high-resolution normalized daily pressure amplitude is higher than what the LMD GCM produces at Gale crater imitating the pressure effects associated with daytime upslope winds that reduce surface pressure and nighttime downslope winds that increase surface pressure amplifying the effects of daily pressure tides (Wilson and Tyler, 2017). An in depth analysis of how well MCD high-resolution values imitate the effects associated to the local circulation (J. Wilson, personal communication) will be presented elsewhere. In all that follows we use MCD high-resolution values.

<figure><img src="image_4.png" /><figcaption>**Figure 4. Normalized amplitudes daily pressure variations.** REMS data is shown in green, values of the MCD standard climatic scenario under the standard resolution of the LMD GCM are shown in blue, values of the MCD</figcaption></figure>
### 3.3. Relevance of REMS-MCD pressure differences: Validation with Viking pressure data

The comparison of observed and climatologic modelled data has the potential to detect interesting meteorological features. A demonstration can be drawn with a similar analysis of a well studied data set: the pressure sensor installed on the Viking Lander 1 (VL1) that landed on Mars at 47.968° W and 22.483° N (Morris and Jones, 1980). Figure 5 shows the comparison of the VL1 and MCD pressure data using the same methodology as in Figure 2. As expected, both data sets fit roughly well during the 2245 sols that VL1 provided pressure data except for the small day-to-day variations due to baroclinic waves present in the VL1 records but not on the MCD curves (the LMD GCM producing the MCD values can predict the signature of baroclinic waves but they are smoothed out in the average climatological values stored in the MCD). However, significant disagreements can be noted in three groups of sols highlighted in Figure 5. These three groups of sols correspond to dust storms known as 1977A, 1977B and 1982A observed in the pressure data of VL1 and Viking Lander 2 (VL2, situated at 6,500 km of distance to the VL1) (Ryan and Sharman, 1981; Tillman 1988).

<figure><img src="image_5.png" /></figure>

**Figure 5.** Comparison between Viking Lander 1 (VL1) pressure data and the MCD climatologic scenario for the VL1 location. Pressure values are in Pascals. VL1 sol is defined as number of sols after landing (VL1 landed on July 20, 1976). Solid thick curves correspond to VL1 daily maximum and minimum pressure data and thin lines correspond to the MCD values (using the climatology scenario). The difference between maximum values (VL1-MCD) and minimum values is shown with a blue and green line respectively. A horizontal line is drawn as a reference at zero differences. Circles highlight the three periods of differences in the pressure field and correspond to the three events of dust storms. Seasons are indicated for the northern hemisphere.
The 1977A dust storm affected the VL1 pressure signal from sols 208 to 229 (Ls=207-220). The 1977B dust storm occurred from sol 312 to 385 (Ls=274-319) and the 1982A dust storm is also visible in the pressure data from sol 2,203 to 2,240 (Ls=200-223). The signatures introduced by those storms are similar to some of the signatures observed with REMS on Figure 2, although the REMS differences with the MCD are smaller than those produced by the 1997A, 1997B and 1982A dust storms. A comparison of this data with Figure 3 also implies that in the event of a global dust storm similar to those observed in 1977 and 1982 the amplitude of the pressure daily variation at Gale crater would be larger than those recorded at the time of the Vikings since they were situated at higher elevations than MSL.

### 3.4. Comparing REMS with MCD model: $\chi^2$ analysis and selection of anomalous groups of sols

The previous comparison relies only on the maximum and minimum values of pressure each sol. A better quantification of how coincident are REMS pressure data and climatic MCD values of pressure can be performed by comparing REMS and MCD pressures along a full sol. This can be done by considering 24 data points per sol separated in LTST where the REMS data can be computed from the mean value of pressure each LTST hour and the MCD data are supplied by the database interface. For each sol a $\chi^2$ representative of the differences between both time series can be computed using the Pearson's chi-squared test (Bevington and Robertson, 2003):

$$ \chi^2 = \sum_i \frac{(P_i^{\text{REMS}} - P_i^{\text{MCD}})^2}{P_i^{\text{MCD}}}, \quad (4) $$

Where $P^{\text{REMS}}$ corresponds to the REMS average observations for each hour, and $P^{\text{MCD}}$ corresponds to the pressure data from MCD at the same Ls and LTST. Figure 6 displays two examples of the comparison of REMS and MCD with low and high values of $\chi^2$. The mean value of the absolute differences of pressures between REMS and MCD varies from 2 to 10 times the corresponding $\chi^2$ value. The advantage of using the Pearson chi-squared test is that similar values of $\chi^2$ found at different seasons with different mean values of pressure represent similar levels of overall agreement between the model and the observations.
<figure><img src="image_6.png" /></figure>

**Figure 6.** Examples of the pressure evolution along two selected sols from REMS (blue) and MCD (red). Differences between REMS and MCD are shown as green lines in the lower panels. Dashed horizontal lines represent differences of ± 5 Pa which are considered to be significative. The example on panel a (left) represents a typical case of REMS-MCD good fit with a low value of $\chi^2$. The example on panel b (right) corresponds to a sol where REMS and MCD values do not match well resulting in a large value of $\chi^2$.

