Aug 1, 1984 - The spatial and seasonal distributions of Martian atmospheric clouds, organized according to simple ..... the Tharsis Ridge and the adjacent plains areas to the south, ..... Columns 3-8 of Table 1 list the conditions which,.
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
0ø
SPRI•
t80"
90 ø SUMIvlE• i
8O
FALL
I
,
0ø
oMoriner 9, ßViking •)?C)o WINTER 3GOo
BO- .:
.• :l ß •,..- . :':
i
i i i i
i i
6O
SUMMER t80"
SPRING
60%.,:i.,•
i
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:
,,
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,
ß
:
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i
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ß
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•)70 ø WINTER :360 ø
FALL
ß
ß
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ß
-60
,.
, ,;;: ' : I
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.:
ß
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-8o
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•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
0ø
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
-
0ø
•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
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i
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,,
,
',
!, •i. .,
ß
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:
I
I
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.: I
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i,
•' ', • :;. .-_o- i ' ,-. -6o-
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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
0ø
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
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,
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'
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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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