COMPARING THE MY25 AND MY34 GLOBAL DUST STORMS USING
THE ENSEMBLE MARS ATMOSPHERE REANALYSIS SYSTEM (EMARS)
N. Polek-Davis, Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, PA, USA (ntp5125@psu.edu), S. J. Greybush, Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, PA, USA, R. J. Wilson, Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA, USA, R. P. McMichael, Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, PA, USA, H. E. Gillespie,
Lunar and Planetary Institute, Universities Space Research Association, Houston, TX, USA.
Introduction: The analysis of dust storms is crucial towards understanding Martian weather and climate. Using the Ensemble Mars Atmosphere Reanalysis System (EMARS; Greybush et al., 2019), this
study builds upon that understanding by conducting a
comparative analysis between two global dust storms.
While regional dust storms occur on Mars every
year, every ~3-5 years one of these storms evolves
into a global dust storm (GDS). The MY 25 GDS began around Ls 177˚ as local dust storms along the
northwestern rim of Hellas Basin, expanded to the
east and north, and encircled the planet by Ls 193˚
(Strausberg et al., 2005; Wolkenberg et al., 2020;
Wang et al., 2015). The MY 34 GDS began around Ls
185˚ in Acidalia around 10˚E, 60˚N, expanded westward and equatorward, and covered Mars by Ls 193˚
(Gillespie et al., 2020). In both MY25 and MY34,
common features include the eastward expansion of
the two storms from local lifting of the tropics, and
the initiation of multiple dust-lifting centers that were
dynamically active throughout the onset and progression of the storm (Bertrand et al., 2020). Despite similar explosive growth and season of occurrence, the
origin locations of these storms differ significantly,
with the MY 25 GDS beginning in the southern hemisphere and the MY 34 GDS in the north. In addition,
the exact circumstances that dictate whether a storm
remains regional or expands globally are not yet

understood. To connect dust in the atmosphere to the
winds that lift it, we examine surface wind stress and
dust opacity. Several comparisons of these factors
have been made between MY 25 and years without
global dust storms (e.g. Montabone et al., 2005; Kass
et al., 2016; Wolkenburg et al., 2020), but no comparisons have yet been made between the MY 25 and MY
34 storms using EMARS.
EMARS assimilates retrievals from the IR Thermal Emission Spectrometer (TES) and Mars Climate
Sounder (MCS) to form a temporal gridded dataset
representing a variety of atmospheric variables.
EMARS dust field and wind stress fields are compared with visible satellite imagery from the Mars Orbital Camera (MOC). Then the dust opacity and wind
stress values are extracted and compared at three different time periods, as well as analyzed relative to
non-global dust storm years in order to identify global
dust storm characteristics and behavior.
Data and Methods: EMARS directly assimilates
temperature retrievals from the Thermal Emission
Spectrometer (TES; Christensen et al., 2001) and
Mars Climate Sounder (MCS; McCleese et al., 2007)
instruments using the Local Ensemble Transform
Kalman Filter (LETKF) method (Hunt et al., 2007)
within the GFDL/NASA Mars Global Climate Model

Figure 1. MOC (left) and EMARS wind stress anomaly (right; N/m2) comparison at MY 25 Ls 186º. Blue
circles highlight areas where EMARS predicts substantial positive wind stress anomalies, and black dotted
lines indicate Martian topography. MOC imagery courtesy of NASA/MSSS.

Figure 2. Latitude-longitude maps of dust opacity differences measured in units of number density and averaged over three different periods (columns). Dotted overlay represents Martian topography. Observed origin
locations of each storm are circled in red. Top row: MY 25 subtracted by MY 24 values. Blue/positive value
areas indicate higher dust opacity in MY 25. Bottom row: MY 34 subtracted by MY 30 values. Blue/positive
value areas indicate higher dust opacity in MY 34.
(MGCM) to provide a best estimate of the Martian atmospheric state at any hour from the middle of MY
24 to the beginning of MY 34 (with a gap in MY27/28
between the TES and MCS eras). For the purposes of
this project, the variables extracted from EMARS are
dust opacity and wind stress. The horizontal dust column opacity fields are strongly driven by the Montabone et al. (2015) dust products, which are originally derived from TES and MCS observations. For
MY 34, the dust maps are derived from the updated
methodology in Montabone et al. (2020), which uses
the 2-D v5.3.2 MCS dust retrievals. MOC imagery
(Malin et al., 1992; Cantor et al., 2007) is then used to
provide an independent view of the dust storm
through visible imagery.
On Mars, dust and wind stress are codependent;
wind stress and convection impact dust lifting, and
dust’s impact on temperature affects convection, wind
patterns, and wind stress (Haberle et al., 2006). There
have been a variety of methods proposed to attempt to
address this relationship in a model (Kahre et al.,
2006; Basu et al., 2004; Newman et al., 2015); however, further development of this relationship in the
context of Mars would be a valuable improvement.
While the MGCM has the functionality to apply a
wind stress lifting scheme, EMARS does not currently employ this option as the wind stress values
must first be validated before such a relationship can
be confidently employed. One of the aims of this research is to validate the wind stress values of EMARS

