Marswrf Convective Vortex and Dust Devil Predictions for Gale Crater

Home , Dust devil, Dust devil tracks

RESEARC H ARTICLE Mars W RF Convective Vortex and Dust Devil Predictions 10.1029/2019J E006082 for Gale Crater Over 3 Mars Years a nd Co mpariso n Key Poi nts: Wit h MS L ‐RE MS Observations • Mars W RF output co mbined with ther modyna mic theory is used to C. E. Ne w man 1 , H. Kahanpää 2 , M. I. Ric hardso n 1 , G. M. Martí nez 3, 4 , predict te mporal a nd spatial tre nds 4 5 of “ d ust devil activity ” i n Gale Crater A. Vice nte ‐Ret ortill o , and M. T. Le m mon • Modeled activity a nd observed 1 2 3 vortex press ure drops are bot h Aeolis Research, Pasadena, C A, US A, Sc hool of Electrical E ngi neeri ng, Aalto U niversity, Espoo, Fi nla nd, Lunar and greatest i n local su m mer, peaki ng Pla netary I nstitute, U niversities Space Researc h Associatio n, Housto n, T X, US A, 4 College of E ngi neeri ng, U niversity of ~ 1 3: 0 0 ‐14:00, a nd s mallest i n wi nter Michigan, Ann Arbor, MI, US A, 5 Space Scie nce I nstit ute, College Statio n, T X, US A • Se nsible heat fl ux drives i ncrease d activity as MS L cli mbs, b ut press ure drop n u mbers i ncrease faster, u nless a t hres hol d activity is use d A bstract Convective vortices and dust devils have been inferred and observed in Gale Crater, Mars, using Mars Science Laboratory ( MS L) meteorological data and ca mera i mages. Rennó et al. (1998, https:// Supporti ng I nfor matio n: doi.org/10.1175/1520 ‐0469(1998)055 <3244:asttfd >2.0.co;2) modeled convective vortices as convective heat • Supporting Infor mation S1 e ngi nes a nd predicted a “ d ust devil activity ” ( D D A) that depends only on local meteorological variables, s p e ci fi cally t he se nsible heat fl ux and the vertical ther modyna mic ef fi cie ncy w hic h i ncreases wit h t he pressure thickness of the planetary boundary layer. This work uses output fro m the Mars W RF General Correspo nde nce to: C. E. Ne w man, Circ ulatio n Model, r u n wit h hig h ‐resol utio n nests over Gale Crater, to predict D D A as a f u nctio n of locatio n, claire @aeolisresearch.co m ti me of day, a nd seaso n, a nd co mpares t hese predictio ns to t he record of vortices fou nd i n MS L's Rover Environ mental Monitoring Station pressure data set. Much of the observed ti me ‐of ‐day a nd seaso nal

Cit ati o n: variatio n of vortex activity is captured, suc h as maxi mu m ( mi ni mu m) activity i n sout her n su m mer ( wi nter), Ne w man, C. E., Kahanpää, H., peaking bet ween 11:00 and 14:00. Ho wever, while t wo daily peaks are predicted around both equinoxes, Ri c h ar ds o n, M. I., M arti n e z, G. M., o nly a late mor ni ng peak is observed. A n i ncrease i n vortex activity is predicted as MS L cli mbs t he nort h west Vice nte ‐Retortillo, A., & Le m mo n, M. (2019). Co nvective vortex a nd d ust devil slopes of Aeolis Mo ns, as observed. T his is attrib uted largely to i ncreased se nsible heat ﬂ u x, d u e t o (i) l ar g er predictio ns for gale crater over 3 mars dayti me s urface ‐t o ‐air te mperature differe nces over hig her terrai n, e n ha nced by reduced t her mal i nertia, years and co mparison with MSL ‐RE MS a nd (ii) t he i ncrease i n drag velocity associated wit h faster dayti me upslope wi nds. Ho wever, t he observed observatio ns. Jour nal of Geophysical Rese arc h: Pl a nets , 1 2 4 , 3442 – 3 4 6 8. increase in nu mber of vortex pressure drops is much stronger than the predicted D D A increase, although a htt ps:// d oi. org/ 10.1029/2019J E006082 better match exists when a threshold D D A is used.

Plain Language Su m mary The dayti me Martian at mosphere produces convective vortices called Received 9 J U N 2019 “ d ust devils ” w he n t hey are d ust ‐ﬁ lled. Vortices produce rapid pressure drops, w hic h have bee n detected i n Accepted 6 N O V 2019 Accepted article online 15 N O V 2019 Gale Crater by Mars Science Laboratory instru ments. Observed vortex pressure drops are co mpared with Published online 28 D E C 2019 vortex activity predicted using a nu merical model, Mars W R F. Because vortices are far s maller than Mars W R F's grid spaci ng, t he model ca n't predict t he m directly. I nstead, t he t heory of Re n nó et al. (1998) is use d to calc ulate “ d ust devil activity ” ( D D A) – a meas ure of vortex activity – based o n t he large ‐s c al e at mosp heric state. Predicted D D A matc hes t he ge neral variatio n of vortex observatio ns wit h ti me of day a nd season, such as maxi mu m ( mini mu m) activity in southern su m mer ( winter), peaking bet ween 11:00 and 14:00. Ho wever, while t wo daily peaks are predicted around both equinoxes, only a late morning peak is observed. Predicted D D A also increases as Mars Science Laboratory cli mbs the slopes of Aeolis Mons, as observed. T his is attrib uted to (i) larger dayti me s urface ‐t o ‐air te mperat ure differe nces at hig her altit udes a nd (ii) faster dayti me upslope wi nds hig her up t he slopes. Ho wever, t he observed i ncrease i n nu mber of vortex pressure drops is muc h stro nger t ha n t he predicted D D A i ncrease, alt houg h t hey matc h better w he n a threshold D D A is used.

1. I ntrod uctio n Co nvective vortices occur duri ng periods of stro ng co nvective heati ng of t he surface, w he n t he grou nd te m- ©2019. T he Aut hors. perat ure exceeds t he air te mperat ure, war mi ng t he air above t he s urface. As t his air rises, existi ng vorticity T his is a n ope n access article u nder t he ter ms of the Creative Co m mons beco mes more vertical a nd i nte nsi ﬁ es, wit h t he air spirali ng aro u nd a lo w ‐press ure regio n t hat develops i n Attrib utio n Lice nse, w hic h per mits use, t he vortex core ( Bal me & Greeley, 2006; Ka nak, 2005; Spiga et al., 2016; Toigo et al., 2003). Rotatio n is ra n- distrib utio n a nd reprod uctio n i n a ny do mly clock wise or co u nterclock wise, a nd vortices ofte n occ ur i n pairs or cl usters ( Bal me & Greeley, 2006). mediu m, provided t he origi nal work is pro perly cite d. They may extend to at least the height of the planetary boundary layer (P B L), which co m monly extends to

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~ 2 – 3 k m i n Eart h's deserts a nd to ~10 – 12 k m over many parts of Mars and signi ﬁ cantly higher in so me regio ns; dust devils wit h heig hts up to 16.5 k m have bee n ide nti ﬁ ed i n so me regio ns ( Fe nto n et al., 2016; Fe nto n & Lore nz, 2015). D ust devils are vortices t hat co ntai n d ust, maki ng t he m visible; t his d ust is raised via stro ng ta nge ntial wi nds arou nd t he vortex core, assisted by t he “ s uctio n effect ” of t he press ure drop a nd ot her t her mop hysical factors (see, e.g., Neakrase et al., 2016 for a revie w of all processes t hat have bee n proposed). Mars vortices, he nce d ust devils, ca n gro w m uc h larger t ha n o n Eart h. He nce w hile t he largest o n Earth may reach a fe w tens of m in dia meter, on Mars they may reach or order a k m in dia meter (Fenton et al., 2016). Detectio n of vortices is typically acco mplis hed by detecti ng t heir press ure drop sig nat ure, wit h wind (speed and direction) and te mperature also measurably affected when a vortex passes close enough to t he se nsors (e.g., Ka ha npää et al., 2016; M urp hy et al., 2016; see also sectio n 2.1). I n t he case of d ust devils, detectio n is also possible via i magi ng or by meas uri ng c ha nges i n i nco mi ng solar radiatio n if t he vortex is d usty e no ug h (e.g., Le m mo n et al., 2004, M urp hy et al., 2016; see also sectio n 2.2). Co nvective vortices are of partic ular i nterest o n Mars beca use d ust devils are t ho ug ht to be o ne of t he pri mary mea ns of i njecti ng d ust i nto t he t hi n Martia n at mosp here, i n w hic h t he t her mal str uct ure a nd he nce circ ulatio n are stro ngly i mpacted by its prese nce ( Gierasc h & Goody, 1972; S mit h, 2004). Note t hat Mars no n ‐c o n- vective vortices have also bee n i nferred fro m nig htti me press ure data (e.g., Ordo nez ‐Exte berria et al., 2018) a nd are li kely a res ult of mec ha nically forced t urb ule nce. Ho wever, t hey are less li kely to raise d ust hig h i nto t he at mosp here or noticeably i ncrease t he at mosp heric dust loadi ng; t he P B L dept h is very s mall ( he nce vertical mixi ng is stro ngly i n hibited) bet wee n approxi mately a n hour after su nrise a nd a n hour before su nset (e.g., Fo nseca et al., 2017), he nce a ny d ust lifte d is li kely ret ur ne d ra pi dly to t he s urface. Observatio ns of active d ust lifti ng ce nters d uri ng d ust stor m o nset s ho w little correlatio n wit h observatio ns of w here a nd w he n d ust devils a nd t heir tracks occ ur ( Ca ntor et al., 2006). Si milarly, t heory a nd modeli ng st udies all s uggest t hat fe wer dust devils s hould occur as t he at mosp here beco mes dustier, w hic h is i n co ntrast to t he rapid ra mpi ng up of d ust i njectio n as d ust loadi ng i ncreases d uri ng stor m o nset (e.g., Ka hre et al., 2006; Ne w ma n et al., 2002a, 2002b). Ho wever, it is esti mated t hat d ust raised by d ust devils or ot her for ms of s mall ‐scale co nvective d ust lifti ng acco u nts for most of t he d ust raised o utside of t he “ stor m season ” ( Ls ~0 – 180°) a nd he nce t hat t hese processes are respo nsible for t he observed te mperat ures at t his ti me of year, w hic h are te ns of K war mer t ha n a “ cl e ar ” at mosp here wo uld be ( Bas u et al., 2004; Ka hre et al., 2006; Ne w ma n et al., 2002a, 2002b).

