# Mars perihelion cloud trails as revealed by MARCI: Mesoscale topographically focused updrafts and gravity wave forcing of high altitude clouds

R. Todd Clancy, Michael J. Wolff, Nicholas G. Heavens, Philip B. James, Steven W. Lee, Brad J. Sandor, Bruce A. Cantor, Michael C. Malin, Daniel Tyler, Aymeric Spiga

► To cite this version:

R. Todd Clancy, Michael J. Wolff, Nicholas G. Heavens, Philip B. James, Steven W. Lee, et al., Mars perihelion cloud trails as revealed by MARCI: Mesoscale topographically focused updrafts and gravity wave forcing of high altitude clouds. Icarus, 2021, 362, 10.1016/j.icarus.2021.114411. insu-03726941

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https://insu.hal.science/insu-03726941v1

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# Mars Perihelion Cloud Trails as revealed by MARCI: Mesoscale Topographically Focussed Updrafts and Gravity Wave Forcing of High Altitude Clouds

R. Todd Clancy<sup>a</sup>, Michael J. Wolff<sup>a</sup>, Nicholas G. Heavens<sup>a</sup>, Philip B. James<sup>a</sup>, Steven W. Lee<sup>a</sup>, Brad J. Sandor<sup>a</sup>, Bruce A. Cantor<sup>b</sup>, Michael C. Malin<sup>b</sup>, Daniel Tyler Jr.<sup>c</sup>, Aymeric Spiga<sup>d</sup>

## Abstract

Daily, global wide angle imaging of Mars clouds in MARCI (MARs Color Imager, (Malin et al., 2008)) ultraviolet and visible bands reveals the spatial/seasonal distributions and physical characteristics of perihelion cloud trails (PCT); a class of high altitude (40–50 km), horizontally extended (200–1000 km, trending W to WSW) water ice clouds formed over specific southern low-to-mid latitude (5S-40S), mesoscale (~50 km) locations during the Mars perihelion, southern summer season. PCT were first reported in association with rim regions of Valles Marineris (Clancy et al., 2009). The current study employs MARCI 2007–2011 imaging to sample the broader distributions and properties of PCT; and indicates several distinct locations of peak occurrences, including SW Arsis Mons, elevated regions of Syria, Solis, and Thaumasia Planitia, along Valles Marineris margins, and the NE rim of Hellas Basin. PCT are present over Mars solar longitudes ($L_s$) of 210–310°, in late morning to mid afternoon hours (10am-3pm), and are among the brightest and most distinctive clouds exhibited during the perihelion portion of the Mars orbit. Their locations (i.e., eastern margin origins) correspond to strong local elevation gradients, and their timing to peak solar heating conditions (perihelion, subsolar latitudes and midday local times). They occur approximately on a daily basis among all locations identified (i.e., not daily at a single location). Based on cloud surface shadow analyses, PCT form at 40–50 km aeroid altitudes, where water vapor is generally at near-saturation conditions in this perihelion period (e.g. Millour et al., 2014). They exhibited notable absences during periods of planet encircling and regional dust storm activity in 2007 and 2009, respectively, presumably due to reduced water saturation conditions above 35–40 km altitudes associated with increased dust heating over the vertically extended atmosphere (e.g., Neary et al., 2019). PCT exhibit smaller particle sizes ($R_{eff}=0.2–0.5 \mu m$) than typically exhibited in the lower atmosphere, and incorporate significant

<footnote><sup>a</sup>Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA</footnote>

<footnote><sup>b</sup>Malin Space Science Systems, 5880 Pacific Center Blvd, San Diego, CA 92121, USA</footnote>

<footnote><sup>c</sup>College of Earth Oceanic and Atmospheric Science, Oregon State University, Corvallis, OR 97331, USA</footnote>

<footnote><sup>d</sup>Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Universités, UPMC Univ Paris 06, PSL Research University, France</footnote>
fractions of available water vapor at these altitudes. PCT ice particles are inferred to form continuously (over ~4 hours) at their PCT eastern origins, associated with localized updrafts, and are entrained in upper level zonal/meridional winds (towards W or WSW with ~50 m/sec speeds at 40–50 km altitudes) to create long, linear cloud trails. PCT cloud formation is apparently forced in the lower atmosphere (≤10–15 km) by strong updrafts associated with distinctive topographic gradients, such as simulated in mesoscale studies (e.g., Tyler and Barnes, 2015) and indicated by the surface-specific PCT locations. These lower scale height updrafts are proposed to generate vertically propagating gravity waves (GW), leading to PCT formation above ~40 km altitudes where water vapor saturation conditions promote vigorous cloud ice formation. Recent mapping of GW amplitudes at ~25 km altitudes, from Mars Climate Sounder 15 µm radiance variations (Heavens et al., 2020), in fact demonstrates close correspondences to the detailed spatial distributions of observed PCT, relative to other potential factors such as surface albedo and surface elevation (or related boundary layer depths).

# 1. INTRODUCTION

A new class of distinct Mars clouds (e.g., see Clancy et al., 2017) was reported in spatial association with Valles Marineris and in temporal/spatial association with peak solar heating of the surface on Mars (Clancy et al., 2009). These clouds exhibit unusual conditions of high formation altitudes (40–55 km), which leads to their entrainment in strong upper level easterly flow to form long linear E-W trails, coupled with local correspondences to mesoscale dynamical conditions. Their southern locations of commonly (but not exclusively) high surface elevations and their perihelion season of maximum solar insolation often encompass peak boundary layer heights on Mars (Hinson et al., 2008; Spiga et al., 2010; Hinson et al., 2019), but their formation altitudes extend well above direct convective forcing altitudes. Nevertheless, their distinct locations of reoccurrence and correlation with peak surface heating (in solar longitude, $L_s$; local time- LT; and sub-solar latitudes) suggest they are forced by local -scale (of order 100 km) regions of extraordinary updrafts.

The current study significantly expands the defined spatial and temporal distributions of these clouds, including distinctions associated with dust storm activity in different Mars years (MY, as defined in Clancy et al., 2000). Although elevated regions bounding Valles Marineris remain especially prominent for cloud frequency and cloud brightness/linear extent, a number of other southern low latitude regions exhibit these clouds. Hence these cloud features are re-designated as perihelion clouds trails (PCT) in this paper to emphasize their broader context with regional dynamical characteristics under peak heating conditions. This broader study includes a much larger population of clouds identified for spatial (latitude, longitude) and temporal ($L_s$, LT, MY) behaviors and distributions, as well as approximate altitudes and cloud opacities and particle sizes. While specific mesoscale simulations are not conducted for PCT formation mechanisms, existing mesoscale model simulations pertaining to enhanced local regions of updraft are discussed. The formation of the clouds at vapor saturation levels well above peak updraft altitudes are attributed to gravity waves (GW) generated by the updrafts. Such a connection to GW forcing is corroborated by recent MCS (Mars Climate Sounder, McCleese et al., 2007) mapping of ~25
km altitude GW amplitudes (Heavens et al., 2020), which present significant spatial correspondences to PCT spatial distributions.

