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The comparison between observed and simulated global dust storms shows one substantial discrepancy. While the simulated storms initiate at a reasonable range of seasonal dates and develop peak air temperatures consistent with the observations, the storms do not terminate as observed. The Hellas storms early in the season [Figure 1c], are much smaller and are propagated in the southern hemisphere by the westerlies. As the westerlies weaken with approaching summer solstice season, these storms decay. However, the storms that initiate around southern summer solstice in the simulation shown in Figure 1b provide a good example. The growth rate and peak temperatures of the storms are consistent with observed behavior. However, albeit limited observations would suggest that after about Ls=260, when

the storms have reached their initial peak, decay ought to set in. This prejudice is based on the fact that both the 1977b and 2001 storms began to decay about 20-30 of Ls after initiation

[Martin and Richardson, 1993; Smith et al., 2002] and that there was no “rounding off” of storm activity at a sustained peak–explosive growth was immediately followed by rapid decay. In contrast, the simulated Ls=230 storm remains active and in a fully developed state from

before Ls=260 until just before Ls=315. Only after Ls=315 does the storm decay. Once it

does start to decay, however, it does so at a rate that is similar to that of observed storms. Thus, after the termination of dust lifting in the storm, the model emulates the observations well, which is consistent with previous storm-decay experiments with GCMs [e.g. Murphy et al., 1995, Wilson and Richardson, 1999]. The problem, then, is not the decay process itself, but “triggering” of the decay phase. The dust source does not shut down quickly in our simulations.

increased dust lifting is an essential part of explosive GDS development, and is well captured by the GCM. However, it has always been apparent that at some point either this feedback must become inactive, or the dust in the source region must deplete. In these simulations we have assumed an infinite supply of dust and hence the dust is never depleted from the lifting centers. It has variously been speculated that the development of strong static stability in the boundary layer associated with high opacity causes a reduction in wind stresses and a cessation of dust lifting. Although the GCM does develop an increasingly stable boundary layer at storm peak, it does not reduce stresses dramatically and does not cut off dust lifting. However, in our preliminary high-resolution simulations (2o_x2.4o _{resolution) we find that we get a realistic} switch off of the storms. The storm peaks are sharp and the decay of the storm starts much sooner than in the low-resolution case. In one simulated year, we even get two big storms in the same year [Figure5]. These results suggest that the interactions between the various phenomena can be accurately captured only if the wind stresses and other fields like temperature, opacity etc. are resolved properly. If the small signatures of critical processes are lost in the coarseness of the grid then we might not be able to accurately simulate all aspects of the GDS.

The GCM storms do eventually decay, and to see how this happens, it is useful to take a different view of storm life cycles. The development of a GDS in the GCM actually corresponds to a transition between two stable climate states (two model steady-state climatic solutions). The years without a GDS demonstrate that the “low-dust” state is a perfectly acceptable steady-state climate solution, which is not particularly surprising. This state is explored further in Basu et al. [2004]. The sustained GDS state shown in Figure 1b corresponds to a “high dust” state, where opacities are maintained at a high value. However, this climate is also in steady state, with injection and deposition of dust globally balanced after the storm peak is attained after Ls=260. This state is steady because as the elevated dust

injection causes the amount of dust in the atmosphere to increase, the deposition rate increases. After the visible dust opacity exceeds 2 or so, further increases in opacity increase dust fall-out more aggressively than they do the circulation and lifting. Thus the global dust opacity increases until the fall-out balances injection. The problem of dust storm initiation was

radiative-dynamic dust lifting feedback, triggered by an initial perturbation. GDS decay is the problem of a forced “high” to “low” dust state transition.

The GCM GDS decay, or the state transition, is triggered by the spin-down of the Hadley circulation in late southern summer. As the Hadley circulation naturally slows as the sub solar point moves equator ward, stresses in the Hadley cell convergence zone (which predominates GDS dust lifting) decrease to a point where lifting stops. There is a feedback here. As the Hadley cell weakens due to changes in solar forcing, the amount of dust lifting and hence opacities are reduced. This causes a further slowing of the Hadley circulation. Eventually, active stress lifting terminates and the GCM drifts back to the “low” dust state. The results of the low-order model of Pankine and Ingersoll [2002] can also be interpreted in this manner. In that case, the model was limited in the range of possible behavior. However, the generation of similar behavior in a full circulation model suggests that storm switch-off may be a significant challenge if internal feedbacks are of dominant importance. Higher resolution simulation of the boundary layer or improved treatment radiative heating may be necessary, but there is no obvious a priori guarantee that such improvement will yield qualitatively different behavior than that currently expressed by the model.

annual cycle. The high wind stresses are not uniformly distributed but are concentrated in specific regions, largely due to topography and the position of the wintertime baroclinic storm tracks. These are active dust lifting regions.

The other option for storm switch-off is source region depletion. Figure 38 shows the maximum wind stress experienced at any point on the planet in a ten year run without wind- stress lifting. This experiment yields a very good simulation of the non-dust storm Martian climate [Basu et al., 2004]. It is clear from Figure 38 that high wind stress is not uniformly distributed, but concentrated in specific regions, largely due to topography and the position of the wintertime baroclinic storm tracks. These high stress regions are the dominant source regions for storms, and thus there is a tendency for dust to be exported from these locations (see Section 3.7). Observationally, these high stress regions are anti-correlated with the bright, low thermal inertia dust deposits [Ruff and Christensen, 2002]. As such, there is reason to expect that the dust deposits in the source regions are not uniformly more than a few millimeters to a few centimeters deep (otherwise they would appear as “dust” in visible and thermal infrared retrievals, which has a low thermal inertia signal). During a large, sustained dust storm, the GCM predicts dust removal in source regions of about this thickness, while the predicted fall- out would take over 10 years to re-supply this much dust. Exhaustion of dust source regions during a dust storm would thus seem plausible.

Some initial simulations have been undertaken with the GCM using exhaustible surface dust deposits. As might be expected, the storms develop sharp temperature and dust opacity maxima as the primary source region depletes. The trend of air temperature in these simulations mimics that of the real atmosphere much better than that of the “inifinite dust source” model, shown in Figure 1. These simulations provide a much richer variety of behavior as the surface distribution of dust becomes an additional source of internal model variability. Discussion of simulations with exhaustable surface dust deposits is defered to a later paper.

We also ran some experiments with changing the dust particle size distribution opacity weightings, i.e replacing the 2 bin dust particle distribution entirely with the larger particle.

much more realistic than the default case. The storm peak was sharper and the Northern spring temperatures were much more accurate than the default simulations with the mix of two particle sizes.