NASA Technical Reports Server (NTRS) 19930010627: Numerical simulations of drainage flows on Mars

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by  NASA Technical Reports Server (NTRS)

LPITechnicalReport 92-08°PartI 19
zonally symmetric numerical modeling of dust storms [2,3] and quantify polar dust deposition magnitudes as a function
has suggested that this mechanism is ineffective at trans- ofvarious model assumptions.
porting significant quantifies of dust beyond middle latitudes. References: [I] Pollack et al. (1979) JGR, 84, 2929-
Recent three-dimensional numerical simulations conducted 2945. [2]Haberle¢_aL(1982) Icarus, 50, 322-367. [3]Murphy
by us [4], in which the full spectrum of atmospheric eddy et al. (1992) JGR Planets, submitted. [4] Murphy et al.
motions are present and capable of transpo_in 8 dust, have (I992) in preparation. [5] PoUack L B. and Toon O. B.
shown that the amount of dust transported into polar regions (1980) Icaras, 50, 259-287. [6]Barnes _I.B. (1990) JGR, 95,
from a southern subtropical source is greatly increased. The 1381-1400. [7] Colbum et al. (1989) Icarus, 79, 159-189.
eddy transport mechanisms suggested in previous works [5,6] [8] Martin T. Z. 0986) Icarus, 66, 2-21. [9] Barnes et al.
appeartobe operating in these simulations. (1992) JGR Planets, subrnitted.
N el1 9 t)-.
The apparent preference fordust storm development dur-
ing northern autumn and winter, when the northseasonal cap
is growing, is interpreted as one reason for the retention of a NUMERICAL SIMUI.ATIONS OF DRAINAGE FLOWS
perennial CO2residual capinthe south, while inthe northall ON MARS. Thomas R. ParishI and Alan D. Howard2,
the COelaid down during thewinter season sublimes away in 1Department of Atmospheric Science, University of Wyo-
the spring. In the north, due to the dust incorporated into the ming, Laramie WY 82071, USA, 2Department of Environ-
cap during its growth, albedos during springthne are lower mental Sciences, University of Virginia, CharlottesviUe VA
than the albedo of the south cap during its spring retreat, 22903, USA.
which develops during a typically less dusty time of the
martian year [7,8]. The less "contaminated" south capreflects Introduction: Data collected by the Viking I_mders
more solar insulation, maintains alower temperature during (VL-1, 23°N; VL-2, 48 "N) have shown that the meteorology
spring and summer, and thus is able toretain a cover of COz of the near-surface martian environment is analogous to de-
ice throughout summer. In the north the lower cap albedo sertlike terrestrial conditions [1]. Geological evidence suchas
results inalarger net radiative flux at the cap surface andthe dunes and frost s_eaks indicate that the surface wind is a
CO2cap is unable to survive the summer. potentially important factor in scouring of the martian land-
Wewish topoint outthat it is notnecessarily the seasonal scape [2]. In particular, the northpolar basin shows erosional
preference fordust storm development alone that conspires to features that suggest katabatic wind convergence into broad
affect the residual cap and layered terrain variations that are valleys near the margin of the polar cap. The pattern of
presently seen. Under present orbital characteristics, southern katabatic wind drainage off the north polar cap is similar to
summer solstice occurs close in time to orbital perihelion, thatobserved on Earthever Antarctica [3] orGreenland.
producing short "hot" summers and long cold winters in the In this paper we will explore the sensitivity of martian
south. This long cold winter resets in amore extensive sea- drainage flows to variations in terrain slope and diurnal
sonal CO2cap inthe south than in the no_. The size of the heating using a numerical modeling approach. The model
cap can have implications for suspended dust reaching the used in this study is a two-dimensional sigma-coordinate
pole, even in the absence of a global dust storm. Baroclinic primitive equation system [4] that has previously been used
waves, which develop due to the large horizontal temperature for simulations of Antarctic drainage flows. Prognostic equa-
gradients at middle to l_gh latitudes of the autumn, winter, tions include the flux forms of the horizontal scalar momen-
and spring hemispheres, are probably capable of liRing dust tum equations, temperature, and continuity. Explicit parame-
from the surface. This liRed dust can then be carried pole- terization ofboth longwave (terrestrial) and shortwave (solar)
ward by these same waves. As the seasonal cap grows, the radiation is included [5]. Turbulent transfer of heat and
distance between the location of dust liRing and the pole in- momentum in the martian atmosphere remains uncertain
creases, and thus the dust must be transported agreater dis- since relevant measurements are essentially nonexistent.
tance if it is to become incorporated into the developing cap Standard terrestrial treatment of the boundary layer fluxes is
at polar latitudes, if in fact it can reach those latitudes [5]. employed [6---8].
