A numerical simulation of thermal convection in the Martian lower atmosphere with a two-dimensional anelastic model.

We have examined the wind velocity and the surface stress associated with thermal convection in dust free Martian atmosphere by using a two-dimensional numerical model. Outbreak of dust storm is a striking phenomenon in the Martian atmosphere. However, it has been well recognized that the Martian general circulation model (GCM) does not produce sufficient surface wind stress to raise dust from the surface under dust free condition. Small-scale wind fluctuations which are not represented in GCM may supplement the necessary surface stress. Daily thermal convection is one of the possible contributers. The numerical model is an two-dimensional anelastic model with radiative and surface processes. The computational domain extends horizontally to 51.2 km and vertically to the height of 20 km. Both the horizontal and vertical grid intervals are 100 m except in the lowermost 100 m, where the vertical resolution is enhanced. The solar flux at the top of the model atmosphere changes diurnally with the northern summer condition (Ls=100° ) at 20° N latitude. The model is integrated for 6 days, and the data of the last day is utilized for analyses. The results of the simulation reveal that the thermal convection in the Martian lower atmosphere is km-size; the vertical and horizontal scales of convective cells are 10 km and several km, respectively. The magnitude of wind velocity associated with the km-size thermal convection is quite large. The values of both horizontal and vertical wind velocity often exceeds 20 m/sec. This magnitude can be understood by considering that the work done by the buoyancy force on a plume (about 2 K higher than the surroundings) is converted to the kinetic energy. The instantaneous maximum value of the surface stress associated with the km-size thermal convection reaches 0.04 Pa, which is equal to the threshold value to raise dust from the surface. The result suggests that GCM can represent dust injection into the Martian atmosphere self-consistently by parameterizing the wind fluctuation associated with the km-size thermal convection in the calculation of surface stress.