MARCI (MARs Color Imager) wide angle imaging from the Mars Reconnaissance Orbiter (MRO) is employed to characterize the broad spatial/temporal distributions of these long, linear cloud forms, based upon the MARCI capabilities of daily and global imaging coverages in 2 ultraviolet (260, 320 nm) and 5 visible (435, 545, 605, 655, 720 nm) bands (Malin et al., 2008; Bell et al., 2009). These MARCI capabilities enabled Wolff et al. (2019) to retrieve global distributions of Mars cloud optical depths on a daily basis over MY28–35 (2006–2019). The wide angle imaging is optimum, among existing Mars imaging datasets, for determinations of PCT spatial/temporal distributions (the current study indicates ~1 PCT globally per Mars sol over the perihelion season). The UV-visible wavelength coverage supports cloud particle size determinations from wavelength-dependent cloud opacity determinations (from cloud ground shadows, which also indicate cloud altitudes). The multiple 100’s of km ENE-to-WSW extents for these linear cloud forms are optimally defined by the ~1500 km field of view (fov, in longitude, continuous in latitude) provided by MARCI imaging.

In addition, context on the background dust, temperature and water vapor conditions present during the formation of PCT is provided by comparisons to contemporaneous (from MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM, Murchie et al., 2007) limb observations (Clancy et al., 2019), Mars Climate Sounder (MCS, McCleese et al., 2007) gravity wave amplitude measurements (Heavens et al., 2020), and Mars Climate Database (MCD, Millour et al., 2014) modeled atmospheric conditions.

## 2. MARCI IMAGING OBSERVATIONS

MARCI daily global imaging began in late 2006 (MY28) and continues to the date of this publication (MY35). As such, it continues to extend the unique global imaging of Mars surface and atmospheric processes begun with the Mars Orbiting Camera (MOC- Malin and Edgett, 2001; Cantor et al., 2002) on the Mars Global Surveyor (MGS) in 1999 (MY24). MARCI imaging characteristics and initial science results are given by Bell et al. (2009) and Malin et al. (2008). Key aspects regarding the current PCT study are the daily global imaging coverage, albeit with coarse spatial resolution (~ 1 and 8 km for visible and UV channels, respectively), and the multi-wavelength coverage as indicated above. MARCI nadir imaging is obtained continuously along its near-polar orbit path and generates contiguous longitudinal coverage between adjacent image strips, although this overlap corresponds to a time difference of ~2 hours (112 minute MRO orbit) and the considerable foreshortening (i.e., reduced spatial resolution) and increased atmospheric path lengths associated with larger emission angles. The fixed local time (~3 pm at low latitude image centers) of the MRO (nearly) sun-synchronous polar orbit supports a consistent data set for atmospheric cloud studies (Wolff et al., 2019), an important consideration given the substantial diurnal variations exhibited by Mars clouds (Smith and Smith, 1972; Michelangeli et al., 1993; Benson et al., 2003; Wilson et al., 2007). The large MARCIfov further provides ~ ±45 minutes LT sampling about the center of eachfov image strip, which
provides a coarse indication of the LT behavior of Mars PCT, particularly when placed in the context of observed PCT longitudinal lengths (i.e., estimated lifetimes).

The current study primarily regards 2007–2011 MARCI observations of PCT, in part to demonstrate their sensitivity to distinct perihelion dust storm conditions during the MY28 (2007) planet encircling dust storm versus the MY29 (2009) more common regional dust storm perihelion seasons (broadly defined here as $L_s=200–310°$). This limited MY range of analyzed MARCI observations reflects the time consuming visual identifications of PCT in daily global MARCI imaging. Nevertheless, it provides a representative sampling of the broad spatial/temporal distributions of PCT, which is the focus of the current study. Clancy et al. (2009) presented a small set of PCT observed in 2007 ($L_s=230–250°$) MARCI images, as well as one MOC imaged PCT from 2005. The identified PCT in this study were primarily limited to two specific regions bordering Valles Marineris. In the current study, MARCI daily global image maps in UV band 7 (320 nm) are employed to characterize daily, global PCT occurrences. The smaller (64x fewer pixels) UV image sizes are more manageable to work with in daily image format and provide optimum cloud surface image contrast due to the very low UV surface albedo of Mars (variable, but mean $A_{Lambert}$$\sim$0.017 at 320 nm; Wolff et al., 2019). For a small subset of brighter PCT (n~30), a multi-band (260, 320, 435, 545, 605 nm) analysis of eastern margin PCT surface shadows are analyzed for cloud altitudes, optical depths, and particle sizes.

## 2.1. Visible PCT Imaging

Figure 1 presents a set of Mars PCT observed in regions away from the classic Valles Marineris locale for PCT. Generally, the Valles Marineris region exhibits PCT with the highest frequency and brightness ($τ$), but they are also prominent on the southwest flanks of Arsia Mons (Figure 1a,b), the elevated Syria and Solis (figure 1c,d) Planitiae, and the northeastern highlands bordering Hellas Basin (figure 1e,f). In nearly half of observed PCT, they occur as a set (2–5) of closely spaced (within 50–100 km), parallel (staggered N-S) linear clouds (Figure 1a,b and 1c,d). PCT generally exhibit distinct brightness variations along their long axis, which is oriented from ENE to WSW. As discussed later, this orientation aligns with the direction of the upper level (above ~40 km) circulation in the perihelion period at low-to-mid southern latitudes. Infrequently, PCT display large-scale ‘outbreaks’ along N-S axes extending over 10’s of degrees in latitude, suggesting correspondence with planetary-scale waves or fronts. This behavior may also be reflected in suggested 2–3 day periodicities in cloud frequency. All of the PCT presented in figure 1 are exhibited in the western portions of the MARCI images, corresponding to local times (LT) less than the 3 pm image center LT. This behavior is shown to be typical of PCT imaged by MARCI. Many of the PCT presented Figure 1 also exhibit cloud shadows offset from their eastern margin origins, an aspect of the high altitudes of PCT and solar incidence angles associated with the 3 pm LT of MARCI imaging. It is these shadows that allow coarse determinations PCT cloud heights, optical depths, and particle sizes, when they are adequately displayed over multiple MARCI filter images.

The set of PCT over the SW flanks of Arsia Mons on March 22 of 2011 ($L_s = 259.3°$ of MY30) are presented in mapped coordinates from the MARCI violet image (Figure 1a), and
in color (Figure 1b) as constructed from MARCI violet, green, and red filter images. The western margins of these PCT extend to the edge of the MARCI wide-angle field of view (fov), although their contrast is reduced by the ~60° projection angles at the MARCIfov margin that degrade spatial resolution and cloud contrast against the lower atmosphere and surface. The full longitudinal extents of these PCT are at least 700 km from their eastern margin origin, which places these clouds among the longer such PCT measured by MARCI. As indicated above and discussed later in this paper, PCT are generally identified in the western, earlier afternoon LT sampled by MARCI imaging (e.g., 2–3pm). Hence, observed PCT lengths are often limited to ~500 km by the MARCI western-halffov. Larger PCT lengths are observed to exist, but additional factors (such as cloud particle lifetimes) tend to degrade PCT brightnesses away from their eastern margin origins (as apparent in Figure 1a,b).

Figure 1c,d presents a set of shorter length PCT over southern Solis Planum, the brightest one exhibiting a full surface shadow. PCT southward of 30S are uncommon, appearing over the eastern margins of the Argyre and Hellas basins (figures 1e,f) as well as the southern boundary of Solis Planum (as in figures 1c,d). The relatively short lengths of these PCT (~150 km) reflect their recent initiations relative to the MARCI imaging. Images of the initial plume formations for PCT do not reveal their characteristic linear forms and so are less easily recognized as such clouds (see Figure 2c). Figure 1e,f presents lee wave characteristics in a PCT imaged above highlands associated with the NE margins of Hellas basin. PCT generally show brightness variations along their horizontal extent, although rarely with as distinctive a wave morphology as seen in figure 1e,f.

