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The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.

A differential absorption light detection and ranging instrument (Differential Absorption LIDAR or DIAL) was installed on‐board the Canadian Coast Guard Ship Amundsen and operated during the winter and spring of 2008. During this period the vessel was stationed in the Amundsen Gulf (71°N, 121–124°W), approximately 10–40 km off the south coast of Banks Island. The LIDAR was operated to obtain a continuous record of the vertical profile of ozone concentration in the lower atmosphere over the sea ice during the polar sunrise. The observations included several ozone depletion events (ODE's) within the atmospheric boundary layer. The strongest ODEs consisted of air with ozone mixing ratio less than 10 ppbv up to heights varying from 200 m to 600 m, and the increase to the background mixing ratio of about 35–40 ppbv occurred within about 200 m in the overlying air. All of the observed ODEs were connected to the ice surface. Back trajectory calculations indicated that the ODEs only occurred in air that had spent an extended period of time below a height of 500 m above the sea ice. Also, all the ODEs occurred in air with temperature below −25°C. Air not depleted in ozone was found to be associated with warmer air originating from above the surface layer. Key Points LIDAR measured tropospheric ozone depletions over sea ice Correlation between mixing ratio and connection to sea ice Strong correlation between mixing ratio and potential temperature

A microphysical model for Mars dust and ice clouds has been applied in combination with a model of the planetary boundary layer (PBL) for the interpretation of measurements by the LIDAR instrument on the Phoenix Mars mission. The model simulates nighttime clouds and fall streaks within the PBL that are similar in structure to the LIDAR observations. The observed regular daily pattern of water ice cloud formation and precipitation at the top of the PBL is interpreted as a diurnal process in the local water cycle in which precipitation of large ice crystals (30–50 μm effective radius) results in downward transport of water vapor within the PBL. This is followed by strong vertical mixing during daytime, and this cycle is repeated every sol to confine water vapor within the PBL.

The LIDAR instrument on the Phoenix mission provided observations of clouds within the Planetary Boundary Layer (PBL) on Mars. In mid to late summer there was a regular Sol‐to‐Sol pattern with cloud formation at around midnight and dissipation before midday. The ice water content (IWC) of the clouds was estimated from the measurements with peak values at 6 am of 1 mg/m3, associated with total column IWC of up to 5 g/m2. The time of cloud formation did not change throughout the second half of the mission. This is consistent with the expected atmospheric cooling, if the observed decreasing trend in the column amount of water occurred mainly within the PBL.

The Phoenix and Mars Reconnaissance Orbiter (MRO) missions collaborated in an unprecedented campaign to observe the northern polar region summer atmosphere throughout the Phoenix mission (25 May to 2 November 2008; Ls = 76°–150°) and slightly beyond (∼Ls = 158°). Five atmospherically related campaigns were defined a priori and were executed on 37 separate Martian days (sols). Phoenix and MRO observed the atmosphere nearly simultaneously. We describe the observation strategy and history, the participating experiments, and some initial results. We find that there is general agreement between measurements from different instruments and platforms and that complementary measurements provide a consistent picture of the atmosphere. Seasonal water abundance behavior matches with historical measurements. Winds aloft, as measured by cloud motions, showed the same seasonally consistent, diurnal rotation as the winds measured at the lander, during the first part of the mission (Ls = 76°–118°). A diurnal cycle recorded from Ls ∼ 108.3°–109.1°, in which a dust front was approaching the Phoenix Lander, is examined in detail. Cloud heights measured on subsequent orbits showed that in areas of active lifting, dust can be lofted quite high in the atmosphere, doubling in height over 2 h. The combination of experiments also revealed that there were discrete vertical layers of water ice and dust. Water vapor column abundances compared to near‐surface water vapor pressure indicate that water is not well mixed from the surface to a cloud condensation height and that the depth of the layer that exchanges diurnally with the surface is 0.5–1 km.