The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

Summary (149 words of referenced text): 46The climate impact of aerosols is highly uncertain owing primarily to their poorly quantified 47 influence on cloud properties. During 2014-15, a fissure eruption in Holuhraun (Iceland) 48 emitted huge quantities of sulphur dioxide, resulting in significant reductions in liquid cloud 49 droplet size. Using satellite observations and detailed modelling, we estimate a global mean 50 radiative forcing from the resulting aerosol-induced cloud brightening for the time of the 51 eruption of around -0.2 W.m -2 . Changes in cloud amount or liquid water path are 52 undetectable, indicating that these aerosol-cloud indirect effects are modest. It supports the 53 idea that cloud systems are well buffered against aerosol changes as only impacts on cloud 54 effective radius appear relevant from a climate perspective, thus providing a strong constraint 55 on aerosol-cloud interactions. This result will reduce uncertainties in future climate 56 projections as we are able to reject the results from climate models with an excessive liquid 57 water path response. 58 59Main Text: (3103 words of referenced text, including concluding paragraph) 60 The 2014-15 eruption at Holuhraun (486 words of referenced text): 61Anthropogenic emissions that affect climate are not just confined to greenhouse gases. 62Sulphur dioxide and other pollutants form atmospheric aerosols that can scatter and absorb 63 sunlight and can influence the properties of clouds, modulating the Earth-atmosphere energy 64 balance. Aerosols act as cloud condensation nuclei (CCN); an increase in CCN translates into 65 a higher number of smaller, more reflective cloud droplets that scatter more sunlight back to 66 space 1 (the ÔfirstÕ indirect effect of aerosols). Smaller cloud droplets decrease the efficiency 67 of collision-coalescence processes that are pivotal in rain initiation, thus aerosol-influenced 68 clouds may retain more liquid water and extend coverage/lifetime 2,3 (the ÔsecondÕ or Ôcloud 69 lifetimeÕ indirect effect). Aerosols usually co-vary with key environmental variables making 70 it difficult to disentangle aerosol-cloud impacts from meteorological variability [4][5][6] . 71Additionally, clouds themselves are complex transient systems subject to dynamical 72 feedbacks (e.g. cloud top entrainment/evaporation, invigoration of convection) which 73 influence cloud response [7][8][9][10][11][12] . These aspects present great challenges in evaluating and 74 constraining aerosol-cloud interactions (ACI) in General Circulation Models (GCM) 13-17 , 75 with particular contentious debate surrounding the relative importance of these feedback 76 mechanisms. 77Nonetheless, anthropogenic aerosol emissions are thought to cool the Earth via indirect 78 effects 17 , but the uncertainty ranges from -1.2 to -0.0 W.m -2 (90% confidence interval) due to 79 i) a lack of characterization of the pre-industrial aerosol state 15,18,19 , and ii) model parametric 80 and structural errors in representing cloud responses to aerosol chan...

SignificanceSimulated clouds over the Southern Ocean reflect too little solar radiation compared with observations, which results in errors in simulated surface temperatures and in many other important features of the climate system. Our results show that the radiative properties of the most biased types of clouds in cyclonic systems are highly sensitive to the concentration of ice-nucleating particles. The uniquely low concentrations of ice-nucleating particles in this remote marine environment strongly inhibit precipitation and allow much brighter clouds to be sustained.

Abstract. A 1200×1200 km2 area of the tropical South Atlantic Ocean near Ascension Island is studied with the HadGEM climate model at convection-permitting and global resolutions for a 10-day case study period in August 2016. During the simulation period, a plume of biomass burning smoke from Africa moves into the area and mixes into the clouds. At Ascension Island, this smoke episode was the strongest of the 2016 fire season.The region of interest is simulated at 4 km resolution, with no parameterised convection scheme. The simulations are driven by, and compared to, the global model. For the first time, the UK Chemistry and Aerosol model (UKCA) is included in a regional model with prognostic aerosol number concentrations advecting in from the global model at the boundaries of the region.Fire emissions increase the total aerosol burden by a factor of 3.7 and cloud droplet number concentrations by a factor of 3, which is consistent with MODIS observations. In the regional model, the inversion height is reduced by up to 200 m when smoke is included. The smoke also affects precipitation, to an extent which depends on the model microphysics. The microphysical and dynamical changes lead to an increase in liquid water path of 60 g m−2 relative to a simulation without smoke aerosol, when averaged over the polluted period. This increase is uncertain, and smaller in the global model. It is mostly due to radiatively driven dynamical changes rather than precipitation suppression by aerosol.Over the 5-day polluted period, the smoke has substantial direct radiative effects of +11.4 W m−2 in the regional model, a semi-direct effect of −30.5 W m−2 and an indirect effect of −10.1 W m−2. Our results show that the radiative effects are sensitive to the structure of the model (global versus regional) and the parameterization of rain autoconversion. Furthermore, we simulate a liquid water path that is biased high compared to satellite observations by 22 % on average, and this leads to high estimates of the domain-averaged aerosol direct effect and the effect of the aerosol on cloud albedo. With these caveats, we simulate a large net cooling across the region, of −27.6 W m−2.