## 2.2. UV PCT Imaging

Figure 1 presents image characteristics of PCT identified in violet and color (RGB) image maps. As indicated above, such MARCI visible filter images yield surface resolutions of order 1 km, whereas the MARCI UV (260, 320 nm) filter images generally exhibit 8 km surface resolutions. UV and violet MARCI images yield the best cloud-surface contrast for PCT identifications. We adopt MARCI 320 nm daily global map images for the purposes of defining PCT spatial/temporal distributions, due to their higher signal/ratios relative to 260 nm imaging and manageable daily map image sizes relative to MARCI violet (435 nm) imaging. Figure 2 presents a set of PCT identifications in MARCI 320 nm daily global image maps. In the top panel, classic PCT forms over southeastern Valles Marineris and northeastern Hellas rim are apparent in a daily global image map from April 20, 2009. In the middle panel, a newly formed PCT over Arsia Mons appears in the center image strip and in its extended growth state ~2 hr later in the adjoining western (left) image strip. PCT are also apparent in the lower right, over the high southwest ridge bounding Solis Planum. The lower panel of figure 2 presents PCT over the northeast Hellas rim on May 20, 2011, and a potential PCT plume initiating over the Solis Planum southwest ridge (on May 13, 2011, circled on figure). Notice that the Hellas PCT shown in the lower left image is the same PCT presented in MARCI violet and color images of figures 1e,f. The low resolution UV PCT images of figure 2 are provided to indicate the degree to which PCT can be identified in terms of location, size, and times within the MARCI UV daily global image maps.
## 2.3. PCT Image Analysis Approaches

The current analysis of spatial and optical characteristics of PCT follows the earlier analysis by Clancy et al. (2009). In particular, MARCI daily global image maps support visual identifications of PCT and, for optimally presented PCT, eastern margin surface shadows as a function of image wavelength provide fairly straightforward (if coarse) measures of cloud height, cloud optical depths, and cloud particle sizes (Montmessin et al., 2007; Clancy et al., 2009; Määttänen et al., 2010). Incorporation of a much broader range of MARCI PCT observations allows characterization of their diagnostic temporal/spatial distributions and so their distinct formation conditions. MARCI near-polar, wide-angle image strips (cross-track fields-of-view,fov, ~30° in longitude) are obtained on successive MRO orbits. The ~2 hour orbit of MRO corresponds to ~27° rotation of Mars between adjacent image strips of ~30° cross trackfov at the equator, which supports the construction of MARCI global daily image maps. An important caveat for time-dependent features such as clouds is that these global image mosaics are a-synoptic, with ~2 hour temporal discontinuities at the individual image strip boundaries. Mars PCT extending beyond the western, 2:15pm (most commonly) or eastern, 4:15pm boundaries of mapped cross-track fovs are typically truncated by these ~ two hour offsets (figure 2a). PCT eastern margin origins are not defined for such truncations at the eastern margins, whereas the full lengths of PCT are not observed where they are truncated at the western margins of the image strips.

The occurrence and locations of PCT are identified in daily, globally mapped images of 320 nm brightnesses, where the optical signatures of PCT are enhanced by their spatially sharp morphologies, optically significant opacities ($\tau$~0.03–0.15), and contrast against low UV surface reflectances (figure 2). These PCT mapping identifications are conducted for perihelion seasons (over $L_S$ ranges of 210–310°) in MY28 (2007) and MY29 (2009), which incorporate planet encircling and regional dust storms, respectively. Searches at earlier and later $L_S$ periods have not indicated significant PCT activity. For each identified PCT, the timing (MY, $L_S$, LT) and location (latitude, longitude) of their eastern margins (if viewed) are recorded, along with the viewed length of the PCT and the presence and quality of eastern origin surface shadows.

Roughly 30 PCT are identified for detailed cloud shadow analyses, as selected from 2007–2011 (MY28–30) MARCI observations. These PCT exhibit bright and sharply defined eastern margin cloud formation origins, which lead to distinct cloud surface shadows in multiple MARCI filter images, allowing both cloud height (from projected shadow-cloud displacement) and optical depth (from cloud solar extinction) as a function of image wavelength (for cloud particle size estimates).

# 3. PCT SPATIAL and TEMPORAL DISTRIBUTIONS

## 3.1. Spatial Behaviors (locations and surface conditions)

Figure 3 presents specific locations of PCT identified in MY 28 and 29, as indicated on portions of the United States Geological Survey (USGS) Mars Orbiter Laser Altimeter (MOLA- Smith et al., 2001)-based topographic map for the most prominent locales of Arsia Mons and Syria Planum (upper), along Valles Marineris and the eastern margin of
Thaumasia Planum (middle), and the northeast margins of Hellas Basin (lower). Additional regions that exhibit PCT less frequently are associated with the southern and eastern highland boundaries of Solis Planum and the northeast rim of Argyre Basin (see figure 14). PCT locations appear particularly confined to specific locations associated with Valles Marineris and Arsia Mons, and more generally are associated with local elevation gradients. The localization of PCT is especially striking along northern and southern rim positions of Valles Marineris and the southwest flank of Arsia Mons. In the case of the former, it was argued that strong topographically controlled (anabatic) updrafts along the canyon walls, frequently simulated by mesoscale modeling (Tyler et al., 2002; Rafkin and Michaels, 2003; Michaels et al., 2006; Spiga and Forget, 2009; Tyler et al., 2017; Leung and Rafkin, 2017), likely contribute dynamical forcing for PCT formation.

Figure 4 presents the frequency distributions for surface albedo (MARCI 720 nm, Steve Lee, personal communication) and elevation (MOLA) corresponding to the eastern margin origins, or cloud forming plume regions, of PCT identified over 2007–2009. The distinct, multiple maxima apparent in these distributions reflect the repeat locations of PCT within relatively confined regions. For example, the high albedo peak (left panel) near 0.34 represents the concentrated locations of PCT on the southwest flank of Arsia Mons, as do the 4–7 km surface elevation peaks in the right panel of figure 4. The surface elevation peak near 0 km reflects, in large part, the set of PCT associated with eastern Valles Marineris. More consequentially, figure 4 indicates that PCT origins are not strongly correlated with surface albedo or elevation, which in turn suggests that local surface heating and the depth of the local boundary layer (which correlates with surface elevation; Hinson et al., 2019) are not controlling factors in generating PCT.

PCT extended lengths, oriented E-W to ENE-WSW, are their most distinctive spatial characteristic. These lengths are discussed in the following section, but relate to PCT cloud particle lifetimes and upper level wind speeds. PCT widths at the eastern margins, where active cloud formation occurs, likely reflect the scale of underlying mesoscale dynamics (updrafts) that force PCT formation at higher altitudes. Individual PCT widths are generally of order 30–60 km, although multiple, closely spaced PCT are observed to span group widths of >100 km (figure 1ab, 2ab, 3a). These widths are marginally resolved by the 8 km UV pixels (e.g., figure 7b), but much better resolved in the visible 1 km scale pixels (figure 1). However, the detailed morphology of these mesoscale PCT cloud forming regions is not resolved in MARCI imaging.