1.1. T heory of Co nvective Vortices a nd D ust Devils Re n nó et al. (1998) ( he ncefort h R98) provided a t heory t hat models co nvective vortices as co nvective heat engines and predict a “ d ust devil activity ” ( D D A) that depends only on local meteorological variables. Note that despite the na me, this theory applies equally to vortices with or without dust. The theory of R98, described i n sectio n 3.2, has bee n used i n several Mars Ge neral Circulatio n Models ( G C Ms) to predict t he a mo u nt of d ust lifti ng by d ust devils; lifti ng set proportio nal to t he D D A appears to s uccessf ully capt ure t he global “ background ” dust a mount in the at mosphere as a function of season when no major dust stor ms are prese nt ( Bas u et al., 2004; Ka hre et al., 2006). T he t heory has also previo usly bee n tested by co mpari ng predictio ns of global models bot h wit h t he spatiote mporal variatio n of d ust devils a nd t heir tracks observed fro m orbit (see sectio n 1.1.1) a nd wit h t he variatio n of i maged d ust devils a nd co nvective vortices (i nferred fro m press ure data) observed fro m t he s urface (see sectio n 1.1.2). The D D A is a measure of the energy that may be harnessed by vortices, but ho w this should be distributed i nto a vortex distrib utio n is not clear at prese nt. For exa mple, s ho uld a n i ncrease i n D D A res ult i n more vortices, or a n i ncrease i n t he maxi mu m vortex stre ngt h, or bot h? To date, co mpariso ns wit h observatio ns have largely foc used o n t he n u mber of vortices (or tracks) detected, b ut sectio n 5.3 also i nvestigates t he relatio n- s hip to pressure drop mag nitude. 1.1.1. Co mpariso ns of T heory Wit h Orbital Observatio ns Ka hre et al. (2006) fou nd t hat t he spatial a nd seaso nal patter ns of D D A predicted usi ng a G C M were broadly co nsiste nt wit h observatio ns of d ust devils a nd t heir tracks fro m orbit ( Fis her et al., 2005): bot h s ho w peak activity during local su m mer in both he mispheres, with A mazonis marked as a particularly active region. Ho wever, orbital data sets at the resolution required have li mited spatial and seasonal coverage and do not typically provide i nfor matio n o n ti me ‐of ‐day variatio n, as observatio ns are ofte n made at t he sa me local ti me each orbit. In particular, afternoon observations are far more co m mon than morning ones, although,

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e.g., t he Colo ur a nd Stereo S urface I magi ng Syste m o n t he Trace Gas Orbiter is able to cover t he f ull dayti me portio n of t he di ur nal cycle d ue to its no n ‐S u n ‐sy nc hro no us orbit ( T ho mas et al., 2017). A f urt her iss ue is t hat s maller d ust devils ca n not be see n fro m orbit a nd d ust devils wit h relatively lo w d ust co nte nt may be missed. Also, d ust devil tracks or eve n d ust devils t he mselves may not be visible over certai n s urfaces, e.g., if t he s urface revealed w he n a layer of d ust is re moved has t he sa me color a nd albedo, as mig ht be true of a t hick dust deposit. T hese argu me nts may explai n w hy Ka hre et al. (2006) fou nd t hat s mall but nonzero D D A values are predicted for so me seasons and locations at which no dust devils or tracks are observed ( Kahanpää et al., 2016), although this may also indicate a threshold D D A is needed for vortices to for m or d ust lifti ng to occ ur; see also sectio n 5.3. I n additio n, if t here is no loftable d ust i n a regio n t he n no d ust devils or trac ks will be see n, eve n if n u mero us stro ng vortices occ ur. T here is no way to i nfer t he prese nce of d ust ‐free vortices fro m orbit, he nce orbital observatio ns are biased agai nst regio ns wit h lo w dust availability. Orbital data sets of d ust devils a nd t heir trac ks t h us co ntai n n u mero us biases a nd are not ideally s uited to testi ng t heories of vortex or d ust devil n u mber, stre ngt h, ti mi ng of occ urre nce, etc. 1.1.2. Prior Co mpariso ns of T heory Wit h La nded (I n Situ) Observatio ns U nlike orbital data sets, i n sit u meas ure me nts do not s uffer fro m most of t he above iss ues. O n t he s urface, bot h d ust ‐fi lled a nd e mpty vortices are most straig htfor wardly detected by meas uri ng t he variatio n of s urface pres- s ure, wit h ra pi d b ut s hort ‐lived press ure drops i ndicati ng t he close passage of a vortex's lo w ‐pr ess ur e c or e ( e. g., M urp hy & Nelli, 2002; Sc ho fi eld et al., 1997). S urface i magi ng of d ust devils ca n t he n be used to cover a wider area; to provi de esti mates of t he s pee d, dia meter, a nd heig ht of vortices; a nd to assess t heir d ust ‐r aisi n g a bilit y. T he mai n disadva ntages of i n situ meas ure me nts are t hat spatial coverage is far more li mited t ha n fro m orbit: d ust devils m ust be visible fro m t he rover or la nder, wit h i ncreasi ng bias to ward detecti ng larger a nd/or d ustier d ust devils as t he dista nce i ncreases; si milarly, vortices ca n o nly be detected i n press ure data if t hey affect pres- s ure at t he la nded locatio n. For press ure detectio ns, it ca n be dif fi c ult to deco nvolve t he effects of stre ngt h, dia- meter, speed of passage, a nd dista nce, especially wit hout reliable wind data or i magi ng, althoug h several efforts have bee n made to do so (e.g., Jackso n et al., 2018; K urga nsky, 2019; Lore nz, 2016). Yet overall, s urface missio ns have t he pote ntial to capt ure t he seaso nal a nd di ur nal variatio n of nearly t he f ull stre ngt h spectr u m of d ust devils a nd vortices i n a si ngle locatio n or regio n. For la nders, i magi ng ca n also provide so me li mited i nfor- matio n o n a ny s patial variatio ns i n d ust devils across t he sce ne, w hile for rovers, s patial variatio ns i n bot h d ust devils a n d d ust ‐free vortices may be measured as t he locatio n c hanges over t he mission. Ka hre et al. (2006) fo u nd t hat t he predicted di ur nal variatio n of D D A at t he Mars Path fi nder ( MP F) landing site usi ng t heir G C M's output was very consiste nt wit h t he diur nal variatio n of dayti me co nvective vortices inferred fro m MPF pressure data ( Murphy & Nelli, 2002), with the peak occurring bet ween noon and 1 p m LTST. Using a different G C M, Chap man et al. (2017) also noted a general agree ment bet ween predicted D D A and the observed diurnal cycle of nu mber of convective vortices at the MP F and also at the Phoenix La nder sites ( Elle hoj et al., 2010), alt ho ug h i n bot h cases t he model appeared to predict a slig htly later peak ti me of day. Ho wever, C hap ma n et al. (2017) found a mis match at t he Viki ng La nder 2 site, for w hich t heir G C M predicted peak D D A in the late afternoon (around 17:00), whereas observations of wind fl uct uatio ns interpreted as being caused by vortices peaked bet ween 10 a m and noon ( Ringrose et al., 2003). They also found a mis match for Gale Crater, with their G C M predicting both a morning and afternoon peak in D D A over t h e fi rst Mars year, w hereas t he average diur nal cycle found fro m pressure drop observatio ns over t he sa me period s ho ws a si ngle peak aro u nd noo n ( Ka ha npää et al., 2016). I n fact, t here appears to be a stro ng observed seaso nal variatio n i n t he di ur nal cycle i n Gale Crater, as disc ussed i n sectio ns 5.1 a nd 5.2 belo w. Prior to t he c urre nt work, Ka ha npää et al. (2016) was t he o nly st udy to have co mpared t he seaso nal variatio n of predicted D D A wit h s urface observatio ns of seaso nal vortex activity, as i nferred fro m t he fi rst year of s urface pressure data take n fro m t he Mars Scie nce Laboratory ( MS L) Curiosity rover. Usi ng output fro m a hig h ‐ resolutio n, nested mesoscale model ( Mars W R F, w hic h is also used i n t he curre nt work, see sectio n 3), t hey found an overall good match bet ween the predicted D D A and observed seasonal cycle. The main exception was arou nd sout her n su m mer solstice, duri ng w hic h ti me far fe wer vortex pressure drops were i nferred t ha n predicted. T he possible ca use of t his mis matc h is disc ussed f urt her i n sectio n 5.1.

1.2. Prior Modeli ng of t he Circ ulatio n i n t he Gale Crater Regio n U nlike prior la ndi ng sites w here d ust devils have bee n st udied, t he Gale Crater site of t he MS L missio n has si g ni ﬁ cant mesoscale topographic relief. While direct observations of circulation are li mited to the MS L

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instru ments, the nature of the circulation has been modeled by several different groups ( Tyler & Barnes, 2013, 2015; Rafkin et al., 2016; Pla ‐Garcia et al., 2016; Ne w man et al., 2017; Fonseca et al., 2017; Ric hardso n & Ne w ma n, 2018). T he stro ng relief ge nerates co mplex mesoscale circulatio ns t hat i nteract bot h wit h t h e l ar g e ‐s c al e fl o ws co m monly observed at other landing sites and with the microscale (P B L) circulatio n a nd structure. Depe ndi ng o n t he ti me of year a nd t he stre ngt h of t he large ‐s c al e fl o ws, t he circ ulatio n that the MSL rover observes can be do minated by fl o ws i nd uced wit hi n t he crater itself. A sig ni fi ca nt tr ue buoyancy slope fl o w is associated wit h near ‐surface upslope (anabatic) dayti me and nightti me do wnslope (katabatic) wi nds ( Tyler & Bar nes, 2013), t ho ug h t his fl o w is si g ni fi ca ntly modi fi ed i n practice by net hydrostatic rebalancing of the at mosphere bet ween lo w and high topography as the near ‐s urface scale heig ht dra- matically c ha nges duri ng t he daily cycle ( Rafki n et al., 2016; Ric hardso n & Ne w ma n, 2018). T he lateral advectio n of war m air “ u p sl o p e ” alo ng wit h adiabatic cooli ng of t his air will te nd to red uce t he t her mal drive for co nvectio n. I n additio n, i ncreased wi nds te nd to ge nerate additio nal near ‐s urface t urb u- le nce t hro ug h s hear, w hic h has t he effect of cooli ng t he s urface t hro ug h i ncreased s urface layer se nsible heat fl uxes; i n more extre me for m, t his ca n be see n i n t he fl o ws over t he walls of Valles Mari neris i n nu merical si mulations, where the diurnal cycle of ground and near ‐surface air te mperatures can be greatly reduced (Spiga et al., 2011). Bot h of t hese mec ha nis ms will te nd to decrease co nvectio n over t he slopes of Aeolis Mo ns. By co ntrast, t he mesoscale circ ulatio n set up wit hi n t he crater d uri ng t he dayti me te nds to co nverge over Aeolis Mo ns a nd over t he crater ri m, wit h do w n welli ng over t he crater tre nc h. W he n it is do mi na nt, t his has the effect of suppressing the P B L depth over the trench ( Tyler & Barnes, 2015) and hence da mping P B L motio ns s uc h as co nvective vortices t here. Ho wever, as described i n so me detail i n sectio n ??, t he mesoscale fl o w is not al ways do minant, and in so me c as es t h e i n fl ue nce of large ‐s c al e fl o ws on Gale Crater beco mes signi fi ca nt ( Ne w ma n et al., 2017; Pla ‐G ar ci a et al., 2016; Raf ki n et al., 2016). It is t h us i m porta nt not to get too attac he d to a si m plistic, i dealize d co nce p- tio n of t he Gale Crater fl o w syste m a nd speci fi cally t he idea t hat t he dayti me bo u ndary layer is al ways s up- pressed i n t he Gale Crater tre nc h duri ng t he dayti me. 1.3. Overvie w of the Work Prese nted Here Dust devils and convective vortices have been observed in great nu mbers in Gale Crater by instru ments on MS L, wit h t heir freq ue ncy varyi ng stro ngly wit h locatio n, ti me of day, a nd seaso n. T his data set t h us provides a n excelle nt test of t heories a nd models t hat predict t he i nte nsity of co nvective vortex activity a nd the a mount of dust raised by the m. This paper focuses on co mparing observations with ther modyna mically derived predictions of D D A, obtained by co mbining the theory of R98 with meteorological variables output fro m the Mars W RF nested mesoscale model, run at a grid spacing of ~1.4 k m over Gale Crater. Sectio n 2 describes ho w MS L i nstru me nts are used to detect vortices a nd dust devils, discusses previously publis hed results, a nd fi nally describes the methods used here to infer vortices fro m pressure data. Section 3 describes the Mars W R F model and ho w its output is used to predict the D D A, as de fi ne d i n R98. Sectio n 4 prese nts Mars W R F predictio ns across Gale Crater as a fu nctio n of local ti me at local su m mer solstice, as well as seaso nal variatio ns at selected ti mes of sol for t he regio n i n w hic h MS L operates. Sectio n 5 co mpares vortices measured by MSL's Rover Environ mental Monitoring Station ( R E MS) with the D D A predicted at t he ti me a nd locatio n of t he observatio ns a nd exa mi nes w hat deter mi nes t he D D A variatio n i n t he model. Sectio n 6 co ncl udes.

