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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. A8, PAGES 6671-6688, AUGUST 1, 1984

The Spatial and SeasonalDistribution of Martian Clouds and Some Meteorological Implications RALPH KAHN

Cornell University,Ithaca, New York

The spatialand seasonal distributions of Martian atmospheric clouds,organizedaccordingto simple morphologiccriteria, exhibit distinctpatterns.When interpretedself-consistently using analogiesto terrestrialcloudformsand Martian temperature,wind direction,and water vapor measurements, patternsof cloudoccurrence providesomelooseconstraintson global-scale near-surface wind speed,static stability,and humidity.We haveexaminedall Mariner 9 and Viking Orbiter imagesfor this study;we concentrate primarilyon the largerViking data set.Wheneverobservations are possible,relativelyhigh near-surface windsappearto follow the seasonalcap edgesduring early fall, late winter, and spring. Moderate to high winds are also inferred in mid-latitudesfor mid southernwinter and in low latitudes, mainly in the steeply sloping areas of Tharsis, near the solstices.At other times, lower winds are suspected. Hemisphericasymmetriesare traced to differencesin atmospherichydration state and to

global dust storm relatedeffectson the atmospheric temperaturestructureand the north polar cap recession schedule. Cloudoccurrence data suggest that the near-surface daytimestaticstabilityis low in the northernhemisphere duringsummerand at low latitudesduringmid northernspring.Relativelyhigh staticstabilityis deducednearthe winterpolesand globallyduringthe duststormseason.In spiteof low atmospherictemperatures, the correspondingly low absolutehumidityapparentlyprecludesthe formation of thickwatericecloudsat highlatitudesin mid to late autumnand winterin bothhemispheres and in southernmid-latitudesduring early winter. During northern spring and summer,saturationconditionsseemto be easilyachievedat mid-latitudes,and condensatecloudsare abundant;the situationis complicatedby atmospheric dustin southernspringand summer.Low latitudesgenerallyappearto be farther from saturation than mid-latitudes,and clouds generallyform more easily in the northern hemispherethan in the southduringcorresponding seasons. 1.

INTRODUCTION

Much of our knowledgeof the global-scalemeteorologyof Mars comesfrom deductionsbasedupon remote observations of cloudsin the Martian atmosphere.Cloud color and seasonal behavior provided ground-basedastronomerswith the first cluesto the nature of global dust stormsand the polar hoods [Slipher, 1962]. Spacecraftreconnaissance by the Mariner 9 and Viking orbitersproduceda wealth of more detailedinformation about cloud morphology and distribution [Masursky et al., 1972; Leovy et al., 1972, 1973a; Bri•7•lsand Leovy, 1974; Bri•7•lset al., 1977, 1979; French et al., 1981]. From this body of work it became evident that clouds in the Martian

atmo-

spherecould be classifiedby morphology.Cloud "types"defined accordingto easilyobservablemorphologiccriteria were found to exhibit distinct spatial distributions and seasonal variations, suggestingthat cloud occurrencedata may be usefulfor probing regional meteorologicalconditions.French et al. [1981] began the systematicstudy of global patternsin cloud occurrence,usinga keyingschemethat distinguishes six morphologictypes, and a data set containing all Mariner 9 and part of the Viking Orbiter images. This paper is an expansionof the earlier work, with a substantially different emphasis;in addition to presentingnew cloud distributionobservations,we have focusedspecifically upon drawing meteorologicalinferencesfrom the cloud occurrencedata. To this end, the coupledspatial and temporal behavior of each cloud type is examined in much greater detail than was done in previousstudies.We have generalized the classificationscheme used by French et al. and have searchedall Mariner 9 and all Viking Orbiter pictures(approximately7,400 and over 50,000images,respectively)for the Copyright 1984by the AmericanGeophysicalUnion.

new catalog.In the next sectionwe review the cloud keying schemeand presentplots of cloud type occurrenceand related statistics.We then interpret these data in light of the combined constraintsof meteorologicalconditionslikely to favor formationof eachcloudtype,accordingto terrestrialanalogy, and the observedwater vapor abundance,temperature,and wind fields on Mars. These results are used to formulate some

generalizationsabout the seasonaland spatial variation of Martian near-surfacewind strength,atmosphericstaticstability, and humidity and to identify areas where more detailed observations and theoretical work could be most valuable. 2.