## 3.2 Temporal Behaviors ($L_s$, MY, LT)

PCT are observed over a ~100° range of Mars solar longitudes ($L_s$) centered on Mars perihelion ($L_s$=251°) and southern summer solstice ($L_s$=270°). This period corresponds to peak (+20K) global surface and atmospheric temperatures as forced by the ~40% orbital variation of solar incident flux over the eccentric Mars orbit. It also corresponds to maximum seasonal (associated with Mars 26° obliquity) insolation at southern low latitude regions where PCT are observed. The perihelion atmospheric conditions associated with PCT formation are discussed in more detail subsequently, but the warm surface and
atmospheric conditions lead to increased dynamical forcing and upper level water vapor abundances, both of which are key to PCT formation.

Figure 5 presents the frequency of PCT identified in MARCI daily global 320 nm image maps, for Mars Years (MY) 28 (bottom panel) and 29 (top panel). In MY29, a PCT is identified every Mars day, on average (1° of Mars $L_s$ ~ 2 Mars days). They appear most frequently around Mars perihelion ($L_s$=251°) to southern summer solstice ($L_s$=270°), during which 2–3 appear daily, often separately over the Arsia Mons, Valles Marineris and Hellas basin preferred locations. In some cases frequency peaks also correspond to multiple PCT over the same region (figure 1a). A regional dust storm occurred in MY29 around $L_s$=235° (dashed vertical line, Montabone et al., 2014; Wolkenberg et al., 2018) and appears to have suppressed PCT formation for ~20 days. Regional dust storms often occur during this early southern summer period and lead to temperatures and dust loading increases to altitudes well above 40 km (Smith, 2004; Guzewich et al., 2013; Montabone et al., 2014; Kass et al., 2016). The MY28 global dust storm was a perihelion dust storm, initiating after perihelion (vertical dashed line in figure 5 lower panel). This major global dust storm began after $L_s$=260° (e.g. Smith, 2009; Montabone et al., 2014) and suppressed PCT formation for the remainder of MY28. Such global dust storms have been shown to exhibit thermal and dust loading increases over such an extended seasonal ($L_s$) interval (50–60° Smith, 2019), and to altitudes well above 50 km (Clancy et al., 2010; Heavens et al., 2018).

Extensive vertical extension of dust loading and atmospheric heating associated with dust solar absorption may well affect PCT formation mechanisms in the lower atmosphere, as discussed subsequently. However, it is clear that water vapor saturation conditions at the 40–50 km altitude region of PCT cloud formation are a major element of the disappearance of PCT during Mars regional and global dust storms. In particular, near-saturation water vapor conditions, present at 40–50 km altitudes over latitudes and the perihelion season of PCT formation (see figure 11), are shifted to much higher altitudes during the MY28 (Heavens et al., 2018; Fedorova et al., 2018) and MY34 (Heavens et al., 2019; Aoki et al., 2019; Neary et al., 2019; Liuzzi et al., 2020; Fedorova et al., 2020) global dust storms. This is likely to be a controlling influence on the disappearance of PCT during large-scale dust storm activity. In the top panel of figure 6, the observed distribution for lengths of PCT are determined from the MARCI daily global image maps for the MY28, 29 periods presented in figure 5. The numbers of identified short-length (<200 km) PCT are biased low due to the difficulty in identifying the characteristic linear form of PCT with such clouds. The longest cloud lengths are biased by the western margin cutoffs associated with the MARCI image strips, a limitation exacerbated by the tendency of PCT to present in the western, early afternoon hours of these image strips.

However, the plotted histogram of PCT lengths can be interpreted more usefully as an indication of the local times (LT) of PCT initiation, with a few simplifying assumptions. The lower panel of figure 6 indicates the observed LT of PCT eastern origins (grey histogram with MARCI image strip center at 3pm LT indicated by dashed line). Notice that the range of LT extent for MARCI image strips is modest, roughly ±45 minutes about the 3pm center LT. Notice, also, that the observed LT of PCT eastern origins are significantly biased to
earlier than 3pm. This follows from the earlier point that PCT are generally observed in the
western portions of MARCI image strips.

The distributed set of black histogram values for LT presents the interpreted LT of PCT initiation, which assumes the observed length of each PCT (top panel) represents the lifetime of the PCT and that lifetime can be estimated with the adoption of a fixed (for all PCT) westward transport velocity. Here, we adopt a 50 m/sec westward velocity, roughly consistent with the Mars Climate Database (Millour et al., 2014) for zonal winds in the season, latitude, and altitude (40–50 km) range of PCT formation. In summary, then, the LT of the PCT eastern margin (cloud generation region), the observed length of the PCT, and a constant velocity for the growth of that length are used to estimate an LT of origin (i.e., zero PCT length) for the PCT.

Under these assumptions, local times for PCT initiation fall largely over 11am-2pm, as indicated by the black LT distribution in the bottom panel of figure 6. However, somewhat earlier (10am) and later (3pm) LT of PCT initiation may occur if the shorter and longer lengths of PCT are under-represented by the length distribution presented in the top panel of figure 6. In either case, the initiation of PCT appears centered on midday, overhead solar heating conditions. A corollary of figure 6 is that PCT last for many hours, both in terms of active cloud generation at their eastern margins and the longevity of cloud particles that are transported westward (or WSW) from this cloud generation region.

4. PCT HEIGHTS

Cloud heights for PCT are notably high for such discrete bright clouds viewed in nadir geometry. Mesospheric (55–75 km) CO2 ice clouds appear comparably bright in the nadir and at higher altitudes than PCT (Montmessin et al., 2007; Clancy et al., 2019), but are composed of the primary Mars atmospheric constituent and occur around the cold aphelion season. Due to their substantial opacities ($\tau_{nadi}=0.02–0.2$), PCT and mesospheric CO2 clouds cast measurable surface shadows, which support geometric (parallax) altitude and extinction opacity determinations (Montmessin et al., 2007; Clancy et al., 2009; Määttänen et al., 2010).

Figure 7 presents a set of MARCI images showing bright PCT with accompanying surface shadows. These portions of non-map-projected image swaths in violet (435 nm, top two and bottom right) and UV (320 nm, bottom left) present foreshortening at the eastern margins of the MARCI wide-angle field-of-view. MARCI backplanes for these calibrated (I/F) image formats include pixel-by-pixel photometric angles and pixel position angles, assuming MOLA surface altitudes. The apparent eastward offsets in associated cloud shadows relative to the bright PCT reflects the 3pm (image strip center) afternoon direction of the Sun and the high altitudes of the PCT (~ 40–50 km, figure 8), which determine the cloud-shadow offsets. Cloud shadow heights are determined for the eastern margins of these PCT cloud brightnesses and shadows, to maximize cloud-shadow spatial correspondences.