Since the southern cap is, at itsmaximum extent, largerthan Model Results: Katabatic wind simulations. A series of
the northern seasonal cap, the north cap (at polar latitudes) numerical experiments has been conducted that focuses on
might be more susceptible to dust contamination than the the relationship between katabatic wind intensity and terrain
south cap, even without dust storms. In fact, numerical slope. The model runs are valid for high-latitude (75°), noc-
simulations [9] suggest that the magnitude of baroclinic Rmml conditions similar to midwinter onthe northpolar cap
waves is larger in the norththanin the south, further increas- in which no solar radiation reaches the ground. A horizontal
ingthe northern hemisphere preference forcap contamination gridconsisting of 20 points with agrid spacing of20 km was
by dnst. used; 15 levels were used in the vertical with higher resolu-
We will present model results detailing the mechanisms tion in the lower atmosphere. The results of five uniform
by which suspended dust is transported into polar latitudes slope runs are presented here. Each model simulation covered
20 WorkshoponthePolarRegionosfMars
a 24-hr period; terrain slopes were set to 0.0005, 0.001, vat), considerably. Th/s again underscores the dominance of
0.002, 0.004, and 0.008. In each _ent, the model at- the radiation budget in forcing the thermal structure onMars.
mosphere was started fromrest to isolate the katabatic wind. Influence of solar forcing on the martian drainage flows.
An initiallapseroteof3°C km-Iwas usedwitha surface To test the sensitivity of the martian slope flows to solar
temperaturoefapproximatel2y20K attheverticarleference forcing, numerical experiments have been conducted in which
level. the full cycle of solar forcing is replicated over sloping
Inallcases,thekatabatiwcindreachedaquasisteadsytate terraixL Results for a constant slope of 0.008 at 75° latitude
with/n the t'a_'t12 hr. The resuR/ng vcrt/cal profiles of wind wil/be described.
speed and temperature for the five martian katabatic wind Five numerical simulations have been conducted covering
simulations after 24 hr are shown in Fig. 1. Curves A-E in the seasonal range of solar declination angles (24°, 12°, 0°,
this figure correspond to terrain slopes of 0.0005, 0.001, -12 °,and -24°); the model equations areintegrated for three
0.002, 0.004, and 0.008 respectively. The intensity and depth complete martian days to allow the model to settle into a
of the katabatic wind (Fig. Ia) appearsensitive to the terrain stable diurnal oscillation. The results presented here are
slope. Note that despite the absence of ambient horizontal takenfrom day2;only minor variations were seen beyond the
pressure grad/ents in the flee atmosphere, akatabatic circu- first dinmal cycle. It is assumed that the polar cap is com-
lation has developed in the lowest 3 km of the atmosphere posed of "dirty" ice with an albedo of 030. ALl simulations
after one martian day. It is clear that the radiative flux diver- start from arest state. Thus the influence of large-scale pres-
gence in the atmosphere acts to influ_ce nearly the entire sure gradients in the flee atmosphere is neglected. This
martian atmosphere. This is considerably different from the implies that all atmospheric motions arise due tOthe long-
Antarctic simulations and isadirect result ofthe thin martian wave radiative cooling or solar heating ofthe sloping terrain.
atmosphere. Even relatively modest values of radiative flux No solar insolation is allowed forthe first 12hr ofmodel in-
divergence can lead to appreciable temperature change over tegration time to allow the drainage flows to become estab-
time and hence the establishment of horizontal pressure gra- lished before model sunrise.