THE SPATIAL AND SEASONALDISTRIBUTION OF MARTIAN

CLOUDS

We separatesurfaceobscurationobservedin Viking and Mariner 9 imagesinto sevencategories,accordingto cloud morphology,usingthe schemedisplayedin Figure 1. The resulting"cloud types"are listedin Table 1, togetherwith brief descriptionsof each and some conjecturesabout probable meteorologicalconditionswhichwould favor formation,based primarilyon analogyto terrestrialclouds[e.g.,Ludlam,1980-[. Thesewill be discussed in more detaillater. Sampleimagesof the Martian cloudtypesare presentedin Figure 2. The entire Viking and Mariner 9 data setswere examinedat leasttwicefor this study,by two independentobservers. There are about 16,500 haze eventsin the catalog and over 1,400 occurrences of the other cloud types.In 5-7% of the casesthe cloudmorphologywas sufficientlyunclearfor the observersto disagreeon the cloud type. In theseinstancesa third observer helpeddecideuponthe designation. The cloudsin eachimage were countedindependentof possibleoccurrenceof parts of cloudsin adjacentimages.However,statistics are compiledby orbit, in part to avoid biasesdue to this possibility(seediscussionof Table 3). Figures 3-11 show the distributions of each of the cloud

Paper number 4A0636. 0148-0227/84/004A-0636505.00

types as a function of latitude and season.(Ls, the seasonal 6671

6672

KAHN: MARS CLOUD DISTRIBUTION AND METEOROLOGICALIMPLICATIONS

CLOUD

KEYING

In eachcasethecenterlatitudefor thepicturein whichthe

SCHEME

Distinct Structure ? N Periodic Structure ? Localized

in

Topocjraphic Feature

•y•Haze

Y

Localized

Fog

Streak Cloud

Sourceof Rising Material ? Y

•Plume

One- Dimensional Periodicity?

cloud was observedis used in plotting the point. This implies a general uncertainty of several degreesof latitude for the actual cloud locations.While longitude information is not displayed in these figures, many of the cloud occurrencesare plotted on latitude-longitude grids by French et al. [1981]. Additional longitude data are presentedin Figures 12 and 13. Here the Viking observationsof thin haze, and moderate and thick haze, are plotted on a mercator and two polar projectionscoveringthe entire planet for each of 10 seasonalperiods of varying lengths. For these figures, images containing the planetary limb have been excludedto avoid confusionabout

ßN

Cloud Street

haze thickness. Associated

Surface

Obstacle?

,

We use the traditional seasonaldivisions, with summer and

Wave Cloud Lee

Wove

Fig. 1. Cloud morphology keyingschemeusedto classifyeach cloud observedin the Mariner 9 and Viking Orbiter images.

variable, rangesfrom 0ø to 360ø over the Mars year, with 0ø defined as the northern spring equinox.) Hazes are divided into three categoriesaccordingto the visibilityof surfacefeatures through the obscuration."Thin" haze (visible optical depthß < 1) occurrenceis shownin Figure 9, "moderate"haze (z • 1) occurrence is givenin Figure 10, and thick haze(z > 1) occurrenceis presentedin Figure 11. For theseplots, images whichcontainthe planetarylimb have beenexcluded,to avoid uncertaintyin the opacitydeterminationdue to high emission angle. An open circle is used for Mariner 9 observations (1972-1973),and a solid circlefor Viking events(1976-1980).

