PARAMETERIZATION OF CAP-EDGE DUST LIFTING OVER THE
SOUTHERN POLAR REGION
K. C. Chow, J. Xiao, Y. M. Wang, State Key Laboratory of Lunar and Planetary Sciences, Macau University
of Science and Technology, Macau (kcchow@must.edu.mo)

Introduction:
While the dust climate on Mars has certain variability such as the rare but severe global or planetencircling dust activities, it appears to have a certain
regular variation over a Martian year as indicated in
some observational studies (Montabone et al. 2015;
Kass et al. 2016). Occurrence of southern highlatitude dust activities (SHLDA) around the southern
solstice period have been observed for many Martian
years (e.g. Toigo et al. 2002; Imamura and Ito 2011;
Douté 2014), and is named as “B storm” in Kass et
al. (2016). These dust events occur near the southern
cap-edge region and play an important role in the
observed dust climate. However, they generally cannot be simulated in the existing Mars general circulation models. In this study, an approach of parameterization has been proposed in which the dust lifting
threshold stress is adjusted in according to the surface temperature difference between the regolith and
the ice in the southern polar region.
The above setting of threshold stress over the
cap-edge grid points is based on the following two
reasons. First, the ground temperature difference in
the southern cap-edge region is significantly strong
in the period between Ls 230° and 270°. It can be
anticipated that the wind stress over the cap-edge
region due to thermal forcing as well as the sublimation flow as mentioned above will also be significant
in this period. Second, some recent studies proposed
a so-called “thermal creep” process on Mars (e.g., de
Buele et al. 2014, 2015; Küpper and Wurm 2015);
which suggests that when solar radiation is strong
over the Martian regolith without ice cover, the CO2
gas condensed in the porous subsurface before (e.g.
at night) will sublimate and burst out to the surface.
This may facilitate the erosion of soil particles over
the ground surface by wind stress and so may decrease the threshold stress for dust lifting.
Setup of Simulations:
The Mars GCM used is the MarsWRF model
(Richardson et al. 2017). The global domain has ~ 5°
resolution (~ 300 km in the equatorial region). The
vertical grid has 52 levels with a model top of 0.0057
Pa (~ 84 km). The basic configuration of the model
is similar to that in Chow et al. (2019), with the
“wide band model” for evaluating the radiation, and
the approach of “interactive dust” is used in all the
simulations.
In the interactive dust approach, dust in the at-

mosphere is provided by dust lifting over the surface.
Two types of dust lifting parameterization are included in the model. One is to parameterize dust lifting associated with dust devils, and dust lifting is
dependent on the surface sensible heat and the temperature difference between surface and the boundary layer top. Another type is the wind stress dust lifting. Dust lifting occurs when the surface wind stress
exceeds a certain threshold stress value, which is
everywhere constant at 0.044 N m-2 in the control
simulation (EXP_CTRL).
The dust schemes consider dust particles having
two sizes of 1 and 2.5 µm in diameter. Supply of dust
particles is assumed to be unlimited over the surface.
In EXP_CTRL, the model with the above configuration was tuned such that it can reasonably reproduce the observed time series of dust optical depth
(e.g. Montabone et al. 2015). To simulate the
SHLDA, another simulation (EXP_CAP) was performed. This simulation is similar to EXP_CTRL
except the threshold stress for dust lifting at the capedge grid point is reduced to about 0.007 N m-2 (16
% of the constant threshold stress) when the temperature difference across the cap edge is greater than a
particular threshold value (115 K).
Results:
When the parameterization of cap-edge dust is
implemented, occurrence of SHLDA can be seen
(Fig. 1b) between Ls 230° and 270°. The effect of
the parameterization is mainly in the dust season.
The column dust optical depth (CDOD) is slightly
higher in the dust season in almost all latitudes.
The SHLDA simulated in EXP_CAP can have a
significant effect on the temperature field (Fig. 2)).
In particular, around Ls 270° the temperature near
the southern polar region is significantly increased
below the altitude of 20 km in EXP_CAP (Fig. 2c).
The higher temperature in the southern polar region
around the southern summer solstice in EXP_CAP is
generally more consistent with observation such as
MCS. It is worth noting that temperature increase in
EXP_CAP is not limited to the southern high-latitude
region, certain increase can also be seen in the northern mid- to high-latitude region at higher altitudes
(Fig. 2c).
The pattern of temperature increase in the southern high-latitude region in EXP_CAP (Fig. 2c) is
generally consistent with the increase in dust mixing
ratio over this region (Fig. 3). The increase in dust
over this region is significant up to the altitude of 10

km. The increase in dust mixing ratio can extend
from the southern hemisphere to the northern subtropical region through the Hadley circulation (Fig.
3).
The distribution of the corresponding dust lifting
flux by wind stress in Ls 240° - 250° (Fig. 4b) approximately shows a wave-number-2 feature, generally with larger flux in the sector 0° E - 90° E where
the mass of CO2 ice is more. Considering the polar
distributions of wind-stress dust lifting flux and the
prevailing westward surface wind field, the distributions of optical depth (Fig. 4a) appear to be in the
downstream of the dust lifting regions, i.e., westward
advection of dust from the dust lifting region.
Conclusion:
By applying the present approach of parameterization, dust events in the southern cap-edge region
have been simulated around the southern solstice
period. As a result, the simulated temperature in the
southern high-latitude region is increased and the
resulting vertical temperature profile is closer to that
from observation.
Despite its simplicity, the approach of parameterization has considered the effect of two major forcing
(thermal forcing and sublimation flow) on the capedge wind. The strength of these two forcing is basically proportional to the temperature difference
across the cap edge.

Richardson, M. I., A. D. Toigo & C. E. Newman,
2007. PlanetWRF: A general purpose, local to global
numerical model for planetary atmospheric and climate dynamics. J. Geophys. Res., 112 (E9).
Toigo, A.D., Richardson, M.I., Wilson, R.J., et
al., 2002. A first look at dust lifting and dust storms
near the south pole of Mars with a mesoscale model.
J. Geophys. Res. 107, 5050–5062.

Acknowledgement:
This study is funded by the grants from the FDCT of
Macau (grant no. 0068/2019/A2).

a

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b

Fig. 1 Time series of MarsWRF simulated zonal-mean CDOD. (a) Control simulation
(EXP_CTRL). (b) Simulation with parameterization
of cap-edge dust in the southern polar region
(EXP_CAP).

a

Fig. 3 Vertical profile of difference (EXP_CAP EXP_CTRL) in zonal-mean dust mixing ratio (10-5
kg kg-1) averaged from Ls 240° to Ls 270°.

b

a
c

Fig. 2 Height-latitude profile of zonal-mean temperature (K) averaged for 5 sols around Ls 270°. (a)
Control simulation (EXP_CTRL). (b) Simulation
with the parameterization of cap-edge dust in the
southern polar region (EXP_CAP). In (a) and (b)
shading intervals are 5 K. (c) Difference between
EXP_CAP and EXP_CTRL (interval 1 K).

b

Fig. 4 Polar projection at Ls 240° - 250° in
EXP_CAP; outermost circles at 30° S. (a) CDOD
(shadings) and topography (contours). (b) Dust lifting flux (10-4 kg m-2 per Sol, shadings) and CO2 ice
(kg m-2, contours). Vectors are surface wind (m s-1).