A systematic comparison of REMS and MCD pressure data using this method is shown in Figure 7. We also display the maximum and minimum pressure of REMS measurements and MCD values for each sol to show the behaviour of the pressure field. Values of $\chi^2$ have a mean value $\bar{\chi}^2 = 2.22$ Pa, and a standard deviation $\sigma$ of 1.48 Pa. These values represent the “mean difference” between the REMS and MCD pressure data and can be caused by a variety of effects including the temporal resolution of the data stored in MCD and the indirect way that the high-resolution mode of MCD incorporates the crater circulation. Sols where $\chi^2$ is greater than $\bar{\chi}^2+2\sigma$ (5.18 Pa) are exceptional and appear only in well-defined groups of sols. All together they represent about 4% of all sols. These cases are highlighted in orange in Figure 7 and represent extreme deviations from the MCD values that cannot be explained solely by the crater circulation not accounted in the MCD simulations or numeric errors due to the temporal interpolations between stored values in the MCD. Although not all the sols of each group have a $\chi^2$ higher than the limit used here, the few exceptions are isolated and the highlighted group of sols are well limited in their start and end by
high values of $\chi^2$. Other regions where $\chi^2$ is relatively large with $\bar{\chi}^2+\sigma<\chi^2<\bar{\chi}^2+2\sigma$ are easily visible (in yellow) in the figure and show a seasonal repeatability that will be discussed later.

<figure><img src="image_7.png" /></figure>

**Figure 7.** Comparison between REMS daily pressure and the MCD simulation for the location of the Gale crater after correction of elevation effects. Upper curves show the daily maximum (red) and minimum (blue) values of pressure. Solid thick curves correspond to REMS data and thin lines correspond to the MCD values. Dots show the $\chi^2$ of the REMS and MCD differences computed as explained in the text. The horizontal blue line shows the mean value of $\chi^2$ ($\bar{\chi}^2 = 2.22$ Pa) and the red line shows the limit for selecting anomalous pressure events where $\chi^2 > \bar{\chi}^2 + 2\sigma$ (5.18 Pa). Highlighted regions appear in orange for these events and in yellow for more moderate levels of $\chi^2$ that verify $\bar{\chi}^2+\sigma<\chi^2<\bar{\chi}^2+2\sigma$. Grey arrows indicate selected winter events discussed on section 5.5.

Note that regions highlighted in **Figure 7** also correspond in general to regions with the highest differences between the maximum and minimum values of pressure shown in Figure 2. Therefore both methods are consistent but the second method allows a better selection of differences between the observations and models. This method is the basis for our systematic selection of periods of interesting atmospheric activity.

### 3.5. Daily components of REMS and MCD pressure variations: Tides, local circulation and modifications by distant storms

Guzewich et al. (2016) decomposed daily pressure data from REMS over 875 sols in harmonic components using a Fourier transform of the daily data (for that purpose REMS data is transformed into average observations for each hour resulting in evenly spaced data in each sol).
They provide a physical description of the daily variations in terms of thermal tides of different periods: diurnal tides with a period of 24 hours, semi-diurnal with a period of 12 hours, terdiurnal with a period of 8 hours and quadidiurnal with a period of 6 hours. Each of these harmonic components obtained from analysis of observations from one location of the planet, consists in fact in the constructive or destructive sum of migrating tidal components (westward propagating and sun-synchronous), non-migrating tidal components (eastward and westward propagating and not sun-synchronous) (Guzewich et al., 2016; Wilson et al., 2017) with additional effects associated to the crater circulation (Haberle et al., 2014; Tyler and Barnes, 2013, 2015, 2017).

We here extend the methodology used by Guzewich et al. (2016) to the 1514 first sols analysing the first four harmonics for both REMS data and MCD simulations. The analysis of the temporal series was done assuming the pressure for each sol can be represented using a truncated harmonic series:

$$P(t) \approx \sum_{i=1}^{i=4} A_i \sin\left(n_i \frac{2\pi}{24}(t + \varphi_i)\right), \quad (5)$$

where $A_i$ are the amplitudes of each tidal harmonic, $t$ is the time given in hours in LTST, $\varphi_i$ are the phases of the different modes also expressed in hours and $n_i$ is the index of the different tide mode from 1 to 4.