The cloud shadow analysis provides surface altitudes for the PCT, which are converted to
aeroid altitudes based on MOLA surface elevations. The distribution of these derived PCT
aeroid heights, presented in the right panel of figure 8, indicates PCT heights fairly consistently fall within the 40–50 km aeroid altitude range, within the ±3 km (1σ) errors of the individual height determinations. Aeroid heights, which follow constant pressure levels to first order, are more representative of the saturation altitudes for PCT, and exhibit a more constant indication of PCT cloud heights, as shown in the left panel of figure 8. Derived PCT surface heights are plotted against local surface elevations, which vary significantly over PCT sites, as indicated in figure 8 (left panel, for limited cloud analysis PCT) and figure 4 (right panel, for all identified PCT). A relatively constant PCT average aeroid height of 46 km (dashed line in figure 8 left panel) appears consistent with the plotted cloud surface height versus local surface height for PCT, within the considerable PCT height errors and among the large variation of surface elevations for these PCT. We note that these altitudes apply only to the eastern margin, cloud-forming regions of PCT. Cloud shadows along the length of PCT are effectively obscured by the PCT themselves (e.g., figure 7a), except at higher southern latitudes (see figures 1c,d and 12).

## 5. PCT OPACITIES and PARTICLE SIZES

PCT nadir optical depths are also derived from eastern margin surface shadows, which effectively yields extinction opacity versus MARCI filter wavelength (Clancy et al., 2009). The relative decrease in apparent surface reflectance (calibrated I/F) across the deepest part of the PCT eastern margin shadows, as corrected by atmospheric (background dust) scattering into the shadow region (a ~40% effect), yields a fairly direct extinction optical depth (Montmessin et al., 2007), albeit with significant uncertainties (1σ~15%). We adopt the atmospheric scattering correction parameter for the ratio diffuse/direct illumination as 0.3, based on radiative transfer calculations assuming a constant background dust optical depth of 0.6 (see Clancy et al., 2009), which is found to be fairly independent of wavelength over the MARCI filter range. Shadow extinction measurements for a given PCT are obtained for the same surface locations among the five MARCI filters employed, at band-center wavelengths of 260, 320, 435, 545, and 605 nm. This wavelength range provides optimum sensitivity for typical PCT particle sizes, in terms of wavelength variations in the cloud particle extinction cross-section versus cloud particle size. In the following, we reference these ice particle sizes in terms of an effective (cross-section weighted average) particle radius ($R_{eff}$), associated with a modified gamma particle size distribution with an effective variance of 0.1 (e.g., Wolff et al., 2017).

Figure 9 presents a sample of cloud-shadow-determined optical depths (left panel), and the number distribution of derived cloud particle sizes (right panel, $R_{eff}$) for 26 PCT presenting cloud shadows appropriate to particle size determinations. The three sets of symbols indicate derived normal cloud optical depths for three PCT, in which the wavelength dependences of these optical depths among the 5 MARCI UV-visible imaging bands are compared over the same wavelength range, to Mie extinction cross-sections for ice particle size distributions with $R_{eff}$ = 0.15, 0.25, and 0.5 µm. Ice particle sizes of PCT are coarsely fit on the basis of these shadow-derived and Mie-calculated wavelength dependences on mean ice particle sizes, which are particularly sensitive to the relatively small ice particle sizes of PCT. As indicated in the right panel of figure 9, the majority of PCT are characterized by $R_{eff}$ of 0.2–0.4 µm, versus $R_{eff}$ of 1–4 µm in the lower atmosphere (e.g., Clancy et al., 2017).
Figure 10 presents the implied water content of PCT versus altitude (left panel), as well as the relatively altitude-independent character of the derived PCT particle sizes (right panel). The implied water content of PCT is given as a fraction of the available water vapor content of the local atmosphere, assuming water vapor densities versus altitude drawn from the LMD MCD simulations for the latitudes and seasons of PCT occurrence. The PCT water abundance, versus altitude, is calculated from the determined R<sub>eff</sub>; aeroid altitude, 435nm optical depth, the Mie cross-section for 435 nm extinction for the determined R<sub>eff</sub>; and an assumed 5 km cloud thickness. The latter assumption is more relevant to estimation of implied PCT particle number densities, since water vapor and water ice mass scale with cloud thickness. The estimated PCT particle number densities range from 2–5 cm<sup>-3</sup> for R<sub>eff</sub>=0.5 µm to 100–300 cm<sup>-3</sup> for R<sub>eff</sub>=0.2 µm, the latter being more typical of PCT as figures 9 and 10 demonstrate. The left panel of figure 10 indicates that, while the PCT fraction of atmospheric water content is perhaps somewhat larger at the higher altitudes of PCT occurrence, that fraction is generally less than half of the estimated atmospheric water vapor content available to form clouds at 40–50 km altitudes.

# 6. CLOUD FORMATION CONDITIONS

The saturation conditions under which PCT form are estimated on the basis of LMD MCD atmospheric temperatures and water vapor abundances at the PCT 40–50 km altitudes and perihelion season of formation. The westward (or WSW) transport of PCT cloud particles to form their cloud trail morphologies are similarly drawn from the MCD wind velocity climatology at these locations and times. Figure 11 presents typical, for season and location, water vapor saturation conditions (left panel) and zonal/meridional winds over the 30–60 km altitude region (right panel), encompassing the 40–50 km altitude range over which the majority of the analyzed PCT occur. Atmospheric water vapor is at or near saturation in the MCD climatology for altitudes above 35 km, such that the MCD simulates weak ice clouds over the 35 to 55 km altitude range (dotted line in figure 10a). Indeed, water ice clouds are commonly presented over this altitude range and season in the MCS profile retrievals (McCleese et al., 2010), as a kind of optically thin ($\tau_{vis} \sim 0.01-0.03$) perihelion equivalent to the lower altitude (~15–35 km), optically thicker ($\tau_{vis} \sim 0.1-0.3$) aphelion cloud belt (e.g., Smith, 2004; Wolff et al., 2019). During large-scale dust storms, this perihelion saturation region is shifted to well above 50 km altitudes, as shown by Aoki et al. (2019); Neary et al. (2019); Liuzzi et al. (2020); Fedorova et al. (2020). As discussed earlier (figure 5), this leads to the suppression of PCT formation during such storms.

The presence of near saturations conditions above 50 km altitude is further demonstrated in the widespread appearance, in CRISM limb observations, of detached water ice clouds in the mesosphere during the perihelion portion of the Mars orbit (50–80 km altitudes, figure 9 and 10 in Clancy et al., 2019). CRISM limb spectra ($\lambda = 0.4 - 4 \mu m$) indicate mesospheric water ice layers with small particle sizes similar to those exhibited by PCT, but with substantially smaller number densities/optical depths. CRISM and MARCI obtain simultaneous observations from the MRO spacecraft, and the wide-angle MARCI imaging allows overlap in MARCI nadir and CRISM limb fields-of-view. Figure 12 demonstrates such overlap, with a set of three CRISM limb viewing paths (thick white lines, scanning in the MRO orbit plane) indicated on a map projected MARCI 425 nm image swath. The MARCI nadir image
reveals a set of PCT appearing along the elevated west margins of the Syria and Solis Planitia. All three CRISM limb views indicate bright detached water ice aerosol layers, at aeroid altitudes of 55 to 67 km and with $R_{eff}$= 0.3 µm particle sizes. The CRISM limb spectra also indicate that fine dust ($R_{eff}$= 0.7 µm) dominates the 40–50 km altitude region below these mesospheric ice layers, suggesting the more broadly distributed bright regions in the MARCI image are associated with mesospheric ice hazes.

