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Textured Dust Storm Activity in Northeast Amazonis–Southwest Arcadia, Mars: Phenomenology and Dynamical Interpretation

N. G. HEAVENS Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia

(Manuscript received 14 July 2016, in final form 19 November 2016)

ABSTRACT

Dust storms are Mars’s most notable meteorological phenomenon, but many aspects of their structure and dynamics remain mysterious. The cloud-top appearance of dust storms in visible imagery varies on a con- tinuum between diffuse/hazy and textured. Textured storms contain cellular structure and/or banding, which is thought to indicate active lifting within the storm. Some textured dust storms may contain the deep con- vection that generates the detached dust layers observed high in Mars’s atmosphere. This study focuses on textured local dust storms in a limited area within Northeast (NE) Amazonis and Southwest (SW) Arcadia Planitiae (258–408N, 1558–1658W) using collocated observations by instruments on board the Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter (MRO) satellites. In northern fall and winter, this area frequently experiences dust storms with a previously unreported ruffled texture that resembles wide, mixed- layer rolls in Earth’s atmosphere, a resemblance that is supported by high-resolution active sounding and passive radiometry in both the near- and thermal infrared. These storms are mostly confined within the atmospheric boundary layer and are rarely sources of detached dust layers. The climatology and structure of these storms are thus consistent with an underlying driver of cold-air-advection events related to the passage of strong baroclinic waves. While the properties of the studied region may be ideal for detecting these structures and processes, the dynamics here are likely relevant to dust storm activity elsewhere on Mars.

1. Introduction rare cases, global dust storms enshroud most of the surface area of the planet in a haze of dust. Dust storms are Mars’s most notable meteorological Observations of the global dust storm in 2001 sug- phenomena. First observed by telescopic observers as gested that global storms are not associated with a single obscured albedo features and then as ‘‘yellow clouds,’’ center of circulation and/or frontal boundary. Instead, their equivalence to Earth’s dust storms was confirmed they consist of multiple local/regional storms that form from the first spacecraft observations (Gifford 1964; along the advancing dust haze itself or as the result of Martin and Zurek 1993). The vast majority of storms are global teleconnections associated with the high loading local: storms do not last from 1 sol (Martian day) to the of dust (Strausberg et al. 2005; Cantor 2007). Regional next and have areas less than 1.6 3 106 km2 (Cantor et al. dust storms can form from merging local storms but are 2001). Local storm frequency decreases with increasing often associated with discrete weather systems, partic- area according to a power law until an area of 1.6 3 ularly fronts (Cantor et al. 2001; Cantor 2007; Wang and 106 km2, beyond which larger storms are more com- Richardson 2015). mon than would be expected from the power law Global storms loft dust up to 75 km above the surface (Cantor et al. 2001). These regional storms (1%–2% of the planet (Conrath 1975; Cantor 2007; Clancy et al. of storms) last for a few sols (Cantor et al. 2001). In 2010; Heavens et al. 2015). This dust strongly heats the atmosphere while preventing sunlight from reaching the surface, which results in strong near-surface cooling Denotes content that is immediately available upon publica- during the day but warming (due to a dust-driven tion as open access. greenhouse effect) at night (Gurwell et al. 2005; Strausberg et al. 2005). Most of the dust forms a well- Corresponding author e-mail: N. G. Heavens, nicholas.heavens@ mixed haze, but some forms structures detached from hamptonu.edu the main haze, known as detached dust layers (DDLs).

DOI: 10.1175/JAS-D-16-0211.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/04/21 04:35 PM UTC 1012 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 74

FIG. 1. Surface properties of the ROI in NE Amazonis–SW Arcadia Planitiae: (a) Topography relative to Mars’s areoid (km) from the MOLA (Smith et al. 2001, 2003). Resolution: 16 points per degree. (b) Reflectivity at 1064 nm based on MOLA observations (Heavens 2 2 2017; available from the author by request). Resolution: 2 points per degree. (c) Daytime thermal inertia (J m 2 s 1/2) from TES ob- servations (Putzig and Mellon 2007; Putzig et al. 2009). Resolution: 20 points per degree. (d) 1 DCI from TES observations (Ruff and Christensen 2002; Ruff 2016). Resolution: 16 points per degree.

DDLs are also observed in Mars’s atmosphere outside 1984; Cantor et al. 2001; Strausberg et al. 2005; Cantor of regional and global dust storm activity, but the dust- 2007). Typically, these descriptions contrast distinct iest and highest-altitude DDLs are predominantly as- turbulent features such as plumes and cells on scales sociated with global and some regional dust storms between the few-kilometer resolution of the imagery (Clancy et al. 2010; Heavens et al. 2011a,b, 2014, 2015). and ;100 km (texture; Guzewich et al. 2015; Kulowski The circulation within local storms is not as well et al. 2016) with indistinct laminar haze. Guzewich et al. characterized. Modeling suggests that dust loading on (2015) argues that texture indicates circulations associ- 10–100-km scales can lead to the formation of organized ated with active dust lifting, because advection and dif- convection powered by the solar heating of dust (Rafkin fusion would homogenize them into hazes on time scales 2009; Spiga et al. 2013). In the simulations of Spiga et al. of hours. Because of the resemblance of some textures (2013), convection in the center of the storm extended to convective clouds on Earth, texture also has been significantly above the planetary boundary layer. In interpreted as an indicator of deep convection as well as simulations by Rafkin (2009), convection was mostly of active lifting (Strausberg et al. 2005). Yet with the confined to the planetary boundary layer. However, this possible exception of near-infrared imaging of a single convection formed a self-sustaining circulation like a local storm (Määttänen et al. 2009), the connection be- tropical cyclone under some conditions (Rafkin 2009). tween dust storm texture and other aspects of mesoscale From study of one local dust storm, Kahn (1995) argued structure and/or vertical mixing in dust storms has not that local dust storms require external forcing for growth been investigated. This study therefore systematically and maintenance as well as for initiation. This hypoth- investigates the relationship between the appearance of esis has not yet been tested systematically. local dust storms in visible imagery and their thermal Observational studies have mostly focused on syn- and aerosol structure in a smooth, low-elevation, and optic to planetary aspects of dust storm structure, but dusty region of interest (ROI) in the dusty Martian the mesoscale structure apparent in visible imagery is plains of Northeast (NE) Amazonis and Southwest often described as well (e.g., Briggs et al. 1979; Kahn (SW) Arcadia Planitiae (258–408N, 1508–1658W).

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FIG. 2. Comparisons of the probability distributions of surface properties in the ROI and over 608S–608N. Probability is expressed as fraction of total area, either of the ROI or of 08–608N. (a) Topography relative to Mars’s areoid (km) (Smith et al. 2001). Binning interval is 2 2 100 m. (b) Reflectivity at 1064 nm (Heavens 2017). Binning interval is 0.01 units. (c) Daytime thermal inertia (J m 1 s 1/2)(Putzig and 2 2 Mellon 2007). Binning interval is 10 J m 2 s 1/2. (d) 1 DCI (Ruff and Christensen 2002). Binning interval is 0.002 units.

One challenge of studying dust storm structure on northern vernal equinox (Clancy et al. 2000; Piqueux Mars is that most observations of an individual dust et al. 2015). storm are limited to a single orbital track, which will b. Properties of the ROI intersect only one part of the storm. It is therefore hard to compare and contrast storm structure in regions The ROI is relatively topographically smooth. There where the visible presentation of storms strongly var- are a few small craters and one unnamed larger crater at ies. In addition, observations by thermal sounders and 298N, 1538W(Fig. 1a). Most of the area falls within a lidar will be affected by variability in topography, al- 500-m altitude range (Figs. 1a, 2a) typical of Mars’s bedo, and thermophysical properties, complicating northern lowlands. The standard deviation of topogra- comparisons of storms of even identical visible ap- phy in part of this area when mapped at ;0.5-km reso- pearance over terrain of variable characteristics. Dust lution is ;20 m (Fenton and Lorenz 2015). storms in the ROI, however, have distinctive visible Reflectivity, thermal inertia, and dust cover index structure. And the ROI has the additional advantage of (DCI) (a measure of dust emissivity in the thermal in- being topographically smooth and having albedo and frared) are proxies for surface dust abundance that are thermophysical properties that vary fairly simply sensitive to different depth ranges (Ruff and Christensen across the region. 2002). Visible and near-infrared reflectivities have skin depths on the order of a few microns (Ruff and 2. Methods and data Christensen 2002). Moving to thermal infrared wave- lengths enables DCI to sample surface dust abundance on a. Martian time the order of tens of microns, while thermal inertia sam- All references to Martian time use a calendar where ples on the length scale of the diurnal thermal wave (on Mars year (MY) 1 began on 11 April 1955, and time the order of centimeters) (Ruff and Christensen 2002). within a given year is expressed as areocentric longitude Away from Mars’s ice caps, surface albedo/reflectivity

Ls: the angle between Mars and the sun relative to the is bimodally distributed (Fig. 2b). Most of the area

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TABLE 1. Sources of observational data for this study. Note that all data are subject to a variety of interruptions in temporal coverage or variations in resolution or other aspects of data quality. The cited references under ‘‘scientific description’’ and the documentation accompanying the ‘‘data source’’ should be consulted for further details. Temporal coverage is based on availability at the data source at the time of last access. ORA FTEAMSHRCSINE V SCIENCES ATMOSPHERIC THE OF JOURNAL Data type Spectral range Spatial resolution Temporal coverage Scientific description Data source

MGS–MOC daily global maps Visible wavelengths 8 points per degree MY 24, Ls ; 1508–MY 28, Malin and Edgett (2001) Wang (2016) Ls ; 1228 Wang and Richardson (2015) MRO–MARCI daily global maps Visible wavelengths 8 points per degree MY 28, Ls ; 1328–MY 30, Bell et al. (2009) Wang (2016) Ls ; 111.88 Wang and Richardson (2015) MRO–MARCI raw imagery UV–near-IR wavelengths 0.7–4 km MY 28, Ls ; 1108–MY 33, Bell et al. (2009) MARCI (2016) Ls ; 758 21 21 MGS–TES radiance 200–1600 cm ; 5–10 cm ;3 km MY 24, Ls ; 1038–MY 27, Christensen et al. (2001) Christensen (2002) resolution Ls ; 818 MGS–TES retrievals Temperature: ;15 mm Along-track: 10–20 km; MY 24, Ls ; 1038–MY 27, Conrath et al. (2000) Christensen (2002) 21 Dust: 1075 cm cross-track: 9 km Ls ; 818 Smith (2004) 2 Water ice: 825 cm 1 MGS–MOLA active radiometry 1064 6 2 nm Spot size: 150 m; horizontal MY 24, Ls ; 1048–MY 25, Smith et al. (2001) MOLA (2016a) spacing: 300 m Ls ; 1868 Neumann et al. (2003) MGS–MOLA passive radiometry 1064 6 2 nm During active sounding: MY 24, Ls ; 1048–MY 28, Sun et al. (2006) MOLA (2016b) 0.34 km 3 3 km (cross-track Ls ; 1168 3 along-track After active sounding: 0.34 km 3 0.5 km (cross-track 3