winter beginningon the solstices,and spring and autumn on the equinoxes,for naming the time bins. However, for Mars the meteorologicalcharacteristicsof each seasonmay be more closelycenteredon the respectiveequinoxesand solsticesthan for earth. (The situation, particularly in high northern latitudes, is complicated by the large eccentricityof the Mars orbit [e.g.,Kieffer, 1976].)We find that the actualdistribution of cloud occurrencesdivides naturally into 10 distinct "seasons,"independentof the namingscheme.Theseare indicated in Figures3-11 with dashedlines. In order to make a fair evaluation

of the variation

of cloud

occurrencewith season,spacecraftimaging coverage effects must be considered.Since spatial and temporal samplingis generallyvery sparse,we assesscoverageby dividing each hemisphereinto three latitude bins (equatorial,00-30ø latitude; mid-latitude, 30ø-60ø; and polar, 600-90ø) and then

countingthenumberof spacecraft orbits(nij)in seasonal bin i

TABLE 1. Cloud Morphology Interpretation Scheme Probable Cloud

Cloud

Type* Lee waves

Probable Wind

Mechanism

Condensates

Trapped forced internal gravity waves

High

Condensates

Kelvin-Helmholtz waves

High

High

Dust

Forced convection

High

High

Condensate or dust

Free convection

Low

form without

Condensates or dust

Local condensation or dust concentration

Moderate to

sharp edges acrossthe long

Dust

Forced

Condensates

Free convection

Moderate

Moderate

Condensates

Local condensation

Low

Low

One-dimensional one side of an obstacle One-dimensional

Speed

Probable Wind Shear

Composition

Description periodic form to

Wave clouds

Possible Formation

High

Probable Static

Stability

Probable Surface Shear Stress

High near the surface

Moderate

periodic form with no associated obstacle

Plumes

Elongated form

High

with at least

one sharp edge acrossthe long

...

Low

Low

axis

Streak clouds

Cloud

streets

Elongated cloud

axis Cloud form with double

...

high convection

high

...

Low

Low

periodicity Localized fogs

Hazes

Obscuration of the surface localized to some surface feature General obscuration

or dust

Condensates

or dust

or dust settling

Condensation concentrations

*See Figure 1.

or

atmosphericdust

---

Low

KAHN'

MARS CLOUD DISTRIBUTION AND METEOROLOGICAL IMPLICATIONS

6673

Fig. 2a

Fig. 2. Examplesof cloudtypes:(a) Lee waves,streakclouds,and thin and moderatehazein northernhigh latitudes duringlate summerand earlyautumn;Viking Orbiter(VO) picture840A16centeredat +62.1ø latitude,220.3ø longitude, taken at 06.51local time at picturecenter,on Ls = 154.1.(b) Plumesand hazein Hellasbasin,with somelocalizedfogsin the surroundingcratersduring southernmidspring;VO picture 157B28,centeredat -56.3 ø latitude,299.9ø longitude, takenas 12.43localtime on Ls= 192.6.(c) Wave cloudsand thick hazesurrounding OlympusMons in early northern summer;VO picture722A74,centeredat + 15.9ø latitude, 127.6ø longitude,taken at 10.59local time on L,- 98.1. (d) Cloud streetsand moderatehazein high northernlatitudesduringmidsummer;VO picture062B53,centeredat +80.3ø latitude,180.0ølongitude,takenat 11.35localtime on Ls - 136.3.

