Huiqun Wang

Martian clouds observed by Mars Global Surveyor Mars Orbiter Camera

We have made daily global maps that cover both polar and equatorial regions of Mars for Ls 135°–360° and 0°–111° using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) red and blue wide-angle swaths taken from May 1999 to January 2001. We study the seasonal distribution of condensate clouds and dust clouds during roughly 1 Martian year using these daily global maps. We present the development and decay of the tropical cloud belt and the polar hoods, the spatial and temporal distributions of lee waves and spiral clouds, and an unusual “aster” cloud above the volcanoes, consisting of rays around a central disk, like the flower. The tropical cloud belt contains mostly fibrous clouds during northern spring/early summer and convective clouds during middle/late northern summer. The detailed development and decay of the tropical cloud belt is nonuniform in longitude. Two distinct stormy periods in late summer precede the formation of the north and south circumpolar hoods. The north polar storms in late summer resemble baroclinic frontal systems on Earth but contain both dust and condensate clouds. Spiral clouds occur only in the northern high latitudes and only during northern spring and summer. The north polar hood displays a stationary wave number two structure during the fall and winter. The south polar hood has fewer streak clouds and lee wave clouds than the north polar hood. During this particular year the lee wave cloud abundance in the south had two peaks (in early fall and late winter), and the lee wave cloud abundance in the north had one peak (in early fall).

Cyclones, tides, and the origin of a cross-equatorial dust storm on Mars

We investigate the triggering mechanism of a cross-equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off-line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally-modified return branch of the Hadley circulation.

Relationship between frontal dust storms and transient eddy activity in the northern hemisphere of Mars as observed by Mars Global Surveyor

We have compiled a catalog of frontal dust storms in the northern hemisphere using Mars Orbiter Camera daily global maps spanning ~2.3 Martian years of Mars Global Surveyor (MGS) observations (from 1999 to 2003). The most vigorous frontal storms that flush dust to the low latitudes occur in early-mid fall and mid-late winter, away from the northern winter solstice. While many streaks are observed in the polar hood during the winter solstice period, no frontal dust storms are observed in the vicinity of the north polar region. We have also analyzed simultaneous MGS Thermal Emission Spectrometer (TES) temperature data and found statistically significant negative temperature anomalies associated with frontal storms. In the lowest scale height of the atmosphere, the geographical and seasonal distributions of temperature standard deviations associated with transient variations agree well with the distributions of frontal storms. The correlation deteriorates with increasing altitude, suggesting that lower-level temperature waves are associated with the frontal dust storms. Specifically, eastward traveling m = 3 waves with periods of 2–3 sols appear to be closely related to the development of flushing frontal storms.

A first look at dust lifting and dust storms near the south pole of Mars with a mesoscale model

Surface wind stresses and dust lifting in the south polar region of Mars are examined with a three-dimensional numerical model. The focus of this study is the middle to late southern spring period when cap-edge dust lifting events are observed. Mesoscale model simulations of high southern latitudes are conducted at three dates within this season (L_s = 225°, 255°, and 310°). Assuming that dust injection is related to the saltation of sand-sized grains or aggregates, the Mars MM5 mesoscale model predicts surface wind stresses of sufficient strength to initiate movement of sand-sized particles (∼100 μm), and hence dust lifting, during all three periods. The availability of dust and/or sand-sized particles is not addressed within this study. Instead, the degree to which the existence of sufficiently strong winds limit dust injection is examined. By eliminating forcing elements from the model, the important dynamical modes generating high wind stresses are isolated. The direct cap-edge thermal contrast (and topographic slopes in some locations) provides the primary drive for high surface wind stresses at the cap edge, while sublimation flow is not found to be particularly important, at these three dates. Simulations in which dust is injected into the lowest model layer when wind stresses exceed a threshold show similar patterns of atmospheric dust to those seen in recent observations. Comparison between these simulations and those without active dust injection shows no signs of consistent positive or negative feedback due to dust clouds on the surface wind stress fields during the late spring season examined here.