Unauthenticated |Downloaded 10/04/21 04:35 PMUTC along-track) MRO–MCS limb radiance Broadband channels: ;5-km (vertical), ;6km MY 28, Ls ; 1118–MY 32, McCleese et al. (2007) MCS (2016b) visible–thermal infrared (horizontal) Ls ; 3348 MRO–MCS retrievals Temperature: ;15 mm ;5 km (vertical), ;33 km MY 28, Ls ; 1118–MY 32, Kleinböhl et al. (2009, 2015) MCS (2016a) 21 Dust: 463 cm (horizontal) Ls ; 3348 2 Water ice: 843 cm 1 OLUME 74 APRIL 2017 H E A V E N S 1015

FIG. 3. Climatology of textured local dust storms in the ROI during MY 24 and MY 29. The dates and data availability of storms are indicated with colored crosses. Sounder data indicate that the storm was observed by TES, MCS, and/or MOLA. The start or end of observations by MOC or MARCI during the relevant year is indicated with colored dashed lines. would be classified as bright, though a dark reflectivity imagery was obtained. Using the United States Geo- feature is present on the northern end of the ROI logical Survey’s (USGS) Integrated Software for Im- (Fig. 2b). This dark feature follows a boundary between agers and Spectrometers (ISIS) package, images were low thermal inertia and intermediate thermal inertia calibrated, photometrically corrected with a Minnaert (Fig. 1c), a surface property that is likewise bimodally function with a k parameter of 0.7 to emphasize the distributed (albeit with a long tail) (Fig. 2c). The low- presence of dust (Cantor 2007), and projected in equi- reflectivity feature also closely maps to a feature of high rectangular coordinates. Raw data with emission or so- DCI (low additive inverse DCI) (Fig. 1d). DCI or its lar incidence angles greater than 808 were excluded from additive inverse is likewise bimodally distributed away the projected image. from the poles (Fig. 2d). Thus, from least dusty to most To complement MOC imagery, calibrated radiances dusty are 1) the eastern part of the low-reflectivity fea- (converted to brightness temperature), retrieved tem- ture; 2) the western and central areas of the low- perature profiles from the surface to 40 km, and reflectivity feature; 3) the higher thermal inertia but absorption-only column opacity retrievals from nadir still high-reflectivity area; and 4) the low thermal inertia observations by the Thermal Emission Spectrometer and high-reflectivity area. (TES) on board MGS were used. Radiance was mea- sured by three pairs of two detectors that formed ob- c. Observational data servational tracks that are ;3 km apart on the surface. A This section will briefly describe each type of data. surface temperature was retrieved from each spectrum These descriptions will include data processing not by the TES team (Christensen et al. 2001). The retrieved covered in the scientific descriptions of the data or in the temperature profiles and column opacity retrievals were documentation where the data is archived. Citations to based on averages of data from measurements by all six scientific descriptions and data sources are included in detectors. TES nadir retrievals were not made over an overview in Table 1. areas with poor thermal contrast between the surface Most of the visible imagery is calibrated, photomet- and the atmosphere, which limited retrievals at night rically corrected, and equirectangularly projected as and over the winter pole. daily global maps from observations by the Mars Orbiter Observations by Mars Orbiter Laser Altimeter Camera (MOC) on board Mars Global Surveyor (MGS) (MOLA) on board MGS were used to complement and the Mars Color Imager (MARCI) on board Mars MOC imagery. MOLA observed in an active radiometry Reconnaissance Orbiter (MRO). However, MARCI mode, where it measured the time of flight and returned images can be higher-resolution than eight points per power of a laser pulse (shot) emitted by the instrument degree (Table 1). In addition, MOC and MARCI must at nadir toward the surface. The product of the re- 2 be processed differently because of camera differences flectivity and two-way atmospheric transmissivity rstatm (Wang and Richardson 2015). Where resolution or (also written as RT) can be derived from each mea- processing differences were of interest, raw MARCI surement using the lidar link equation (Neumann et al.

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FIG. 4. Examples of textured dust storms in the ROI (season of observation indicated in the panel titles) during MYs 24 and 29.

2003). For surface returns where the surface reflectivity that Eq. (1) may underestimate column opacity over the rs is known, the column opacity at 1064 nm is ROI during dust storms Heavens (2017). The impact of this bias on this study would lead to underestimate of the 1 RT column opacity variations described in section 3. t 52 log . (1) 1064 MOLA’s passive radiometry mode measured Mars’s 2 rs brightness (presumably reflected sunlight) at 1064 nm. MOLA cannot differentiate between dust and ice. These data were converted to a Minnaert model-based When independent information suggests that dust is the normal albedo (k 5 0.7) and recalibrated so that they dominant source of opacity, MOLA opacity can be could be compared with the 1064-nm reflectivity map of converted to TES equivalent by dividing by a factor of Heavens (2017). The apparent brightness of Mars at 2.6 with an uncertainty of up to 30% (Montabone et al. 1064 nm is relatively insensitive to opacity at low opacity 2015; Heavens 2017). The analogous conversion for but quite sensitive at opacities greater than 1 (Clancy water ice can be accomplished by dividing by ;3 et al. 2003; Szwast et al. 2006; Heavens 2017), so passive (Clancy et al. 2003; Smith 2004). radiometry may be a useful indicator of relative opacity Reflectivity at 1064 nm was estimated from the 0.583 variations in dust storms during the day. 0.58 map of Heavens (2017, 2016). Under clear conditions Limb radiance observations and retrievals of vertical over most surfaces, laser pulse returns exceeded the digital temperature, dust opacity, and water ice opacity profiles range of the MOLA detector, which resulted in RT being a (from the surface to 80 km) from nadir/off-nadir and minimum estimate. For saturated returns, opacity esti- limb observations by MCS on board MRO were used to mated from Eq. (1) is an upper bound, while the lower complement MARCI imagery. Vertical profiles are re- bound is zero. As the strength of MOLA’s laser di- trieved on pressure coordinates, which can be refer- minished, saturationwaslesscommon(Neumann et al. enced to an altitude grid relative to the surface and 2003). The uncertainty in saturated and unsaturated re- derived from the pointing of the instrument. The not- turns is fully discussed by Heavens (2017), but note that the yet-publicly-available version-5 retrievals (Kleinböhl 1s uncertainty in column opacity is #60.1 for unsaturated et al. 2017) are presented in this study, but it has been surface returns. As a result of uncertainty about aerosol verified that using version 4 (listed in Table 1) does not forward scattering to the MOLA detector, it is possible materially change the results.

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FIG. 5. Examples of dust storms with ruffled texture from MY 24 (season of observation indicated in the panel titles). (a)–(c) Images of entire storms and (d)–(f) magnified images from the boxes in (a)–(c), respectively.

Retrieved opacities were converted to density- boundary layer properties when unperturbed by the scaled opacity, a quantity proportional to mass mix- local dust storm. The DSASS assumes a globally uni- ing ratio within uncertainties about aerosol properties. form visible dust opacity of 5 and so is appropriate for The density-scaled opacity in meters squared per estimating a dust storm’s effects on convective kilogram is 12 000 times the dust mass mixing ratio in boundary layer height h. parts per million (Heavens et al. 2011a, 2015). The Hourly binned meteorological data from the watericeopacitymustbedividedbyafactorof1.5 Viking Lander 2 (VL2)duringMYs12and13pro- when comparing with TES to account for the differ- vided guidance about surface wind speed and di- ence between absorption-only and extinction opacities rection (Hess et al. 1977; Tillman and Johnson 1997). (wavelengths are similar) (Smith 2004). MCS dust VL2 observed near 488N, 1348E a site well outside the opacity must be multiplied by a factor of 2.7 to be ROI but in a similarly low-elevation area in the compared with TES (Montabone et al. 2015), 7.3 to northern midlatitudes. Thus, this is the dataset of compare with a visible opacity at 660 nm (Heavens surface wind observations most easily compared with et al. 2011a), and therefore ;7.8 to compare with the ROI. opacity at 1064 nm [following Ockert-Bell et al. (1997) e. Focus of the study/dust storm survey of the ROI and Clancy et al. (2003)]. This study focuses on textured local dust storms d. Climatological guidance in support of within the ROI (or mostly so) during the early to interpretation midafternoon [1300–1600 local solar time (LST)], as Data from the Mars Climate Database (MCD), ver- determined by survey of MOC and MARCI imagery sion 5.2 (a reference atmospheric database based on during two Mars years, one during the MGS era (MY GCM output), was downloaded for two scenarios: cli- 24), and one during the MRO era (MY 29). Textured matology average solar scenario (CASS) and dust local dust storms that appear directly associated with storm average solar scenario (DSASS) (Forget et al. frontal boundaries were excluded, because these 1999; Millour et al. 2015; MCD 2016). The CASS data storms produce greater dust cloud cover outside the are appropriate guidance for the circulation and ROI than inside it. This focus optimizes the

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FIG. 6. Examples of dust storms with ruffled texture from MY 29 (season of observation indicated in the panel titles). (a)–(c) Images of entire storms in imagery processed by Wang and Richardson (2015) (8 points per degree) and boxes outlining the bounds of the insets displayed in the panels directly below. (d)–(f) MARCI imagery processed as described in section 2 at a resolution of 12 points per degree. The images in (d)–(f) have been stretched to enhance the contrast between light and dark areas, and the contrast of all panels has been uniformly increased.

observing of textured local dust storms to periods The exception is the unusually early storm observed from which the fullest complement (as measured by at Ls 5 147.538 in MY 29 (Fig. 3). During MY 29, local number and functionality of individual instruments) and regional dust storm activity (normally minimal in of spacecraft observations were available and global northern spring and summer) commenced earlier than dust storm activity was absent. normal (;Ls 5 1358)(Smith 2009; McCleese et al. 2010; Note that observations from MGS instruments did Heavens et al. 2011a). Such ‘‘early season dust storm not begin until early northern summer of MY 24. The activity’’ was not observed during MY 24 (Smith 2009) potential for textured local dust storm activity in the but would have been observable in TES observations

ROI prior to the commencement of observations by (from Ls 5 1048) before MOC daily global mapping MGS will be considered in the next section. commenced. In fact, at least two storms were ob- served just beyond the eastern margin of the ROI in

the Ls 5 1408Ls 5 1408–1508 period (Cantor et al. 2006). 3. Results However, the storm at Ls 5 147.538 in MY 29 is well within the ROI. The potential link between early-season a. Climatology dust storm activity and local dust storm seasonality in the All but one of the textured local dust storms in the ROI ROI suggests that it is unlikely that storms occurred in occur during two periods: Ls 5 1908–2308 (early northern MY 24 prior to MOC daily global mapping. Several fall) and Ls 5 2908–3408 (the middle of northern winter) storms occurred in MY 24 after Ls 53278, so it is possible (Fig. 3). The first period is far less active than the second that a few storms were missed during the interruption in period in MY 24, but both periods are similarly active in MARCI coverage in MY 29 (Fig. 3). However, the fun-

MY 29. Activity is minimal around Ls 5 2708 (northern damental climatology is unaltered by the gaps in winter solstice). coverage.

