Modelling dust distributions in the atmospheric boundary layer on Mars

Abstract

A time and height dependent eddy diffusion model is used to investigate possible scenarios for the size distribution of dust in the lower atmosphere of Mars. The dust is assumed to either have been advected from a distant source or to have originated locally. In the former case, the atmosphere is assumed to initially contain dust particles with sizes following a modified gamma distribution. Larger particles are deposited relatively rapidly while small particles are well mixed up to the maximum height of the afternoon boundary layer and are deposited more slowly. In other cases, a parameterization of the dust source at the surface is proposed. Model results show that smaller particles are rapidly mixed within the Martian boundary layer, while larger particles (r > 10 µm) are concentrated near the ground with a stronger diurnal cycle. In all simulations we assume that the initial concentration or surface source depend on a modified gamma function distribution. For small particles (crosssectional area weighted mean radius, r eff = 1.6 µm) distributions retain essentially the same form, though with variations in the mean and variance of the area-weighted radius, and the gamma function can be used to represent the particle size distribution reasonably well at most heights within the boundary layer. In the case of a surface source of larger particles (mean radius 50 µm) the modified gamma function does not fit the resulting particle size distribution. All results are normalised by a scaling factor that can be adjusted to correspond to an optical depth for assumed particle optical scattering properties.

Temperature, pressure, and wind instrumentation in the Phoenix meteorological package

“…As a result, models of the Martian boundary layer will play an important role in data assimilation and interpretation as they have for Viking and Pathfinder [ Savijärvi , 1999]. Taylor et al [2007] present some model results on dust distributions within the boundary layer for 70°N, while other coupled boundary layer and dust modeling and large eddy simulation studies are ongoing. Mars global circulation models [e.g., Moudden and McConnell , 2005] and model databases [ Lewis et al , 1999] have been used to help in the determination of the meteorological conditions we expect to find at the Phoenix site.…”

The vertical distribution of dust in the Martian atmosphere during northern spring and summer: Observations by the Mars Climate Sounder and analysis of zonal average vertical dust profiles

“…The opacity as a function of the volumetric number density, N v , is [e.g., Taylor et al , 2007] In , Q ext can be treated as a function of the integrated dust size distribution rather than radius and can be extracted from the integral along with π and N v , such that The value of Q ext used by the retrieval algorithm is 0.35, which is obtained from Mie theory as described by Kleinböhl et al [2009a].…”

“… Conrath [1975] attributed the vertical distribution to the competing effects of sedimentation and vertically uniform vertical eddy diffusion. This simple picture has been complicated by (1) the possibility of additional removal processes such as the enhancement of sedimentation by the condensation of volatiles on dust particles [ Nelli and Murphy , 2002]; (2) modeling of vertical transport above the boundary layer due to dynamical processes such as the thermal tides [ Wilson and Hamilton , 1996]; (3) more detailed treatment of mixing within the boundary layer [ Taylor et al , 2007]; (4) explicit consideration in models of dust particle size and its variability [ Kahre et al , 2008]; and (5) consideration of the sources of dust such as mountain slope circulations [ Lee et al , 1982; Rafkin et al , 2002] and dry convective vortices (“dust devils”) [ Kahre et al , 2006; Cantor et al , 2006; Greeley et al , 2006]. Of these processes, dust devils have been the principal focus of investigation.…”

A model of dust in the Martian lower atmosphere

“…T07 showed that we can expect a rapid decrease in particle concentration with height at the afternoon boundary layer height in cases without a local source of particles. However, the use of a closed PBL diurnal cycle meant that their model is insufficient to make quantitative comparisons with Phoenix lander data.…”