dients. Figure 2 illustrates the diurnal course of the surface tem-
Figure lb illustrates the vertical temperature profile inthe perature, wind speed, and wind direction overthe 0.00g slope
lowest 3 km for the five cases considered. Pronounced in- martian terrain for solar declinations of 24°, 12°, 00, -12 °,
version conditions prevail over adeep atmospheric layer. The and -24°, corresponding to curves A, B, C, D, and E respec-
exponential shapes of the temperature profiles are similar to tively. Here the martianday is divided into 24 martian hours.
those seen overthe interior ofAntarctica although the vertical Note that the Sun never sets during the midsummer period
scale of the martian profiles are much greater than found on (Fig. 2a, curve A) and never rises for the winter case. The
Earth. Note that no significant difference is seen in the ground temperature (Fig. 2b) undergoes diurnal oscillations
thermal structure for the five cases despite the fact that the of 30 K in summer, with the magnitude of the oscillation
terrain slopes and intensity of the katabatic wind regimes decreasing with the approach of the autum_ the diurnal
PO00 , , , , , , , , , , , , , , I000
o) b)
1800 900
1600 1 800
1400 700
E
1200 _ 600
t--v-- _ooo _: 500
(.9 (.9
L"IO- 800 _ 400
600 300
400 200
200 Ioo
o, : l l _ Oli
o 4 8 12 16 20 24 28 170 180 190 200 210 220
WIND SPEED (ms-1) TEMPERATURE (K)
Fig.1. Verticalprofilesof(a) windspeedandCo)temperatureinthelowest3ianafter24-hrintegrationofcomtant sloperuns.
LPI Technical Report 92-08, Part I 21
ground temperature oscillation amounts to 15K at the equi- katabatic wind some 30°tothe right of the fall line ofthe ter-
nox. Maximum temperatures appearan hour or so afterlocal rain forthe winter katabatic wind case. The wind directions
noon. Wind speeds at the first sigma level corresponding to show surprisingly little variation with time except for the
approximately 22 m above the surface (Fig. 2c) show marked midsummer declination angle of 24° (curve A). Note that
diurnal trends during smm_er and equinox periods. upslope flow is modeled during the early aItemoon hours of
Maximum wind speeds occur in the early morning hours co- the summer case. The effect of solar insolation appears to
inciding with a minimum in the solar insolation in midsum- retard but not overcome the katabatic forcing in all but the
mer or just before sunrise in other simulations. Note that the summertime case. This emphasizes the robust nature of the
simulated midsummer katabatic wind maximum of approxi- martian katabatic circulation.
mately 10.5 ms -l (reached in the early morning hours) is Although the most pronounced diurnal changes occur near the
3 ms -1 less than seen for the other cases. This reflects the surface, significant oscillations can be traced well into the
insolation from the midnight Sun, which retards development atmosphere. In particular, the strong solar heating of the ter-
of the katabatic wind. Wind directions atthe first sigma level rain results in a well-mixed boundary layer that extends up-
throughout the diurnal course for the five numerical simula- ward of 2 kin or so by early afternoon. Figure 3 illustrates the
tions are shown in Fig. 2d. The downslope direction is 180 ° vertical structure of temperature and wind speed in the lowest
for these simulations. The Coriolis force acts to deflect the 3 km for the five 0.008 slope experiments at 0200 LT and
160 I I I I I I I i l I I I 23( Illltllll I I l
o) b)
140 22,_
22O
120 [
2 51
//IZ
I00
I--
:E 80
W
:E
60
F-
4O
20
0
0 4 8 12 16 20 24 4 8 12 16 20 24
TIME (hrs) TIME (hrs)
15 111111111111 I i i i I I t i I I l
-C) _A ,f_ , 320
13
28O
C
-" II 240
E
v
Q 9 20O
W g
LU
EL
7 _ 160
W /A
Q I'r
Z 120
g-
5
80
5
4O
I
I IllllI_lJllll OI I t l | I I I • I I !
0 4 8 12 16 20 24 0 4 8 12 16 20 24
TIME (hrs) TIME (hrs.)
Fig. 2. Diurnal variation of(-,) solar insolation re.aching the ground, Ca)ground temperature, (¢) wind speed, and (d) wind direction atthe lowest
sigma level fo¢ the solar cycle simulations over the 0.008 slope.