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TABLE 2. List of storms found by the survey defined in section 2e. Both the Ls (8) in which the storm is observed and the mean Ls (8)of the daily global map [as given in the databases associated with Wang and Richardson (2015)] are listed. The assessment of ruffled texture counts only the unambiguous cases at the best available resolution.

MY Ls (storm) Ls (daily global map) Sounder data? Ruffled? 24 210.11 210.15 Yes Yes 24 219.57 219.22 No Yes 24 312.53 312.22 No Too small to tell 24 314.89 314.65 No Yes 24 317.85 317.65 Yes Yes 24 323.67 323.43 No Too small to tell 24 324.23 323.98 Yes Yes 24 331.05 331.04 No No 24 332.75 332.65 No No 24 333.33 333.08 No Yes 24 334.45 334.25 Yes Too small to tell 24 335.54 335.17 Yes No 24 337.75 337.51 Yes Yes 29 147.53 147.4 Yes No 29 199.02 199.2 Yes Yes 29 203.30 203.2 No Yes 29 209.51 209.4 Yes No 29 210.79 210.6 Yes No 29 212.64 212.8 Yes No 29 225.36 225.5 Yes No 29 227.95 227.8 Yes No 29 291.80 291.9 Yes No 29 300.50 300.3 Yes No 29 302.31 302.5 Yes Yes 29 304.76 304.8 Yes No 29 312.56 312.4 No No 29 320.81 320.8 Yes Yes 29 322.57 322.8 No No 29 327.15 327.0 Yes Yes b. Visible presentation of storms The clearest examples of ruffled texture are from three storms during MY 24 (Fig. 5). The MOC imagery Storms vary in both morphology (shape and extent of resolves alternating bright and shadowed bands with the storm as a whole) and texture (cloud-top appearance in some curvature or bifurcation. The wavelength of this terms of structure, lack of structure, and contrast with the banding is ;30 km (4 times the resolution of the image). surrounding surface). Storms can be up to 1500 km long In Fig. 5d, the direction of banding is southwest– and 500 km wide (Fig. 4a), an isolated cloud no more than northeast, while it is approximately south–north in 100 km in any dimension (Fig. 4b), or quasi rectangular and Figs. 5e and 5f. Some storms may have ruffled texture of intermediate size (typical) (Fig. 4c). The northern that is only partially resolved at the resolution of the boundary of most storms is the dark reflectivity feature in imagery, as could explain the appearance of two small the ROI (Figs. 4a–c,e). Some storms appear to curve areas of alternating bright and shadowed oval clouds in around this feature (Figs. 4a,c), while others are aligned Fig. 4c. Other storms are too small for texture to be along a straight line parallel to it (Fig. 4b). However, one assessed (Fig. 4b). storm expanded to the north of it (Fig. 4d), and a few In the MARCI imagery of Wang and Richardson storms are observed far to the south of it (Fig. 4f). (2015), no storms have a ruffled texture, but several Storms also have a variety of textures. The storm in have a honeycomb texture (made up of rectangular cells Fig. 4d resembles the cumuliform dust clouds previously with bright edges) not seen in the MY 24 storms imaged reported at Mars (Strausberg et al. 2005): it has a puffy or by MOC. At higher resolution and with a simpler pho- smoky texture with some embedded clouds of ;100-km tometric correction, one example of honeycomb texture diameter. However, the most distinctive texture of storms (Fig. 6a) becomes ruffled texture with wavelengths up to in the ROI consists of linear features or bands that are 60 km and some embedded oval clouds (40 km 3 20 km) approximately aligned in parallel. This ruffled texture can (Fig. 6d). So poorly resolved ruffled texture may explain be seen in the northeast of the storm in Fig. 4a. the honeycomb appearance of other storms. In addition,

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FIG. 7. Surface temperature retrievals from TES near and over dust storms in the ROI in the context of MOC imagery. (a)–(d) Context images with the positions of TES retrievals indicated by the markers. (e) Surface temperature vs latitude (see legend). two storms with nonhoneycomb texture (Figs. 6b,c) also cases, dust storms are associated with a maximum in appear ruffled (east–west orientation) at higher resolu- water ice opacity (Figs. 9e,f). In other cases, water ice tion (Figs. 6e,f). Ruffled texture is dominant in 12 of opacity changes little in the dust storm (Figs. 9g,h). 29 (41%) storms in the ROI (Table 2). However, in all cases, dust is the dominant source of 1064-nm opacity inside the storm and is likely to be the c. Horizontal structure dominant source of 1064-nm opacity outside of it. Storms produce noticeable drops in surface temper- These high opacities (equivalent to .2.6 at 1064 nm) atures underneath their clouds, the consequence of limit the utility of MOLA active sounding measure- strongly reduced surface insolation in an atmosphere ments in storm centers. Typical surface reflectivities in with a radiative relaxation time scale 40–60 times the ROI ’ 0.4 (Fig. 2b). As a result of absorption of the smaller than Earth’s (Goody and Belton 1967; Read MOLA laser, it is difficult to measure RT values less et al. 2015) and the low thermal inertia (Fig. 1c). There than 0.02 (Neumann et al. 2003). So 1064-nm opacities are typically breaks in TES observations/retrievals of greater than 1.5 will be difficult to measure. storms (Figs. 7a–d) and little data poleward of 458N, but Available MOLA observations oriented approxi- the extant data associate the storms with anomalous mately perpendicular to ruffled texture (e.g., the storms temperature minima of 10–30 K relative to a general in Figs. 9a,b) show that the ruffled texture is associated poleward decrease in temperature (Fig. 7e). The small- with the fine structure in column opacity. In one storm est drops are associated with the observations on storm (Fig. 10), hints of a secondary peak in dust opacity on the margins (Figs. 7c,e). Similar drops are observed by MCS southern margin of the storm are observed in TES re- in the MY 29 storms (Fig. 8). There is a 30-K drop in the trievals (Fig. 9f). However, MOLA active sounding re- unusually low-latitude storm in Fig. 8c (Fig. 8e), while solves four (and part of another) oscillations (with the unusually early storm is associated with a weak amplitude up to 0.7) in column opacity on the southern poleward surface temperature gradient (Figs. 7e, 8e). margin of the storm (Fig. 10b) and two oscillations in The storms also are associated with high TES infrared opacity on the northern margin of the storm (Figs. 10b,c). dust column opacities (Figs. 9e–h). Dust opacity gener- The minima of similar oscillations in opacity may be ally exceeds 1 in storm centers (Figs. 9e,f,h) and is visible at 37.38,37.78,and38.08N(Figs. 10b,c). The first greater than 0.5 on storm margins (Figs. 9g). In some set of oscillations corresponds to a region where the

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FIG. 8. Surface temperature retrievals from MCS near and over dust storms in the ROI in the context of MOC imagery. (a)–(d) Context images with the positions of MCS retrievals indicated by the markers. (e) Surface temperature vs latitude (see legend). ruffled texture is hard to distinguish in visible imagery oscillations in column opacity as well as another dusty (Fig. 10a). The wavelengths of the oscillations on the area outside of the ruffled texture at 428N(Figs. 11b,c). southern margin of the storm shorten to the north and Passive radiometry indicates that the area is dimmer are even smaller on the northern margin of the storm. than normal to the northeast of the storm (a possible Passive radiometry also suggests that this part of the shadowing effect), but there is a brightness oscillation storm (31.58–348N) is similarly bright to the surface. associated with the dusty area at 428N(Fig. 11d). Yet there are small variations in brightness that cor- TES observed this storm to the west of MOLA, relate with the peaks in opacity (Figs. 10b,d). As con- yielding the only continuous TES radiance observations ditions become too opaque for active sounding perpendicular to ruffled texture (Fig. 9c). TES surface (northward of 348N), oscillations in reflectivity in pas- temperatures vary the most where the oscillations in sive radiometry continue and the reflectivity increases column opacity are observed (Figs. 12b,d). Accounting (Fig. 10d), in line with the ruffled texture and increased for detector arrangement, four troughs in surface tem- brightness of the visible image (Fig. 10d). The extended perature are resolved (Fig. 12c). These troughs are dimness of the area north of the storm relative to the deeper the farther west the observations are made. The surface may indicate the shadow of the storm or un- northernmost three of the troughs are at the approxi- certainties in the surface reflectivity map. mate latitude of the column opacity peaks (Figs. 12b,d). The wavelengths of the oscillations (and thus the scale Therefore, minima in surface temperature closely align of the ruffled texture) were quantified by measurement with maxima in column opacity. The approximate of their peak-to-trough distance in latitude (corrected by wavelength of the ruffled texture here is 24 km. ;58 orientation of the ruffles off of east–west) and d. Vertical structure doubling. The first four oscillations on the southern margin have wavelengths of 70, 42, 32, and 24 km. The The vertical extent of the storms can be inferred from two resolved oscillations on the northern margin have their impact on the thermal structure and vertical dust wavelengths of 10 and 9 km. distribution of the atmosphere. While the dust storms In another storm (Fig. 11), MOLA observations are studied here have a significant impact on surface tem- on the northeastern side of the storm, just on the margin perature, they only impact atmospheric thermal struc- of dust clouds with ruffled texture (Fig. 10). MOLA ture within the first atmospheric scale height. TES active sounding observes three (or perhaps up to five) retrievals at the highest pressure level typically reported