Cloud-tracked winds for the first Mars Global Surveyor mapping year

We have measured winds using cloud motion in consecutive Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide-angle global map swaths taken during the first mapping year (L s 135°–360°–111°). We present a total of ∼11,200 wind vectors collected in the north polar region during L_s 135°–195° (late summer/early fall) and L_s 20°–55° (mid spring) and in the south polar region during L_s 337°–10° (late summer/early fall). For cases with good coverage, we also present the derived mean zonal and meridional winds and the associated eddies. The speed of the zonal winds in 60°N–70°N increases at ∼0.6 m/s/°L_s in late northern summer, and that in 60°S–70°S increases at a rate of ∼0.7 m/s/°L_s in late southern summer. The latitudinal distribution of zonal wind within 50°N–75°N from mid northern summer to early northern fall indicates that winds at higher latitudes are generally weaker than those at lower latitudes, but the rate of increase with time is faster at higher latitudes. There is a cyclonic gyre in the 90°W–0°–30°E sector in the north polar region. There are large-scale waves in the weekly mean meridional wind and in the biweekly mean eddy momentum flux and eddy kinetic energy fields in the north polar region from mid to late summer. The cloud-tracked winds in the north are generally consistent with winds calculated by general circulation model at the water condensation level derived from MGS Thermal Emission Spectrometer (TES) observations but appear stronger than the gradient winds derived from TES assuming no flow at the surface.

High-resolution sunspot observations

Abstract A high-resolution six-hour sequence for a stable round sunspot 22 deg from disk center reveals the following properties: 1. 1)|The inflow of penumbral material to the umbra is limited to the inner half of the penumbra. 2. 2)The outer half of the penumbra shows the Evershed effect outward flow, extending beyond the penumbra. Both proper motions and Doppler shifts of these elements are observed. The Evershed flow peaks in discrete elements, which are clearly associated with regions of stronger and more horizontal magnetic field in the outer penumbra. 3. 3)From the apparent reversal of field sign in the penumbra, we estimate that field lines emerge at angle of at least 22° to the horizontal throughout the penumbra. 4. 4)The MMF (moving magnetic features) are generally associated with bright K-line elements. While some are bipolar and most show the penumbral polarity associated with horizontal field, there are many exceptions. 5. 5)The “orphan penumbra” shows motions similar to the normal penumbra.

Major dust storms and westward traveling waves on Mars

Westward traveling waves are observed during major dust storm periods in northern fall and winter. The close correlation in timing makes westward traveling wave one of the signature responses of the Martian atmosphere to major dust storms. Westward traveling waves are dominated by zonal wavenumber m = 1 in the middle atmosphere and are typically characterized by long wave period. They are associated with significant temperature perturbations near the edge of the north polar vortex. Their wind signals extend to the low latitudes and the southern hemisphere. Their eddy momentum and heat fluxes exhibit complex patterns on a global scale in the middle atmosphere.

Cross‐Equatorial Flushing Dust Storms and Northern Hemisphere Transient Eddies: An Analysis for Mars Year 24

The relationship between frontal/flushing dust storms and northern hemisphere synoptic period transient eddies in Mars year 24 is examined in this paper. Frontal dust storms are observed roughly continuously during the presolstice (early/middle fall) and postsolstice (middle/late winter) time periods, but flushing dust storms that cross the equator are confined to shorter seasonal windows on both sides of the solsticial pause. In the lower atmosphere, the timing of cross-equatorial flushing dust storms correlates better with eddy temperature than with eddy meridional wind; in the middle atmosphere, it correlates better with eddy meridional wind than with eddy temperature. This is because both the lower atmosphere eddy temperature and the middle atmosphere eddy meridional wind are dominated by zonal wave number m = 3 eastward traveling waves during the cross-equatorial flushing dust storm periods. Frontal dust storms do not seem to be limited to any particular wave mode, but cross-equatorial flushing dust storms appear to be closely related to m = 3 eastward traveling waves, at least in Mars year 24. The effectiveness of m = 3 waves in this regard is partially due to their amplitudes but more importantly due to their seasonal distributions and latitudinal positions. During the time periods when m = 3 waves are strong, the m = 3 waves are also located at lower latitudes, closer in distance to the fairly strong southward mean meridional wind in the low latitudes. Dust in frontal dust storms at high latitudes can be easily entrained into the low-latitude circulation and be efficiently transported southward.