An assessment of the impact of local processes on dust lifting in martian climate models

Abstract

International audienceSimulation of the lifting of dust from the planetary surface is of substantially greater importance on Mars than on Earth, due to the fundamental role that atmospheric dust plays in the former’s climate, yet the dust emission parameterisations used to date in Martian global climate models (MGCMs) lag, understandably, behind their terrestrial counterparts in terms of sophistication. Recent developments in estimating surface roughness length over all Martian terrains and in modelling atmospheric circulations at regional to local scales (less than O(100 km)) presents an opportunity to formulate an improved wind stress lifting parameterisation. We have upgraded the conventional scheme by including the spatially varying roughness length in the lifting parameterisation in a fully consistent manner (thereby correcting a possible underestimation of the true threshold level for wind stress lifting), and used a modification to account for deviations from neutral stability in the surface layer. Following these improvements, it is found that wind speeds at typical MGCM resolution never reach the lifting threshold at most gridpoints — winds fall particularly short in the southern midlatitudes, where mean roughness is large. Sub-grid scale variability, manifested in both the near-surface wind field and the surface roughness, is then considered, and is found to be a crucial means of bridging the gap between model winds and thresholds. Both forms of small-scale variability contribute to the formation of dust emission ‘hotspots’: areas within the model gridbox with particularly favourable conditions for lifting, namely a smooth surface combined with strong near-surface gusts. Such small-scale emission could in fact be particularly influential on Mars, due both to the intense positive radiative feedbacks that can drive storm growth and a strong hysteresis effect on saltation. By modelling this variability, dust lifting is predicted at the locations at which dust storms are frequently observed, including the flushing storm sources of Chryse and Utopia, and southern midlatitude areas from which larger storms tend to initiate, such as Hellas and Solis Planum. The seasonal cycle of emission, which includes a double-peaked structure in northern autumn and winter, also appears realistic. Significant increases to lifting rates are produced for any sensible choices of parameters controlling the sub-grid distributions used, but results are sensitive to the smallest scale of variability considered, which high-resolution modelling suggests should be O(1 km) or less. Use of such models in future will permit the use of a diagnosed (rather than prescribed) variable gustiness intensity, which should further enhance dust lifting in the southern hemisphere in particular

Atmospheric Science with InSight

“…Relatively large friction velocity are also noticed as a result of passing convective cells in the scene. The histogram corresponds to the mapped values: friction velocity associated with turbulence could well exceed the mean value associated with the background wind (Fenton and Michaels 2010;Mulholland et al 2015; convective cells. The aeolian migration of surface structures at the InSight landing site is further discussed in Sect.…”

Large-Eddy Simulations of Dust Devils and Convective Vortices

“…As is the case on the Earth, progress is being made that may soon lead to parame-terizations in GCM and RCM for lifting by dust devils (and, more generally speaking, by CBL structures). These would be based on CBL statistics drawn from LES (Fenton and Michaels, 2010;Mulholland et al, 2015) rather than thermodynamical arguments, as is the case in existing GCM parameterizations (Renno et al, 1998;Newman et al, 2002) (see in this issue for further discussions). Given that dust is a major driver of the Martian climatic system (e.g.…”

Parameterization of Rocket Dust Storms on Mars in the LMD Martian GCM: Modeling Details and Validation

“…Dust storms, which occur on local‐, regional‐, and planet‐encircling scales, are regularly observed on Mars, with the largest storms occurring from southern spring to summer (often termed the “dust storm season”; Cantor, ; Cantor et al, ; Wang & Richardson, ). Many types of regional and larger storms, resulting from lifting over large areas, have been numerically modeled in traditional Global Climate Models (GCMs; Basu et al, , ; Kahre et al, ; Mulholland et al, , ; Newman & Richardson, ; Newman et al, ). In some of these simulations, dust with detached structures are simulated, for instance, as shown in Newman et al ().…”

“…For a long time, taking into consideration the combined effects of gravitational sedimentation and vertical mixing, dust has been assumed to be well mixed in the lower atmosphere up to a level in the troposphere and to decrease Dust storms, which occur on local-, regional-, and planet-encircling scales, are regularly observed on Mars, with the largest storms occurring from southern spring to summer (often termed the "dust storm season"; Cantor, 2007;Cantor et al, 2001;Wang & Richardson, 2015). Many types of regional and larger storms, resulting from lifting over large areas, have been numerically modeled in traditional Global Climate Models (GCMs; Basu et al, 2004Basu et al, , 2006Kahre et al, 2006;Mulholland et al, 2013Mulholland et al, , 2015Newman & Richardson, 2015;Newman et al, 2002). In some of these simulations, dust with detached structures are simulated, for instance, as shown in Newman et al (2002).…”