2. Vortex and Dust Devil Observations Made by MSL MS L carries several instru ments that have been used to detect convective vortices and/or dust devils: a meteorological se nsor suite i n t he for m of R E MS a nd ca meras i n t he for m of t he rover's navigatio n ca meras ( Navca ms) and mast ‐mounted science ca mera ( Mastca m). 2.1. Vortex Observatio ns with R E MS The R E MS suite includes sensors for measuring at mospheric pressure, te mperature, relative hu midity, U V r a di ati o n ﬂ ux, and wind speed and direction. Measure ments are perfor med at a rate of 1 Hz for typically up to te n ho urs per sol, w here a sol is a Mars solar day. T his i ncl udes 5 mi n utes of observatio ns at t he start of every ho ur i n Local Mea n Solar Ti me ( L MS T) pl us typically eig ht 1 ‐h o ur “ exte nded blocks ” per sol, w here a n ho ur is de ﬁ ned as 1/24 of a sol a nd a mi nute as 1/60 of a n hour. T he cade nce of R E MS exte nded blocks is

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Fi g ure 1. (a) Gale Crater topograp hy a nd t he locatio n of t he rover traverse. (b) T he rover traverse over t he ﬁ rst 3 Mars years of t he MS L missio n, plotted over a Mars Reconnaissance Orbiter Context Ca mera i mage. Cyan ovals contain all rover locations for a single MS L year. The MS L locations are sho wn with different colors depe ndi ng o n seaso n as t he lege nd box i n (b) i ndicates. T here are very fe w red dots i n MS L years 1 a nd 2, d ue to t he rover movi ng little aro u nd s u m mer solstice i n t hose years.

describe d i n detail i n, e.g., Ne w ma n et al. (2017), b ut basically res ults i n all ho urs of t he sol bei ng observe d at least o nce at 1 Hz i n every 6 ‐sol period and al most daily coverage of the hour containing noon Local True Solar Ti me ( L TS T), i.e., w he n solar i nsolatio n peaks. W he n so me part of a co nvective vortex passes over t he rover, its sig nat ure may be see n i n (i) a rapid, s hort ‐ lived pressure drop due to t he vortex's lo w ‐press ure core. Its sig nat ure may possibly also be see n i n (ii) a rapid, si multa neous c ha nge i n wi nd directio n as t he rotati ng vortex moves across t he se nsor positio n, (iii) rapid, si multa neous air te mperature fl uct uatio ns associated wit h t he war m core of t he vortex a nd t he mixi ng of air aro u n d it, a n d/or (iv) a n associate d, relatively s hort ‐lived decrease i n U V fl ux if t he vortex co ntai ns s uf- fi cie nt d ust t hat it blocks so me solar radiatio n fro m reac hi ng t he s urface. T he latter ca n be used to esti mate t he a mo u nt of d ust prese nt i n t he vortex, alt ho ug h a sig ni fi ca nt a mou nt of dust must sit i n t he at mosp heric col u m n above t he rover to ca use s uc h effects, w hic h is likeliest for larger d ust devils t hat pass bet wee n t he rover a nd t he S u n directio n. If a co nvective vortex does not pass directly over t he rover, ma ny of t hese observatio ns are more m uted a nd ca n be i mpossible to differe ntiate fro m ge neral t urb ule nce i n t he area, alt ho ug h t hey may be used to support t he argu me nt for vortex passage w he n a pressure sig nal is detected. W hile t he te mporal (di ur nal a nd seaso nal) coverage of R E MS meas ure me nts is very good, t heir spatial coverage is li mited to t he locatio n of MS L. Fort u nately, MS L is a rover. T h us by April 2018, it had not o nly mo n- itored three co mplete Mars years but had also driven more than 18.5 k m (approxi mately 9 k m as the cro w fl ies) fro m its origi nal locatio n i n t he tre nc h of t he crater. As s ho w n i n Figure 1, over t his period, MS L crossed through the Bagnold Dunes and began to cli mb the slopes of Aeolis Mons. This means that the R E MS data set also co ntai ns i nfor matio n o n t he spatial variatio n of vortex activity, albeit o nly alo ng t he r over's traverse.

2.2. Dust Devil Observations With Navca m/ Mastca m R E MS measure ments provide infor mation about vortex activity at the rover location, but it is dif ﬁ c ult t o extract infor mation about their size or motion (especially when si multaneous wind data are absent) or ho w much dust they contain. By contrast, MS L's ca meras can be used to i mage dust devils directly, yet no clear sightings of dust devils were made in the ﬁ rst 2 years of t he MS L missio n. U ntil s hortly after sol

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1520, Navca m d ust devil searc hes looked nort h across t he crater tre nc h. I n retrospect, t his appears to have bee n a poor c hoice, as t he D D A is predicted to be ge nerally lo w t here co mpared to ot her directio ns visible fro m that location (see section 4.1), although dust availability may also have been an issue. Subsequent searches looking to ward Aeolis Mons proved far more successful, although Navca m 360° surveys are perfor med reg ularly i n additio n to lo nger movies to avoid directio nal bias creepi ng i n agai n. I n fact, as t he rover has cli mbed higher, it has beco me co m mon to see dust devils when i maging to the north, as this no w looks back do w n t he slopes. Ho wever, t he res ult is t hat t he list of d ust devils fo u nd i n s urveys o nly exte nds back as far as sol 1607, ~2.4 years into the mission. The period since that ti me no w exceeds one Mars year but i ncludes a global a nd regio nal dust stor m i n M Y34, bot h of w hic h stro ngly affected at mosp heric dust opaci- ties but were not included in the present Mars W RF si mulations. Given the strong i mpact of at mospheric dust opacity on both the predicted D D A ( which decreases as opacity increases; Ne w man et al., 2002a) and observed vortex and dust devil occurrence ( which strongly decreased during the M Y34 global stor m; Guze wich et al., 2018), a co mparison with i maging is therefore deferred to a future paper in which Mars W R F is forced wit h realistic observed d ust opacity maps for a give n year, rat her t ha n usi ng a d ust scenario t hat represe nts a no nspeci fi c year wit h no major stor ms. 2.3. Previo usly P ublis hed Res ults Steakley and Murphy (2016), Kahanpää et al. (2016), and Ordonez ‐Exte berria et al. (2018) have all previo usly exa mi ned vortex activity i n Gale Crater usi ng R E MS data. I n Steakley a nd Murp hy (2016), t he sig natures of 245 co nvective vortices wit h pressure drops exceedi ng 0.3 Pa were fou nd i n t he fi rst 707 sols of t he missio n, eq uivale nt to a n average of ~1 vortex per sol after acco u nti ng for t he i nco mplete te mporal sa mpli ng of R E MS measure ments. They also found that pressure drops peaked bet ween 11:00 and 13:00 L TS T, with vortices more freque nt duri ng local spri ng a nd su m mer t ha n i n fall a nd wi nter. Approxi mately the sa me period was investigated by Kahanpää et al. (2016), who found si milar results in ter ms of the ti ming and seasonal behavior of pressure drops exceeding 0.5 Pa. In addition, they co mpared these results to D D A predictions made using output fro m the Mars W RF model (see section 3 for more details). They found that the D D A calculated fro m Mars W RF output predicted the seasonal variation of observed press ure drops ( > 0.5 Pa) relatively well, except for a n overpredictio n of t he peak aro u nd local s u m- m er s olsti c e ( Ls ~ 2 7 0°), w hi c h t h e y attri b ut e d t o t h e l o c all y hi g h t h er m al i n erti a of t h e s p e ci fi c area w here t he rover was located over m uc h of t his period, as disc ussed i n sectio n 5.1. Fi nally, t hey also i nterpreted a vortex b urst o n sol 664 as possibly bei ng d ue to a fro nt passi ng over Gale Crater. T wo full years of R E MS data were studied by Ordonez ‐Exteberria et al. (2018), w ho fo u nd 635 dayti me pres- s ure drops exceedi ng 0.5 Pa over 1417 sols, correspo ndi ng to a n average rate of 1.2 eve nts per sol w he n cor- rected for sa mpling. These drops were most prevalent near local noon, but they also noted several weak eve nts not correlated wit h stro ng s urface ‐t o ‐air te mperature differe nces, as well as noti ng a large nu mber of nightti me pressure drops, which were interpreted as being due to mechanically forced turbulence. Nig htti me pressure drops were especially stro ng i n regio ns of roug h or steep topograp hy a nd s ho wed seasonal dependences which may link the m to variations in local ‐ a n d r e gi o n al ‐scale wi nds. Fi nally, t hey noted roughly a doubling in the nu mber of day and night pressure drops fro m the fi rst to seco nd Mars year of t h e missi o n. 2.4. Methodology Used i n This Work A met hod si milar to t hat described by Ka ha npää et al. (2016) is used to ide ntify passi ng vortices. Movi ng through the R E MS pressure data set in 20 ‐s slidi ng wi ndo ws, a vortex ca ndidate is ide nti fi e d if eit her of t he follo wi ng criteria are met: • Mini mu m pressure > 0.5 Pa lo wer than the mean of the previous and next 20 ‐s i ntervals • Mini mu m pressure > 0.3 Pa lo wer a n d mean pressure > 0.1 Pa lo wer than the mean of the previous and next 20 ‐s i ntervals Note t hat t he fi rst of t hese criteria was not applied by Ka ha npää et al. (2016). T he criterio n was added to ide ntify relatively stro ng pressure drops wit h very s hort duratio n. Suc h eve nts appeared to be co m mo n dur- i ng t he seco nd a nd t hird Martia n year of t he MS L missio n, u nlike duri ng t he fi rst year. T he seco nd criterio n e nables t he detectio n of pressure drops wit h duratio ns lo nger t ha n 60s, w hic h is a n adva ntage co mpared to the methods applied by Steakley and Murphy (2016) and Ordonez ‐Exteberria et al. (2018). A vis ual

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i nspectio n of every pressure drop is t he n perfor med to ide ntify a nd re move a ny t hat do not look vortex ‐li k e or w hic h are “ double cou nti ng ” of t he sa me vortex by t he algorit h m. Fig ure S1 s ho ws so me exa mples of pressure drops retained and re moved during this step. In co m mon with Kahanpää et al. (2016), weak pressure drops appearing si multaneously with the R E MS U V sensor passing into or co ming out of the shado w of rover structures are re moved as t hese eve nts are appare ntly caused by t he “ s hado w effect ” of the R E MS pressure se ns or, fi rst reported by Harri et al. (2014) a nd noted also by Ordo nez ‐Exteberria et al. (2018). Nevert heless, by co ntrast wit h Ka ha npää et al. (2016), pressure drops wit h mag nitude exceedi ng 0.8 Pa are retai ned. Magnitudes and half maxi mu m durations of the vortex candidates are deter mined by fi tti ng a li near co mbi- natio n of a Lore ntzia n f u nctio n a nd a li ne to t he R E MS press ure data as described by Ka ha npää et al. (2016). Vortices t hat arrive i n pairs or larger “ b ursts ” are for the most part handled individually; ho wever a fe w events with multiple pressure mini ma are fi tted with a linear co mbination of t wo Lorentzian functions. After t his, eve nts wit h a mag nitude s maller t ha n a t hres hold value are discarded. I n co ntrast to Ka ha npää et al. (2016) a nd Ordo nez ‐Exteberria et al. (2018), t his st udy uses a t hres hold of 0.6 Pa i nstead of 0.5 Pa, the rationale being that the pressure data of MS L years 2 and 3 are so meti mes noisier than those of year 1, co mplicati ng t he ide nti fi catio n of press ure drops s maller t ha n 0.6 Pa. For co mplete ness, t he distrib utio n of pressure drops is also fou nd usi ng a met hod very si milar to t hat of Ordo nez ‐Exteberria et al. (2018) a n d usi ng a t hres hold of 0.5 Pa. W hile t his seco nd met hod results i nfe wer lo ng duratio n, large pressure drops bei ng discovered, t he overall patter n of di ur nal a nd seaso nal variability is very si milar usi ng eit her met hod (see secti o n 5.1).