[1] The Tropical Warm Pool-International Cloud Experiment (TWP-ICE) provided extensive observational data sets designed to initialize, force, and constrain atmospheric model simulations. In this first of a two-part study, precipitation and cloud structures within nine cloud-resolving model simulations are compared with scanning radar reflectivity and satellite infrared brightness temperature observations during an active monsoon period from 19 to 25 January 2006. Seven of nine simulations overestimate convective area by 20% or more leading to general overestimation of convective rainfall. This is balanced by underestimation of stratiform rainfall by 5% to 50% despite overestimation of stratiform area by up to 65% because of a preponderance of very low stratiform rain rates in all simulations. All simulations fail to reproduce observed radar reflectivity distributions above the melting level in convective regions and throughout the troposphere in stratiform regions. Observed precipitation-sized ice reaches higher altitudes than simulated precipitation-sized ice despite some simulations that predict lower than observed top-of-atmosphere infrared brightness temperatures. For the simulations that overestimate radar reflectivity aloft, graupel is the cause with one-moment microphysics schemes whereas snow is the cause with two-moment microphysics schemes. Differences in simulated radar reflectivity are more highly correlated with differences in mass mean melted diameter (D m ) than differences in ice water content. D m is largely dependent on the mass-dimension relationship and gamma size distribution parameters such as size intercept (N 0 ) and shape parameter (m). Having variable density, variable N 0 , or m greater than zero produces radar reflectivities closest to those observed.

[1] Observations made during the TWP-ICE campaign are used to drive and evaluate thirteen cloud-resolving model simulations with periodic lateral boundary conditions. The simulations employ 2D and 3D dynamics, one-and two-moment microphysics, several variations on large-scale forcing, and the use of observationally derived aerosol properties to prognose droplet numbers. When domain means are averaged over a 6-day active monsoon period, all simulations reproduce observed surface precipitation rate but not its structural distribution. Simulated fractional areas covered by convective and stratiform rain are uncorrelated with one another, and are both variably overpredicted by up to a factor of $2. Stratiform area fractions are strongly anticorrelated with outgoing longwave radiation (OLR) but are negligibly correlated with ice water path (IWP), indicating that ice spatial distribution controls OLR more than mean IWP. Overpredictions of OLR tend to be accompanied by underpredictions of reflected shortwave radiation (RSR). When there are two simulations differing only in microphysics scheme or large-scale forcing, the one with smaller stratiform area tends to exhibit greater OLR and lesser RSR by similar amounts. After $10 days, simulations reach a suppressed monsoon period with a wide range of mean precipitable water vapor, attributable in part to varying overprediction of cloud-modulated radiative flux divergence compared with observationally derived values. Differences across the simulation ensemble arise from multiple sources, including dynamics, microphysics, and radiation treatments. Close agreement of spatial and temporal averages with observations may not be expected, but the wide spreads of predicted stratiform fraction and anticorrelated OLR indicate a need for more rigorous observation-based evaluation of the underlying micro-and macrophysical properties of convective and stratiform structures.

Citation: Shipway BJ, Hill AA. 2012. Diagnosis of systematic differences between multiple parametrizations of warm rain microphysics using a kinematic framework. Q.

Ten 3-D cloud-resolving model simulations and four 3-D limited area model simulations of an intense mesoscale convective system observed on 23-24 January 2006 during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are compared with each other and with observed radar reflectivity fields and dual-Doppler retrievals of vertical wind speeds in an attempt to explain published results showing a high bias in simulated convective radar reflectivity aloft. This high-bias results from ice water content being large, which is a product of large, strong convective updrafts, although hydrometeor size distribution assumptions modulate the size of this bias. Making snow mass more realistically proportional to D 2 rather than D 3 eliminates unrealistically large snow reflectivities over 40 dBZ in some simulations. Graupel, unlike snow, produces high biased reflectivity in all simulations, which is partly a result of parameterized microphysics but also partly a result of overly intense simulated updrafts. Peak vertical velocities in deep convective updrafts are greater than dual-Doppler-retrieved values, especially in the upper troposphere. Freezing of liquid condensate, often rain, lofted above the freezing level in simulated updraft cores greatly contributes to these excessive upper tropospheric vertical velocities. The strongest simulated updraft cores are nearly undiluted, with some of the strongest showing supercell characteristics during the multicellular (presquall) stage of the event. Decreasing horizontal grid spacing from 900 to 100 m slightly weakens deep updraft vertical velocity and moderately decreases the amount of condensate aloft but not enough to match observational retrievals. Therefore, overly intense simulated updrafts may additionally be a product of unrealistic interactions between convective dynamics, parameterized microphysics, and large-scale model forcing that promote different convective strengths than observed.