Context. Recent observations indicate potentially carbon-rich (C/O>1) exoplanet atmospheres. Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/O ratio as additional parameter but carbon-rich cloud formation modeling is a challenge for the models applied. The determination of the habitable zone for exoplanets requires the treatment of cloud formation in chemically different regimes. Aims. We aim to model cloud formation processes for carbon rich exoplanetary atmospheres. Disk models show that carbon-rich or near-carbon-rich niches may emerge and cool carbon planets may trace these particular stages of planetary evolution. Methods. We extend our kinetic cloud formation model by including carbon seed formation and the formation of, and MgS[s] by gas-surface reactions. We solve a system of dust moment equations and element conservation for a pre-scribed Drift-Phoenix atmosphere structure to study how a cloud structure would change with changing initial C/O 0 =0.43 . . . 10.0. Results. The seed formation efficiency is lower in carbon-rich atmospheres than in oxygen-rich gases due to carbon being a very effective growth species. The consequence is that less particles will make up a cloud for C/O 0 >1. The cloud particles will be smaller in size than in an oxygen-rich atmosphere. An increasing initial C/O ratio does not revert this trend because a much greater abundance of condensible gas species exists in a rich carbon environment. Cloud particles are generally made of a mix of materials: carbon dominates if C/O 0 >1 and silicates dominate if C/O 0 <1. 80-90% carbon is reached only in extreme cases where C/O 0 =3.0 or 10.0. Conclusions. Carbon-rich atmospheres would form clouds that are made of particles of height-dependent mixed compositions, sizes and numbers. The remaining gas-phase is far less depleted than in an oxygen-rich atmosphere. Typical tracer molecules are HCN and C 2 H 2 in combination with a featureless, smooth continuum due to a carbonaceous cloud cover, unless the cloud particles become crystalline.
We investigate the general stability of 1D spherically symmetric ionized Bondi accretion onto a massive object in the specific context of accretion onto a young stellar object. We first derive a new analytic expression for a steady state two temperature solution that predicts the existence of compact and hypercompact Hii regions. We then show that this solution is only marginally stable if ionization is treated self-consistently. This leads to a recurring collapse of the Hii region over time. We derive a semi-analytic model to explain this instability, and test it using spatially converged 1D radiation hydrodynamical simulations. We discuss the implications of the 1D instability on 3D radiation hydrodynamics simulations of supersonic accreting flows.
The predictability of precipitation type in a January 2017 winter storm over the northeast US and southeast Canada is examined using a convective-scale initial-condition ensemble with the Weather Research and Forecasting (WRF) model. Real-time forecasts of the event by Environment and Climate Change Canada predicted 15-25 cm of snow accumulation in Montreal, Quebec. However, the initial 4 h of the event saw 5-8 mm of freezing rain instead, followed by 7 cm of snow. While the total liquid-equivalent precipitation was consistent with the forecast, the unexpected freezing rain caused significant disruption in the Montreal region. The fraction of freezing precipitation (freezing rain and/or ice pellets) over the initial 4 h in Montreal varied greatly across the ensemble, with some members producing nearly all snow and others producing nearly all freezing precipitation. In members with larger fractions of freezing precipitation (as opposed to snow), the cyclone’s midlevel trough was displaced slightly to the northwest, and its downstream (eastern) edge was narrower, the latter of which was traced back to model initialization. These differences increased the midlevel southerly flow into southern Quebec, which both enhanced the horizontal warm advection and decreased the vertical cold advection leading up to the event. The consequent midlevel warming over Montreal in these members produced an above-zero layer that melted falling precipitation, leading to freezing upon contact with the ground. This case study highlights the value of convective-scale ensembles for identifying mechanisms by which initial, synoptic-scale uncertainties lead to high-impact, localized errors in precipitation type.
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