Dust devil height and spacing with relation to the martian planetary boundary layer thickness

Abstract

In most remote and unmonitored places, little is known about the characteristics of daytime turbulent activity. Few processes render the optically transparent atmospheres of Earth and Mars visible; put more plainly, without clever instruments it is difficult to “see the unseen”. To address this, we present a pilot study of images of martian dust devils (DDs) testing the hypothesis that DD height and spacing correlates with the thickness of the planetary boundary layer (PBL), h. The survey includes Context Camera (CTX) images from a 580×590km2 area (196–208°E, 30–40°N) in northern Amazonis Planitia, spanning ∼3.6 Mars Years (MY) from Ls=134.55°, MY 28 (13 November 2006) to Ls=358.5°, MY 31 (28 July 2013). DD activity follows a repeatable seasonal pattern similar to that found in previous surveys, with a distinct “on” season during local summer, beginning shortly before the northern spring equinox (Ls=0°) and lasting until just after the northern fall equinox (Ls=180°).DD heights measured from shadow lengths varied considerably, with median values peaking at local midsummer. Modeled PBL heights, constrained by those measured from radio occultation data, follow a similar seasonal trend, and correlation of the two suggests that the martian PBL thickness is approximately 5 times the median DD height. These results compare favorably to the limited terrestrial data available.DD spacing was measured using nearest neighbor statistics, following the assumption that because convection cell widths have been measured to be ∼1.2±0.2h (Willis, G.E., Deardorff, J.W. [1979]. J. Geophys. Res. 84(C1), 295–302), a preference for DD formation at vertices of convection cells intersections could be used to estimate the PBL height. During local spring and summer, the DD average nearest neighbor (ANN) ranged from ∼1 to 2h, indicating that DD spacing does indeed correlate with PBL height. However, this result is complicated by two factors: (1) convection cell spacing estimates indicate that the observed DD distributions undersample the possible grid of cell vertices in each CTX image, and (2) comparison of the ANN with a random distribution shows that the most closely-spaced DD distributions exhibit some clustering; we propose that this clustering demonstrates that many of the observed DDs are located along convection cell walls, in addition to cell vertices.