Mars perihelion cloud trails as revealed by MARCI: Mesoscale topographically focused updrafts and gravity wave forcing of high altitude clouds
Résumé
Daily, global wide angle imaging of Mars clouds in MARCI (MARs Color Imager, (Malin et al., 2008)) ultraviolet and visible bands reveals the spatial/seasonal distributions and physical characteristics of perihelion cloud trails (PCT); a class of high altitude (40-50 km), horizontally extended (200-1000 km, trending W to WSW) water ice clouds formed over specific southern low-to-mid latitude (5S-40S), mesoscale (∼50 km) locations during the Mars perihelion, southern summer season. PCT were first reported in association with rim regions of Valles Marineris (Clancy et al., 2009). The current study employs MARCI 2007-2011 imaging to sample the broader distributions and properties of PCT; and indicates several distinct locations of peak occurrences, including SW Arsia Mons, elevated regions of Syria, Solis, and Thaumasia Planitia, along Valles Marineris margins, and the NE rim of Hellas Basin. PCT are present over Mars solar longitudes (LS) of 210-310 °, in late morning to mid afternoon hours (10 am-3 pm), and are among the brightest and most distinctive clouds exhibited during the perihelion portion of the Mars orbit. Their locations (i.e., eastern margin origins) correspond to strong local elevation gradients, and their timing to peak solar heating conditions (perihelion, subsolar latitudes and midday local times). They occur approximately on a daily basis among all locations identified (i.e., not daily at a single location). Based on cloud surface shadow analyses, PCT form at 40-50 km aeroid altitudes, where water vapor is generally at near-saturation conditions in this perihelion period (e.g. Millour et al., 2014). They exhibited notable absences during periods of planet encircling and regional dust storm activity in 2007 and 2009, respectively, presumably due to reduced water saturation conditions above 35-40 km altitudes associated with increased dust heating over the vertically extended atmosphere (e.g. Neary et al., 2019). PCT exhibit smaller particle sizes (Reff=0.2-0.5 μm) than typically exhibited in the lower atmosphere, and incorporate significant fractions of available water vapor at these altitudes. PCT ice particles are inferred to form continuously (over ∼4 h) at their PCT eastern origins, associated with localized updrafts, and are entrained in upper level zonal/meridional winds (towards W or WSW with ∼50 m/s speeds at 40-50 km altitudes) to create long, linear cloud trails. PCT cloud formation is apparently forced in the lower atmosphere (≤10-15 km) by strong updrafts associated with distinctive topographic gradients, such as simulated in mesoscale studies (e.g., Tyler and Barnes, 2015) and indicated by the surface-specific PCT locations. These lower scale height updrafts are proposed to generate vertically propagating gravity waves (GW), leading to PCT formation above ∼40 km altitudes where water vapor saturation conditions promote vigorous cloud ice formation. Recent mapping of GW amplitudes at ∼25 km altitudes, from Mars Climate Sounder 15 μm radiance variations (Heavens et al., 2020) in fact demonstrates close correspondences to the detailed spatial distributions of observed PCT, relative to other potential factors such as surface albedo and surface elevation (or related boundary layer depths).