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FIG. 9. Horizontal opacity structure of example storms in MY 24. (a)–(d) Context images with the positions of TES retrievals indicated by the black markers. (e)–(h) Retrieved TES dust and water ice column opacities in the vicinity of the dust storm shown in the panel directly above. in this area (and therefore nearest the surface) only Therefore, the impact of the storm on the thermal have a negative poleward slope (Fig. 13). There is no structure is limited to the first atmospheric scale height evidence for atmospheric heating by dust, nor for the or less. The TES atmospheric temperature retrievals temperature depression seen in the surface temperature may not capture the entirety of this cooling, because retrieval (Fig. 13). they are smoothed at a resolution of 0.75 scale heights The limited vertical extent of the storms is also im- (Conrath et al. 2000). plied by TES radiance data. Consistent with the surface The impact of storms on thermal structure also can be temperature retrievals, areas outside a storm or within characterized using MCS data by estimating a back- less-dusty areas inside a storm have uniformly higher ground temperature field from interpolation to the brightness temperatures in continuum emission outside north and south of a dust storm (the red points in 21 the 15-mm (667 cm ) band of CO2 (Fig. 14). In addition, Fig. 15a) and differencing it from a temperature re- less dusty areas have uniformly higher brightness tem- trieval near the dust storm center (the blue point in peratures inside the 15-mm band of CO2, though Fig. 15a). This storm cools the surface by ;30 K brightness temperatures in less dusty areas are ,5K (Fig. 15c) but has a cooling impact of 1–2 K in the first 2 warmer within 30–40 cm 1 of the band center (Figs. 14b,d). scale height above the surface (Fig. 15c). The storm has First, this contrast in spectra suggests that the dust no apparent impact on the local temperature field storm has an overall negative effect on atmospheric (Fig. 15b), so the larger temperature differences at temperature. Second, the much smaller difference pressures lower than 100 Pa are probably due to atmo- between dust storm and non–dust storm spectra in the spheric variability unrelated to the dust storm. While it is

15-mmbandofCO2 suggests that this negative effect is possible that the dust storm cooling is poorly resolved fairly shallow. Contribution functions for atmospheric because of the broad weighting functions associated temperature in the 15-mmbandofCO2 peak at 500 Pa with MCS retrievals at low altitudes (Kleinböhl et al. 2 at ;40 cm 1 from the band center and at 300 Pa 2009), the temperature impact of the dust storm still 2 at ;30 cm 1 from the band center (Conrath et al. 2000). appears to be restricted to the first atmospheric Therefore, most cooling is at pressures higher than scale height. between 300 and 500 Pa. Estimated surface pressures in Dust in most storms is confined to the first atmo- the vicinity of these dust storms are ;800 Pa (Fig. 14). spheric scale height as well. A typical example is shown

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FIG. 10. (a) MOC image of a portion of a ruffled storm (MY 24, Ls 5 317.858) with the location of MOLA returns indicated with black crosses. (b) The 1064-nm aerosol column opacity in the vicinity of the storm estimated from MOLA active sounding data. Missing data are a result of nonsurface returns, from which a column opacity cannot be estimated. Black dots indicated saturated returns, while red crosses indicate unsaturated returns. (c) As in (b), but focused on the northern margin of the storm. (d) Normal reflectivity in the vicinity of the storm estimated from MOLA passive radiometry data. The black line indicates the estimated surface reflectivity at 0.58 resolution (Heavens 2017). in Fig. 16. Far to the south of the storm, there is a DDL demonstrates that the dust in the storm is confined be- with mass mixing ratio of up to 30 ppm, but mass mixing low 8 km. Note that if the particle size of dust were larger ratios over the storm are less than 10 ppm (Fig. 16). in the storm than assumed, the ratio of 1064 nm to MCS Retrievals over the storm are as low as 8 km above the dust opacity would decrease (Clancy et al. 2003; surface, and the total 1064-nm column opacity inferred Kleinböhl et al. 2009), resulting in lower inferred 1064-nm from the MCS dust retrievals is ;0.35, much less than column opacity and higher expected dust mass mixing the .1.5 or .2.6 implied by MOLA and TES observa- ratio than calculated here. tions of the centers of dust storms in the ROI during MY A few storms may extend to higher altitudes. A dust 24. Thus, a 1064-nm column opacity of ;2 must be mass mixing ratio of ;25 ppm is observed 20 km above confined below 8 km. A well-mixed dust distribution to the surface on the southern margin of one storm some height zc has an opacity of approximately (Figs. 17a,b). Mass mixing ratios of up to 100 ppm are ð observed 40 km above the surface in the unusually early z dt c 2z dust storm, while a DDL of 30–40 ppm is observed to the t 5 r z 3 exp , (2) 0 r north of this storm (Figs. 18a,b). Visible imagery and the 0 0 H failure of the surface temperature retrieval suggest total t r where H is the atmospheric scale height, (d z/ )0 is the column opacity is high (Figs. 8a,e). Yet this storm may r surface dust density-scaled opacity, and 0 is the surface not have been a vertically continuous dust cloud; the air density. Note that the dimensions of opacity dtz are failure of MCS retrievals at 40 km and below could be inverse length. Assuming a 1064-nm column opacity of due to an unusually thick DDL confined around 40 km 2, surface pressure of 800 Pa, a surface temperature of lying over a storm confined to the boundary layer. In 220 K, and an atmospheric scale height of 10 km, the addition, the absence of retrieval information below expected dust mass mixing ratio in the storm is 29 ppm. 40 km does not allow the storm’s temperature impact to That such a high dust mass mixing ratio is not observed be determined.

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FIG. 11. (a) MOC image of a portion of a ruffled storm (MY 24, Ls 5 324.238) with the location of MOLA returns indicated with black crosses. (b) The 1064 nm aerosol column opacity in the vicinity of the storm estimated from MOLA active sounding data. Missing data are a result of nonsurface returns, from which a column opacity cannot be estimated. Black dots indicate saturated returns, while red crosses indicate unsaturated returns. (c) As in (b), but focused on the region of closest proximity between the storm and MOLA observations. (d) Normal reflectivity in the vicinity of the storm estimated from MOLA passive radiometry data. The black line indicates the estimated surface reflectivity at 0.58 resolution (Heavens 2017).

In another case, a DDL is observed over a dust storm, surface emission. As aerosol opacity decreases with but the bulk of the dust storm’s dust still appears to be height, so does emission from aerosol in A5. Some ad- confined below 8 km (Figs. 19a–c). Directly attributing ditional radiance may come from scattering into the the DDL to the storm is not straightforward. Figures 16a limb or from the surface into the field-of-view wings of and 17b show that DDLs are common on the southern the detector (Kleinböhl et al. 2009, 2011, 2015). And, margin of the ROI. Observations on the sol prior to the thus, A5 radiance should follow latitudinal surface dust storm show a DDL in the ROI at 258N(Fig. 20a)of temperature trends. DDLs change this simple picture by similar magnitude to that observed over the storm reducing or even reversing the decay of opacity with (Fig. 20b), yet the layer on the prior sol does not appear height, resulting in additional emission that can be seen near a textured dust storm. Caution thus must be exer- as a weaker (or even reversed) lapse rate in brightness cised in using DDLs as the sole indicator of dust storm temperature or a horizontally contrasting feature vertical mixing. The most that can be said in favor of (Heavens et al. 2015). Such a feature can be seen at deeper vertical mixing is that the storm was mostly 30–40 km above the surface in Fig. 21b between 278 confined to the first atmospheric scale height, but a few and 378N, the location of the DDL resolved in Fig. 19a. powerful updrafts may have existed within it. The unusually early storm (centered at 358N) is associ- However, comparison of the limb radiance data ated with an unusually weak gradient of brightness tem- around both storms (Fig. 21) suggests that the unusually perature above the surface that horizontally contrasts early storm was associated with a continuous, deeply with the surrounding atmosphere sufficiently well to mixed layer of dust rather than a narrow DDL. Radi- form a bump. Detailed interpretation of this feature ance observations in the dust-sensitive A5 channel would require a radiative transfer model (Fig. 21a), but normally decrease steeply and uniformly with height, the considerable emission between the surface and 35 km because the atmosphere emits weakly in A5 unless implies dust opacity is high throughout this range, not aerosol is present. At or below the surface, A5 observes just near the surface and 40 km. Radiance data in

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FIG. 12. (a) MOC image of a portion of a ruffled storm (MY 24, Ls 5 324.238) with the location of TES radiance and MOLA obser- vations, as indicated by the legend. The notation x 1 y indicates detectors x and y. (b) Surface brightness temperature (K) retrieved from TES radiance observations in the vicinity of the storm. Marker colors are as in the legend. (c) The 1064-nm aerosol column opacity in the vicinity of the storm estimated from MOLA active sounding data. Saturated returns are the low-opacity data. (d) Normal reflectivity in the vicinity of the storm estimated from MOLA passive radiometry data. The black line indicates the estimated surface reflectivity at 0.58 resolution (Heavens 2017). temperature-sensitive channels (omitted for brevity) do shear, whereby they become two-dimensional con- not show a significant atmospheric temperature effect. vective rolls (Young et al. 2002). The ruffled texture certainly appears two-dimensional. There is no way to determine whether the ruffles are 4. Discussion regions of alternating vertical wind direction. However, In this section, it is proposed that most of the dust if dust lifting rates do not spatially vary, areas of con- storms in the ROI are structurally and dynamically vergent upward flow will be dustier than areas of di- analogous to wide mixed-layer rolls in Earth’s atmo- vergent downward flow. Thus, the opacity variations in sphere. Alternative hypotheses and exceptional cases these storms are consistent with the hypothetical vertical also will be considered. The applicability of this result to wind structure. Moreover, most of these storms are the rest of Mars then will be discussed. confined to the first atmospheric scale height, strongly suggesting that they are a boundary layer phenomenon. a. Ruffled texture: Mediated by rolls or gravity waves? Indeed, the dust storm with the greatest vertical extent

Ruffled texture resembles cloud streets in Earth’s at- (the unusually early storm of Ls 5 147.538, MY 29) is mosphere, which are ‘‘quasi-two-dimensional [struc- unambiguously nonruffled (Fig. 4d). tures] (i.e., nearly linear),’’ ‘‘exhibit coherent up- and Yet not all quasi-two-dimensional structures are downdrafts,’’ and ‘‘are a common feature in the atmo- cloud streets. Kahn (1984) identified two other types spheric boundary layer (ABL) under windy conditions’’ on Mars: lee waves and wave clouds. Lee waves, in the (Young et al. 2002,p.997).Impliedin‘‘quasi-two- sense of Kahn (1984), are one-dimensional periodic dimensional’’ is that they are periodic. Cloud streets clouds generated by gravity wave trains associated arise from the alignment of three-dimensional con- with obstacles such as craters or volcanoes. Wave vection cells toward the direction of the mean wind clouds resemble lee waves but are not associated with

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FIG. 13. Temperature variability near four example dust storms. Each panel shows TES retrievals of temperature (K) at the surface and 783 Pa (red and black markers), as well as the estimated surface pressure (Pa) (blue line). obstacles. They thus also result from gravity wave Kahn (1984) was that the latter have ‘‘double peri- trains, just ones excited by sources other than topog- odicity,’’ likely referring to the ‘‘string of pearls’’ ap- raphy (e.g., Fritts and Alexander 2003). The distinc- pearance of some cloud streets when individual tion between wave clouds and cloud streets made by cumuli are organized into lines (Young et al. 2002).

