The planetary boundary layer (PBL) is the portion of a planetary atmosphere closest to the surface and is controlled primarily by surface-atmosphere interactions. During the day on Earth, the PBL is about 1-3 kilometers tall, with convection (wind originating from temperature differences) occurring throughout. This convection forms sand dunes and dust devils in deserts and dusty environments. On Earth, the wavelengths of sand dunes are indicative of the time-averaged PBL depth, and the heights of dust devils are comparable to the PBL depth. The venusian PBL is poorly observed, so to determine how near-surface venusian air moves, we examine computer simulations, which represent our understanding of how air moves in general.
Maxence Lefèvre from the University of Oxford applied a large-eddy simulation (LES), a computer simulation used to assess motion in the PBL, to Venus. The LES approximated the solution to the equations of motion for venusian air, representing the conservation of momentum, mass, and energy. The LES was given boundary conditions appropriate for Venus at noon and midnight on its low plains, which are prevalent over much of the planet, and high terrain at Ovda Regio. Lefèvre found noontime PBL depths of 2 kilometers over the low plains and 7 kilometers over the high terrain, with negligible convection at midnight in a boundary layer less than 0.5 kilometers deep. In the high terrain, vertical winds can reach 1.3 meters per second, with possible dust devils up to 1 kilometer wide and 5 kilometers tall, lasting up to several hours. Furthermore, he found that the average depth of the PBL over the low terrain was 0.5 kilometers, about the wavelength seen at the two large dune fields on Venus. The large-scale heating, representing topographic effects and differences between daytime and nighttime winds, accounted for most of the differences between the PBLs of low plains and high terrain. READ MORE