Figure 8 shows the amplitudes $A_i$ of the different harmonic modes for REMS (points) and MCD simulations (lines) and the phases $\varphi_i$ of the different modes. Amplitudes of the different tides in REMS data and MCD simulations are generally similar and the differences concentrate in the regions where sharp changes in the REMS pressure values appear. Phases are mostly similar except for the terdiurnal mode, which is out of phase between REMS and MCD by about 3 hr which is close to half the period of the terdiurnal mode (8 hour). This means that this mode is close to be almost completely out of phase in the model with respect to the observed data.

Haberle et al. (2014) discuss the possible detection in the daily pressure amplitude of effects associated to the expected local circulation at the crater Gale. Daytime upslope flow along Mount Sharp and the walls of the crater rim should lower surface pressures with an opposite effect during nighttime raising surface pressure in a cycle that has the same phase as the diurnal tide enhancing the daily variation of surface pressure. The first and second harmonics in REMS pressure data and MCD show a similar (but not equal) behaviour except for the particular groups of sols selected
through this comparison. The comparison in Figure 8 of REMS pressure harmonics with harmonics obtained from MCD high-resolution allows to conclude that the model fits fairly well the overall behaviour of the daily oscillation. A clear demonstration of this point can be seen from the large differences in daily normalized pressure amplitudes in Figure 4 between the high-resolution and low-resolution modes of MCD. An in depth analysis of the global tides and local circulation effects at Gale crater is given by Wilson et al. (2017).

The episodes of REMS – MCD differences selected in the previous section are not accompanied by significant changes in the phase of the different tidal modes, only their amplitudes. Zurek (1981) and Wilson and Hamilton (1996) investigated thermal tides in the Martian atmosphere noting that diurnal migrating Kelvin waves are particularly responsive to the dust distribution (Wilson et al. 2008; Wilson et al. 2017). Zurek (1981) investigated how the dust storm forcing can affect pressure measurements at a remote site using Viking observations and Guzewich et al. (2016) using REMS observations compared with a model simulation of a distant dust storm. Haberle et al. (2014) further discussed a dust storm event identified through its signature in the harmonics shaping the daily surface pressure measured by REMS over the first 100 sols.
<figure><img src="image_8.png" /><figcaption>**Figure 8.** Amplitudes of the pressure harmonics and phases. Top panel: Amplitudes of the first four frequencies of the Fourier transform of the surface pressure. Lower panel: Phases (measured in LTST) for the first four frequencies of the Fourier transform of the surface pressure. The different harmonics are marked by colors: diurnal (blue), semidiurnal (cyan), terdiurnal (green) and quadidiurnal (red). In the two panels dots correspond to Fourier transform of REMS pressure data, and solid lines to the same Fourier transform of MCD pressure data. The discontinuity in phases of the terdiurnal mode is only apparent, as the green dots cross the phase values of 8 hr equivalent to the period of this mode. The lower part of both diagrams (magenta large dots and blue and red lines) are shown for comparison and taken from Figure 7. Arrows indicate selected winter events discussed on section 5.5.</figcaption></figure>
4. Dust opacity

Previous studies of Viking (Pollack et al., 1979; Pollack et al., 1993) and REMS pressure
data (Guzewich et al., 2016) have assessed the decisive role of atmospheric dust in enhancing the
amplitudes of daily pressure tides as a result of their effect on the atmospheric radiative heating and
cooling and the enhancement of the atmospheric temperature variations. In particular, Guzewich et
al. (2016) compared the amplitudes of the diurnal and semi-diurnal tides with the amount of dust at
Gale crater finding a nearly perfect correlation for both modes. The amount of dust in that work was
obtained from radiance measurements on sky images obtained by the Mastcam camera on MSL
using a methodology similar to that performed by the Mars Exploration Rovers (MER) (Lemmon et
al., 2015). Additional measurements of the temporal evolution of dust opacity at Gale crater have
been taken from published data from the REMS ultraviolet photodiodes (Smith et al., 2016)
providing similar data with a better time sampling (typically one value per sol). Further dust
measurements were obtained during the time covered by our study by the Mars Climate Sounder
(MCS; McCleese et al., 2007) instrument on MRO. However these measurements are obtained in
limb geometry in the direction of MRO's orbital motion "in-track" (Kleinböhl et al., 2009) or in
limb geometry at 90° "cross-track" (Kleinböhl et al., 2013) resulting in a low temporal resolution in
Gale crater to provide a good match to the REMS daily pressure data.