FIG. 14. Differences between two TES spectra [converted to brightness temperature (K)] within and near two example dust storms. (a),(c) The blue and red circles indicate the locations of the spectra in MOC imagery. The distinction (a) between inside the storm and outside the storm and (c) between more or less dusty conditions is based on surface temperature. (b),(d) The difference between the spectra at the red and blue points in (a) and (c), 21 respectively. The vertical dotted lines indicate 630 and 640 cm from the center of the 15-mmCO2 band.

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FIG. 15. Estimated effect of a local dust storm on an MCS temperature profile: (a) Surface temperature data near the surface (black crosses). The red-circled data (and the higher-altitude data at the same locations) are used to estimate the background temperature structure by linear interpolation. The blue-circled data (and the higher- altitude data at the same location) are taken to be the dust storm temperature structure. (b) MCS retrieved tem- perature (K) structure in the vicinity of the dust storm. (c) Difference between the dust storm temperature profile and the background temperature profile (K).

Confusingly, recent work by Kulowski et al. (2016) streets can interact with and even generate gravity classifies textured dust storm activity in terms of waves above the cloud layer (Balaji et al. 1993; Young ‘‘pebbled,’’, ‘‘puffy,’’ and ‘‘plume-like.’’ The storm in et al. 2002; Magalhaes et al. 2011; Melfi and Palm 2012). Figs. 5a and 5d is cited as an example plume-like Moreover, mismatch between the length scales of storm, and the ROI is mapped as a hotspot of plume- gravity waves and boundary layer convection can result like storm activity. in suppression of convection in the wave troughs (Balaji Nevertheless, ruffled texture is a novel Martian cloud and Clark 1988). Thus, the propagation of gravity wave type that does not fit into the Kahn (1984) scheme and trains through even disorganized boundary layer con- may be improperly classified by Kulowski et al. (2016). vection might look like convective rolls. One difference Periodicity distinguishes ruffled texture from the elon- between these phenomena is how they interact with the gated, sharp-edged plume clouds and elongated, smooth- wind. The long axis of a convective roll aligns with the edged streak clouds previously reported in dust storms by boundary layer wind shear vector, while gravity wave Kahn (1984). Lee-wave clouds are common on Mars trains would propagate along the mean wind shear in the (Pickersgill and Hunt 1982; Kahn 1984). On one hand, inversion above the boundary layer. Thus, if rolls and they have wavelengths of 30–60 km, like ruffled texture. waves were simultaneously present, their alignment On the other hand, they are only known as condensate would require strong rotational shear perpendicular to clouds (Kahn 1984). Moreover, their dynamics cannot the long axis direction of the ruffled texture (Young explain the ruffled texture. There is no topography of et al. 2002). sufficient size, amplitude, and direction in the ROI for the Gravity waves are an imperfect explanation for the ruffled texture to be an obstacle to form them (Fig. 1a). length scales of ruffled texture. On one hand, the The ruffled texture more strongly resembles wave clouds wavelength change across the storm in Fig. 10 could be and cloud streets, which, too, were only observed in due to resonant interaction between gravity waves and condensate clouds by Kahn (1984). boundary layer convection as wind and stability condi- Wave clouds and cloud streets can be difficult to dis- tions change as a result of the storm (Melfi and Palm ambiguate. The convective rolls that make up cloud 2012). In this case, the wavelength l is found by

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FIG. 16. Dust distribution in the vicinity of the labeled dust storm. (a) Dust distribution in the vicinity of the dust storm expressed as mass mixing ratio (ppm) estimated from MCS retrievals. The location of the storm center indicated by surface temperature data is marked with a black circle. Areas where data are interpolated beyond the normal spacing of MCS retrievals have been covered by white space. (b) The lowest altitude above the surface at which data are reported for MCS dust retrievals in the vicinity of the dust storm. The datum for the center of the storm is marked with a black circle. (c) Integrated opacity of the dust retrievals in the vicinity of the dust storm converted to 1064-nm opacity. The datum for the center of the storm is marked with a black circle.

2pjV j Gravity waves also cannot explain east–west-trending l 5 sh , (3) N ruffled texture. The MCD predicts strong westerly winds above the boundary layer during the two active periods where Vsh is the boundary layer–free atmosphere wind of dust storm activity (Figs. 22a,b), exactly perpendicu- ä ä ä 21 shear, N is the Brunt–V is l frequency [0.008 s at the lar to what would be expected. In cases of north–south- typical season, latitude, and altitude of the storms (Ando trending ruffled texture of 20–30-km wavelength (e.g., et al. 2012)]. Ruffled texture wavelengths range from Fig. 6f), however, a role for gravity waves cannot be 70 km on the southern margin to 24 km in the interior of excluded. this storm (typical for the interiors of storms) to 9 km on Roll convection, however, can explain all of the the northern margin. Thus, an increase in stability variability in the orientation of the ruffled texture. The (higher N) and/or decrease in wind shear to the north east–west-trending ruffled texture would result from could explain the change in wavelength. strong westerlies associated with the jet mixing down to On the other hand, the implied wind shear at the the surface. Sufficient shear in the jet will orient the 21 storm’s southern margin would be 89 m s . Near- rolls. The MCD suggests that background northerly 21 surface winds of 30–40 m s are required to initiate winds can be stronger than westerly winds at the sur- saltation of sand particles on Mars, while direct lifting face (Fig. 22b). If this were true for the extreme winds would require even higher winds (Read et al. 2015). A during dust storms, diagonally oriented ruffled texture strong westerly jet is present above the boundary layer (Fig. 5d) could be explained. Strong northwesterly or throughout most of northern fall and winter (Fig. 22a,b). southwesterly surface winds would explain the north– So it might be possible to generate high enough shear south-trending ruffled texture (Fig. 6f), because these with easterly winds. However, the MCD predicts pre- storms would experience southerly or northerly vailing southerly or southwesterly flow near the surface shear aloft. (Figs. 22a,b). Moreover, during the two main periods of Roll convection also can explain the length scales of dust storm activity, easterly winds are rare and weak ruffled texture. Cloud streets can be classified in terms of while the prevailing winds are strong and broadly aspect ratio, the ratio of roll spacing to h. The aspect westerly at the location of VL2 (Fig. 23). ratio of ‘‘classical’’ cloud streets is 2–4. However, cloud

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FIG. 17. Dust distribution in the vicinity of the labeled dust storm. (a) Dust distribution in the vicinity of the dust storm expressed as mass mixing ratio (ppm) estimated from MCS retrievals. The location of the storm center indicated by surface temperature data is marked with a black circle. Areas where data are interpolated beyond the normal spacing of MCS retrievals have been covered by white space. (b) The lowest altitude above the surface at which data are reported for MCS dust retrievals in the vicinity of the dust storm. The datum for the center of the storm is marked with a black circle. (c) Integrated opacity of the dust retrievals in the vicinity of the dust storm converted to 1064-nm opacity. The datum for the center of the storm is marked with a black circle. streets often have broader wavelengths than the classical the ocean (Young et al. 2002), but for which there is no value (Young et al. 2002; LeMone and Meitin 1984; data for h . 2.5 km. Melfi and Palm 2012). The MCD (under CASS condi- b. Possible triggers for ruffled dust storms tions) estimates h in the ROI to be 2.5–3.8 km when dust storms are most common (Fig. 22c). Scaling based on Wide mixed-layer rolls on Earth are associated with median dust devil heights would imply that h in the ROI cold-air outbreaks over lakes or oceans (Young et al. varies between a few hundred meters and 3 km during 2002). A similar association of dust storms with post- northern fall and winter under relatively clear condi- frontal cold-air advection in the ROI is plausible. The tions (Fenton and Lorenz 2015). In addition, h might be survey already excludes dust storms associated with 75% smaller under high dust opacities (Davy et al. large frontal boundaries, so all of the dust storms sur- 2009), which is consistent with the MCD under DSASS veyed could be postfrontal. In addition, the location and conditions (h ; 0.75 km) (Fig. 22c). Thus, the aspect climatology of the storms suggests some connection ratio of the ruffled texture in the storm in Fig. 10 could between the storms and Northern Hemisphere baro- range from 2.6 to 20 (assuming the boundary layer is clinic wave activity. Note that storm activity is restricted perturbed by the dust storm: h 5 3.5 km) and from 12 to to two distinct periods, one in northern fall and the other 93 (assuming it is h 5 0.75 km). The variability across the in northern winter, with a break around northern winter storm could be explained by the boundary layer col- solstice (Fig. 3). A similar ‘‘solsticial pause’’ in the in- lapsing as the storm advanced southward, which would tensity of Mars’s Northern Hemisphere baroclinic wave imply the aspect ratio ranges from 12 at the northern activity and/or Northern Hemisphere dust storm activity margin (DSASS conditions) to 20 at the southern mar- has been observed by a variety of techniques (Wilson gin (CASS conditions). And this argument easily could et al. 2002; Wang 2007; Lewis et al. 2016; Mulholland be extended to the other storms of ruffled texture. These et al. 2016). In addition, the association of the strongest length scales are most consistent with ‘‘wide mixed-layer wind events at VL2 with westerly flow (Fig. 23d) is po- rolls,’’ which have aspect ratios of up to 18 (Young et al. tentially consistent with postfrontal cold-air advection 2002). Moreover, there is strong linear relationship be- and would explain the east–west trend of much of the tween h and aspect ratio for wide mixed-layer rolls over ruffled texture.

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FIG. 18. Dust distribution in the vicinity of the labeled dust storm. (a) Dust distribution in the vicinity of the dust storm expressed as mass mixing ratio (ppm) estimated from MCS retrievals. The location of the storm (as indicated by comparison with visible imagery and surface temperature retrieval failure) is marked with black circles. Areas where data are interpolated beyond the normal spacing of MCS retrievals have been covered by white space. (b) The lowest altitude above the surface at which data are reported for MCS dust retrievals in the vicinity of the dust storm. The data for the storm are marked with black circles. (c) Integrated opacity of the dust retrievals in the vicinity of the dust storm converted to 1064-nm opacity. The data for the storm are marked with black circles.