such that the dust-wind relationship can be utilized
within EMARS for future work.
For this study we are most interested in the anomalous meteorological factors contributing to global
dust storm formation, rather than repeatable patterns
of dust lifting and dust opacity. We can therefore define the non-GDS data for TES and MCS as an average of their respective data during the periods within
which these observations are available: MY 24 and
MY 26 for TES, and MY 30 and MY 31 for MCS.
We then subtract the non-GDS data from the GDS
years for each respective instrument in order to highlight the anomalous data and remove any biases from
differences in instrument characteristics and assimilation of these datasets. Finally, EMARS anomaly
fields can be compared between the two storms.
MOC Imagery and EMARS Comparison: We
compared MOC imagery to EMARS dust (not shown)
and wind stress anomalies (Figure 1) at a variety of
times (only one time depicted in Figure 1). EMARS
dust compares favorably to MOC imagery in most locations during the growth phase of the MY25 GDS.
In Figure 1, we can also see a generally favorable
comparison of positive wind stress anomalies with areas of potentially active dust lifting: near Hellas, Argyre, and in the Northern high latitudes. The wind
stress anomalies at these high latitudes are likely associated with baroclinic wave activity. Many, but not

Figure 3. Latitude-longitude maps of wind stress differences measured in N/m2 and averaged over three different periods (columns). Dotted overlay represents Martian topography. Top row: MY 25 subtracted by MY
24 values. Blue/positive value areas indicate higher wind stress in MY 25. Bottom row: MY 34 subtracted by
MY 30 values. Blue/positive value areas indicate higher wind stress in MY 34.
all, areas of likely dust lifting are captured by positive
wind stress anomalies.
Dust Storm Comparison: Both the dust opacity
(Figure 2) and the wind stress (Figure 3) display major
differences between non-GDS and GDS years. Figure
2 shows the total dust opacity difference between MY
24 and 25, both in the TES era; as well as MY 30 and
34, both in the MCS era. Within EMARS, the dust
disparity is most clearly seen at 50° E, 60° S for MY
25, and 10° E, 40° N for MY 34. This matches up with
observations of the storms’ original lifting centers
(Montabone et al., 2005; Gillespie et al., 2020). The
dust opacity then increases in both intensity and
spread for both cases, with the MY 25 GDS expanding northeast, and the MY 34 GDS progressing southwest. The latitudinal movement is likely due to the
Hadley circulation of the Martian atmosphere, expanding in different directions depending on the initial location of the dust. Once the dust has been transported sufficiently northwards, the jet stream carries
it eastward to allow it to encircle the planet (Gillespie
et al., 2020). Figure 3 shows that both storms show
positive wind stress anomalies in the region of initial
lifting, but that the MY25 storm has more significant
wind stress anomalies that extend across a larger portion of the planet. This encourages exploration of the
triggering of dust lifting centers at additional locations.
Conclusions: This work compares EMARS dust
opacity and wind stress anomalies for MY25 with

MOC visible imagery and notes a reasonable correspondence to areas of likely active dust lifting. Dust
opacity anomalies for MY25 and MY34 show the differences in progression for the two storms, despite beginning at a very similar time of year. While both
storms feature wind stress anomalies near the origin
points for the storms, differences between the two
storms indicate stronger anomalies in MY25, with a
greater connection to multiple lifting centers across
the planet. Instrumentation-related differences in
EMARS between the TES and MCS eras, particularly
how the instruments view the atmosphere (and penetrate the veil of dust to view the lower atmosphere)
and how this information is assimilated, should also
be considered in comparative analyses of these
storms.
Acknowledgments:
This work has been partially supported by NASA
MDAP grant 80NSSC20K1054. We acknowledge the
teams involved in creating the TES, MCS, MOC, and
MARCI data, the Montabone et al. (2015) dust scenarios, and the EMARS dataset.
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