3. Using Mars W RF Mesoscale Output to Predict D D A in Gale Crater 3.1. The Mars W RF Model and Its Con fi g uratio n for T his Work Mars W R F is a m ultiscale model of t he Martia n at mosp here, described i n Ric hardso n et al. (2007), Toigo et al. (2012), Ne w man and Richardson (2015), and Ne w man et al. (2019). This work uses a very si milar model con- fi guration to that described in Ne w man et al. (2017), in which Mars W R F was run as a global 2° model with fi v e “ neste d ” hig her resolutio n regio ns roug hly ce ntered o n MS L's la ndi ng site, wit h eac h nest s maller t ha n its pare nt a nd wit h t hree ti mes t he horizo ntal resol utio n; see Fig ure 14 of t hat paper. I n t his work, ho wever, o nly fo ur nests are used, givi ng fi ve do mains in total and a resolution of ~1.4 k m in the inner most nest (do mai n 5), w hic h covers all of Gale Crater. I n additio n, t he prese nt work pri marily uses res ults fro m vertical grid B described in Ne w man et al. (2017), as it was found to produce the best match to winds and aeolian features within Gale Crater. Ho wever, so me results are also sho wn that use vertical grid A, which more fi nely resolves the lo west portion of the boundary layer, to de monstrate the i mpact of model setup (see secti o n 5.4). T h e ti m e ‐varyi ng, t hree ‐di mensional at mospheric dust distribution seen by Mars W RF's radiative transfer sche me is prescribed using the Mars Cli mate Database Mars Global Surveyor dust scenario, which was developed to be represe ntative of a year wit hout a ny major dust stor ms ( Forget et al., 2001; Le wis et al., 2001). T welve si m ulatio ns, eac h lasti ng eig ht sols wit h t he fi rst t he n discarded as a “ s pi n u p ” sol, were r u n at areoce ntric solar lo ngit ude, Ls = 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, a nd 330°, to f ully sa mple t he a n n ual cycle of solar forci ng. Most of the spatially variable surface properties co me fro m Mars Global Surveyor Mars Orbiter Laser Alti meter and Ther mal E mission Spectro meter observations ( Christensen et al., 2001). These include M OL A topographic height (S mith & Zuber, 1996), the M OL A intrashot surface roughness map ( Garvin et al., 1999), and maps of albedo, ther mal inertia, and e missivity (Putzig & Mellon, 2007). Additional hi g h ‐resolution topography data, fro m a Mars Express, High Resolution Stereo Ca mera instru ment Digital Terrain Map, were used over Gale Crater for do main 5 ( Neuku m et al., 2004; Jau mann et al., 2007). Although so me local surface properties have been measured by MSL (e.g., ther mal inertia inferred fro m t he di ur nal cycle of s urface te mperat ure), t hese are not necessarily represe ntative of t he s urface properties even a fe w meters a way, whereas vortex pressure drops detected by R E MS may be caused by vortices that passed several h u ndred meters fro m t he rover a nd origi nally for med at a n eve n greater dista nce. D ue to dri- vability require ments and science interest, the surface properties measured along the rover traverse are so meti mes not eve n represe ntative of t he typical local regio n arou nd it, alt houg h a lack of vortices over a long period when MSL explored “ Yello wknife Bay ” is indeed most likely attributable to much higher

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t her mal i nertias over t hat e ntire regio n (see sectio n 5.1). Overall, ho wever, co mbi ni ng locally a nd re motely measured surface properties appropriately into an i mproved map of the region around MSL would be very c halle ngi ng. I n additio n, t he overall patter n of D D A predicted i n Gale Crater is fo u nd to be more co ntrolled by topograp hy t ha n by variatio ns i n albedo or t her mal i nertia (sectio n 4.2.2). He nce, despite t he so me w hat lo w resol utio n, o nly maps of s urface properties derived fro m orbital data are used i n t his work.

3.2. Dust Devil and Vortex Theory and Co mbination with Mars W RF Output In R98, convective vortices (and hence dust devils) are modeled as convective heat engines. The D D A is d e ﬁ ne d as:

Λ ≈ max ð 0 ; η F s Þ ; ( 1)

w here F s is t he s urface se nsible heat ﬂ ux (t he heat i np ut to t he base of t he vortex) a nd η is t he vertical t her- modyna mic ef ﬁ cie ncy of t he d ust devil co nvective heat e ngi ne (t he fractio n of t he i np ut heat t ur ned i nto mechanical work). After so me manipulation, η is give n as:

η ≈ 1 − b ; ( 2)

w here

p χ þ 1 − p χ þ 1 b ≡ s t o p ; ( 3) χ p s − p t o p ðχ þ 1 Þ p s

and where p s is t he a mbie nt s urface press ure, p t o p is t he a mbie nt press ure at t he top of t he co nvective bo u ndary layer, a n d χ is t he ratio of t he u niversal gas co nsta nt a nd t he heat capacity at co nsta nt press ure, eq ual to 0.25 for Mars. T he P B L top is calculated i nside Mars W R F's bou ndary layer sc he me as t he locatio n w here t he b ulk Ric hardso n n u mber falls to a critical val ue of 0.5 ( Ho ng & Pa n, 1996). In Mars W RF, the sensible heat ﬂ ux is give n by:

F s ¼ C d ρ u T s urf − T air ; ( 4)

w here C d is a drag coef ﬁ cie nt t hat depe nds o n t he stability of t he near ‐surface at mosphere, ρ is air de nsity, u *is drag velocity, a n d T s urf a n d T air are t he s urface a nd 1.5 ‐m layer air te mperat ures, respectively. Bot h t he se nsible heat ﬂ ux a nd u *are calculated inside Mars W RF's surface layer sche me (Zhang & Anthes, 1982), which uses Monin ‐Obukhov si milarity and accounts for four stability categories: stable, mechanically i nduced turbule nce, u nstable forced co nvectio n, a nd u nstable free co nvectio n.

4. Vortex a nd Dust Devil Predictio ns Across Gale Crater Usi ng Mars W RF This section presents D D A predictions across Gale Crater and explores ho w the D D A and its contributing ter ms are affected by s urface properties a nd t he circ ulatio n. Sectio ns 4.1 a nd 4.2 foc us o n local (i.e., so ut her n) s u m mer solstice, Ls = 270°, t he ti me of peak solar forci ng a nd greatest vortex activity i n Gale Crater, wit h seasonal variations explored in section 4.3. Section 5 then co mpares MS L observations of vortex pressure drops with predicted D D A along the rover traverse over 3 Mars years, using the context provided in sectio n 4 to i nter pret t hose res ults.

4.1. Overvie w of Predicted D D A at Souther n Su m mer Solstice Figure 2 s ho ws t he predicted D D A across t he crater at Ls~270° over eig ht ti me periods. Predicted D D A outside t hese periods is negligible (belo w 0.02). I n ter ms of te mporal variatio n, t he predicted D D A peaks i n t he 1 2: 0 0 – 14:00 period, with al most none predicted bet ween 08:00 and 09:00 and only slightly more bet ween 16:00 a nd 17:00. I n ter ms of spatial variatio n, t he lo west predicted D D A occ urs i n t he crater tre nc h to t he N, N E, and N W of the MS L landing site and to the S E and S of Aeolis Mons, although there is also a band of lo w predicted D D A that runs east ward across the northern slopes of Aeolis Mons, starting roughly fro m t he MS L la ndi ng site. T he hig hest predicted D D A occurs largely o n t he N W, S W, a nd S E crater ri m (as well as i n so me regio ns o utside t he crater) a nd i n a nort h ‐so ut h strip r u n ni ng t hro ug h most of t he wester n side of t he

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Fig ure 2. Predicted D D A (color contours) over Gale Crater for nine periods (sho wn in L TS T) in local su m mer ( Ls~270°), calculated fro m Mars W R F do main 5 out- p ut usi ng eq uatio n (1). Topograp hy is s ho w n as black co nto urs (co nto ur i nterval 500 m). T he MS L la ndi ng site (at 137.4417° E, 4.5895°S) is i n t he ce nter of t he w hite di a m o n d.

crater trench. There is also moderate D D A predicted over most of Aeolis Mons and in the S W quadrant of t he tre nc h. Fro m a n MS L perspective, based o n t hese plots (see also t he “ zoo med ‐i n ” plots disc usse d i n sectio n 4.3), o ne might expect that looking fro m the landing site to ward the slopes of Aeolis Mons or to the S W, bet ween about local noon and 15:00, would have provided the best chance of observing dust devils in near ‐fi el d MSL i maging. Conversely, looking any where bet ween the N W and N E (i.e., to ward the nearest portion of the crater ri m), or to ward the slopes of Aeolis Mons roughly due east of the landing site, would provide t he worst c ha nce, regardless of ti me of day. It is t herefore u nfort u nate t hat MS L's early d ust devil i magi ng foc used o n looki ng across t he plai ns of t he tre nc h ro ug hly to t he nort h; t his may have co ntrib uted to t he lack of d ust devils i maged i n t he fi rst portio n of t he missio n, as disc ussed i n sectio n 2.2. Ho wever, t he c ha ngi ng positio n of MS L over t he co urse of t he missio n (see Fig ure 1) also bro ug ht t he rover hig her up t he N W slopes of Aeolis Mons, into the region with moderate predicted D D A, which would be expected to provide more ne ar ‐fi eld i magi ng of d ust devils i n several directio ns a nd more detectio ns of i n sit u vortex press ure drops.

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Fi g ure 3. D D A (top ro w) a nd its t wo co ntrib utors, se nsible heat ﬂ ux ( middle ro w) a nd vertical t her modyna mic ef ﬁ cie ncy (botto m ro w), at sout her n su m mer solstice ( Ls ~270°) i n t hree dayti me periods: 10:00 – 11:00 (left col u m n), 12:00 – 13:00 ( middle colu m n), a nd 14:00 – 15:00 (right colu mn) L TS T. Topography and white dia mond are as described i n Fig ure 2.

4.2. Co ntrols o n the Predicted D D A at Souther n Su m mer Solstice Figure 3 sho ws the D D A and ter ms contributing to it – the vertical ther modyna mic ef fi cie ncy a nd se nsible h e at fl u x – for t hree of t he ti me periods s ho w n i n Fig ure 2: 10:00 – 1 1: 0 0, 1 2: 0 0 – 13:00, a nd 14:00 – 15:00 L TS T. Fro m t hese plots, it is clear t hat t he patter n of D D A has stro ng si milarities to t he patter n of se nsible heat fl u x. I nside t he crater, bot h are lo west over m uc h of t he tre nc h (especially t he regio ns picked o ut i n sectio n 4.1 as having lo w D D A) and are highest over much of Aeolis Mons and in a north ‐so ut h strip at ~137° E. Ho wever, the D D A mini mu m that stretches east across the northern slopes of Aeolis Mons is directly linked to the strong mini mu m in ther modyna mic ef fi ciency there. Note that the sensible heat fl ux peaks in the 1 2: 0 0 – 13:00 plot, whereas the ther modyna mic ef fi cie ncy peaks i n t he 14:00 – 15:00 plot. T his is beca use t he for mer has a faster response to changes in solar heating, as it largely involves the surface heating up. Ho wever, the ther modyna mic ef fi ciency relates to the P BL depth, which takes so me ti me to increase

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Fig ure 4. As in Figure 3 but no w sho wing the vertical ther modyna mic ef ﬁ cie ncy (top ro w), its mai n co ntrib utor, t he press ure t hick ness of t he P B L ( middle ro w), and the P BL thickness in k m (botto m ro w).

follo wi ng i ncreased surface war mi ng. He nce t here is typically a delay of about 2 hours bet wee n t he ti me of peak forcing (around local noon) and peak P BL depth. The different ti mings are discussed further in secti o n 5. 4.2.1. Vertical T her mody na mic Ef ﬁ cie ncy a nd Its Co ntrib utors T he top t wo ro ws of Figure 4 s ho w t he vertical t her mody na mic ef ﬁ cie ncy a nd its mai n co ntrib utor, t he pres-

s ure t hick ness of t he P B L P s urf ‐P t o p (see equatio n (3)), a nd de mo nstrate ho w closely t he spatiote mporal variatio n i n t he for mer follo ws t hat of t he latter. Overall, i n t his seaso n, t he press ure t hic k ness of t he co nvective boundary layer within the crater is al ways greatest in the trench and gro ws signi ﬁ ca ntly as t he day progresses, with the thinnest boundary layer over Aeolis Mons and increasing only moderately over the ti mes s ho w n. Bou ndary layer dept h is more ofte n prese nted i n heig ht coordi nates (botto m ro w of Figure 4), w hic h displays a very si milar patter n. I nteresti ngly, t his is al most t he reverse of t he spatial distrib utio n noted i n, e.g., Tyler and Barnes (2015), which focused on an earlier season: Ls~150°, the ti me of MSL's landing.