22.WorkshoponthePolar]_eglonos.fMars
1200 LT. The temperature profile for the early morning at local noon for the five solar declinations suggest that the
(Fig. 3a) shows an inversion structure for each simulation katabatic wind regime in the lower atmosphere is consider-
including the midsummer case (curve A) in which the Sun re- ablyreducedby solarradiationduringthenonwinterperiods.
mains above the horizon for the entire martian day. Although The katabatic wind signature is still present in all but the
midsummer case.Littlediurnalvariationisseenabove Ikin.
relatively minor diurnal changes are seen above Ikin,
seasonal temperature differences are evident. The midsum- Summary: Numerical simulations suggest katabatic
mer thermal structure suggests a near-adiabatic profile above winds are ubiquitous features of the nocturnal lower bound-
the inversion. The atmospheric stability above the katabafic ary layerathigh latitudeson Mars. The drainageflowsarc
layer increases as the intensity of solar radiation decreases analogous to thoseseen over the Antarcticcontinent,and
have comparable scalesofwind speeds and depths.Model
such that by the equinox, inversion conditions prevail. The
thermal structure of the atmosphere at noon (Fig. 3b) indi- experimentssuggestthatIongwave radiativecoolingisthe
cates adiabatic conditions prevail up to around 2 km during dominant forcingmechanism forthemartiankatabaticwinds
midsummer. The wind speeds (Figs. 3c,d) for the 0.008 slope and isresponsiblefor establishinga horizontalpressure
show well-developed katabatic wind profiles during the early gradientinadeep layeroverslopingterrain.The intcusityof
morning hours,althoughtheintensityofthedrainageflowis theinducedcirculationisdependent on theslopeoftheun-
rexiucexclonsiderablyforthemidsummer casc.Wind speeds derlyingterrain.
3000 I I | i I I I I I t I 3000 I i I I I 1 I I
o) b)
2600 2600
2200 2200
1800 1800
t--
1400 1400
I.tJ
"r-
I000 I000
6OO 6O0
20o 2OO
i I
182 186 190 194 198 202 06 180 190 200 210 220
TEMPERATURE (K) TEMPERATURE (K)
5000 t _ , , , , , , , , , , i 3000 , , J , , , , ' J ' t , ,
i
-c) -_ -d)
2200 - 2200
-g 21680000 ' _. 2t860000 I
i-- I•",-r ,4
3:
(-9
,ooo ,ooo
Lu]
T
600 _. 1_, 600
t400 C_Blt_ ID 1400 B__.___ ),E
200 __ 200
l i I I
0 4 8 i_ 16 20 24 0 4 8 12 16 tO 24 28
WIND SPEED (m s-I) WIND SPEED (ms-I)
Fig. 3. Vertical profiles oftemperatureand windspeed inthe lowest3km overthe0.00g slope at0200 LT(left) and 1200 LT(right).
LP[TechnicalReport 92-08,PartI 23
Model experiments incorporating the solar cycle show the The ice-shoved features observed in the Bringer Hills
katabatic wind to be completely suppressed only during the Oasis were up to 83 m long, 2 m high, and 4 m wide. Ice-
midsummer daytime simulation. The adjustment time for the shoved ridges up to 300 m long and 10 m high have been
development of the katabatic wind is quite short; during the reported [1]. Other unique landforms associated with polar
early morning hours of midmmmaer the drainage flows are beaches are f_ost cracks and mounds, patterned ground.
able to develop. This implies that once the Sun sets, the de- pingos,pittedbeaches,coastalstriated bedrock,and
velopment ofthe katabatic wind isvery rapidand near-steady ventifacts. Investigations of ice-covered lakes in Antarctica
conditions prevail injust afew hours. has also disclosed information that may have important
References: [I] Hess S. L. etal. (1977) JGR, 82, 4559- exobiological implications [2-4], namely the discovery of
4574. [2] Howard A. D. (1981) NASA TM-82385, 333-335. modern, cold-water, blue-green algal stromatolltes that are
[3] Parish T. R. and Bromwich D. H. (1987) Nature, 328, adapted to extremely cold temperatures, fresh-to-saline water,
51-54. [4]Anthes R. A. and Warner T. T. (1978) Mon. Wea. and low light intensifies, and the fact that an ice cover acts as
Rev., 106, I045-I078. [5] Parish T. R. and Walght IC T. both insulating blanket and protective seal for the liquid
(1987)Mon. Wea.Rev., 115, 2214-2226. [6]Brost R. A. and water located below. The ice cover's "sealing effect" allows
Wyngaard :I.C.0978) J. Atmos. Sci., 35, 1427-1440. [7]Busch the liquid water toretain biologically important gases that are
N.E. etal.(1976)J. Appl. Meteor., 15,909-919. [8]Businger J. dissolved in the water column.