(i = 1,--', 10) for which any images were acquired within latitude bin j (j- 1,---, 6). For the Viking missionsthese data are shownin Table 2. The numberof imagesin eachbin

compiledby orbit and are givenin Table 3.) Careful examination of Figures3-13 togetherwith Table 3 yieldsa numberof patternswhich provide cluesto the variation of meteorologi(ri•)is givenin Figure14.The no andro weregenerated using cal conditionson Mars. It is especiallyinstructiveto compare the BetterImage RetrievalPrograms(BIRP) [Arvidsonet al., systematicallythe relative occurrenceof differentcloud types 1979].We thenobtainan estimateof the probability(Pw,)of for fixedlatitudebin and season,changesin the frequencyof a observing a givencloudtype(k) in eachbin by dividingthe givencloud type in one latitude bin with changingseason,and number of orbits in each bin for which any occurrenceof differencesin the distributionof cloud types at different latiduring a particularseason. cloudtypek wasfoundby n0. In this way we estimatethe tudesand in differenthemispheres number of days on which certain meteorologicalconditions In the remainder of this section we review these trends. In the occurredrather than the number of cloudsthat happenedto followingsectionwe considersomeof the possiblemeteorobe imagedat particular times.An estimateof the confidencein logicalimplicationsof our observations.

thisestimateis givenby the standarderror Sp•ikdefinedas

Spuk =(Puk(1-Pøk))

2.1.

(1)

The Pw,andSpw , aregivenin Table3. (WhileFigure14 is produced for each comparisonwith Figures 3-13, statisticsof greaterrelevanceto the study of meteorologicalconditionsare

Autumn

2.1.1. Mid and high latitudes. Cloud occurrencepatterns in mid and high latitudesare quite similarin eachhemisphere, but the early autumn changestend to occur later and to be lesspronouncedat lower latitudes.Coverageof high latitudes

6674

KAHN.'

MARS CLOUD DISTRIBUTION

AND METEOROLOGICAL

IMPLICATIONS

Fig. 2b

is poor from mid to late autumn through midwinter, and there is no imagingduring polar night. In high northern latitudes we see a significant decreasein lee waves,wave clouds,streak clouds,thin haze, and especially cloud streets over midsummer

values and an increase in thick

tend to be similar but less pronounced.The exceptionis in late southern autumn, when there is a general hemispheric decrease in cloudiness

but an increase

in low-latitude

thin

hazes. Cloud streets appear suddenly during early autumn, particularly in Labyrinthus Noctis, as first noted by Briggset al. [1977] (see also French et al. [1981]), reaching elevations between4 and 6 km above the surface.They are not found in

haze during early autumn. The few existing Viking images suggestthat high-latitudeearly autumn conditionsmay persist at least through midautumn and possibly into early winter. Thin and moderate hazes replace thick haze in mid-latitudes by late autumn. In southern high latitudes, there is a large increasein the

tude, 145ø longitude) are also the sites for early morning brighteningat this time [Briggs et al., 1977]. Only thin hazes are common in low latitudes during autumn. They tend to

occurrence of lee waves, wave clouds, and thin and moderate

cluster around the Tharsis volcanoes, with thicker hazes near

haze between late summer and early autumn, and streak clouds remain common during this period. Streak clouds were found in the Hellas basin during this seasonby Mariner 9 [Leovy et al., 1973a], but the Viking data show no strong longitudinal variations for these clouds [French et al., 1981]. According to Figure 12a and 13a, however, there is a concentration of hazesin and to the west of the low-lying Hellas and Argyre basinsin early southernfall. In late fall (40ø < Ls < 80ø) a large general decreasein the frequencyof occurrenceof all high-latitude cloud types is observed,while at mid-latitudes the data may suggesta small

the peaks [Leovy et al., 1973a; Figures 12 and 13]. In early autumn the hazesare concentratedover the elevatedregions, but during midautumn and early winter they fill the sur-

decrease at this time.

2.1.2. Low latitudes. In low latitudes, seasonalchangesin haze occurrence take place later than at mid-latitudes and

late autumn.LabyrinthusNoctisand Memnonia(-15 ø lati-

rounding area as well. 2.2.

