We are using radio occultation (RO) measurements from Mars Global Surveyor to investigate the nighttime structure and dynamics in the lower atmosphere of Mars. High-resolution temperature profiles retrieved from the RO data contain unique information about nocturnal mixed layers (NMLs) – detached layers of neutral stability that form at night in response to radiative cooling by a water-ice cloud layer. Basic properties of the NMLs and constraints on their spatial distribution and seasonal evolution can be obtained through analysis of the RO profiles. We have examined more than 3000 RO profiles in a latitude band centered on the Phoenix landing site (234°E, 68°N), where nighttime water-ice clouds were observed by the LIDAR instrument (Whiteway et al., Science 325, 68-70, 2009). NMLs appear routinely in the western hemisphere in RO observations at 5 h local time from early summer of MY27. There is a close resemblance in both thickness (a few km) and altitude (about 4 km above the surface) to the cloud layer observed at the same local time by the Phoenix LIDAR in MY29. The NMLs confirm that radiative cooling by the Phoenix cloud is sufficient to trigger convective instability, as predicted by a Large Eddy Simulation (Spiga et al., Nat. Geosci. 10, 652-657, 2017). We have also analyzed more than 800 RO profiles from the northern tropics near summer solstice of MY28. Tropical NMLs are largely confined to regions of elevated terrain, where the daytime convective boundary layer is deep. At 4 h local time, the top of the NML is about 10 km below the peak of Olympus Mons. The spatial distribution of the NMLs appears to be influenced by diverse processes ranging from topographic circulations to planetary-scale thermal tides. In addition, we are using a Mars Global Circulation Model and Large Eddy Simulations to interpret the RO results. Goals of the modeling effort include: to identify the atmospheric processes that control the formation of nocturnal water ice clouds; to understand the spatial distribution of the clouds and their evolution with time of day and season; and to assess the impact of NMLs on the nighttime weather and water transport in the lowest scale height above the surface.

Mars Global Surveyor (MGS) orbiter observed a planet-encircling dust storm (PDS) in Mars year (MY) 25 from Ls=176.2-263.4°. We present an examination of Mars Orbiter Camera (MOC) dust storms and transient baroclinic eddies identified from Fast Fourier Synoptic Mapping (FFSM) of Thermal Emission Spectrometer (TES) temperatures for the first two phases of the storm: precursor, Ls=176.2- 184.7°, and expansion, Ls=184.7-193°. FFSM analysis of TES 3.7 hPa thermal data shows the presence of eastward traveling waves at 60° S with a period of about three sols. We hypothesize that these waves are transient baroclinic eddies that contributed to the initiation of precursor storms near Hellas. Integration of FFSM and MOC MY 24, 25, and 26 data shows interesting temporal and spatial associations between the evolution of eddies and storms, including: 1) comparable periodicities of travelling waves and pulses of storm activity, and 2) concurrent eastward propagation of both eddies and storms. These results suggest a causal relationship between baroclinic eddies and local storm initiation. Based on our analysis of these MGS data, we propose the following working hypothesis to explain the dynamical processes responsible for PDS initiation and expansion. Six eastward-traveling transient baroclinic eddies triggered the MY 25 precursor storms in Hellas during Ls=176.2–184.6° due to the enhanced dust lifting associated with their low-level wind and stress fields. This was followed by a seventh eddy that contributed to expansion on Ls=186.3°. Increased opacity and temperatures from dust lifting associated with the first three eddies enhanced thermal tides which supported further storm initiation and expansion out of Hellas. Constructive interference of eddies and other circulation components including sublimation flow, anabatic winds (daytime upslope), and diurnal tides may have contributed to storm onset in, and expansion out of Hellas.