**Figure 9** shows the dust opacity measurements from Mastcam images and from modelling the values of the REMS UV photodiodes. The dust opacity measurements are shown in comparison with the dust optical depth given by the MCD climatologic scenario. The data are compared with the differences between REMS and MCD pressure data using the χ² values computed on section 3.2 and the amplitudes of the first two frequencies of the Fourier transform. There are three factors to note: (1) There is an overall good agreement between the amount of dust given by the climatologic scenario of MCD and the observations of dust at Gale crater. This is prescribed in MCD from the statistics of dust measurements over the planet over eight Martian years (MY24 to MY32 equivalent to data from April 1999 to July 2013 which slightly superimposes with the start of the MSL mission in August 2012) (Montabone et al., 2015). (2) Differences between the climatologic dust opacity and the measurements also concentrate in the regions previously identified and where the climatologic pressures differ from the measured values. (3) There is a strong correlation between pressure differences, the optical depth measured by REMS and Mastcam, and the amplitudes of the first two frequencies of the Fourier transform as previously reported by Guzewich et al. (2016) over a smaller data set.
Locally, dust variations could in theory affect the MCD prediction of the pressure in Gale crater if they significantly change the near surface atmospheric temperature compared to the MCD prediction. This temperature is used to interpolate pressure from the coarse GCM grid altitude to the detailed MOLA local altitude (much below in Gale Crater). If the near surface daytime atmosphere is dustier and colder than expected, then one would expect the high resolution MCD to underestimate the scale height, and thus overestimate the local surface pressure at the MSL landing site.

<figure><img src="image_9.png" /><figcaption>**Figure 9.** Comparison of pressure data with dust optical depth. Top: Amplitudes for the diurnal (blue dots) and semidiurnal (cyan dots) pressure tides. Middle: Dust optical depth from Mastcam (red circles) and REMS UV measurements (green circles) and the assumption in the MCD (continuous green line). Bottom: differences between REMS and MCD pressure values from the $\chi^2$ calculation in section 3.2 (magenta dots) and regions of interest (vertically shadowed regions). The blue and red lines are as in Figure 7. Arrows indicate selected winter events discussed on section 5.5.</figcaption></figure>

Differences in pressure values corresponding to high values of $\chi^2$ are correlated with differences in dust optical depth between the in-situ measurements and the climatologic model. In
some cases these differences appear in the same sol. However, in most cases the peaks of dust opacity over Gale crater appear some sols after the maximum values of $\chi^2$ over the pressure field. These apparent delays between the atmospheric pressure response and the dust at Gale crater are difficult to quantify with accuracy because in many sols there are no measurements of the optical depth while in a few others there is not enough pressure data to compute the value of $\chi^2$. A rough order of magnitude estimation of the time-scale of these delays is about 1-10 sols. We interpret these delays as the combination of the fast response of the pressure to regional dust storms through their effect on nearly resonant diurnal Kelvin waves (Wilson et al., 2008b, 2017) and the longer time it takes for dust from these storms to arrive to Gale.

# 5. Characterization of the high-dust and anomalous pressure periods

In this section we describe the events that generate the largest differences between MCD and REMS pressure data. In order to characterize the presence of dust or clouds (condensates) that could be related to the pressure discrepancies, we examined images of Gale crater obtained by the MARCI instrument. A systematic analysis of MARCI Weather Reports (available at

http://www.msss.com/msss_images/subject/weather_reports.html) guided this analysis. Selected periods of time were analyzed by downloading MARCI raw images from PDS-NASA and navigating and calibrating the images using the Integrated Software for Imagers and Spectrometers (ISIS). Events caused by dust and water clouds are discussed separately.

## 5.1. Dust events

The six regions highlighted in orange in Figures 7 to 9 concentrate the strongest differences between REMS and MCD pressure data. The first event corresponds to the dust event identified in early analysis of REMS pressure data by Haberle et al. (2014). Analysis of MARCI images shows that in each of these six events, local and regional dust storms can be responsible of the pressure differences between REMS and MCD. No condensate clouds were detected in MARCI images over Gale crater during these events and all of them occurred during the dust storms Martian period (Ls~180°-360°; Kahre et al., 2017). We also tested the possibility that some of these events could be caused by local upload of dust from dust devil activity at Gale crater comparing the data in Figure 9 with the frequency of dust devils and convective vortices determined in Ordonez-Etxeberria et al. (2018). This investigation showed no significant correspondence implying that enhancements of
dust at Gale crater are caused by nearby dust storms and no from local dust devil activity.

The mean characteristics of these events, as well as the region where a possible source of dust is observed in MARCI images causing the pressure variation are summarized in Table 1.