Winter

2.2.1. Mid and high latitudes. Very few pictures were taken polewardof 60ø latitude during northern winter by the Viking orbiters.Existingobservations suggestthat only thin haze and streak clouds occur frequently during early winter and midwinter in both hemispheresand in late northern winter. Briggs and Leovy [1974] studied the Mariner 9 data set,which coversmid and high northern latitudesbeginningin late northernwinter (L• • 320ø).They found occasional,dif-

KAHN: MARS CLOUD DISTRIBUTION AND METEOROLOGICAL IMPLICATIONS

6675

Fig. 2c

fuse,low-lying hazes poleward of 60ø latitude, which they be- clouds,cloud streets,and moderatehazesappear suddenlyin lieve are probably composedof CO2 ice. They also discovered mid-latitudeslate in winter (Ls • 325ø),and a local dust storm a wide range of mid-latitude cloud phenomena at this time, was found in Chryse Planitia [James and Evans, 1981]. The including frontal systemswith associated wave and streak sudden appearance of mid-latitude clouds in the late winter clouds propagating in a prograde direction, local dust storms wasfirstnotedby Bri•7•7s et al. [1977].' (probably associatedwith frontal activity) in Chryse Planitia In mid' southernlatitudes,lee waves,wave clouds,and and near 47ø latitude, 158ø longitude (see also Leovy et al. streak cloudsare found, and hazesare rare in early winter; a [1972]), a huge lee wave cloud behind the crater Milankovich, proliferation of streak clouds and hazes occurs in midwinter, cloud streets reminiscent of terrestrial snow-producing cloud near L• = 125ø. Hazes tend to be moderate and to concentrate forms, and at least one optically thick cloud in Alba Patera. around the low-lying Hellas and Argyre basins.In the south Most of thesecloudswere found within the lowestscaleheight polar region, occasionallee wavesand wave cloudsoccur over of the atmosphere,but at least one isolated mid-latitude cloud the cap late in winter (seealsoLeovyet al. [1973a]). reached 30-km

altitude.

Our data indicate that in mid-latitudes, only a few streak cloudsand all types of hazesoccur in early winter to midwinter, with thinner hazesbecomingmore common by mid northern winter. Some of these may constitute the telescopically observednorth "polar hood" in this season.Briggsand Leovy [1974] also note that some clouds which may be identified with the north polar hood begin to form around Ls = 180ø and persistuntil Ls -• 360ø. The classicalground-basedobservations [Martin, 1975] may include surfacefrost and may be sensitiveto very thin hazes seen through large slant paths. Year-to-year variations further complicate any comparisons with the classical observations.

As with the Mariner 9 data, lee waves, wave clouds, streak

2.2.2.

Low

latitudes.

Northern

low

latitudes

show

the

samepattern of cloudsand hazesas mid-latitudesthrough the winter season, but the mid-latitude lee waves, wave clouds, and streak clouds which form in late winter are absent.

In the south, wave and streak clouds, cloud streets, and

hazes occur preferentially at low latitudes, particularly over the Tharsis Ridge and the adjacentplains areas to the south, in early winter. Wave clouds, thin hazes, and streak clouds may be more numerousthan in late autumn,althoughthere is a strong picture coverageeffect.In early winter, low-latitude wave clouds and hazes are strongly clustered around the Tharsis Ridge [see French et al., 1981; Figures 12 and 13]. The wave clouds tend to reside on the steeplysloping edgesof the ridge,with the roll axesorientednormal to the topograph-

6676

KAHN: MARS CLOUD DISTRIBUTION AND METEOROLOGICAL IMPLICATIONS

.i

Fig. 2d

ic contours. High-altitude (30 km) clouds are found primarily near dawn [Briggs et al., 1977]. Most clouds disappear by Ls = 125ø. Only hazes remain common through late winter and early spring, as most of the cloud activity moves to midlatitudes.

