Large Eddy Simulation of vertical vortices in highly convective Martian boundary layer

The Phoenix lander, a NASA scout mission, is scheduled to arrive on Mars near 70 degrees North in May 2008 and operate during the northern summer (2008). Among other experiments, it will make continuous meteorological measurements of pressure and temperature, plus some wind and humidity measurements. Phoenix will use a vertically pointing lidar provided by the Canadian Space Agency (CSA) to observe profiles of dust and water ice particles. It is hoped to use the lidar data to determine boundary-layer depths assuming a detectable drop in aerosol level at the top of the daytime convective boundary layer. Suspended atmospheric dust is an important driver of the boundary-layer circulation and climate system of Mars. It is also an important factor in Earth's atmosphere. Convective Boundary Layers (CBLs) generate a variety of dynamical structures including vertical vortices which provide a mechanism for dust lifting into the atmosphere. Dust devil tracks have been observed at the 65-72 latitude band of Mars' North Polar Region by the Mars Global Surveyor (MGS) Mars Orbiter Camera Narrow-Angle (MOC-NA). These satellite images indicate evidence of dust devils and strong winds in the Phoenix landing site environment.

In this study, Large Eddy Simulations (LES) of planetary boundary layers are performed to compare the physical characteristics of simulated vertical vortices to those of observed dust devils. LES allows time-evolving simulations of turbulence and convection in a three-dimensional computational box and has been successfully used for a wide range of terrestrial atmospheric problems. Our LES model is based on the NCAR LES, developed by Peter Sullivan and others, adapted and developed for Martian applications. As a necessary part of preparation for the analyses of data from the mission, this study examines the possible formation and maintenance mechanisms for vertical vortices in the highly convective Martian and terrestrial boundary layers. The simulations performed include both terrestrial and Martian atmospheric CBLs to compare characteristics of dust devil-like vortices of each planet, in situations with and without ambient wind.