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W hile so me observatio ns have s ho w n t hat P B L dept h o n Mars is larger w here t he pressure is lo wer, suc h as over hig h topograp hy (e.g., Hi nso n et al., 2008, 2019), t he nat ure of t he so ut her n s u m mer circ ulatio n at Gale Crater prod uces a n i nversio n of t his res ult. T he ca use of t he spatial variatio n i n P B L dept h a nd its depe n- de nce o n seaso n is disc ussed f urt her i n sectio n 4.3. 4.2.2. S urface Se nsible Heat Fl ux a nd Its Co ntrib utors Fig ure 5 s ho ws t he mai n co ntrib utors to t he s urface se nsible heat ﬂ u x – ne ar ‐s urface air de nsity ρ , drag vel o-

cit y u *, a nd t he s urface ‐t o ‐air te mperat ure differe nce T s urf ‐T air (see eq uatio n (4)) – i n t hree dayti me periods. Se nsible heat fl u x is als o i n fl ue nced by differe nces i n t he drag coef fi ci e nt C d , w hic h varies wit h t he stability of t he bo u ndary layer, b ut t he additio nal variatio ns i ntrod uced are less t ha n a fe w perce nt. Fro m a co mpariso n of all fo ur ro ws of Fig ure 5, t he patter n of predicted s urface se nsible heat fl u x is m ost i n fl ue nced by t he

variatio n of t he s urface ‐t o ‐air te mperat ure differe nce, follo wed by t hat of u *. Near ‐s urface air de nsity varies spatially by less t ha n a factor of t wo over t he w hole do mai n i n t he 10:00 – 11:00 period a nd by eve n less as t he day progresses a nd he nce has little i mpact o n t he patter n of se nsible heat fl ux via its prese nce as a m ulti plier i n eq uatio n (4). O n t he co ntrary, t here appears to be a stro ng negative correlatio n bet wee n t he patter n of near ‐s urface air de n- sity a nd se nsible heat fl ux, b ut t his is largely d ue to t he crater topograp hy w hic h stro ngly i n fl ue nces bot h. Alt houg h surface te mperature stro ngly co ntrols near ‐s urface air te mperat ures via radiative heati ng a nd se n- si ble heat fl ux, dayti me air te mperat ures meas ured at t he sa me heig ht, x , above t he local s urface will be cooler for a n air parcel at heig ht x above t he slopes of Aeolis Mo ns t ha n for o ne at heig ht x above t he crater tre nc h. T he air parcel above t he hig her gro u nd is cooler d ue to horizo ntal (alo ng co nsta nt geopote ntial s urfaces) mix- i ng wit h neig hbori ng air parcels t hat are at hig her altit udes above t heir o w n local s urface (a nd are t h us colder

d ue to t he ge nerally negative dayti me lapse rates i n t he real at mosp here). T his res ults i n larger T s urf ‐T air o ver Aeolis Mo ns t ha n over t he crater tre nc h a nd he nce closely li nks surface topograp hy to t he D D A.

Ot her s urface properties, s uc h as t her mal i nertia a nd albedo, also have a n i mpact o n t he patter n of T s urf ‐T air sho wn in the botto m ro w of Figure 5. The top ro w of Figure 6 sho ws the albedo and ther mal inertia maps used by Mars W R F i n t he do mai n 5 nest. Aeolis Mo ns has t he lo west t her mal i nertias i nside t he crater, wit h the next lo west being in a thin strip running approxi mately north ‐south at ~137° E. Inter mediate values occ ur over most of t he so ut her n tre nc h, wit h t he hig hest val ues of t her mal i nertia over most of t he nort her n tre nc h a nd i n a n area j ust to t he S E of Aeolis Mo ns. T he s urface albedo is also lo w i n a nort h ‐s o ut h stri p at ~137° E a nd ~138.5° E, as well as over t he sout her n half of Gale Crater. Figure 7 co mpares results fro m t he origi nal si m ulatio n wit h o ne usi ng u nifor m albedo a nd t her mal i nertia across do mai n 5. T he variable s urface pr o p erti es i n t h e ori gi n al si m ul ati o n a p p e ar r es p o nsi bl e f or (i) t h e n ort h ‐so ut h strip of hig h s urface te mperatures a nd hig h surface ‐t o ‐air te mperature differe nces at ~137° E, due to lo wer albedo a nd t her mal i nertia ca usi ng t he s urface to war m more rapidly after s u nrise, a nd for (ii) slig htly red uci ng t he s urface ‐t o ‐air t e m- perature differe nce i n t he nort her n crater tre nc h, due to hig h albedos a nd t her mal i nertias t hat result i n lo wer dayti me surface te mperatures. Conversely, high albedos and lo w ther mal inertias over Aeolis Mons appear to largely ca ncel o ut a nd to have little net effect o n t he overall patter n of predicted D D A. Overall, t he n, t he bulk of t he patter n of surface ‐t o ‐air te mperature differe nce i nside Gale Crater appears largely insensitive to the albedo and ther mal inertia map. In particular, Aeolis Mons has the greatest values of

T s urf ‐T air , wit h s maller val ues occ urri ng i n t he crater tre nc h, regardless of w het her varyi ng or u nifor m s urface t her mal properties are used. T hus t he topograp hic co ntrol o n air te mperature, described above, appears to do mi nate over t he i n ﬂ ue nce of ot her s urface properties o n s urface te mperat ure.

T he dayti me patter n of u *is a res ult of co nstr uctive a nd destr uctive i nterfere nce bet wee n (i) local dayti me sl o p e fl o ws i nside t he crater, up t he slopes of Aeolis Mo ns a nd t he crater ri m; (ii) regio nal dayti me slope fl o ws across the dichoto my boundary where Gale Crater sits, up the slope fro m approxi mately N to S (or N N E to SS W); a nd (iii) t he global circ ulatio n at t his ti me of year, w he n t he Hadley circ ulatio n is stro ngest and consists of up welling in the southern he misphere, do wn welling in the northern he misphere, and a ret ur n fl o w fro m N to S across the equator. The resulting early afternoon circulation for this region and for Gale Crater i n particular is s ho w n i n t he botto m ro w of Figure 6, i n t he for m of wi nd vectors at 1.5 m above t he surface i n do mai ns 4 a nd 5 of t he model. Regio nally, nort herly (i.e., fro m t he nort h) wi nds do m- i nate, d ue to t he global a nd regio nal dayti me fl o ws bei ng i n t he sa me directio n i n t his seaso n. T here are so me si g ns of t h e fl o w diverti ng to go aro u nd Gale Crater, b ut it also breaks i nto t he crater over t he N W ri m, pe ne- trati ng part way across t he tre nc h t here. By co ntrast, local dayti me upslope wi nds over muc h of t he nort her n

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Fig ure 5. As i n Figure 3 but no w s ho wi ng se nsible heat ﬂ ux (top) a nd its mai n co ntrib utors: near ‐s urface air de nsity (seco nd ro w), drag velocity (t hird ro w), a nd s urface ‐t o ‐air te mperat ure differe nce (botto m).

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Fi g ure 6. Top ro w: (a) Albedo a nd (b) t her mal i nertia maps used i n Mars W R F do mai n 5 (see sectio n 3.1 for details). Botto m ro w: Mars W R F topography (colors) a nd wind vectors (arro ws) at 1.5 m above the surface in (c) do main 4 and (d) do main 5 at 2 p m L TS T at southern su m mer solstice.

ri m slopes appear to oppose the entry of wind across the rest of the northern ri m and into the northern trench. Local dayti me upslope ﬂ o ws on the steeper, north side of Aeolis Mons ( which sticks up high above the altitude of the northern ri m) reinforce the larger ‐scale nort herly ﬂ o w as it passes over the middle a nd hig her slopes, wit h upslope wi nds o n t he ge ntler sout her n slopes too weak to oppose it except i n o ne s mall regio n. Co mpari ng to t he t hird ro w of Figure 5, it is clear t hat t he nort h westerlies by t he N W

crater ri m, a nd stro ng nort herlies over muc h of Aeolis Mo ns, do mi nate t he patter n of u *i nsi de t he crater. He nce overall, it appears t hat t he c ha nge i n se nsible heat ﬂ ux alo ng t he rover traverse (fro m t he la ndi ng site i n t he crater tre nc h to Aeolis Mo ns' slopes) is predo mi na ntly a res ult of Gale Crater's topograp hy, w hic h pro- d uces bot h (a) air de nsity variatio ns drivi ng s urface ‐t o ‐air te mperature differe nces t hat peak over Aeolis Mons and (b) slope winds that also peak over much of Aeolis Mons, with the effect of albedo and ther mal inertia maps being only secondary. 4.2.3. Cause of the Spatiote mporal Behavior of D D A i n Souther n Su m mer i n Gale Crater In su m mary, during southern su m mer in Gale Crater, the spatial pattern of sensible heat ﬂ ux is co ntrolle d largely by ther mal processes and wind patterns driven by the crater topography (and secondarily by albedo

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Fig ure 7. Co mpari ng co ntrib utors to se nsible heat fl ux, as well as se nsi ble heat fl ux a nd D D A, at noo n at so ut her n s u m mer solstice, for a si m ulatio n wit h (left t wo colu mns) observed and (right t wo colu mns) unifor m ther mal inertia and albedo maps, in Mars W RF do main 5. Color bars are as in Figures 3 (for D D A), 5 (for se nsible heat fl ux a nd co ntrib uti ng variables), a nd 6 (for s urface property maps).

a nd t her mal i nertia variatio ns), w hile t he patter n of t her mody na mic ef ﬁ cie ncy is ca used by t he s uppressio n of t he P B L over Aeolis Mo ns co mpared to t he crater tre nc h i n t his seaso n. Ho wever, t he latter is predicted to vary co nsiderably wit h ti me of year, as explored f urt her i n sectio n 4.3. Overall, t he variatio n i n se nsible heat

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fl ux largely dictates t he patter n of D D A, wit h t he t her mody na mic ef fi cie ncy acti ng to di mi nis h t he peak over Aeolis Mons and weaken the mini mu m in the crater trench. As describe d at t he to p of sectio n 4.2, t he se nsible heat fl ux peaks closer to local noo n, w hile t her mody na mic ef fi ciency peaks after ~14:00. The net result is a single peak in predicted D D A bet ween ~noon and 14:00 in t his seaso n. I n so me ot her seaso ns, ho wever, t his later peak i n t her mody na mic ef fi cie ncy, pl us a late i ncrease i n se nsible heat fl ux d ue to stro ng after noo n wi nds, res ults i n a do uble ‐peak str uct ure bei ng predicted (see s e cti o n 5. 2. 1).