2.3. Spring

2.3.1. Mid and high latitudes. Lee waves, wave clouds, and streak clouds occur less frequently and move poleward betweenlate winter and early spring in both hemispheres(see ß Moriner 9,

Wove Clouds Lee Woves



SPRI•

t80"

90 ø SUMIvlE• i

8O

FALL

I

,



oMoriner 9, ßViking •)?C)o WINTER 3GOo

BO- .:

.• :l ß •,..- . :':

i

i i i i

i i

6O

SUMMER t80"

SPRING

60%.,:i.,•

i

,,

:

,,

:

i

.,' •o

' .i

,

ß

:

; ß

ß

:

3o-

i

'i

ß

3O

i

Ii

o- i : !.t.,, !-

i

'

ß

-3O

ß

i

ß

: i

ß

i ! !

"!i:": '%: :":

ß

o

i

i i

ß ßß

ß Viking

•)70 ø WINTER :360 ø

FALL

ß

ß

!. i

ß

-60

,.

, ,;;: ' : I

I I

.:

ß

Iß I

ß I :'.'. I I

-8o



FALL

•z5 ß •0'

90 ø •NTER

zoo" 245'

t8CY' •i•

'

-•o

I

45- eo'

,o!,

zao-•5 ß

O*

270 ø StaMen 360*

Season ( Ls ) ß

Fig. 3. The distributionof lee wavesin the Mariner 9 and Viking Orbiter imagingdata, plotted accordingto latitude and season(Ls).

45'

FALL

i, 90"

I

'

i

i-

i' i

125 ø

t60 ß

WaftER

200 ß

180•

245 ß

semi;

29•

27Ce

325"

SUMMER 3•

S•S0. {Ls!

Fig. 4. The distributionof wave cloudsin the Mariner 9 and Viking Orbiter imagingdata, plottedaccordingto latitudeand season (Ls).

KAHN:

MARS CLOUD DISTRIBUTION AND METEOROLOGICAL IMPLICATIONS

Streak Clouds



Cloud Streets

90 ø SUMMER

SPRING

t80 e

(7'

FALL

sPRING •.

90 ø

SUMMER t80 ø FALL

80f•" ß ',' L..'- '",'

60'½•

,

i _

!

60

'

':

I i !

3O

i- . :.

,,

,... ::7{• •i-

•. ß

-60 . -SOlO*

,

ß

ß

:,

Mariner 9 (none), Vikmng 270 ø WINTER 360 ø

8O

.:

O-

6677

45ø

FALL

90 •

WINTER t80 •

SPRING 2TO •

-30

-

-60

-

-80

-



•UMMER 360 •

•, ii•:• I • I i

FALL

WIN•R

1•



Seem (Ls)

i ,

s•i•

27•

(Ls)

•R

3•

Fig. 5. The distribution of streak clouds in the Mariner 9 and Viking Orbiter imagingdata, plotted accordingto latitude and season (Ls).

Fig. 7. The distribution of cloud streets in the •arincr 9 and Viking Orbiter ima•ng data, plotted accordingto latitude and season

Table 3). Occasionalplumesand cloud streetsare found at the equatorward side of the mass of cloud that residesover the retreatingcap edge,as shownin Figure 15. In the north, early spring streak clouds and thin haze tend to clusterin Acidalia Planitia (mid and high latitudes between0ø and 50ø longitude) (Figure 12; French et al. [1981]); the data do not strongly suggestlongitudinal asymmetriesin the other cloud type distributions. Lee waves and most streak clouds disappear by midspring.A similar seriesof eventswas observedby Mariner 9; the polar hood clouds dissipatedinto faint clouds in early northern spring, and only streak clouds and haze remained around the north polar cap by L s • 50ø [Leovy et al., 1973a]. Cloud distributionsare about 10ø of latitude more equatorward in the southernhemispherethan correspondinglyin the north, and lee waves and wave clouds disappear 20ø of Ls earlier. While thin hazes and some wave clouds persist through late northern spring, in the south, hazes become thicker and more frequent (particularly in the plains area south of Tharsis), then thinner, yet more common, and more uniformlydistributed.