A. et al. (1971)£ Atmos. Sci., 28, 181-189. Several paleolac_e basins have been located and
mapped on Mars [5,6]. The last vestiges of these martian
lakes, which eventually frozethroughoutbecause the influx
ANTARCTIC LAKES (ABOVE AND BENEATH THE of meltwater ceased,are expected to be found at high lati-
ICE SHEET): ANALOGUES FOR MARS, J.W. Rice Jr., tudes. Provided that the ice cover was covered with the ap-
Astrogeology Branch, U.S. Geological Survey, Flagstaff AZ propriate sediment thicimess [7], these paleolake remnants
86001, USA. would form amassive lens of buried ice. It is proposed that
this lacustrine ice lens would be composed of interlayered
The perennial ice-covered lakes of the Antarctic are con- fluvial/lacustrine sediment and ice. This layering would be
sidered to be excellent analogues to lakes that once existed created bythe influx of sediment brought in bymultiple flow
on Mars. Field investigations of ice-covered lakes, paleo- episodes from channels located along the periphery of the
lakes, and polar beaches have been conducted in the Buager basin [8--10]. Aeolian deposits would also contribute to the
Hills Oasis, Eastern Antarctica. These studies will alsobeex- ice cover mantling. More investigative studies and field work
tended to the Dry Valleys, Western Antarctica, and the willbeconducted ontheseproblems.
Arctic. Sub-Ice-Sheet Lakes: Sub-ice lakes have been discov-
Important distinctions have been made between ice-cov- ered [11] under the Antarcticice sheet using radio echo
ered and non-ice-covered bodies of water in terms of the sounding. Theselakes occur in regions oflowsurfaceslope,
geomorphie signatures produced. Field investigations have low surface accumulations, and low ice velocity, and occupy
revealed that the classical lacustrine landforms created by bedrock hollows.
non-ice-covered lakes (spits, bars, berms, cusps, tombolos, The development of Radio Echo Sounding ('RES) in the
and wave-cut platforms) are absent in an ice-covered lake late 1950s was driven by the necessity to measure ice thick-
regime. The features mentioned above are the result of the ness in a rapid, accurate, and continuous manner. RES
direct coupling of wind and the free water surface. The ice provides information on electrical properties in ice, enables
cover acts as a geomorphically protective agent. Therefore, the study of ice-sheet surface form, thickness, internal
the shores of ice-covered bodies of water are low-energy en- structure, dynamics, thermodynamics, and basal conditions
vironments, i.e., poorly sorted, due to restricted ornonexist- and processes [12].
ent wave action. Most of the lakes beneath the Antarctic ice sheet are lo-
The most notablelandforms produced by ice-covered cated nearDome C in Eastern Antarctic[a11]. Several very
lakes are ice-shoved ridges. These features form discrete seg- large lakes, up to g000 krn2,have been discovered [12]. RES
mented ramparts of boulders and sediments pushed up along studies do not allow the depth of these lakes to be deter-
the shores of lakes/seas. The shorelines are generally pla- mined; however, the minimum thickness ofa fresh-water
nated with therampartsdefining theinneredgeof theshore- layercanbeestimated by the skin depth necessary for radio
line. These ridges usually have a heterogeneous veneer of reflection [12]. Some of these lakes may have a minimum
boulders, pebbles, sand, and gravel mantling an ice core. The depth of6.5 m.
ice core normally melts out and leaves behind its mantle of The sub-ice lakes of Antarctica may have formed more
material in the form of irregular discontinuous ridges. The ice than 5m.y. ago [11]. This age is based upon deep-sea cores
core can persist for years if it is sufficently insulated by its taken in the RossSeathat indicate that the main Antarctic ice
mantle ofmaterial. sheet 1_ changed little in size since a retreat some 5 m.y.