The 10–40S extent of PCT at 40–50 km altitudes and water ice layers at 50–70 km altitudes that is presented in figure 12 is a somewhat unusual observation in that identified correspondences between MARCI PCT and CRISM mesospheric aerosol layer measurements are uncommon. The CRISM limb observations have very restricted temporal and spatial (longitude, in particular) coverages. However, MARCI observations do not suffer this limitation and the latitudinal extent of PCT in figure 12 is unusual. Large-scale correspondences among contemporaneous PCT are suggested here, but also in close PCT groupings (Figures 1 and 2) and large day-to-day fluctuations in PCT frequency (figure 5). But they are not clearly established in this current study. A more intensive mapping study employing 8x higher resolution 435 nm daily maps would improve PCT detections towards establishing or discounting such relationships. Similarly, while there is no obvious evidence of dust lifting in the presence of PCT, the employed image analysis is not very sensitive to diffuse dust opacity increases of ≤50%.

PCT lengths of ~600 km indicate that ice cloud particles formed at their eastern margins are stable to loss by sublimation and excessive gravitational settling over ~3 hour timescales, assuming westward transport in a 50–60 m/sec meridional circulation (figure 11). As indicated in Clancy et al. (2009), the small PCT ice particle sizes will descend gravitationally on the order of 1–2 km over this timescale. Their stability to sublimation on this timescale is more difficult to estimate, but the near saturation conditions of water vapor at PCT altitudes (figure 11) suggests they can be stable to sublimation loss, as does their obvious persistence in the MARCI imaging. Consequently, a plume and downwind trail model for PCT is generally consistent with the known atmospheric conditions for their observed locations and season of occurrence.

## 7. POTENTIAL DYNAMICAL ORIGINS

The midday formation of PCT ice particles at 40–50 km altitudes over specific locations implies lower atmospheric dynamical forcing that is persistent over 2–4 hour timescales and extends its influence to ~4 scale heights above the convective boundary layer. Here we consider two distinct dynamical processes that have been separately studied by numerous recent investigations and may be intrinsically connected to PCT formation, topographically forced (anabatic flow) mesoscale updrafts and gravity waves. A connection to diurnally dependent mesoscale updrafts within Valles Marineris was originally proposed to force PCT along bounding eastern regions of this feature (figure 3c, Clancy et al., 2009), although such circulations were not expected to reach PCT heights. In the current study, we propose that gravity waves are forced by these mesoscale updrafts to generate PCT at 40–50 km altitudes, and that such mesoscale updrafts (and attending gravity waves) occur over a number of other specific topographic regions present at southern low-to-mid latitudes (primarily 10S-30S).
Figure 13 presents a schematic interpretation for this proposed origin of Mars perihelion cloud trails.

## 7.1 Lower atmospheric updrafts

Diurnally forced topographic flows lead to daytime upslope (anabatic) flows along topographic gradients, as investigated by numerous mesoscale simulations, over Olympus Mons (Michaels et al., 2006; Spiga and Forget, 2009), craters (Rafkin and Michaels, 2003; Spiga and Forget, 2009; Tyler and Barnes, 2015; Tyler et al., 2017), and Valles Marineris (Tyler et al., 2002; Rafkin and Michaels, 2003; Spiga and Forget, 2009). For example, Valles Marineris daytime upslope winds peak at 25–40 m/sec at the chasm walls, based on the mesoscale simulations of Spiga and Forget (2009), Rafkin and Michaels (2003), and Toigo and Richardson (2003), for the $L_s = 320^\circ$ season. The latter two mesoscale analyses of Valles Marineris were conducted in support of Mars Exploration Rover mission operations. In the case of the Tyler and Barnes (2015) mesoscale simulations in support of Mars 2020 operations, simulated boundary layer temperatures have been favorably compared to retrieved temperature profiles from Mars Express radio occultations (Hinson et al., 2019).

Tyler and Barnes (2015) also conducted mesoscale simulations of the topographic flow regime at Melas Chasma, a western portion of Valles Marineris over which PCT are observed (figure 3c). This mesoscale simulation exhibits a distinct front of vertical updraft, which is offset ~50 km from the chasm rim and peaks well above the surface (10–15 m/sec at ~4 km). Meridional winds converge with the topographic flow under this updraft, with order 10–15 m/sec surface winds 10’s of km away from the narrow (~20 km) updraft region. In many spatial respects, the Tyler and Barnes (2015) mesoscale simulations of Melias Chasma (and crater) topographic circulations (anabatic flow) provide dynamical support for the inferred localized updraft regions associated with PCT cloud formation. However, the Melas Chasma simulated updrafts occur along the length of the chasm walls, whereas PCT appear at localized points offset from the Valles Marineris rim (and at other locales). Tyler and Barnes (2015) do note that uneven topography along the chasm walls can lead to distinct regions of focused updraft velocities, which is apparent in their presented simulation, as well as others (e.g., Spiga and Forget, 2009).

We note that these topographic circulations are diurnally forced, the simulated vertical updraft fronts offset from the Melas Chasma walls are associated with anabatic winds and presented at ~3 pm, similar to MARCI PCT identifications. We also note that the Tyler and Barnes (2015) simulation was for northern spring equinox ($L_s = 5^\circ$), well away in season from PCT occurrence. The general topographic circulations are not inherently seasonal, but the simulated updrafts offset from the crater and chasma rims are influenced by the global/ regional wind fields in which they are embedded. For example, regional surface winds oriented towards the Melas Chasma outflow play a role in the position and amplitude of the simulated updrafts presented in Tyler and Barnes (2015). As discussed above, the seasonality of PCT occurrence is primarily related to seasonality in atmospheric water vapor saturation conditions at 40–50 km altitudes. The topographically forced updrafts over these regions likely persist over a more extended seasonal range, as modulated by the southern latitude, prevailing near surface winds.
## 7.2. Gravity waves

All of these mesoscale studies indicate substantial vertical velocities that, in the case of volcano upslope winds in particular, can initiate substantial cloud formation with strong diurnal behaviors (Michaels et al., 2006; Spiga and Forget, 2009). PCT occur at much higher altitudes than currently modeled topographic cloud systems, at 40–50 km versus 10–25 km altitudes. Hence, it is necessary to identify indirect cloud forcing at higher altitudes, associated with topographically forced updrafts at lower altitudes. Gravity wave (GW) forcing is one such mechanism that is likely triggered by vertical updrafts at lower altitudes, which leads to vertically propagating internal waves with accompanying thermal wave amplitudes. GW and their associated thermal perturbation amplitudes increase with altitude as the inverse of atmospheric density until they become unstable to breaking and local momentum deposition, the altitude of which depends on GW phase speeds and local horizontal wind fields (e.g. Collins et al., 1997; Medvedev et al., 2011; Yiğit et al., 2015). Minimum temperature phases (sometimes called ‘cold pockets’) in gravity waves propagated to Mars mesosphere (60–70 km) have been proposed to generate observed CO₂ clouds at low latitudes around Mars northern summer ($L_s = 0-140°$ Clancy and Sandor, 1998; Spiga et al., 2012; Listowski et al., 2014). Recent Mars GCM simulations of Mars GW vertical propagation, versus season and latitude, suggest that low latitude GW are more vigorously propagated into the mesosphere during the Mars southern versus northern summer season (Kuroda et al., 2019). Hence, minimum temperatures in vertically propagating GW at 40–50 km altitudes present a plausible process to generate PCT formation over topographically forced updrafts in the lower atmosphere (0–10 km) around perihelion (which coincides with southern summer in the current epoch). Furthermore, the perihelion (southern summer) season presents a baseline of near-saturation water vapor conditions at 40–50 km altitudes (figure 11), also a key parameter in generating PCT.