<table><caption>**Table 1.** Summary of events analysed in this work. Notes: <sup>a</sup>Optical depth from REMS UV data. <sup>b</sup>Optical depth from Mastcam images. Events are grouped with either a * or a ** symbol to indicate seasonal similarities.</caption><thead><tr><th>Event</th><th>Period [sol]</th><th>Period [L<sub>s</sub>]</th><th>Duration (Sols)</th><th>Date (first sol) yy/mm/dd</th><th>Sol [highest τ]</th><th>Highest τ</th><th>Dust storm location</th><th>Distance (km)</th></tr></thead><tbody><tr><td>1*</td><td>83-104</td><td>197.7-210.6</td><td>21</td><td>12/10/30</td><td>109</td><td>1.33<sup>a</sup></td><td>Hellas</td><td>4,000</td></tr><tr><td>2**</td><td>258-274</td><td>308.8-318.5</td><td>16</td><td>13/04/27</td><td>270</td><td>1.31<sup>b</sup></td><td>Chryse - Acidalia</td><td>10,000</td></tr><tr><td>3*</td><td>754-773</td><td>198-210.9</td><td>19</td><td>14/09/19</td><td>801</td><td>1.62<sup>a</sup></td><td>Utopia</td><td>3,500</td></tr><tr><td>4</td><td>852-857</td><td>261.9-266.4</td><td>5</td><td>14/12/29</td><td>853</td><td>1.65<sup>a</sup></td><td>Elysium Mons</td><td>1,800</td></tr><tr><td>5**</td><td>938-969</td><td>315-334.6</td><td>31</td><td>15/03/28</td><td>977</td><td>1.33<sup>a</sup></td><td>Tempe and Acidalia</td><td>10,000</td></tr><tr><td>6*</td><td>1411-1454</td><td>190.9-220</td><td>43</td><td>16/07/25</td><td>1480</td><td>1.45<sup>b</sup></td><td>Utopia and Isidis Planitia</td><td>2,500</td></tr></tbody></table>

Some of these dust events occur at almost the same L<sub>s</sub> and show a similar pattern in the pressure behaviour (events 1, 3 and 6 in early Spring and events 2 and 5 in Summer). We focus our discussion on the descriptions of event 3 as representative of early Spring events, event 5 as representative of Summer dust storms, and event 4, which is unique and different to other cases.

## 5.2. Early Spring events: Event 3

Both, MCD and MSL data show that in early southern Spring the local conditions at Gale crater include an increase of dust optical depth in agreement with the predicted start of the dust season (Kahre et al., 2017). MARCI images and weather reports for early Spring show the development of different dust storms.

Event 3 presents common characteristics with events 1 and 6, although the source of the dust arriving at Gale crater is different in each case. In the dates of event 3 a prominent dust storm develops northward of Utopia in the north latitudes at about 3,500 km from MSL. The dust storm starts on sol 754 (19 September 2014; Figure 10a) and spreads over the next few sols sending dust material towards equatorial latitudes and largely dissipating by sol 760 (Figure 10b). However, it is not clear how much dust may enter Gale crater from this storm (Figure 10b) since the overall dust at
Gale crater follows its expected evolution over that period of time (Figure 9). Figure 11 shows the daily REMS pressure measurements and MCD values for these sols. Pressure differences are accentuated at the time the storm develops and concentrate in the afternoon hours. The differences decrease gradually as the storm dissipates.
<figure><img src="image_10.png" /><figcaption>**Figure 10.** MARCI images of dust storm related to event 3. Maps are based on the superposition of several MARCI images. a) Dust storm between Deuteronilus and Utopia Planitia on sol 754 (19 September 2014). b) Some sols later the dust lifted by the storm has dissipated through large areas including the equatorial latitudes and Gale crater (highlighted with a white circle) on sol 760 (25 September 2014).</figcaption></figure>
<figure><img src="image_11.png" /></figure>

**Figure 11.** REMS Pressure measurements (blue) and MCD values (red) on event 3. a) Sol 754 coincident with the onset of the dust storm and characterized by sudden pressure discrepancies between measurements and MCD expectations. b) Sol 760 when the storm has largely spread still contains significant differences between REMS and MCD values but with a lower and diminishing value of $\chi^2$. Differences between REMS and MCD pressure values are given in the bottom panels. Dashed lines represent differences of ±5 Pa.

A sudden change in the behavior of the pressure can also be observed some sols after the event and not accompanied by high values of $\chi^2$. In the great majority of sols the daily pressure maximum is found close to dawn at 8.00 LTST (Figures 6 and 11). However in events 1, 3 and 6 the maximum daily pressure moves to 0:00 to 3:00 LTST a few sols after the event. Figure 12 examples this behavior for sol 787 which occurs after the pressure event 3. Figure 13 shows this variation of daily maximum pressure as function of time presenting the difference between the peak pressure during the first hours of each sol (~0–3 LTST) and the peak close to dawn (~6–9 LTST). In most cases this difference is negative but it becomes positive in three well defined cases corresponding to the late stages of events 1, 3 and 6. The fact that these changes are accompanied by sharp increases in the dust opacity measured in Gale crater from Mastcam and REMS UV (Figure 13) makes us hypothesize that this behavior corresponds to the arrival of dust over Gale crater. This kind of quick changes in the tide phase is consistent with changes in the interference
pattern between the migrating tide and the diurnal Kelvin wave (Wilson and Hamilton, 1996).
Examples of transient Kelvin wave amplification are shown in Wilson et al. (2008b).

