Characterizing the sensitivity of daytime turbulent activity on Mars with the MRAMS LES: Early results

Abstract

Background: Daytime convectively-driven boundary layer turbulence produces structures on scales smaller than a few kilometers that can only be resolved in modeling by large-eddy simulations (LES). These structures, when superimposed on larger-scale dynamically- and topographically-driven flows, describe the wind regime experienced by aeolian features on the surface of Mars. Because convective turbulence produces the small percentage of strong wind gusts that may exceed the saltation stress threshold, such activity must be considered in order to properly predict particle entrainment, particularly in an environment in which mean winds are generally much weaker than the saltation stress threshold. We present early results from large-eddy simulations that characterize the daytime turbulent wind distributions under varying local conditions. Method: We have run several large-eddy simulations using the Mars Regional Atmospheric Modeling System (MRAMS) at two locations on Mars, the Viking Lander 1 and Phoenix landing sites. Most simulations were defined at Ls=120o, northern summer, but one set of simulations took place at Ls=300o, northern winter. We have tested a number of initial wind conditions, including cases with no mean wind, a weak wind that is uniform with height, wind speed shear that increases with height, and two cases of strong uniform winds with superimposed speed shear. The resulting friction velocity distributions have been fit to a two-parameter Weibull probability density function (PDF), providing parameters that could help to quantify the influence of convective turbulence on wind distributions. Conclusion: Daytime convective turbulence considerably widens the distribution of friction velocities, increasing the probability that winds will exceed the stress threshold and mobilize sand grains on Mars. The amount of widening increases with insolation as determined by latitude, season, and local time. Although no initial wind is necessary to produce a high speed wind tail that exceeds the local saltation stress threshold, initial winds can add momentum to the model domain that can be mixed down to the surface further increasing the likelihood of strong near-surface winds. In most cases tested thus far, friction velocity fits to a two-parameter Weibull distribution produce shape parameter values ranging from 2.5 to 3, which are larger than those previously used, and scale parameter values approximately equal to the mean friction velocity. Preliminary estimates indicate that increasingly complex and realistic initial winds exhibit friction velocity distributions that d epart significantly from the Weibull distribution, with shape parameter values that range up to ~7. Describing the PDF of LES friction velocities with a Weibull distribution has known inadequacies, represents to date the best available simple analytic description of wind distributions for the modeling community. Therefore, these fit parameters are best used with caution, especially in high wind events, and applied only to conditions similar to those modeled.