4.3. Seaso nal Variatio ns i n t he Patter n of Predicted D D A i n Gale Crater T his sectio n exa mi nes t he predicted seaso nal a nd spatial variatio ns across t he N W quadra nt of Gale Crater, w hic h co ntai ns t he MS L traverse. Figure 8 zoo ms i n o n t hat portio n of do mai n 5 to exa mi ne i n more detail t he spatial variatio n at fo ur ti mes of year: Ls = 0°, 90°, 180° a nd 270°. Se nsible heat fl ux a nd vertical t her mody na mic ef fi cie ncy peak at differe nt ti mes of sol (see disc ussio n early i n sectio n 4.2) a nd he nce are s ho w n i n Fig ure 8 at t he ti mes w he n t his peak typically occ urs (12:00 – 13:00 a nd 14:00 – 15:00, respectively), wit h D D A s ho w n at bot h ti mes of sol. Fig ures S2, S3, a nd S4 additio nally s ho w res ults at t hree ti mes of sol for, respectively, D D A, se nsible heat fl ux, a nd vertical t her mody na mic ef fi ci e n c y. As expected, t he seaso n wit h t he weakest solar heati ng, local wi nter ( Ls = 90°), has t he lo west predicted D D A (left colu mn), peaking near the top of Aeolis Mons at both ti mes sho wn. The largest D D A over this region are predicted in local su m mer (Ls = 270°), with inter mediate D D A at the t wo equinoxes (Ls = 0 and 180°). Ho wever, the peak values in su m mer occur close to the western edge of the region, at least 10 k m fro m the rover traverse, out of the range of dust devil i maging and much too far a way to affect the R E MS press ure sig nal. T hese peaks are clearly tied to i ncreases i n t he se nsible heat fl u x t h er e (t hir d c ol u m n), as disc usse d i n sectio n 4.2. I n t he regio n closer to t he rover traverse, se nsible heat fl ux is ge nerally stro nger i n all seaso ns over Aeolis Mo ns t ha n i n t he crater tre nc h. T he patter n of s urface ‐t o ‐air te m perat ure differe nce is very si milar i n all seaso ns, as it is largely deter mi ned by t he crater topograp hy a nd ot her s urface properties. T his mea ns t hat seaso nal variatio ns i n t he patter n of se nsible heat fl ux are largely dictated by seaso nal variatio ns i n t he patter n of drag velocity and hence winds. Around su m mer solstice, for exa mple, the region where MSL has begun cli mbing the slopes of Aeolis Mons has particularly weak wind speeds (see third ro w of Figure 5, where a “ tongue ” of lo wer wi nd speeds exte nds fro m t he tre nc h up t he slopes). T hus, at least i n t his si mulatio n,

u *is predicted to i ncrease alo ng MS L's ro ute less t ha n it wo uld have had t he rover cli mbed, e.g., a more wester n slope. Sectio n 5 f urt her disc usses t he relative i mporta nce of t he wi nd fi eld i n capt uri ng observed variatio ns i n vortex activity d ue to MS L's c ha ngi ng positio n. As discussed in section 4.2.1, in su m mer the dayti me P BL depth, hence vertical ther modyna mic ef fi - ciency, is predicted to be s maller over the Aeolis Mons slopes than in the crater trench. Ho wever, the opposite is largely true over the three other seasons sho wn in Figure 8. In fact, the dayti me P B L depth is predicted to peak over Aeolis Mons fro m Ls~330 to 180° in the 12:00 – 13:00 period (see Figure S4) and fro m Ls~30 to 150° in the 14:00 – 15:00 period. This is consistent with a nu mber of studies carried out for MSL's landing season (Ls~150°), which predicted that the PBL depth would be larger over Aeolis Mons than in the trench (e.g., Tyler & Barnes, 2013). The change in pattern with season is due to w het her t he crater circ ulatio n is pri marily drive n by regio nal or local (crater i nterior) fl o ws. Around local winter solstice (Ls = 90°), for exa mple, the winds measured by MSL were strongly linked to local upslope (do wnslope) fl o ws onto (off) Aeolis Mons ( Ne w man et al., 2017; Richardson & Ne w man, 2018) wit h no clear regio nal i n fl uence. Strong, sy m metric dayti me fl o ws up these slopes would be more likely to produce a higher dayti me P BL depth over Aeolis Mons. Around local su m mer solstice (Ls = 270°), ho wever, available MSL wind and aeolian observations (Baker et al., 2018; Viúdez ‐Moreiras et al., 2019) a nd modeli ng (e.g., Rafki n et al., 2016) s uggest t hat stro ng regio nal a nd larger ‐s c al e fl o ws pe netrat- ing into the crater fro m the north have more i mportance. As a result, the circulations inside and outside t he crater – es pecially its nort her n half – are far more co n nected. T his i n tur n results i n a n i ncreased P B L depth on the northern ri m and trench in su m mer co mpared to the usually “ suppressed ” crater i nteri or bou ndary layer (e.g., Rafki n et al., 2016) but co nversely disrupts t he upslope wi nds t hat were respo nsible for increasing the P BL depth over Aeolis Mons in winter.

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Fig ure 8. D D A fro m noon to 13:00 ( fi rst colu m n) a nd 14:00 to 15:00 (seco nd colu m n), se nsible heat fl ux fro m noo n to 13:00 (t hird colu m n), a nd vertical t her mody na mic ef fi ciency fro m 14:00 to 15:00 (fourth colu mn), predicted by Mars W RF in a zoo med ‐i n regio n of do mai n 5, averaged over 7 sols for eac h of t he fo ur seaso ns. Top ro w, Ls = 0°; seco nd ro w, Ls = 90°; t hird ro w, Ls = 180°; botto m ro w, Ls = 270°. T he w hite boxes co ntai n MS L's traverse over t he fi rst 3 Mars years of the MSL mission.

Overall, if MS L continues to scale the slopes of Aeolis Mons for another 3 years, Figure 8 suggests that increasing nu mbers of vortex pressure drops would be observed in most seasons. In local su m mer, ho wever, t his will be d ue to t he i ncreased se nsible heat ﬂ ux, as t he vertical t her mody na mic ef ﬁ ci e n c y i n t h at s e as o n is expected to decrease q uite stro ngly as MS L cli mbs t he slopes f urt her.

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5. Direct Co mparison Bet ween Mars W RF Predictions and MSL Observations A quantitative prediction of the vortex activity observed by MS L can be made by sa mpling the predicted D D A at t he rover locatio n for eac h Ls a nd L TS T hour. T his ca n t he n be co mpared directly wit h observatio ns of vortices a nd d ust devils to assess t he model's perfor ma nce a nd – if t here is agree me nt – t o i nf er r e as o ns f or t he observed seaso nal a nd i ntera n nual c ha nges. Sectio n 5.1 provides t he statistics of vortex pressure drops over t he ﬁ rst 1980 sols (nearly 3 Mars years) of the mission, extracted using the method described in sectio n 2.4. Because MS L la nded at ~4.6°S at Ls~150°, s hortly before local (i.e., sout her n) spri ng equi nox ( Ls = 180°), eac h Mars year ( Y Rs 1 – 3) is broke n i nto fo ur wide periods ce ntered o n local (i) spri ng eq ui nox ( Ls = 135 – 225°), (ii) s u m mer solstice ( Ls = 225 – 315°), (iii) fall eq ui nox ( Ls = 315 – 45°), a n d (iv) wi nter solstice ( Ls = 45 – 135°). Section 5.2 co mpares these statistics with those obtained fro m Mars W R F output and de monstrates that the model predicts many aspects of the observed vortex activity. Section 5.3 exa mines t he relatio ns hip bet wee n D D A a nd vortex nu mbers as a fu nctio n of vortex stre ngt h, w hile sectio n 5.4 exa m- i nes t he i mpact of si m ulatio n set up o n res ults.

5.1. T he Statistics of Observed Press ure Drops i n 12 Periods Spa n ni ng Sols 1 to 1980 The stacked histogra ms in Figure 9 sho w the inferred nu mber of pressure drops per hour for each hour bet ween 07:00 and 17:00 L TS T, for four seasonal ti me periods in the fi rst 3 years of MS L's missio n. T hese were obtained by nor malizing the nu mber of observed pressure drops by the nu mber of measure ments in each period. The pressure drops are also binned according to the magnitude of the detected pressure drop of eac h eve nt, usi ng four logarit h mic mag nitude bi ns t hat cover t he ra nge fro m t he detectio n t hres hold of 0.6 Pa to the maxi mu m vortex pressure drop magnitude detected over this period of just under 5.6 Pa. The res ults for t he fi rst 2 years are consistent with those presented in previously published work (see sectio n 2.3) a nd are also co nsiste nt wit h res ults for all 3 years obtai ned usi ng a met hod very si milar to t hat of Ordo nez ‐Exteberria et al. (2018) (see Text S1 a nd Fig ures S5 a nd S6). In most Mars years, the largest nu mber of vortex pressure drops occurs in the period surrounding su m mer solstice a nd t he s mallest i n t he period s urro u ndi ng wi nter solstice, wit h o ne mai n exceptio n. I n Y R1, t he period around su m mer solstice was relatively lo w in both nu mber and magnitude of pressure drops co mpared to the period surrounding spring equinox. As suggested to explain si milar results found in Kahanpää et al. (2016), this may have been due to the rover sitting at one location fro m MSL sol 166 to 272 (Ls~250 – 317°), w hic h is very si milar to t he Y R1 s u m mer solstice period (de fi ned as Ls = 225 – 315°) s ho w n i n t he sec- o nd plot i n t he top ro w of Fig ure 9. T his locatio n ( “ Yello wk nife Bay ” ) was measured to have a much higher t her mal i nertia t ha n nearby regio ns ( Martí nez et al., 2014), w hic h wo uld have prod uced a slo wer rise i n s urface te mperature during the day, reduced the dayti me sensible heat fl ux, a nd very likely res ulted i n t he reduced vortex activity observed by R E MS. By contrast, the ther mal inertia maps used in Mars W RF are derived fro m orbital data and do not capture such local variations; thus Mars W R F did not predict such a decrease i n t he D D A (see disc ussio n i n sectio n 3.1). T wo ot her periods also co me close to bei ng exceptio ns. I n Y R3, a co mparable nu mber of pressure drops were measured around winter solstice as around the previous spring equinox. Ho wever, this is most probably a res ult of t he stro ng i ncrease i n vortex activity as t he rover cli mbed rapidly hig her o n t he slopes; i.e., t here were likely far more press ure drops recorded i n t he period aro u nd t he f oll o wi n g s pri n g ( n ot c o nsi d er e d i n t his study). Seco nd, t he period arou nd fall equi nox i n Y R2 has a s maller nu mber of pressure drops overall t ha n the period around that su m mer solstice, but there is a greater peak in nu mber of drops bet ween 11:00 and 13:00. This is due to several sols when the at mosphere was ano malously convective around local noon, as also noted by Ordonez ‐Exteberria et al. (2018). Typically, t he n u mber of 1 ‐hour measure ment periods con- t ai ni n g N vortices (s ho w n i n Fig ure S7) decreases logarit h mically wit h N u p t o N ~10 a nd t he n decli nes more slo wly, b ut a “ hu mp ” around N ~18 correspo nds to s uc h especially restless periods. In most seasons and Mars years, the peak nu mber of observed vortex pressure drops occurs bet ween 11:00 and 14:00 L TS T. It appears to ske w earlier in this period around the equinoxes ( fi rst a nd t hird colu m ns) a nd later arou nd t he solstices (seco nd a nd fourt h colu m ns), alt houg h agai n t he seaso n spa n ni ng local su m- mer i n Y R1 is a no malo us here. Fi nally, t here is a stro ng overall year ‐t o ‐year increase in the nu mber and mag nitude of observed vortex pressure drops for most periods a nd ti mes of day, wit h t he greatest activity occurring when MS L sits highest on the slopes of Aeolis Mons.

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Fig ure 9. R E MS observed pressure drops, separated according to the magnitude of the pressure drop (stacked histogra ms; colored bars), co mpared to the D D A at MS L's locatio n predicted by Mars W R F (dark gree n li nes) as a f u nctio n of ti me of sol over fo ur seaso nal ra nges eac h spa n ni ng 90° of Ls, a nd ce ntered o n Ls = 180°, 270°, 0°, a nd 90°. T he Poisso n ( N 0. 5 ) co u nti ng error is also s ho w n (black bars).