Recently, Mars Climate Sounder 15 µm radiances have supported global mapping of the distribution of GW amplitudes below PCT altitudes (Heavens et al., 2020). Figure 14 compares the global (southern hemisphere) distribution of identified (2007–2001) PCT to MCS GW amplitudes at ~25 km altitudes (roughly the 0.5 mbar pressure-level sounding level of 15 µm nadir sounding). PCT and GW exhibit strong spatial correspondences with observed PCT locations. However, enhanced GW amplitudes are presented over Pavonis and Ascraeus Mons, the southern Solis Planum ridge and eastern Hesperia Planum, where PCT observations are not, or less frequently, observed. PCT require water vapor saturation conditions and GW forcing propagation to 40–50 km altitudes, either of which may not be as strongly present over these regions. In particular, the southern Solis Planum region does not favor water saturation at PCT altitudes in the MCD climatology, and LMD GCM simulations present more limited GW propagation above the northern Pavonis and Ascraeus portions of Tharsis Montes (Spiga et al., 2012).

GW forcing at low latitudes during this season was attributed to daytime boundary layer convection driven by strong surface heating and resulting superadiabatic lapse rates by Heavens et al. (2020). Convection of this type excites gravity waves when the updraft encounters stable air at the top of the boundary layer (Stull, 1976; Spiga and Forget, 2009). However, PCT are not a likely result of such convective forcing, because they are not clearly
tied to high surface elevations (where boundary layers are vertically deep and gravity waves
are likely to be large enough to avoid total internal reflection and trapping in the boundary
layer) or dark albedo surfaces (where heating would be strongest). That PCT correlate with
specific topographic gradients where mesoscale updrafts are predicted (e.g., Figure 3)
suggests that these updrafts may be an effective source of GW forcing, exciting GW with
non-zero phase speeds in the same way as boundary layer convection but at horizontal scales
significantly greater than 5–10 km simulated by Spiga and Forget (2009). The non-zero
phase speeds and specificity to particular topographic geometries of GW of this class also
would make them different than GW excited by air being forced over an obstacle.
Consequently, such GW forcing is important to distinguish observationally and in model
parameterizations of GW, because they will propagate differently and have different
dynamical effects on higher atmospheric levels than GW excited by boundary layer
convection or orographic GW, as classically understood.

8. CONCLUSIONS

Perihelion Cloud Trails (PCT) are a visually striking Mars cloud form that occurs around the perihelion season of Mars when global temperatures are elevated and water ice clouds are generally present only as high, thin hazes. Hence, PCT stand out against weak background ice hazes at this time (Wolff et al., 2019) as mesoscale (width at eastern cloud forming margin ~50 km), long (up to 1000 km, E-W or ENE-WSW), high-altitude (40–50 km, aeroid), and bright (τ<sub>vis</sub>~0.1) clouds. MARCI daily global imaging, with UV and visible image passbands, reveals the spatial, seasonal, and diurnal characteristics of these clouds; as well as their typically small particle sizes (R<sub>eff</sub> = 0.2–0.3 µm) and large particle number densities (~300–100 cm<sup>-3</sup>). PCT occur repeatedly over quite specific southern low latitude regions (0–40S) with topographic gradients mostly associated with Valles Marineris, SW Arsia Mons, NE Hellas rim, and Syria, Solis, and Thaumasia Planitia elevated margins, at an average frequency of 1–3 per day globally.

Their spatial specificity indicates they are likely forced by topographically influenced
mesoscale updraft regions acting over midday hours (10am-3pm) in the lower atmosphere.
These localized updrafts generate gravity waves (GW) that in turn initiate ice cloud
formation in the water-vaporsaturated 40–50 km altitude region. This saturation dependence
leads to their disappearance in regional and global dust storms. The background zonal/
meridional winds at the 40–50 km altitudes of PCT ice cloud formation rapidly (~50 m/sec)
transport ice cloud particles to the W or WSW to create the observed cloud trails. There is
substantial mesoscale modeling evidence for topographically forced mesoscale updrafts
roughly consistent with PCT spatial behaviors (Tyler and Barnes, 2015), and recent MCS
measurements of persistent enhancements in gravity wave amplitudes (at ~25 km altitudes,
Heavens et al., 2020) over many PCT locations.

If the above dynamical interpretations of PCT are correct, then PCT present a remarkable
visual identification of spatially fixed regions of strong vertical updrafts providing a
mesoscale source of vertically propagating GW. Clearly, direct mesoscale modeling of such
updrafts and the resulting formation of PCT at 40–50 km is a difficult enterprise. But PCT-
identified locales of strong updrafts can provide a measure of observational support for
detailed mesoscale simulations of topographically influenced circulations, a topic of considerable importance in determining lander entry and surface environmental conditions (Toigo and Richardson, 2003; Rafkin and Michaels, 2003; Spiga and Forget, 2009; Tyler and Barnes, 2015; Rafkin et al., 2016).

## Acknowledgements

We are indebted to the excellent MRO and the Malin Space Science System (MSSS) operations staff for the collection and processing of the MARCI image data presented and analyzed here. Primary grant support for this work was provided by the NASA MDAP Program (under NASA contract award numbers NNX15AQ07G and 80NSSC19K1215). Partial contract support for this work was provided by the NASA MRO mission (under MSSS sub-contract 06-0152) and by NASA CoI funding for the NOMAD/ExoMars TGO mission (under JPL subcontract 1551112).

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<figure><img src="image_3.png" /><figcaption>**Figure 1:** MARCI violet (435 nm, left panels) and color (RGB, right panels) map projected images of Mars PCT; for SW Arsia Mons (top images, $L_s = 259.3^\circ$, MY30, G09_021804_2593_MA_00N122W_110322), southern Solis Planum (center images, $L_s = 277.1^\circ$, MY29, B10_013350_2771_MA_00N77W_090601), and NE Hellas (lower images, $L_s = 296^\circ$, MY30, G11_022561_2960_MA_00N269W_110520). A set of 3-4 PCT over Arsia Mons extends faintly to the western margin of the MARCI imagefov. Similarly, faint parallel PCT accompany the bright Solis Planum PCT. The NE Hellas PCT exhibits a distinctive wave structure.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2:**</figcaption></figure>

Portions of MARCI UV (320 nm) daily global image maps presenting PCT in various stages of growth and presentation. The top image demonstrates classic PCT cloud forms south of Melas Chasma, and twin parallel PCT over the NE rim of Hellas Basin. All three PCT western margins are clipped by the subsequent (~2 hours later) MARCI image swath. In the center image, a Syria Planum PCT appears in its early growth stages in the center image swath, and at its subsequent (~2 hours later, circled) western extent of >1200 km. The bottom images present the low resolution UV image of the Hellas PCT shown in the lower panels of figure 1, and what appears to be a PCT in its earliest stages of plume development in Solis Planum (circled).
<figure><img src="image_5.png" /><figcaption>**Figure 3:**</figcaption></figure>