<figure><img src="image_12.png" /></figure>

**Figure 12.** REMS pressure measurements (blue) and MCD values (red) after event 3 with a displacement of maximum pressures towards midnight. Differences between REMS and MCD pressure values are given in the bottom panel. Dashed lines represent differences of ±5 Pa.

<figure><img src="image_13.png" /></figure>

**Figure 13.** Temporal behaviour of daily pressure local maximums compared with dust measurements. Blue line (left axis) shows the difference between the peak pressure during the first hours of each sol (LTST ~ 0 – 3 hr) and the
peak close to dawn (LTST~6-9 hr). The upper dots and curve show the dust opacity in Gale crater (right axis). Sharp increases in the local dust abundance tend to produce less negative values of this pressure difference and three significant cases are highlighted in the figure close to events 1, 3 and 6.

Therefore, this event 3 seems to be caused by the stochastic behavior of normal dust storms that develop in this season increasing the regional dust in discrete events and the local dust more gradually. While the pressure predicted, and the dust abundance data used in MCD overall agree with REMS observations, the spikes of activity caused by individual storms explain the differences between model and data. Dust arrival to Gale crater might be accompanied by a migrating maximum daily pressure towards midnight from the typical daily maximum location at the end of the night. This behavior is common to the three early spring events (events 1, 3 and 6 in table 1).

### 5.3. Summer dust storm events: Event 5

Events 2 and 5 occur close to the mid-Summer in the Southern hemisphere and are accompanied by high values of dust opacity not present in the MCD climatology. We use event 5 as an example of the behavior observed in both cases.

Event 5 on table 1 extends from sol 938 to 969 and $L_s$ from 315° to 335° and is in fact a double event in which differences between REMS and MCD concentrate at the start and end of this period (event 2 is a single event very similar to each of these two cases). The temporal location of REMS and MCD discrepancies is not coincident with the presence of dust storms close to Gale crater. Instead, an inspection of MARCI weather reports (Malin et al., 2015) shows that a large dust storm develops in Acidalia (almost at the opposite equatorial location of the Gale crater), crosses the equator through Cryse, and expands across Margaritifer, Solis, Aonia, and the Argyre Basin still thousands of kilometers away from Gale crater. Figure 14 shows the dust storm at its peak of activity on sol 938 (March 29, 2015). On sol 953 REMS and MCD pressures are again very similar. A few sols later, on sol 968 REMS and MCD data differ significantly again following the onset of a new distant dust storm that started on sol 966 (April 25, 2015) and covered large parts of Tempe and Acidalia planitites. MCD and REMS data become similar again on sol 969. After that, the dust opacity at Gale crater as measured by Mastcam and REMS UV data increases significantly from sol 971 to 994 without resulting in large discrepancies in the pressure signals.

Figure 15 show characteristic daily pressures during this event. Extreme differences between pressure measurements and MCD climatic values are found well correlated with the dust optical
opacity shown in Figure 9 with diminishing REMS-MCD differences for lower amounts of dust opacity.

<figure><img src="image_14.png" /></figure>

**Figure 14.** A large dust storm covering large portions of the equator from Acidalia to Argyre Basin observed from MARCI on sol 938 (March 29, 2015). In spite of its distance to Gale crater this large dust storm seems related to the pressure event 5.
<figure><img src="image_15.png" /><figcaption>**Figure 15.** Pressure differences between REMS and MCD pressure data during the double event 5. (a) Sol 934 just before the onset of the storm in Acidalia; (b) Sol 940 with a fully developed dust storm; (c) Sol 953 with a recovery of the normal situation and (d) Sol 968 at the time of the second dust storm. Colors are as in Figure 6.</figcaption></figure>
5.4. A localized small and nearby dust storm: Event 4

The shortest pressure event found in our pressure analysis corresponds to a small group of
sols from 852 to 857 at the transition from Spring to Summer in the South hemisphere and without
a clear seasonal repeatability. We warn however that the same seasonal period is not covered well
enough with REMS data from the previous or later Martian year. Guzewich et al. (2016) briefly
described this event and concluded that the dust storm had an immediate effect in the amplitude of
the diurnal tide and a small impact in the semidiurnal tidal response of the atmosphere. MARCI
images (Malin et al., 2014b) show the development of a modest regional dust storm on sol 852
(December 29) about 1,800 km north of Gale crater. The storm develops west of Elysium Mons and
expands southwards in a few sols as shown in Figure 16. The storm injects dust inside Gale crater in
a time scale of a single sol after its onset as it can be seen on Figure 16 and Figure 9 where a clear
and short increase of the dust opacity values found by MSL Mastcam and REMS UV sensors is
found for sol 853. Figure 17 shows the evolution of REMS and MCD pressure data for these sols.
When the storm develops on sol 852 still far from Gale crater a pressure signal is detected in the
REMS-MCD analysis. This suggests that analysis of pressure signals and comparisons with models
may detect the onset of nearby dust storms earlier than the analysis of orbital data.
<figure><img src="image_16.png" /></figure>