2.3.2. Low latitudes. In northern spring,low latitudesare generallydevoid of cloudsexceptthin hazes,which residepreferentiallyover the Tharsis Ridge and increasein thicknesslate in the season.Mariner 9 also saw longitudinal asymmetriesin the distribution of low-latitude cloudsin late spring.In particular, AM brightening occurred over the Tharsis Ridge, Olympus Mons, and Amazonis Planitia, and thick clouds at 20-

to

within

30-km

elevation

as well

as streak

2 or 3 km of the surface around

oMariner 9, ßViking

8 ß

;'

;

:

:

:

•o•

ß

3o-

;

ß

'

'

I •

'

i

"

i

'

,,

,

',

!, •i. .,

ß

'!i;

-so d•. "80" O'

:

I

I

i

I o

:! , i, [

"

"" I

.: I

I

!

i,

•' ', • :;. .-_o- i ' ,-. -6o-

',

I.

' !i

,, I

10

"'

,, =, •

'"' ; ':. -] , .:.,! I. 'f;',.' ',

• : ,, ';Ii..: ,, I. ½', :, ' =[

, .•:

1:•5"t•Y" 21D(P2:45 ø 290"3•5ø FALL 90ø WINTERt80' SP•NG270ø SU•,•ER360ø Seoson

Fig. 6. The distributionof plumesin the Mariner 9 and Viking Orbiter imagingdata, plotted accordingto latitude and season(Ls).

and

haze

[Leovy et al., 1973a]. In the south, hazes become thicker and then thinner, as was the case at higher latitudes, and wave clouds, streak clouds, and plumes(aligned parallel to the topographiccontours)are found, particularly in the plains areas south of Tharsis, in late spring. 2.4.

Summer

2.4.1. Mid and high latitudes. Hazes are very common in mid and high northern latitudes during summer and become progressivelymore optically thick as autumn approaches.

LocolizedFog

Plumes

clouds

the Tharsis volcanoes

o IVloriner9

(:7 SPRING90= SUMMER t•' ß

i

30

I

'I

.-_o . •' ' ' -30

,

i

: i

= " =

I

.i

,

!'

= i,

, -Viking

FALL •70ø WINTER :360 ø

:

i I

', ,

'I 'I

'I :'

'

'[• o

, : ', ,, I

I

, ,

.... I

i•o

,

, "''!','= .i•,,• !

! ''

.o

' -'- ø:'

-•o : :. ' ,,.: : •" '•i :!.': • ,•' a•x• I'21 -8o ' 25-•:, ,asa45z9•3z50 0ø FALL 90e WINTER t80ø SPRING 27•:) e SUMMER 360ø Seoson(Ls) Fig. 8. The distribution of localized fogs in the Mariner 9 and Viking Orbiter imagingdata, plottedaccordingto latitudeand season (Ls).

6678

KAHN: MARS CLOUD DISTRIBUTION AND METEOROLOGICALIMPLICATIONS

Th,n Hoze



SPRING

90 ø SUMMER

t80"

Vikmng

FALL

ThickHoze

27CP WINTER360 ø

80

'

t80 o

FALL

270 o WINTER 3GOo

80

i

6O

Viking

C• SPRING 90 o •MER

I

•o ' ' '

i

"' i

,

II ,, ,,

ß

3O

ß

,, '• ....,

II

'.•

.-_

!.

i

0

o,

I ,..• "ß! i.I ',I ,,

-30

-6O

-80

, (:•

FALL

II 8(7

-80 125'

t6(7

245'

20(7

90 • WINTER 180•

29(•

325'

4•

SPRING 270 e SUMMER

FALL

Seoson(Ls)

8•

t25' t60' 245 •270 e290' 325'•60 WINTER tt•) 2•OCP e SPRING SUMMER Seeson(Ls)

Fig. 9. The distribution of thin hazesin the VikingOrbiterimagFig. 11. The distributionof thick hazesin the Viking Orbiter ing data, plottedaccordingto latitudeand season(Ls).All images imagingdata, plottedaccordingto latitudeand season(Ls).All

containingthe planetary limb have bccn omitted. Shadedareas arc denselyfilled with data points.

imagescontainingthe planetary limb have been omitted. Shaded areasarc denselyfilled with data points.