5.2. Mars W RF ‐Predicted D D A at t he Rover Locatio n Figure 9 also sho ws Mars W RF predictions of D D A for the sa me ti me ‐of ‐day ra nge a nd seaso nal periods for which the R E MS pressure drop data are sho wn. Each panel is produced by stepping through that seasonal period sol by sol, sa mpli ng t he mi n ute ‐b y ‐mi nute D D A predictio ns i n t he model at MS L's locatio n for t hat sol (i nterpolati ng bet wee n results for t he t wo Mars W R F si mulatio ns t hat are closest to it i n Ls), a nd t he n averagi ng t he res ults at eac h ti me of day over all sols co ntai ned i n t he period. Co mpari ng t he observatio ns and Mars W R F predictions sho ws that the general seasonal and diurnal variation is largely captured in the variatio n of D D A predicted usi ng Mars W R F output. T he ge neral tre nd of i ncreased vortices fro m year to year for most seasons and ti mes of sol is also captured by the model; ho wever, the observed nu mber of vortex pressure drops increases faster than the predicted D D A as MSL cli mbs up the slope of Aeolis Mons. This is disc usse d f urt her i n sectio n 5.3. For bot h t he observed nu mber of pressure drops a nd predicted D D A, t he peak ge nerally occurs i n t he period co ntai ni ng su m mer solstice, is reduced i n t he periods arou nd bot h equi noxes, a nd is s mallest i n t he wi nter solstice periods. T he exceptio ns are arou nd Y R1 su m mer solstice, w he n observed activity is lo w (see discus- sio n i n sectio n 5.1), a nd aro u nd Y R3 wi nter solstice, w he n t he observed activity is co mparable to t hat aro u nd t he precedi ng spri ng eq ui nox. T he latter is also predicted by Mars W R F, ho wever, a nd (as disc ussed i n sectio n 5.1) is li kely d ue to t he year ‐t o ‐year increase as MSL cli mbs Aeolis Mons. Both peak D D A and nu mber of vortices occur arou nd t he solstices i n t he 13:00 to 15:00 period (agai n, except for t he su m mer solstice of Y R1, when observations dip) and occur earlier around the equinoxes. Ho wever, the clear predicted “ d o u bl e ‐pea k ” str uct ure i n D D A aro u nd spri ng eq ui nox, w hic h is also visible aro u nd fall eq ui nox, is ge nerally not see n i n t he observed n u mber of vortices or press ure drop mag nit udes. By co ntrast, t here is a hi nt of a

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Fig ure 10. Mars W RF ‐predicted daily variatio n of: D D A; se nsible heat ﬂ ux a nd its pri mary co ntrib utors (s urface ‐t o ‐air te mperat ure differe nce, drag velocity u *, a n d air de nsity); a nd vertical t her mody na mic ef ﬁ cie ncy a nd its pri mary co ntrib utor (t he press ure t hick ness of t he P B L); calc ulated for t he rover locatio n a nd seaso n correspo ndi ng to t he releva nt sols, for t he sa me seaso nal periods as i n Fig ure 9.

d o u bl e ‐peak structure in the observations around su m mer solstice (particularly in Y R3, whereas in the model it is most noticeable i n Y R1). In su m mary, there is general agree ment but also so me signi fi cant differences bet ween the predicted D D A a nd observed vortex activity. Ho wever, give n t he several areas of agree me nt, it is possible to dig i nto t he model results to exa mi ne w hat may be respo nsible for t he observed vortex variability wit h seaso n, ti me of day, a nd locatio n o n Mars. 5.2.1. Co ntrols o n t he Seaso nal a nd Year ‐t o ‐Year Variatio n of D D A As discussed above and sho wn in Figure 9, Mars W RF generally does a good job of capturing the observed seasonal variation of pressure drops. Figure 10 sho ws the diurnal variation of each relevant variable in Mars W RF for each seasonal period at the rover location and de monstrates the relative con- trib utio n of se nsible heat fl ux and vertical ther modyna mic ef fi cie ncy. T he for mer typically peaks s hortly before or around noon, whereas the latter peaks bet ween ~14:30 and 16:00, shifting later as the season shifts fro m spring to su m mer to fall. These results reveal that the peak D D A occurs around su m mer solstice d ue largely to a stro nger after noo n se nsible heat fl ux than around the equinoxes, which in turn is d ue largely to a stro nger s urface ‐t o ‐air te mperat ure differe nce at t his ti me of year. Fi nally, aro u nd wi nter solstice, t he s urface ‐t o ‐air te mperature difference and P B L depth are both far lo wer than in other seasons at most ti mes of sol, with no afternoon peak in predicted P BL thickness, which results in the lo west D D A bei ng predicted for t his seaso n relative to ot her ti mes of year. Fig ure 10 also de mo nstrates – as disc usse d i n sectio n 4.3 – t hat a n i ncrease i n se nsible heat fl u x fr o m y e ar t o year, not t her mody na mic ef fi cie ncy, is respo nsible i n most seaso ns for t he predicted i ncrease i n D D A as MS L cli mbs up t he slope of Aeolis Mo ns. O nly aro u nd fall (a nd, to a lesser exte nt, spri ng) eq ui nox is a n appreci- able increase predicted in peak P BL depth (hence ther modyna mic ef fi cie ncy) i n t he after noo n fro m year to year, w hereas peak se nsible heat fl ux is pre dicte d to i ncrease sig ni fi c a ntl y – and monotonically – fro m year to year i n every seaso n. Differe nces bet wee n years are disc ussed f urt her i n sectio n 5.3.

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5.2.2. Co ntrols o n t he Ti me ‐of ‐Day Variatio n of D D A As sho wn by equations (1) and (4), energy is available for vortex for mation (hence nonzero D D A values occur) w he n t he se nsible heat fl ux is positive, i.e., w he n t he surface is war mer t ha n t he at mosp here above it. T he model i n fact predicts no D D A before 07:50 or after 17:00 i n a ny seaso n. T hese predicted ti me periods of no nzero D D A co ntai n all of t he observed dayti me pressure drops exceedi ng 0.6 Pa (see Figure 9). Fig ure 10 reveals t hat t he predicted do uble ‐peak str uct ure i n so me seaso ns is so me w hat d ue to t he differe nt ti mi ngs of t he pea k i n s urface ‐t o ‐air te mperature difference and ther modyna mic ef fi cie ncy b ut pri marily d ue to t he do uble ‐peak str uct ure i n drag velocity i n so me seaso ns. S urface ‐t o ‐air te mperat ure differe nce peaks either in the hour before noon (around both equinoxes in all years and around su m mer solstice in Y R3) or in the hour after noon, whereas ther modyna mic ef fi ciency peaks at ~15:00 around spring equinox and su m mer solstice, up to a n hour later arou nd fall equi nox, a nd is fl at fro m ~10:30 – 15:30 aro u nd wi nter solstice. I n additio n, drag velocity peaks stro ngly i n t he late mor ni ng a nd mid ‐after noo n, wit h a di p i n t he early after noo n, aro u nd bot h eq ui noxes, partic ularly spri ng. T he net effect is bot h a late mor ni ng a nd early to mid ‐ afternoon peak in the predicted D D A in many years and seasons, especially around spring and to a lesser exte nt fall eq ui nox. Fig ure 9 allo ws t he daily ti mi ngs of observed peaks i n vortex activity to be co mpared wit h t hose predicted by Mars W R F as a function of season. The best agree ment occurs around winter solstice, when both the model and R E MS data sho w a nearly sy m metric distribution about the noon – 13:00 period, especially i n Y R3. Around su m mer solstice, by contrast, the model predicts a less sy m metric distribution, with D D A peaking bet ween ~12:00 and 15:00 and a hint of a secondary peak bet ween ~10:00 and 11:00 that fades fro m year to year a nd is go ne by Y R3. T his is visible i n t he observatio ns too, alt ho ug h i n Y R3, t he mor ni ng peak is still prese nt, a nd i n Y R1, t he nu mber of pressure drops >0.6 Pa peaks i n t he late mor ni ng rat her t ha n after noo n. Ho wever, as noted i n sectio n 5.1, Y R 1 suffered fro m relatively fe w R E MS measure me nts a nd t he rover sit- ti ng i n a locatio n wit h a n a no malo usly hig h t her mal i nertia for over a h u ndred sols, so t he data set is ha m- pered by lo w statistics a nd possibly biased by local s urface properties. Around spring equinox, the morning and afternoon peaks are predicted to be rather si milar in magnitude, wit h a disti nct dip bet wee n t he m s hortly after noo n. Ho wever, t he observatio ns s ho w o nly a hi nt of a t wo ‐ peak structure, with a secondary maxi mu m in the afternoon in Y R2 but no sign of this at all in Y Rs 1 or 3. Around fall equinox, t wo peaks are again predicted, but with a s maller dip bet ween the m which s hri nks f urt her wit h positio n up t he slope (i.e., wit h year). As i n spri ng, t here is barely a hi nt of t wo ‐pea k structure in the observations, and this disappears by Y R3. The net effect around both equinoxes is an

overprediction of the afternoon D D A, pri marily due to an apparently erroneous peak in u *i n t he afternoon (see yello w lines in Figure 10). Interestingly, despite using output fro m a different model, run at far lo wer resolution (5°x5° grid spacing), Chap man et al. (2017) predict a si milar double ‐peak str uct ure in D D A when averaging over the whole of Y R1. Ho wever, given the strong sensitivity of winds to the crater topograp hy ( w hic h is not resolved i n t heir model) a nd t he fact t hat t he double ‐peak behavior changes w he n Mars W R F is set up differe ntly (see sectio n 5.4), t his agree me nt is likely coi ncide ntal.

5.3. Seaso nal a nd Year ‐t o ‐Year C ha nges as a F u nctio n of Vortex Stre ngt h T his sectio n no w ig nores di ur nal variability a nd foc uses o n t he total n u mber of press ure drops per sol. T his enables easier co mparison with Mars W R F predictions of seasonal and interannual variations but also provides i mproved statistics for looki ng at t he matc h as a fu nctio n of pressure drop mag nitude. Accordi ng to R98, the maxi mu m tangential wind velocity around a convective vortex is proportional to the square root of its ce ntral pressure drop. T he ce ntral pressure drop ca n he nce be used as a measure of vortex stre ngt h. As t he vortices do not pass directly over MS L, t he detected pressure drop mag nitudes are s maller t ha n t he act ual ce ntral press ure drops of t he vortices. Nevert heless, t he te mporal distrib utio n of t he detected press ure drop magnitudes re ﬂ ects t he distrib utio n of t he vortex stre ngt hs. Figure 11a sho ws the total inferred nu mber of vortex pressure drops >0.6 Pa per sol and Mars W R F predictio ns of mea n D D A as a fu nctio n of Ls over t he 3 years used i n t his work, agai n bi n ned i nto four broad seasons in each year. This clearly de monstrates the degree to which the D D A underpredicts the year ‐t o ‐ye ar increase in the nu mber of vortices observed by MS L. Ho wever, if the observations and the D D A are plotted using a mini mu m D D A value of 0.17, the match is much better ( Figure 11b). This is especially true if the

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Fig ure 11. R E MS total daily inferred pressure drops, separated according to the drop magnitude (histogra ms), co mpared to the mean D D A over the 9 hours during w hic h co nvective vortices are observed (08:00 – 17:00) at MS L's locatio n predicted by Mars W R F (dark gree n li nes), as a fu nctio n of Ls over 3 Mars years, averagi ng over broad (90° – wide) Ls ba nds. I n t he left col u m n, t he D D A axis starts at 0.0, w hereas i n t he rig ht, it starts at 0.17.

su m mer solstice period i n Y R1 is discou nted as a no malous, as discussed i n sectio n 5.1. T his suggests t hat a threshold D D A may apply belo w which no vortices occur (or at least none strong enough to be detected by the methods used here) and that D D A may be better able to predict the total nu mber of vortices if the existence of such a threshold is taken into consideration. To exa mine this in a quantitative manner, ho wever, suc h a D D A t hres hold s hould be applied to eac h s hort ti me period (e.g. every hour of every sol), rather than being applied to the mean D D A value over a period lasting >100 sols (as is effectively sho wn here). Figure 11 also sho ws the co mparison bet ween D D A and the inferred nu mber of vortex pressure drops >1.05 Pa (seco nd ro w), >1.83 Pa (t hird ro w), a nd > 3.2 Pa (fo urt h ro w). T he matc h is i ncreasi ngly poor as t he pressure drop magnitude – he nce stre ngt h of t he vortex – i ncreases. T his suggests t hat a hig her t hres hold D D A may apply to stronger vortices and/or that the relationship bet ween vortex strength and D D A may not be linear. Co mparing Figures 11d, f, and h with Figure 11b ( which is do minated by the most abundant, i.e., weakest, vortices) s uggests t hat i ncreasi ngly hig h t hres holds (so me w hat eq uivale nt to starti ng t he y ‐a xis at a higher value in Figure 11d, higher again in Figures 11f, and still higher in Figure 11h) might i mprove t he matc h to observatio ns of i ncreasi ngly stro ng vortices. If o ne ass u mes t hat a stro nger vortex ( wit h a deeper ce ntral pressure drop a nd he nce hig her ta nge ntial wi nd speeds) is required to raise dust fro m t he surface,

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Fig ure 12. As i n Figure 9 but no w usi ng output fro m a Mars W R F si mulatio n usi ng vertical grid A ( Ne w man et al., 2017). See text for more details.

o ne mig ht expect t he te mporal variatio n of i maged d ust devils to matc h t hat of larger press ure drops (e.g., Figures 11e or 11g, rather than 11a) and thus to have a threshold ‐dependent and/or nonlinear relationship with D D A also. This would be consistent with the ﬁ ndings of Kahre et al. (2006) that M E R Spirit observed zero dust devils in so me seasons, whereas the predicted D D A never dropped to zero at that location. The relationship bet ween predicted D D A and dust devils i maged by surface ca meras will be explored i n f ut ure work.