Moreover, cold-air advection on Mars probably would Young et al. 2002), which might be stronger in marine generate stronger convection than another type of high cold-air outbreaks becauseofhighlatentheatfluxes wind event. The character and intensity of daytime from the ocean surface. Latent heating is minimal in Martian boundary layer convection depends on the the Martian atmosphere, but specific radiative heating near-surface superadiabatic layer (Petrosyan et al. by dust can be of comparable magnitude (Heavens 2011). Advecting colder air above the surface would et al. 2011b). Large amounts of dust mixed into the strengthen the superadiabatic gradient and help main- atmosphere can warm atmospheric temperatures by tain it against the boundary layer processes that coun- tens of kelvins (Wilson 1997; Strausberg et al. 2005; teract it: atmospheric heating by convective updrafts Rafkin 2009; Spiga et al. 2013; Kass et al. 2016), and and absorption of infrared radiation from the surface high concentrations of dust at small scales might cause (Petrosyan et al. 2011). Segal et al. (1997) even argued heating rates up to hundreds of kelvins per hour that the Martian surface and the terrestrial ocean would (Fuerstenau 2006; Heavens et al. 2011b). Thus, the respond analogously to cold-air outbreaks by main- smooth, dusty surface of the ROI supplies a potential taining their surface temperature against relatively large heating source to the atmosphere analogous to the sensible heat fluxes. In the case of the ocean, this sta- potential heating source that cold winds moving over a bility in surface temperature is due to its high thermal warm ocean supplies to the atmosphere. Yet most inertia. In the case of Mars, it is due to the dominance of storms in the ROI seem to lack a warm, dust-heated radiative fluxes over sensible heat flux in the surface core. It is possible that the dust storms are being heated energy budget. The potentially low impact of Mars’s by dust, but this energy either has been advected away cold-air outbreaks on surface temperature also could from the storm or cannot be resolved vertically or explain why there is no sharp boundary in surface tem- horizontally by the observations. Interplay between perature to the north of the storms, just a continuation dynamics and observational uncertainty is also possi- of a similar temperature gradient to the one to the south ble. Spiga et al. (2013) simulates a storm in which there of the storm (Fig. 13). is a region of strong dust heating lying over a region of One theoretical explanation for the scale of wide adiabatic cooling associated with convective ascent mixed-layer rolls is latent heating (Chlond 1992; within an altitude range of ;5km: a situation that

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FIG. 19. Dust distribution in the vicinity of the labeled dust storm. (a) Dust distribution in the vicinity of the dust storm expressed as mass mixing ratio (ppm) estimated from MCS retrievals. The location of the storm center indicated by surface temperature data is marked with a black circle. Areas where data are interpolated beyond the normal spacing of MCS retrievals have been covered by white space. (b) The lowest altitude above the surface at which data are reported for MCS dust retrievals in the vicinity of the dust storm. The datum for the center of the storm is marked with a black circle. (c) Integrated opacity of the dust retrievals in the vicinity of the dust storm converted to 1064-nm opacity. The datum for the center of the storm is marked with a black circle. could not be resolved by infrared atmospheric As these storms seem to be more common in MY 29 sounders like TES or MCS. (when observations were at later average local time), it The ruffled storms’ restricted vertical extent and po- is possible that lifting has stopped or is much reduced tentially cool interiors suggest that they are not ther- from peak activity at earlier local time, resulting in modynamically self-sustaining. They are maintained breakdown of the convective rolls and merger of bands entirely by external forcing from the hypothetical cold- of ruffled texture. Boundary layer collapse under high air advection. Once the large-scale advection weakens, dust opacity may play a role as well. The intermediate the advecting airmass warms, or convection is otherwise stage in this process may be shown in Figs. 6a,b and suppressed, the storm will dissipate. 6d,e. c. Other textures d. Applicability to Martian dust storm dynamics generally Not all storms are ruffled (Table 2). In the case of the storm of Ls 5 147.538,MY29(Fig. 4d), its early date This study focused on the ROI mainly because several (Fig. 3), puffy/smoky texture, deep vertical mixing unusual-looking storms occurred there in MY 24. In- (Figs. 18 and 21a), and the unusually weak background deed, the ROI itself is unusual. North Amazonis Planitia gradient in surface temperature (Fig. 8) and deep ver- is the part of Mars with the highest known dust devil tical mixing suggest that its dynamics are entirely dif- activity as well as where the largest-diameter dust devils ferent. This storm may result from dust-heated free are observed (Cantor et al. 2006; Fenton and Lorenz convection in an environment of strong surface con- 2015). Dust devil activity peaks in northern summer but vergence with weak wind shear and weaker stability is minimal in northern fall and winter, a climatology aloft: ‘‘a rocket dust storm’’ in the sense of Spiga roughly opposite to that of local dust storms (Cantor et al. (2013). et al. 2006; Fenton and Lorenz 2015). Explanations for A few nonruffled storms are too small to evaluate high dust devil activity here include its topographic (Fig. 4b). Winds may have been marginal for dust lifting. smoothness, low altitude (high atmospheric density to Other storms have morphologies and climatology like lift dust), and high abundance of loose dust (Cantor et al. ruffled storms but do not have ruffled texture (Fig. 4e). 2006; Fenton and Lorenz 2015).

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FIG. 20. Detached dust layering on the sol prior to and the sol of the Ls 5 327.15 dust storm of MY 29. (a) MOC imagery in the vicinity of the dust storm on the previous sol, on which has been plotted the magnitude (ppm) and approximate geopotential/areopotential height (km) of the peak mass mixing ratio estimated from MCS retrievals (colored dots). The color of the dot indicates the height, and the size indicates the magnitude. (b) As in (a), but for the sol of the storm.

Dust availability is probably the surface property of organized and distinct. Smooth topography may be key the ROI that most explains its unusual dust storm ac- as well. Topographic features will generate waves that tivity. Where dust is abundant, dust storm formation interfere with periodic patterns generated by other mechanisms requiring dust will operate at peak effi- processes or else generate features like lee-wave clouds ciency. Moreover, textural features in dust storms are that may be confused with or obscure dust storm tracers of the circulation (though not necessarily pas- structures. sive ones). If dust were not present or only mobilized in Therefore, while the ROI may be optimal for ob- some parts of the area, the circulation might still exist serving ruffled texture and the underlying dynamics of but the structure of the storm would look far less local dust storms generated by cold-air outbreaks, both

21 FIG. 21. Observed A5-channel (centered at 463 cm ) brightness temperature (K) observed by MCS in the vicinity of two dust storms, as labeled. The equivalent retrieved dust fields are in Figs. 18 and 19.

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FIG. 22. MCD predictions of (a),(b) vertical winds and (c) CBL height for the location and season labeled. In the legend, W–E and S–N indicate zonal and meridional winds, respectively, and according to the usual sign convention. the texture and the dynamics it signifies are likely oc- occur in the other areas as well. Moreover, the associa- curring elsewhere. Kulowski et al. (2016)’s multiyear tion of dust storm activity with baroclinic activity is well survey of MOC imagery identifies plume-like texture in known. Forcing by frontal boundaries is often consid- areas other than the ROI. If the ruffled texture was ered (e.g., Cantor et al. 2001; Cantor 2007; Hinson and classified as plume-like in the ROI, ruffled texture may Wang 2010; Wang et al. 2011), but intense, postfrontal

FIG. 23. Scatterplots of Viking Lander 2 observations of surface winds for the years, seasonal ranges, and the range of local times indicated. Each marker indicates an individual hourly binned wind measurement.

Unauthenticated | Downloaded 10/04/21 04:35 PM UTC 1034 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 74 cold-air-advection events are likely of comparable for Mars. Third, it demonstrates that a variety of in- importance. teresting aspects of dust storm dynamics are confined to the first atmospheric scale height, necessitating the de- velopment of improved observational techniques to 5. Summary and conclusions better understand them. Finally, this work connects a Here was presented a study of local dust storm char- type of dust storm on Mars with cloud streets on Earth, acteristics in a small area of Mars that is likely highly raising further questions about why the aspect ratios of favorable for the formation of dust storms and the ob- some rolls greatly exceed the values predicted by clas- servation of discernible structure within them. This sical theory but providing a set of new test cases for study is unprecedented in the number and diversity of testing hypotheses old and new. observational datasets it applies to Martian local dust storms. The most common texture in these storms is a Acknowledgments. We thank A. Zalucha, A. Spiga, previously unreported ruffled texture characterized by G. Young, and an anonymous reviewer for helpful elongated, periodic linear variations in dust opacity. comments on this manuscript. This work was sup- These features are proposed to be structurally and dy- ported by NASA’s Mars Data Analysis and Solar namically analogous to the cloud streets on Earth, par- System Workings Programs (NNX14AM32G and ticularly the wide, mixed-layer rolls that are observed NNX15AI33G). This research has made use of over oceans and lakes during postfrontal cold-air out- the USGS Integrated Software for Imagers and breaks. While the study area is optimal for the in- Spectrometers (ISIS). vestigation of dust storm characteristics, the dynamics and even textures inferred in the area are likely not unique to it. Postfrontal cold-air advection is likely an REFERENCES important driver of local dust storm activity wherever Ando, H., T. Imamura, and T. Tsuda, 2012: Vertical wavenumber baroclinic activity on Mars is strong. It is also possible spectra of gravity waves in the Martian atmosphere obtained that ruffled texture in some storms could be explained by from Mars Global Surveyor radio occultation data. J. Atmos. Sci., 69, 2906–2912, doi:10.1175/JAS-D-11-0339.1. interactions with gravity waves. Balaji, V., and T. L. Clark, 1988: Scale selection in locally forced Some storms in the area lack ruffled texture but re- convective fields and the initiation of deep cumulus. J. Atmos. semble storms with ruffled texture in most other char- Sci., 45, 3188–3211, doi:10.1175/1520-0469(1988)045,3188: acteristics. They therefore may be due to weaker SSILFC.2.0.CO;2. outbreak events and/or represent the decay phase of ——, J. L. Redelsperger, and G. P. Klaasen, 1993: Mechanisms for the mesoscale organization of tropical cloud clusters in GATE phase ruffled storms. One storm in the area occurs unusually III.PartI:Shallowcloudbases.J. Atmos. Sci., 50, 3571–3589, early in the year, mixes dust unusually deeply, and has a doi:10.1175/1520-0469(1993)050,3571:MFTMOO.2.0.CO;2. puffy/smoky texture. This storm likely developed in an Bell, J. F., and Coauthors, 2009: Mars Reconnaissance Orbiter environment of strong surface convergence with mini- Mars Color Imager (MARCI): Instrument description, cali- mal vertical shear as opposed to the strong vertical shear bration, and performance. J. Geophys. Res., 114, E08S92, doi:10.1029/2008JE003315. environment implied by the ruffled storms. All of these Briggs, G. A., W. A. Baum, and J. Barnes, 1979: Viking Orbiter storms cool the surface, and most appear to have a imaging observations of dust in the Martian atmosphere. negative or neutral impact on atmospheric temperature, J. Geophys. Res., 84, 2795–2820, doi:10.1029/JB084iB06p02795. implicating them as cold-core weather systems that must Cantor, B. A., 2007: MOC observations of the 2001 Mars be sustained by external forcing. planet-encircling dust storm. Icarus, 186, 60–96, doi:10.1016/j.icarus.2006.08.019. This study therefore has three implications for atmo- ——, P. B. James, M. Caplinger, and M. J. Wolff, 2001: Martian dust spheric studies on Mars and one for atmospheric science storms: 1999 Mars Orbiter Camera observations. J. Geophys. in general. First, it identifies a class of dust storms with Res., 106, 23 653–23 687, doi:10.1029/2000JE001310. interesting textures that form in an area with relatively ——, K. M. Kanak, and K. Edgett, 2006: Mars Orbiter Camera simple and idealizable surface topography and thermo- observations of Martian dust devils and their tracks (Sep- tember 1997 to January 2006) and evaluation of theoretical physical properties. These storms are therefore ideal vortex models. J. Geophys. Res., 111, E12002, doi:10.1029/ targets for mesoscale simulations and idealized models 2006JE002700. of dust storm structure. These simulations also could Chlond, A., 1992: Three-dimensional simulation of cloud street confirm or invalidate the analogies between terrestrial development during a cold air outbreak. Bound.-Layer Me- and Martian atmospheric dynamics upon which this teor., 58, 161–200, doi:10.1007/BF00120757. Christensen, P. R., 2002: Mars Global Surveyor: TES Data Prod- analysis has often relied. Second, this study proposes a ucts. NASA Planetary Data System Geosciences Node, tentative large-scale dynamical mechanism for these accessed 12 August 2016. [Available online at http://pds- storms, which can be tested against reanalysis datasets geosciences.wustl.edu/missions/mgs/tes-tsdr.html.]

