Formation of cirrus clouds depends upon the availability of ice nuclei to begin condensation of atmospheric water vapor. While it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective has not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, while sulfate/organic particles are underrepresented and elemental carbon and biological material are essentially absent. Further, composition analysis combined with relative humidity measurements suggest heterogeneous freezing was the dominant formation mechanism of these clouds. One Sentence Summary:The majority of cirrus clouds may form via heterogeneous freezing on mineral dust and metallic aerosol, not homogeneously or on elemental carbon or biological particles.

We use a Lagrangian, one‐dimensional cloud model to simulate ice cloud formation and dehydration along trajectories in the tropical tropopause layer (TTL). Time‐height curtains of temperature along the trajectory paths are extracted from meteorological analyses. The temperatures are adjusted near the tropopause such that the spatial average cold point temperature matches tropical radiosonde measurements. Temperature perturbations due to Kelvin waves, Rossby gravity waves, and high‐frequency gravity waves are superimposed. The cloud model tracks the growth and sedimentation of individual ice crystals. Ice number densities in the cloud simulations without waves range from <0.001 to ∼0.2 cm−3; when clouds form, they dehydrate the air but generally do not reduce the water vapor mixing ratio down to ice saturation. Wave‐driven temperature perturbations result in higher cloud frequencies and cause higher ice number densities (>1 cm−3) and smaller crystals (1–10 μm radius), resulting in less sedimentation but still effective dehydration overall. Inclusion of wave‐driven temperature oscillations decreases the final TTL H2O mixing ratio somewhat primarily because the wave perturbations decrease the tropical average cold point tropopause temperature by ∼0.75 K. Ultimately, air rising through the TTL is effectively dehydrated with or without wave perturbations. In general, the model results suggest that the final water vapor mixing ratios are primarily controlled by the minimum temperatures encountered by parcels and that they are relatively insensitive to factors such as the wave‐driven temperature variability, the supersaturation threshold for ice nucleation, and the rate of ascent through the tropopause layer. However, the frequency and geographical distribution of cloud formation is very sensitive to these parameters. On average, the clouds dehydrate the air along trajectories down to mixing ratios ∼10–40% higher than the temperature minimum saturation mixing ratio. The simulations predict efficient freeze‐drying of air by cloud formation within the TTL: For the December–January 1995/1996 period simulated the average final H2O mixing ratios at the tropopause (370–380 K potential temperature) range from 2.5 to 3.2 ppmv. These values are somewhat lower than the estimates of the stratospheric water vapor entry value from satellite and in situ measurements; hence an additional source of water (such as injection by deep convection) may be required to explain the observed tropical tropopause humidity.

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The transport of air from the planetary boundary layer (PBL) into the Asian Summer Monsoon anticyclone is investigated using backward trajectories initiated within the anti‐cyclone at 100 mb and 200 mb during August 2011. Transport occurs through a well‐defined conduit centered over the southern Tibetan plateau, where convection lofts air parcels into the anticyclone. The conduit, as a dynamical feature, is distinct from the anticyclone. Thus, while the anticyclone influences transport through the upper troposphere and lower stratosphere, it does not by itself define a transport pipeline through that region. To quantify model sensitivities, parcel trajectories are calculated using wind fields from multiple analysis data sets (European Centre for Medium‐Range Weather Forecasts, National Center for Environmental Prediction's Global Forecasting System, and NASA's Modern‐Era Retrospective Analysis for Research and Applications [MERRA]) and from synthetically modified data sets that explore the roles of vertical motion and horizontal resolution for discrepancies among these calculations. All calculations agree on the relative contributions to PBL sources for the anticyclone from large‐scale regions with Tibetan Plateau and India/SE Asia being the most important. However, they disagree on the total fraction of air within the anticyclone that was recently in the PBL. At 200 mbar, calculations using MERRA are clear outliers due to problematic vertical motion in those data. Large differences among the different data sets at 100 mbar are more closely related to horizontal resolution. It is speculated that this reflects the importance of deep, small‐scale convective updrafts for transport to 100 mbar.

[1] Micrometeorites that ablate in the lower thermosphere and upper mesosphere are thought to recondense into nanometer-sized smoke particles and then coagulate into larger dust particles. Previous studies with one-dimensional models have determined that the meteoric dust size distribution is sensitive to the background vertical velocity and have speculated on the importance of the mesospheric meridional circulation to the dust spatial distribution. We conduct the first three-dimensional simulations of meteoric dust using a general circulation model with sectional microphysics to study the distribution and characteristics of meteoric dust in the mesosphere and upper stratosphere. We find that the mesospheric meridional circulation causes a strong seasonal pattern in meteoric dust concentration in which the summer pole is depleted and the winter pole is enhanced. This summer pole depletion of dust particles results in fewer dust condensation nuclei (CN) than has traditionally been assumed in numerical simulations of polar mesospheric clouds (PMCs). However, the total number of dust particles present is still sufficient to account for PMCs if smaller particles can nucleate to form ice than is conventionally assumed. During winter, dust is quickly transported down to the stratosphere in the polar vortex where it may participate in the nucleation of sulfate aerosols, the formation of the polar CN layer, and the formation of polar stratospheric clouds (PSCs). These predictions of the seasonal variation and resulting large gradients in dust concentration should assist the planning of future campaigns to measure meteoric dust.