Other cloud typesare rare in early summer,but wave clouds centratein the Tharsisarea and becomerare by midseason.

and streakcloudsappearin midsummer and last until early autumn.Cloud streetsappearwith great regularityover the permanentnorth polar cap. Spiral clouds,a rarely observed phenomenon, are found immediatelysurroundingthe per-

Cloud streets in the north and thick haze in the south follow a

similar pattern of occurrence.During mid and late summer,

onlymoderateandthin hazesarefound,andwithdiminishing frequency, in both hemispheres. Thesetrendsgenerallyfollow

manentnorth polar cap edgein early summerto midsummer, the sequence of eventsat higherlatitudes.

betweenLs= 104ø and 130ø [Gierasch et al., 1979].Theyare recordedin Figure 5 as a form of streak clouds.

3.

DISCUSSION

In the south,mid- and high-latitudehazesearly in the

The cloudtypedistributions presented in the previoussecsummer(Ls = 2700-290ø)are thick. Moderatehazespredomition show some distinct seasonal and spatial trends.By drawnate betweenLs = 290ø and 305ø, and thin hazesare common ing comparisonswith terrestrial cloudswe use thesetrends to untilLs= 325ø,whenthe atmosphere becomes generallyclear. constrainour knowledgeof Martian meteorologicalconWave and streakclouds,plumes,localizedfogs,and lee waves ditions.Columns3-8 of Table 1 list the conditionswhich, are found in great abundanceearly in the season,but all basedupon terrestrialanalogy,seemmostlikely to produce exceptthe streakcloudsbecomerare by late summer. 2.4.2. Low latitudes. At low latitudes, wave clouds are very common, and streak clouds and plumes are found fre-

quentlyin both hemispheres duringearlysummer.They con-

thecloudformsfoundon Mars.According to thisinterpretive scheme, cloudswith any periodicinternalstructure(leewaves, wave clouds,and cloud streets)are probablycomposedof condensates, whileothercloudtypesmay containdustand/or

condensates. Clouds likely to be formed by free convection Moderote Hoze 0 = SPRING 90 • SUMMER t(•)P

hrt,•-'

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(cloudstreetsand someplumes)requirelow staticstability andare probablyfavoredby low to moderatewindspeedand low surfaceshearstress.Dust cloudsproducedby forcedconvection(somestreakcloudsand someplumes)requirehigh surfaceshearstressin the dustsourceregions. Terrestrialleewavesform underconditionsof highdaytime static stability and high wind shear.Their Martian counter-

partshavebeenstudiedin detailby Pirraglia[1976] and by Briggsand Leovy [1974].

Waveclouds(alsoreferredto as "billows"in the literature) formin low Richardson numberflowson earthand require highwindshearand an inflectionpointin the verticalprofile of horizontalwindvelocity[Ludlam,1980]. Martian

cloud streets resemble cumulus rows which are

quite commonon earth, particularlyin the subtropics. The 45' 8• t25' t• • 24• 2• •5' terrestrial analogs are individual cumulus plumes, organized • FALL • WINTER t• •N• 27• SU•ER•6• into rows with the long axesparallelto the wind vectorand •• (•) with a row spacingequal to severaltimes the size of the Fig. 10. The distribution of moderatehazesin the VikingOrbiter imagingdata, plottedaccording to latitudeand season(L,). All individualplumes.Suchcloudfieldstypicallyformwhenthere imagescontainingthe planetary limit have bccn omitted. Shaded is low staticstabilityin a layer subjectto moderatewind and areasare denselyfilledwith data points.

moderatewind shear[Ludlam,1980].

KAHN.' MARS CLOUD DISTRIBUTION AND METEOROLOGICALIMPLICATIONS

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