**Figure 16.** a) Regional dust storm developing close to Gale crater west of Elysium Mons on sol 852 (29 December 2014). b) A single sol later the dust lifted by the storm has traversed the equatorial latitudes injecting dust inside Gale crater. The position of Gale crater is highlighted with a white circle.
<figure><img src="image_17.png" /><figcaption>**Figure 17.** Pressure differences between REMS and MCD pressure data during event 4. Sols selected show the normal situation on sol 851 (a); quick onset of the pressure perturbation on sols 852 (b) and 853 (c) and a recovery of the normal situation on sol 857 (d). Colors are as in Figure 6.</figcaption></figure>
## 5.5. Water clouds and Winter Events

We now discuss southern Winter events that appear inside the yellowish highlighted regions in Figures 7 to 9. These yellow regions correspond to slightly lower levels of discrepancies between REMS and MCD and seem linked to seasonal effects. There are two particular periods of time where the discrepancy between REMS and MCD minimizes quickly and for relatively short periods of time. These are the events highlighted with arrows on Figures 7 to 9. Their temporal duration and main characteristics are summarized on Table 2. Both arise at the end of the season of equatorial clouds which roughly extends from Ls=60 to Ls=120.

<table><caption>**Table 2.** Summary of winter events analysed in this work.</caption><thead><tr><td>Event</td><td>Period [sol]</td><td>Period [L<sub>s</sub>]</td><td>Duration (sols)</td><td>Date (central date) yy/mm/dd</td></tr></thead><tbody><tr><td>WE1</td><td>650-675</td><td>141.4-154.47</td><td>25</td><td>14/06/04</td></tr><tr><td>WE2</td><td>1323-1348</td><td>143.7-156.9</td><td>25</td><td>16/04/26</td></tr></tbody></table>

Inspection of MARCI images for these periods of time shows that in the two events the atmosphere over Gale crater is relatively clear without dust or clouds above it. However, thick clouds sometimes pass over Gale crater and can be observed on MARCI images. The passing of these clouds seem to improve the match between REMS and MCD with clouds on sols 661 and 1331 being particularly thick over the locations of MSL (Malin et al., 2014a, 2016). We use WE2 as an example of both events. Figure 18 shows images of Gale crater just before and in the middle of this event showing first clear sky conditions, and later clouds passing above the crater. Figure 19 shows the pressure behavior for this event for the same sols suggesting some correlation with the presence of thick clouds above Gale crater minimizing differences between REMS and MCD data.

Clouds are difficult to produce accurately in any GCM but they play a key role in the radiative transfer of the Martian atmosphere, impacting its thermal structure, its circulation, and the water cycle itself. Clouds tend to radiatively warm the atmosphere during daytime (Wilson et al. 2008a, Madeleine et al. 2012) and should thus enhance thermal tides playing a similar role as the one played by dust. The version of the LMD GCM used to build the MCD 5.2 contains recent improvements in the water cycle (Navarro et al. 2014). A comparison of the clouds produced by the model and cloud opacities measured by the TES instrument can be seen in Figure 2 in Navarro et al. (2014) and shows that the LMD GCM produces fewer clouds at the end of the aphelion cloud season than what the observations show. Thus, MCD results should be more representative of
clearer sky conditions and the Winter Events present an interesting puzzle that cannot be solved
solely on the basis of a direct comparison with the MCD. Instead, a close look to Figure 7 shows
that both events are characterized by a drop of the daily maximum pressure. This suggests that the
observed clouds appear as a consequence of dynamics not fully accounted for in the model with
upwelling and adiabatic cooling lowering surface pressures. Thus, the observed clouds are probably
a consequence of dynamics and not causing the pressure drops. The time-scale of these events (25
sols) also points to dynamics acting at a synoptic scale. So why does a model that underestimates
the cloud abundance fit better the observations when clouds appear in Gale? Locally, clouds as
those observed above Gale for these two events can affect the surface and near surface temperature,
and in particular limit the night time cooling resulting in higher night time pressures (Wilson et al.
2007). As for dust (see Section 4), if clouds are too thin in the MCD, the night time scale height will
be underestimated and the surface pressure at the bottom of Gale crater overestimated.