Individual PCT eastern margins (proposed mesoscale updrafts) are indicated on USGS MOLA-based elevation maps (Smith et al., 2001), for three low southern latitude regions where they most frequently occur; in association with Arsia Mons and Syria Planum elevated regions (top panel), along Valles Marineris and beyond the eastern elevated border of Thaumasia Planum (Nectaris Fossae, middle panel), and the NE rim margins of Hellas Basin (bottom panel). These PCT locations are drawn from visual inspections of MARCI 320 nm daily global maps over MY28 and 29.
<figure><img src="image_6.png" /><figcaption>**Figure 4:**</figcaption></figure>

The frequency distributions for surface albedos (MARCI 720 nm- Steve Lee, personal communication, 2020) and surface elevations (MOLA- Smith et al., 2001), associated with the eastern margins (proposed mesoscale updrafts) of PCT identified in MY 28 and 29. No clear correspondence with surface albedo or local surface elevation is identified for PCT updraft, cloud forming locales.
<figure><img src="image_7.png" /><figcaption>**Figure 5:**</figcaption></figure>

The frequency of global PCT occurrence, as identified in MARCI 320 nm daily global image maps, is plotted versus season ($L_s$) for MY28 (lower panel) and MY29 (upper panel). Roughly 1–3 PCT are observed over all Mars southern latitudes daily, with notable absences during the $L_s$~230° regional dust storm in MY29 and the $L_s$~265° global dust storm in MY28. This PCT sensitivity to large scale dust lifting likely reflects atmospheric heating by dust lifted to the PCT formation altitudes of 40–50 km, thereby reducing vapor saturation conditions necessary to cloud formation.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6:**</figcaption></figure>

The observed frequency distribution of observed PCT lengths (top panel, km), as identified in MARCI 320 nm daily global image maps in MY28 and 29. The bottom panel presents the observed local time (LT) of PCT eastern margin (updraft) regions as a grey shaded histogram, where the ~3 pm LT for MARCI image swath centers is identified by a dashed line. Assuming the observed PCT lengths reflect a constant 50 m/sec westward transport of cloud particles formed at 40–50 km altitudes above these eastern updraft margins, we estimate the initial LT of PCT origins as the black histogram plot.
<figure><img src="image_10.png" /><figcaption>(a) left- NE Hellas at $L_s$ = 274.1°, 28May2009; right - Syria Planum at $L_s$ = 273.9°, 27May2009</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(b) Gangi/Capri Chasma at $L_s$ = 237.7°, 16Feb2011; left- 320 nm image, right- expanded 435 nm image</figcaption></figure>

**Figure 7:**

A set of PCT and their offset eastern margin surface shadows are selected from non-map-projected (a) MARCI violet (435 nm) MY29 images over NE Hellas rim (left top panel, $L_s$ = 274.1°, B09_013291_2742_MA_00N266W_090528) and Syria Planum (right upper panel, $L_s$ = 273.9°, B09_013285_2739_MA_00N103W_090527); and (b) UV (320 nm, left lower panel) and violet (right lower panel, expanded region) images over eastern Valles Marineris ($L_s$ = 237.7°, MY30, G08_021366_2377麦克00N043W_110216). MARCI visible (violet) images present 8x greater spatial resolution compared to MARCI UV images, showing the more northern PCT and shadow between the Ganges and Capri chasms. The PCT-surface shadow offsets and surface shadow I/F decreases support PCT altitudes and optical depth/particle size determinations, respectively.
<figure><img src="image_12.png" /><figcaption>**Figure 8:** Cloud heights from the local surface (measured, left panel) and as aeroid heights (adjusted, right panel) are obtained from analysis of 27 PCT surface shadows measured over 2007–2011. PCT altitudes are generally determined to ±3 km (1σ) and predominately observed over 40–50 km aeroid altitudes.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 9:**</figcaption></figure>

PCT average ice particle sizes ($R_{eff}$) are derived from the comparison of MARCI filter (wavelength) dependence of surface extinction measurements (from cloud shadows, symbols) to Mie reference extinction as a function of cloud particle size (lines, $R_{eff}$). The observed distributions of PCT ice particle sizes (right panel) were obtained from 26 shadow measurements over 2007–2011, and peak strongly over small ($R_{eff} = 0.2–0.4$ µm) particle sizes.
<figure><img src="image_14.png" /><figcaption>**Figure 10:** Estimated PCT water content, relative to available water vapor abundance (from the MCD, Millour et al., 2014), is calculated from the cloud shadow analyses for PCT optical depths and particle sizes (figure 9). PCT typically incorporates ≤40% of available atmospheric water, which is not clearly a function of PCT altitude (left panel). Similarly, derived PCT ice particle sizes ($R_{eff}$) do not clearly depend on measured PCT altitudes (right panel). Implied PCT ice number densities are 2–5 cm⁻³ for $R_{eff}=0.5$ µm to 100–300 cm⁻³ for $R_{eff}=0.2$ µm.</figcaption></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>Figure 11:</figcaption></figure>

Water vapor saturation conditions (upper panel) and zonal/meridional winds (lower panel) associated with PCT ice formation regions (40–50 km heights, southern low latitudes, perihelion season) are drawn from the MCD model climatology (Millour et al., 2014). The MCD predicts water vapor saturation conditions (dashed and solid lines, left panel) as well as weak ice aerosols (dotted line) over the range of PCT observed altitudes at this time. MCD zonal and meridional winds are consistent with rapid westward, to WSW, transport of PCT ice clouds, as observed.
<figure><img src="image_17.png" /><figcaption>**Figure 12:** An extensive (5S-40S, 90W-115W) portion of a MARCI violet (435 nm), mapprojected image swath (G09_09_021922_2651_00N103W_110331) exhibits a bright Syria Planum PCT (12S), as well as a set of weaker PCT at 30S-40S latitudes. All of these PCT are oriented over the N-S ridge forming the western boundaries of Syria and Solis Planitia. Solid white lines shifted ~5° E of these PCT indicate coincident CRISM limb scans that indicate extensive mesospheric (50–75 km) ice haze layers, which also appears as bright hazes in the MARCI image swath. CRISM limb spectra indicate mesospheric water ice particle sizes similar to those derived for PCT (Clancy et al., 2019).</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Schematic for**
**Perihelion Cloud Trail**
**(PCT) Formation**</figcaption></figure>

**Figure 13:**

This PCT schematic description reflects the uncertain topographic forcing of lower scale height updrafts, its generic forcing of vertically propagating, amplitude-increasing gravity waves, their forcing of PCT ice clouds at 40–50 km altitudes, and the entrainment of those clouds in strong westward zonal (and weaker southward) flow at these altitudes. The small PCT ice particle sizes are expected to remain stable to gravitational settling over their westward extensions.
<figure><img src="image_19.png" /><figcaption>**Figure 14:**</figcaption></figure>

Southern hemisphere map distributions of all MY28,29 observed PCT (lower panel) and MCS measured gravity wave (GW) amplitudes are compared, for the perihelion/southern summer $L_s$ range of 220–300°. The ~25 km level GW map results derive from MCS surface-viewed 15 µm radiances over MY31–34 (Heavens et al., 2020). The mapped PCT eastern margin origins, presumed sites of strong vertical updraft and GW generation to create PCT ice formation at 40–50 km altitudes, generally correspond to MCS identified regions of high GW amplitude. However, additional regions of high GW amplitudes (e.g., Pavonis and Ascraeus Mons) do not lead to PCT, perhaps due to less favorable saturation or GW propagation conditions at PCT altitudes.