Unauthenticated | Downloaded 10/04/21 04:35 PM UTC APRIL 2017 H E A V E N S 1035

——, and Coauthors, 2001: Mars Global Surveyor Thermal Emis- ——, 2017: The reflectivity of Mars at 1064 nm: Derivation sion Spectrometer experiment: Investigation description and from Mars Orbiter Laser Altimeter data and application surface science results. J. Geophys. Res., 106, 23 823–23 871, to climatology and meteorology. Icarus, doi:10.1016/ doi:10.1029/2000JE001370. j.icarus.2017.01.032, in press. Clancy, R. T., B. J. Sandor, M. J. Wolff, P. R. Christensen, M. D. ——, and Coauthors, 2011a: The vertical distribution of dust in the Smith, J. C. Pearl, B. J. Conrath, and R. J. Wilson, 2000: An Martian atmosphere during northern spring and summer: intercomparison of ground-based millimeter, MGS TES, and Observations by the Mars Climate Sounder and analysis of Viking atmospheric temperature measurements: Seasonal and zonal average vertical dust profiles. J. Geophys. Res., 116, interannual variability of temperatures and dust loading in the E04003, doi:10.1029/2010JE003691. global Mars atmosphere. J. Geophys. Res., 105, 9553–9571, ——, and Coauthors, 2011b: Vertical distribution of dust in the doi:10.1029/1999JE001089. Martian atmosphere during northern spring and summer: ——, M. J. Wolff, and P. R. Christensen, 2003: Mars aerosol studies High-altitude tropical dust maximum at northern summer with the MGS TES emission phase function observations: solstice. J. Geophys. Res., 116, E01007, doi:10.1029/ Optical depths, particle sizes, and ice cloud types versus lati- 2010JE003692. tude and solar longitude. J. Geophys. Res., 108, 5098, ——, M. S. Johnson, W. Abdou, D. M. Kass, A. Kleinböhl, D. J. doi:10.1029/2003JE002058. McCleese, J. H. Shirley, and R. J. Wilson, 2014: Seasonal and ——,——,B.A.Whitney,B.A.Cantor,M.D.Smith,andT.H. diurnal variability of detached dust layers in the tropical McConnochie, 2010: Extension of atmospheric dust Martian atmosphere. J. Geophys. Res. Planets, 119, 1748–1774, loading to high altitudes during the 2001 Mars dust storm: doi:10.1002/2014JE004619. MGS TES limb observations. Icarus, 207, 98–109, ——, and Coauthors, 2015: Extreme detached dust layers near doi:10.1016/j.icarus.2009.10.011. Martian volcanoes: Evidence for dust transport by mesoscale Conrath, B. J., 1975: Thermal structure of the Martian atmosphere circulations forced by high topography. Geophys. Res. Lett., during the dissipation of the dust storm of 1971. Icarus, 24, 36– 42, 3730–3738, doi:10.1002/2015GL064004. 46, doi:10.1016/0019-1035(75)90156-6. Hess, S. L., R. M. Henry, C. B. Leovy, J. A. Ryan, and J. E. Tillman, ——,J.C.Pearl,M.D.Smith,W.Maguire,P.R.Christensen, 1977: Meteorological results from the surface of Mars: Viking S. Dason, and M. S. Kaelberer, 2000: Mars Global Surveyor 1 and 2. J. Geophys. Res., 82, 4559–4574, doi:10.1029/ Thermal Emission Spectrometer (TES) observations: At- JS082i028p04559. mospheric temperature during aerobraking and science Hinson, D. P., and H. Wang, 2010: Further observations of phasing. J. Geophys. Res., 105, 9509–9519, doi:10.1029/ regional dust storms and baroclinic eddies in the north- 1999JE001095. ern hemisphere of Mars. Icarus, 206, 290–305, Davy, R., P. A. Taylor, W. Weng, and P.-Y. Li, 2009: A model of doi:10.1016/j.icarus.2009.08.019. dust in the Martian lower atmosphere. J. Geophys. Res., 114, Kahn, R., 1984: The spatial and seasonal distribution of Martian D04108, doi:10.1029/2008JD010481. clouds and some meteorological implications. J. Geophys. Fenton, L. K., and R. Lorenz, 2015: Dust devil height and spacing Res., 89, 6671–6688, doi:10.1029/JA089iA08p06671. with relation to the Martian planetary boundary layer thick- ——, 1995: Temperature measurements of a Martian local dust ness. Icarus, 260, 246–262, doi:10.1016/j.icarus.2015.07.028. storm. J. Geophys. Res., 100, 5265–5275, doi:10.1029/94JE02766. Forget, F., and Coauthors, 1999: Improved general circulation Kass, D. M., A. Kleinböhl, D. J. McCleese, J. T. Schofield, and models of the Martian atmosphere from the surface to above M. D. Smith, 2016: Interannual similarity in the Martian at- 80 km. J. Geophys. Res., 104, 24 155–24 176, doi:10.1029/ mosphere during the dust storm season. Geophys. Res. Lett., 1999JE001025. 43, 6111–6118, doi:10.1002/2016GL068978. Fritts, D. C., and M. J. Alexander, 2003: Gravity wave dynamics in Kleinböhl, A., and Coauthors, 2009: Mars Climate Sounder limb the middle atmosphere. Rev. Geophys., 41, 3571–3589, profile retrieval of atmospheric temperature, pressure, and doi:10.1029/2001RG000106. dust and water ice opacity. J. Geophys. Res., 114, E10006, Fuerstenau, S. D., 2006: Solar heating of suspended particles and doi:10.1029/2009JE003358. the dynamics of Martian dust devils. Geophys. Res. Lett., 33, ——, J. T. Schofield, W. A. Abdou, P. G. J. Irwin, and R. J. de Kok, L19S03, doi:10.1029/2006GL026798. 2011: A single-scattering approximation for infrared radiative Gifford, F. A., 1964: A study of Martian yellow clouds that display transfer in limb geometry in the Martian atmosphere. movement. Mon. Wea. Rev., 92, 435–440, doi:10.1175/ J. Quant. Spectrosc. Radiat. Transfer, 112, 1568–1580, 1520-0493(1964)092,0435:ASOMYC.2.3.CO;2. doi:10.1016/j.jqsrt.2011.03.006. Goody, R., and M. J. Belton, 1967: A discussion of Martian ——, ——, D. M. Kass, W. A. Abdou, and D. J. McCleese, 2015: No atmospheric dynamics. Planet. Space Sci., 15, 247–256, widespread dust in the middle atmosphere of Mars from doi:10.1016/0032-0633(67)90193-6. Mars Climate Sounder observations. Icarus, 261, 118–121, Gurwell, M. A., E. A. Bergin, G. J. Melnick, and V. Tolls, doi:10.1016/j.icarus.2015.08.010. 2005: Mars surface and atmospheric temperature during ——, A. J. Friedson, and J. T. Schofield, 2017: Two-dimensional the 2001 global dust storm. Icarus, 175, 23–31, radiative transfer for the retrieval of limb emission measure- doi:10.1016/j.icarus.2004.10.009. ments in the Martian atmosphere. J. Quant. Spectrosc. Radiat. Guzewich, S. D., A. D. Toigo, L. Kulowski, and H. Wang, 2015: Transfer, 187, 511–522, doi:10.1016/j.jqsrt.2016.07.009. Mars Orbiter Camera climatology of textured dust storms. Kulowski, L., H. Wang, and A. D. Toigo, 2016: The seasonal and Icarus, 258, 1–13, doi:10.1016/j.icarus.2015.06.023. spatial distribution of textured dust storms observed by Mars Heavens, N. G., 2016: Reference surface reflectivity for Mars at Global Surveyor Mars Orbiter Camera. Adv. Space Res., 59, 1064 nm and data for recalibration of MOLA passive radi- 715–721, doi:10.1016/j.asr.2016.10.028. ometry, version 2. Mendeley data, accessed 1 November 2016, LeMone, M. A., and R. J. Meitin, 1984: Three examples of fair- doi:10.17632/smb69k52st.2. weather mesoscale boundary-layer convection in the

