Nighttime convection in water-ice clouds at high northern latitudes on Mars

Abstract

We investigate water-ice clouds and their influence on the temperature structure of the Martian atmosphere at high northern latitudes in early summer. New results are obtained through coordinated analysis of two types of data from Mars Global Surveyor: atmospheric profiles retrieved from radio occultation (RO) measurements and wide-angle images from the Mars Orbiter Camera (MOC). Some RO profiles contain a layer of neutral static stability, which indicates the presence of convective mixing at a local time (about 5 h) when it does not usually occur. These nocturnal mixed layers (NMLs) were observed frequently in early summer of Mars year 27 at latitudes of 53–72°N and longitudes of 210–330°E. The base of a typical NML is 3 km above the surface, about the same height as the nighttime cloud layer detected by the Phoenix LIDAR in early summer of Mars year 29 at 234°E, 68°N. The depth of the NMLs ranges from less than 1 km to more than 5 km. Comparisons with nearly simultaneous MOC images demonstrate that NMLs are closely associated with water-ice clouds. There is a dense cluster of NMLs within the annular cloud that appears every year in early summer between Alba Mons and the north polar residual ice cap. The lighting conditions at this location and season allowed MOC to observe the annular cloud on most orbits, at 118-min intervals. Its appearance varies dramatically with local time, becoming more symmetrical and better organized at night and dissipating to a crescent shape during the day. According to high-resolution numerical simulations (Spiga et al., 2017), including a large-eddy simulation at the Phoenix landing site, NMLs form when radiative cooling by water-ice aerosols causes convective instability; the mixed layer is forced from above by negative buoyancy. Our results strongly support this conclusion. In addition, MOC images from midsummer contain eastward-moving frontal clouds. Temperature profiles within these clouds show signs of near-surface advection of warm air, which reduces the static stability of the lower atmosphere and contributes, along with cloud radiation, to the formation of an NML.