When clouds pass above the crater both the model and observations are better reconciled,
most likely for their effect on the local scale height as discussed above. However, the detectability
of passing clouds through their possible signature in a pressure station is not clear and may require a
comparison with improved models. A more detailed study of the detectability of clouds from
pressure data could be done by comparing with MSL observations of water-ice clouds recently
reported by Kloos et al. (2018). An alternative explanation is that the events detected might be a
particularly high amplitude pressure perturbation from baroclinic waves as both events appear in
sols characterized by large deviations from REMS smooth data (see Figure 7 in this paper and
Figure 2 in Haberle et al., 2018). In that case clouds could still be produced by the dynamic
perturbation caused by the wave.
<figure><img src="image_18.png" /></figure>

**Figure 18.** MARCI images of Winter Event 2 (WE2). a) Image of the Gale crater on sol 1317 (20 April 2016) with clear skies. b) Gale crater on sol 1334 (7 May 2016) with clouds from the equatorial belt over the crater.
<figure><img src="image_19.png" /><figcaption>**Figure 19.** Pressure differences between REMS and MCD pressure data during Winter event 2. a) Sol 1317 with clear sky conditions. b) Sol 1334 with clouds above Gale crater and a better match between MCD and REMS pressure data. Colors are as in Figure 6.</figcaption></figure>

**6. Summary and conclusions**

We compared REMS pressure observations corresponding to more than 2 Martian years (1514 sols) with climatic values of pressure obtained from the MCD for the same $L_s$ and location of the MSL rover. This comparison has the potential to detect interesting meteorological features in terms of the effects caused by nearby regional dust storms, local dust abundance and passing water clouds. REMS pressure daily data and pressure values from MCD at the MSL location overall fit well during the 1514 sols examined validating the climatologic values contained in the MCD. Significant differences were concentrated in six groups of sols with apparent seasonal repeatability. These group of sols generally present higher abundances of dust than what it would be expected from average climatic values. Part of the dust is produced by discrete numbers of regional dust storms. A comparison of the pressure data with values of dust opacity found at Gale crater from the MSL Mastcam instrument and the REMS UV photodiodes was also presented. The sources of dust
for each event were explored by closely inspecting MARCI images available online as Mars Weather Reports and the most interesting cases were selected and navigated at high resolution.

Here follow our main conclusions from this work.

* REMS pressure data is well reproduced by the high-resolution MCD operating in standard conditions. The high-resolution MCD results are extrapolated from model simulations at a lower spatial resolution using an atmospheric scale height based on the low-resolution MCD temperature at 1 km above the surface. The selection of *H* at 1 km gives a value that mimics effects of slope winds in most locations including the crater circulation at Gale. This tuning of the pressure extrapolation designed to mimic the effect of slope winds in most locations seems to work properly, and, at least at Gale crater, well accounts for the effects of the crater circulations. A comparison of MSL dust opacity values derived from Mastcam images (Smith et al., 2016) and REMS UV data (M. Lemmon, personal communication) with the dust abundance included in the MCD (Montabone et al., 2015) also shows a very good agreement in Gale crater.

* Differences between REMS and MCD pressures are found in discrete groups of sols. Comparison with orbital images of the planet and dust measurements obtained by REMS suggest that some of these groups of sols are characterized by variations of dust abundance inside Gale crater, while others events occur at the same time as the development of large regional dust storms which can be located even at distances as large as 10,000 km. This suggests that dust storms, even distant ones, leave measurable traces in the pressure data that can be used to identify dust storm activity. Guzewich et al. (2016) show simulations of how dust storms may affect the pressure measurements obtained in Gale crater and Wilson et al. (2017) discuss the physical mechanisms for such an action at distance through nearly resonant diurnal Kelvin waves.

* In the South winter season, at the end of the Martian Aphelion cloud belt, MCD and REMS pressure data present small differences that decrease in events of a duration of about 25 sols when water clouds pass above Gale crater. Because of this temporal duration these events could be linked to global dynamics changes in the cloud belt at a synoptic scale. The interpretation of the effect of water ice clouds on a ground pressure station is not clear and may merit further research and at least another possibility of explaining these events in terms of baroclinic waves (Haberle et al., 2018) cannot be disregarded.
* The detailed pressure response at the surface of Mars to pressure tides, local and distant dust storms and cloud systems depends on the season and particular location on Mars surface. An examination of pressure data collected by a single pressure sensor cannot fully disentangle the combined effects of all these factors even with the help of existing models. However, a network of pressure sensors on Mars surface like those on the Mars Science Laboratory, Insight and Mars 2020 compared with model simulations like those available on MCD might disentangle their combined effects.

**Acknowledgements:** We are grateful to the REMS and MSL teams for their work in this mission and in particular to M. Smith and M. Lemmon for their derivation of dust optical opacities used in this work. We are also grateful to G. Martínez for providing pressure data of previous missions on Mars and to Dan Tyler and John Wilson for insightful reviews of this manuscript that greatly enhanced the manuscript. This research has made use of the USGS Integrated Software for Imagers and Spectrometers (ISIS). This work was supported by the Spanish project AYA2015-65041 (MINECO/FEDER, UE) support, Grupos Gobierno Vasco IT-765-13 and UPV/EHU UFI11/55.
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