5.4. I mpact of Si m ulatio n Set up o n Res ults Given so me of the mis matches bet ween the observed and modeled vortex activity, one may ask whether the Mars W R F predictio ns or t he t heory of R98 are respo nsible. As discussed i n sectio n 5.3, t he prese nt study does suggest t hat a t hres hold D D A may be needed to co n nect R98 to pressure drop statistics, but t his study is not well suited to deter mining the accuracy of R98. A better approach would be to perfor m terrestrial experi ments in which the local and regional at mospheric state can be measured and predicted with much higher accuracy than is available for Mars, and co mpared to more co mplete and extensive measure ments of vortices a n d d ust devils. T his sectio n t herefore foc uses o n pote ntial Mars W R F errors. As noted i n sectio n 3.1, all res ults s ho w n above used o utp ut fro m a si m ulatio n t hat has previo usly bee n s ho w n to represe nt wi nds a nd aeolia n feat ures i nside t he Gale Crater reaso nably well. Ho wever, it does so by usi ng a vertical grid wit h rat her poor vertical resol u- tio n i n t he lo wer bo u ndary layer ( Ne w ma n et al., 2017). It t h us makes se nse to begi n i nvestigati ng t he so urce of t he mis matc hes – especially t he ti mi ng of pea k vortex activity – by exa mi ni ng t he res ults usi ng a differe nt vertical grid i n t he model, speci ﬁ cally, vertical grid A of Ne w ma n et al. (2017) w hic h has t hree additio nal model layers centered belo w ~150 m. Figure 12 sho ws the match to observed pressure drops as a function of ti me of sol and season using vertical grid A and may be co mpared directly to Figure 9 sho wing grid B results. The main differences are in the ti mes of peak D D A, with Figure 12 not predicting the erroneous

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mi d ‐after noo n peak aro u nd t he eq ui noxes see n i n Fig ure 9. I n t he origi nal si m ulatio n, as s ho w n by Fig ure 10

, t he late mor ni ng peak is largely d ue to a peak i n u *( he nce se nsible heat fl ux) at t his ti me, w hile t he mid ‐ after noo n peak is a co mbi natio n of a seco nd peak i n u *and the gro wing P B L depth (hence vertical ther mody na mic ef fi ciency). The ne w si mulation does not have this strong double ‐peak str uct ure i n u *and hence does not predict t he erro neous mid ‐afternoon peak (see Figure S8). Ho wever, the ne w si mulation predicts peak D D A at equi nox bet wee n 13:00 a nd 14:00, w hic h is later t ha n t he observed peak i n nu mber of vortices. Overall, there is no clear i mprove ment in the match to diurnal variability when using the ne w si mulation and no i mprove ment to seasonal or year ‐t o ‐y e ar v ari a bilit y at all. The next step beyond what has been done here would be to use dust opacity maps based on regional (e.g., Mo ntabo ne et al., 2015) a nd local (e.g., Guze wic h et al., 2018) observatio ns of at mosp heric dust co nte nt, in order to vary Mars W RF's at mospheric dust distribution more realistically. Work on this is under way, i ncludi ng modeli ng t he i mpact of t he global a nd regio nal dust stor ms i n M Y 34, t he latter of w hic h was also experie nced by t he I nSig ht la nder a nd recorded i n its more se nsitive measure me nts of pressure. T he results of these Mars W RF si mulations will be used to study the i mpact of dust opacity changes on the predicted D D A, w hic h will be co mpared wit h t he observed vortex pressure drops a nd dust devil statistics at bot h t he MS L a nd I nSig ht la ndi ng sites d uri ng t he stor m(s). Ideally, future work would also co mpare observed vortex pressure drops wit h t he explicit si mulatio n of vortices i n a Large Eddy Si mulatio n ( L ES) of Gale Crater. W hile Mars W R F may also be ru n i n t his way, t his is a dif fi cult experi me nt to perfor m due to t he need for t he L ES to i nclude t he w hole of Gale Crater to properly c a pt ur e t h e l o c al t o p o gr a p hi c i n fl ue nces, a nd yet be r u n at ~50 m grid spaci ng i n order to capt ure t he major- ity of t urb ule nt str uct ures a n d resolve larger co nvective vortices. I n practice, a n area fo ur ti mes t he size of t he crater wo uld be needed to avoid bo u ndary co nditio n iss ues a nd edge effects; he nce t his wo uld req uire ~6,000 grid poi nts i n lo ngit ude a nd i n latit ude, a very expe nsive si m ulatio n. I n additio n, t he L ES wo uld need to be drive n wit h ti me ‐varyi ng regio nal wi nd pro fi les taken fro m a lo wer resolution Mars W RF nested mesoscale si mulation. Finally, even at such a high resolution, vortices s maller than ~200 m dia meter would not be properly resolved. Ho wever, t he be ne fi ts of bei ng able to model t he P B L more explicitly t ha n i n a mesoscale si mulation (in which a P B L subgrid ‐scale para meterization is required) and to co mpare MS L observations directly wit h model predictio ns (e.g., t he distributio n of vortex pressure drop mag nitudes, ti mi ngs, etc.) make t his idea very appeali ng.

6. Co ncl usio ns The ther modyna mic theory of R98 is co mbined with output fro m the Mars W RF at mospheric model, run at a grid spaci ng of ~1.4 k m, to predict t he variatio n of d ust devil (or eq uivale ntly, co nvective vortex) activity, D D A, for Gale Crater, Mars. This is then co mpared with the statistics of dayti me surface pressure drops >0.6 Pa measured by R E MS on the MSL rover over 3 Mars years, which are associated with the passage of co nvective vortices. I n t he R98 t heory, D D A is proportio nal to t he se nsible heat fl ux, w hic h i ncreases wit h t he s urface ‐t o ‐air te mperature difference and drag velocity, and to the vertical ther modyna mic ef fi ci e n c y of t he P B L, w hic h i ncreases wit h t he pressure t hick ness of t he P B L. MS L la nded at ~4.5°S at Ls~150°, so eac h Mars year is broke n i nto fo ur wide periods ce ntered o n so ut her n (i) spri ng equi nox ( Ls = 135 – 225°), (ii) s u m mer solstice ( Ls = 225 – 315°), (iii) fall e q ui nox ( Ls = 315 – 45°), a n d (iv) wi nter solstice ( Ls = 45 – 135°). Except for Y R1, w he n t here were large data gaps a nd t he rover re mai ned i n a n a no malo usly hig h t her mal i nertia regio n for over a h u ndred sols, t he greatest n u mber of i nferred vortex occurrences and largest pressure drop magnitudes were observed in the period around su m mer solstice. Mars W RF output also predicts peak D D A around su m mer solstice, which is attributed to both high s urface ‐t o ‐air te mperature differe nces fro m mid ‐morning through mid ‐afternoon and large boundary layer t hick nesses i n t he mid ‐after noo n. I n ter ms of diur nal variatio ns for t his seaso n, Mars W R F output predicts peak D D A bet ween noon and ~15:00, with a s mall secondary peak bet ween 10:00 and 11:00 in Y R1 only, whereas R E MS pressure drops suggest more of a t wo ‐pea k str uct ure i n all years. Si g ni fi cant nu mbers of vortices are also observed and predicted around both equinoxes, but while observations sho w a peak in vortex activity bet ween 11:00 and 13:00, Mars W RF predicts both peak D D A bet ween 11:00 a nd 12:00 a nd a seco nd peak i n t he midafter noo n after ~14:00, especially aro u nd spri ng. I n t he model,

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t he late mor ni ng peak is largely d ue to a peak i n u *( he nce se nsible heat fl ux) at t his ti me, w hile t he midafternoon peak is a co mbination of a second peak in u *and the gro wing P BL depth (hence vertical ther modyna mic ef fi ciency). Another Mars W RF si mulation without the strong double ‐peak str uct ure i n u *does not predict the erroneous midafternoon peak; ho wever, the predicted D D A still peaks around equinox bet ween 13:00 a nd 14:00, w hic h is later t ha n t he observed peak i n n u mber of vortices. Fi nally, t he period aro u nd wi nter solstice has t he fe west d ust devils observed a nd predicted i n Y Rs 1 a nd 2, with Mars W RF output sho wing both the s mallest sensible heat fl ux (largely d ue to a s maller s urface ‐t o ‐air te mperature differe nce) a nd P B L dept h, wit h no midafter noo n peak i n t he latter as occurs i n all ot her seasons. The winter solstice period has the most sy m metric diurnal distribution of pressure drops around ~noon – 13:00, as predicted by t he model w hic h s ho ws t his bei ng d ue to t he relative fl at ness of t he P B L dept h wit h l o c al ti m e. Observations sho w an increase in nu mber and also magnitude of vortex pressure drops fro m year to year in all seaso ns, as t he rover made its way o ut of t he tre nc h a nd up t he lo wer slopes of Aeolis Mo ns, wit h a year ‐ t o ‐year i ncrease also fo u nd i n t he predicted D D A. T he rate of t he year ‐t o ‐year i ncrease i n n u mber of vortex pressure drops >0.6 Pa is underpredicted by the model if vortex nu mbers are assu med to be si mply proportional to D D A. Ho wever, the match may be far better if a threshold D D A is chosen belo w which no pressure drops are expected (or are too s mall i n mag nit ude to be detected by t he met hods used here). T his res ult, as well as the greater mis match bet ween D D A variation and variation in nu mber of larger magnitude pressure drops, suggests t hat t he relatio ns hip bet wee n D D A a nd vortex pressure drop statistics is likely t hres hold ‐ dependent and/or nonlinear. T he model does not ge nerally attrib ute t he year ‐o n ‐year increase in D D A to an increase in P B L depth over t he mou nd (as i n so me previous work). I ndeed, a detailed study of ho w t he P B L dept h is predicted to c ha nge wit h rover positio n s ho ws it o nly i ncreasi ng slig htly arou nd t he equi noxes as MS L cli mbs t he slopes, a nd Ackno wledg ments basically static fro m year to year i n t he ot her seaso ns. I nstead, t he greater D D A a nd vortex occurre nce as T he lead a ut hor wis hes to dedicate t his wor k to her late fat her, Alla n F. the rover cli mbed Aeolis Mons are pri marily attributed to an increase in surface ‐t o ‐air te mperature and Ne w man, who al ways provided loving u *as MS L moved higher on the N W slope. Both of these are found to be pri marily a function of the crater support for and insightful co m me nts on her researc h, fro m her t hesis t hro ug h to topography. The surface ‐t o ‐air te mperature difference increases higher up the slope due to cooler near ‐ t h e fi rst draft of t his pa per. C. E. surface dayti me air te mperatures caused by mixi ng wit h cooler adjace nt hig h ‐altit u de air, wit h a n a d ditio nal Ne w man and M. I. Richardson were s mall effect d ue to lo wer regio nal t her mal i nertias over t he slopes. I n t he case of u *, t he rover experie nces supported by MSL grant 1449994. The co ntributio n of H. Ka ha npää i n t his stro nger peak dayti me upslope wi nds as it cli mbs t he slopes. study was funded by The Finnish The spatial variation in predicted D D A may also explain why no dust devils were i maged for about 2 Cultural Fou ndatio n (gra nt 00180446). G. M. Martí nez a nd A. Vice nte ‐ years at the start of the mission, when i maging ca mpaigns were looking a way fro m the mound to ward Retortillo were s upported by Jet a region of lo wer predicted D D A. Dust availability may also have been a factor, but the results of this Propulsion Laboratory grant 1449038, study suggest that seasonal D D A predictions would be very helpful for directing i maging ca mpaigns for L u nar a nd Pla netary I nstit ute ( L PI) Co ntrib utio n 2236; L PI is operated by future landed missions, particularly those landing in regions with strong topography or other major U niversities Space Researc h s urface property co ntrasts. 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