Unauthenticated | Downloaded 10/04/21 04:35 PM UTC 1036 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 74

tropics. Mon. Wea. Rev., 112, 1985–1998, doi:10.1175/ Montabone, L., and Coauthors, 2015: Eight-year climatology of 1520-0493(1984)112,1985:TEOFWM.2.0.CO;2. dust optical depth on Mars. Icarus, 251, 65–95, doi:10.1016/ Lewis, S. R., D. P. Mulholland, P. L. Read, L. Montabone, j.icarus.2014.12.034. and R. J. Wilson, 2016: The solsticial pause on Mars: 1. Mulholland, D. P., S. R. Lewis, P. L. Read, J. B. M. Madeleine, A planetary wave reanalysis. Icarus, 264, 456–464, and F. Forget, 2016: The solsticial pause on Mars: 2. Mod- doi:10.1016/j.icarus.2015.08.039. elling and investigation of causes. Icarus, 264, 465–477, Määttänen, A., and Coauthors, 2009: A study of the properties of a doi:10.1016/j.icarus.2015.08.038. local dust storm with Mars Express OMEGA and PFS data. Neumann, G. A., D. E. Smith, and M. T. Zuber, 2003: Two Mars Icarus, 201, 504–516, doi:10.1016/j.icarus.2009.01.024. years of clouds detected by the Mars Orbiter Laser Altimeter. Magalhaes, J. M., I. B. Araújo, J. C. B. da Silva, R. H. J. Grimshaw, J. Geophys. Res., 108, 5023, doi:10.1029/2002JE001849. K. Davis, and J. Pineda, 2011: Atmospheric gravity waves in Ockert-Bell, M. E., J. F. I. Bell, J. B. Pollack, C. P. McKay, and the Red Sea: A new hotspot. Nonlinear Processes Geophys., F. Forget, 1997: Absorption and scattering properties of the 18, 71–79, doi:10.5194/npg-18-71-2011. Martian dust in the solar wavelengths. J. Geophys. Res., 102, Malin, M. C., and K. S. Edgett, 2001: Mars Global Surveyor Mars 9039–9050, doi:10.1029/96JE03991. Orbiter Camera: Interplanetary cruise through primary mis- Petrosyan, A., and Coauthors, 2011: The Martian atmospheric sion. J. Geophys. Res., 106, 23 429–23 570, doi:10.1029/ boundary layer. Rev. Geophys., 49, RG3005, doi:10.1029/ 2000JE001455. 2010RG000351. MARCI, 2016: PDS Imaging Node: Data Archive. NASA Plane- Pickersgill, A. O., and G. E. Hunt, 1982: A comparison of observed tary Data System Imaging Node, accessed 24 May 2016. lee-waves on Earth and Mars. Weather, 37, 98–108, [Available online at http://pds-imaging.jpl.nasa.gov/data/mro/ doi:10.1002/j.1477-8696.1982.tb03571.x. mars_reconnaissance_orbiter/marci/.] Piqueux, S., S. Byrne, H. H. Kieffer, T. Titus, and C. J. Hansen, Martin, L. J., and R. W. Zurek, 1993: An analysis of the history of 2015: Enumeration of Mars years and seasons since the be- dust activity on Mars. J. Geophys. Res., 98, 3221–3246, ginning of telescopic exploration. Icarus, 251, 332–338, doi:10.1029/92JE02937. doi:10.1016/j.icarus.2014.12.014. McCleese, D. J., and Coauthors, 2007: Mars Climate Sounder: An Putzig, N. E., and M. T. Mellon, 2007: Apparent thermal inertia and investigation of thermal and water vapor structure, dust and the surface heterogeneity of Mars. Icarus, 191, 68–94, condensate distributions in the atmosphere, and energy bal- doi:10.1016/j.icarus.2007.05.013. ance of the polar regions. J. Geophys. Res., 112, E05S06, ——, ——, T. L. Heet, and J. G. Ward, 2009: MGS Mars TES doi:10.1029/2006JE002790. derived thermal inertia maps v1.0, mgs-m-tes-5-timap- ——, and Coauthors, 2010: Structure and dynamics of the Martian v1.0. NASA Planetary Data System Geosciences Node, lower and middle atmosphere as observed by the Mars Cli- accessed 1 November 2016. [Available online at http://pds- mate Sounder: Seasonal variations in zonal mean tempera- geosciences.wustl.edu/mgs/mgs-m-tes-5-timap-v1/mgst_9001/ ture, dust, and water ice aerosols. J. Geophys. Res., 115, data/global_ti_day_2007.img.] E12016, doi:10.1029/2010JE003677. Rafkin, S. C. R., 2009: A positive radiative-dynamic feedback MCD, 2016: Mars climate database v5.2: The web interface. LMD mechanism for the maintenance and growth of Martian du CNRS, accessed 1 November 2016. [Available online at dust storms. J. Geophys. Res., 114, E01009, doi:10.1029/ http://www-mars.lmd.jussieu.fr/mcd_python/.] 2008JE003217. MCS, 2016a: MRO MCS derived data records (DDR), version 4 Read, P. L., S. R. Lewis, and D. P. Mulholland, 2015: The physics of (August 2015). NASA Planetary Data System Atmospheres Martian weather and climate: A review. Rep. Prog. Phys., 78, Node, accessed 1 November 2016. [Available online at http:// 125901, doi:10.1088/0034-4885/78/12/125901. atmos.nmsu.edu/data_and_services/atmospheres_data/ Ruff, S., 2016: TES dust cover index. Arizona State University, MARS/mcs.html.] accessed 1 November 2016. [Available online at http://www. ——, 2016b: MRO MCS reduced data records (RDR). NASA mars.asu.edu/;ruff/DCI/dci_lo_ice_dust_16ppd_shifted. Planetary Data System Atmospheres Node, accessed 1 No- vicar.] vember 2016. [Available online at http://atmos.nmsu.edu/ ——, and P. R. Christensen, 2002: Bright and dark regions on Mars: data_and_services/atmospheres_data/MARS/mcs.html.] Particle size and mineralogical characteristics based on Melfi, S. H., and S. P. Palm, 2012: Estimating the orientation and Thermal Emission Spectrometer data. J. Geophys. Res., 107, spacing of midlatitude linear convective boundary layer fea- 2-1–2-22, doi:10.1029/2001JE001580. tures: Cloud streets. J. Atmos. Sci., 69, 352–364, doi:10.1175/ Segal, M., R. W. Arritt, and J. E. Tillman, 1997: On the potential JAS-D-11-070.1. impact of daytime surface sensible heat flux on the dissipation Millour, E., and Coauthors, 2015: The Mars Climate Database (MCD of Martian cold air outbreaks. J. Atmos. Sci., 54, 1544–1549, version 5.2). Extended Abstracts, European Planetary Science doi:10.1175/1520-0469(1997)054,1544:OTPIOD.2.0.CO;2. Congress 2015, Nantes, France, European Planetary Science Smith, D. E., and Coauthors, 2001: Mars Orbiter Laser Altimeter: Congress, P23, ESPSC2015-438. [Available online at http:// Experiment summary after the first year of global mapping meetingorganizer.copernicus.org/EPSC2015/EPSC2015-438.pdf.] of Mars. J. Geophys. Res., 106, 23 689–23 772, doi:10.1029/ MOLA, 2016a: Mars Global Surveyor: MOLA PEDRs. NASA 2000JE001364. Planetary Data System Geosciences Node, accessed 8 July ——, G. Neumann, R. E. Arvidson, E. A. Guinness, and S. Slavney, 2015. [Available online at http://pds-geosciences.wustl.edu/ 2003: Mars Global Surveyor laser altimeter mission experi- missions/mgs/pedr.html.] ment gridded data record: MGS-M-MOLA-5-MEGDR-L3- ——, 2016b: Mars Global Surveyor: MOLA PRDRs. NASA V1.0. NASA Planetary Data System, accessed 1 November Planetary Data System Geosciences Node, accessed 8 July 2016. [Available online at http://pds-geosciences.wustl.edu/ 2015. [Available online at http://pds-geosciences.wustl.edu/ mgs/mgs-m-mola-5-megdr-l3-v1/mgsl_300x/meg016/megt90- missions/mgs/prdr.html.] n000eb.img.]

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Smith, M. D., 2004: Interannual variability in TES observations Wang, H., 2007: Dust storms originating in the northern of Mars during 1999–2003. Icarus, 167, 148–165, doi:10.1016/ hemisphere during the third mapping year of Mars Global j.icarus.2003.09.010. Surveyor. Icarus, 189, 325–343, doi:10.1016/j.icarus.2007.01.014. ——, 2009: THEMIS observations of Mars aerosol optical ——, 2016: Mars Daily Global Map Archive. Harvard- depth from 2002–2008. Icarus, 202, 444–452, doi:10.1016/ Smithsonian Center for Astrophysics, accessed 17 Novem- ; j.icarus.2009.03.027. ber 2016. [Available online at https://www.cfa.harvard.edu/ Spiga, A., J. Faure, J. B. Madeleine, A. Määttänen, and F. Forget, hwang/mdgm/; MARCI imagery available at https://www.cfa. ; 2013: Rocket dust storms and detached dust layers in the harvard.edu/ hwang/mdgm/marci.] Martian atmosphere. J. Geophys. Res., 118, 746–767, ——, and M. I. Richardson, 2015: The origin, evolution, and doi:10.1002/jgre.20046. trajectory of large dust storms on Mars during Mars years Strausberg, M. J., H. Wang, M. I. Richardson, S. P. Ewald, and 24–30 (1999–2011). Icarus, 251, 112–127, doi:10.1016/ A. D. Toigo, 2005: Observations of the initiation and evolution j.icarus.2013.10.033. of the 2001 Mars global dust storm. J. Geophys. Res. Planets, ——, A. D. Toigo, and M. I. Richardson, 2011: Curvilinear features 110, E02006, doi:10.1029/2004JE002361. in the southern hemisphere observed by Mars Global Sur- Sun, X., G. A. Neumann, J. B. Abshire, and M. T. Zuber, 2006: veyor Mars Orbiter Camera. Icarus, 215, 242–252, doi:10.1016/ Mars 1064 nm spectral radiance measurements determined j.icarus.2011.06.029. from the receiver noise response of the Mars Orbiter Laser Wilson, R. J., 1997: A general circulation model of the Martian Altimeter. Appl. Opt., 45, 3960–3971, doi:10.1364/ polar warming. Geophys. Res. Lett., 24, 123–126, doi:10.1029/ AO.45.003960. 96GL03814. Szwast, M. A., M. I. Richardson, and A. R. Vasavada, 2006: Surface ——, D. Banfield, B. J. Conrath, and M. D. Smith, 2002: Traveling dust redistribution on Mars as observed by the Mars Global waves in the northern hemisphere of Mars. Geophys. Res. Surveyor and Viking orbiters. J. Geophys. Res., 111, E11008, Lett., 29, 29-1–29-4, doi:10.1029/2002GL014866. doi:10.1029/2005JE002485. Young, G. S., D. A. R. Kristovich, M. R. Helmfelt, and R. C. Tillman, J. E., and N. C. Johnson, 1997: Viking Lander 2 binned Foster, 2002: Rolls, streets, waves, and more: A review of and splined data. Arizona State University, accessed 17 No- quasi-two-dimensional structures in the atmospheric bound- vember 2016. [Available online at http://www-k12.atmos. ary layer. Bull. Amer. Meteor. Soc., 83, 997–1001, doi:10.1175/ washington.edu/k12/mars/data/vl2/.] 1520-0477(2002)083,0997:RSWAMA.2.3.CO;2.

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