Mixed-phase clouds represent a three-phase colloidal system consisting of water vapor, ice particles, and coexisting supercooled liquid droplets. Mixed-phase clouds are ubiquitous in the troposphere, occurring at all latitudes from the polar regions to the tropics. Because of their widespread nature, mixed-phase processes play critical roles in the life cycle of clouds, precipitation formation, cloud electrification, and the radiative energy balance on both regional and global scales. Yet, in spite of many decades of observations and theoretical studies, our knowledge and understanding of mixed-phase cloud processes remains incomplete. Mixed-phase clouds are notoriously difficult to represent in numerical weather prediction and climate models, and their description in theoretical cloud physics still presents complicated challenges. In this chapter, the current status of our knowledge on mixed-phase clouds, obtained from theoretical studies and observations, is reviewed. Recent progress, along with a discussion of problems and gaps in understanding the mixed-phase environment is summarized. Specific steps to improve our knowledge of mixed-phase clouds and their role in the climate and weather system are proposed.

Abstract. The VAMOS 1 Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) was an international field program designed to make observations of poorly understood but critical components of the coupled climate system of the southeast Pacific. This region is characterized by strong coastal upwelling, the coolest SSTs in the tropi- cal belt, and is home to the largest subtropical stratocumulus deck on Earth. The field intensive phase of VOCALSREx took place during October and November 2008 and constitutes a critical part of a broader CLIVAR program (VOCALS) designed to develop and promote scientific activities leading to improved understanding, model simulations, and predictions of the southeastern Pacific (SEP) coupled ocean-atmosphere-land system, on diurnal to interannual timescales. The other major components of VOCALS are a modeling program with a model hierarchy ranging from the local to global scales, and a suite of extended observations from regular research cruises, instrumented moorings, Published by Copernicus Publications on behalf of the European Geosciences Union. R. Wood et al.: VOCALS operationsand satellites. The two central themes of VOCALS-REx focus upon (a) links between aerosols, clouds and precipitation and their impacts on marine stratocumulus radiative properties, and (b) physical and chemical couplings between the upper ocean and the lower atmosphere, including the role that mesoscale ocean eddies play. A set of hypotheses designed to be tested with the combined field, monitoring and modeling work in VOCALS is presented here. A further goal of VOCALS-REx is to provide datasets for the evaluation and improvement of large-scale numerical models. VOCALSREx involved five research aircraft, two ships and two surface sites in northern Chile. We describe the instrument payloads and key mission strategies for these platforms and give a summary of the missions conducted.

Measurements on the UK Met Office C‐130 within a distinct biomass burning plume during the Southern AFricAn Regional science Initiative (SAFARI 2000) show an increase in the single scattering albedo as the aerosol ages, from 0.84 at source to 0.90 in the aged regional haze in 5 hours. Condensation of scattering material from the gas phase appears to be the dominant mechanism; the change in black carbon morphology, from a chain to clump like structure, does not significantly affect the bulk aerosol single scattering albedo.

SUMMARYTwo types of partially absorbing aerosol are included in calculations that are based on intensive aircraft observations: biomass burning aerosol characterized during the Southern AFricAn Regional science Initiative (SAFARI 2000) and mineral dust aerosol characterized during the SaHAran Dust Experiment (SHADE). Measurements during SAFARI 2000 reveal that the biomass burning aerosol layer is advected over the South Atlantic ocean at elevated altitudes above the marine boundary layer which is capped by semi-permanent stratocumulus cloud sheets. Similarly, the mineral dust is measured at elevated altitudes during SHADE resulting in transport above cloud for distances of several thousands of kilometres. We perform theoretical calculations of the effect of these partially absorbing aerosol layers on satellite retrievals of cloud effective radius and cloud optical depth, and show that, in these cases, retrievals of cloud optical depth or liquid water path are likely to be subject to systematic low biases. The theoretical calculations suggest that the cloud effective radius may be subject to a significant low bias for Moderate resolution Imaging Spectrometer (MODIS) retrievals that rely on the 0.86 and 1.63 µm radiance pair for an overlying aerosol layer of either biomass burning aerosol or mineral dust. Conversely, the cloud effective radius may be subject to a significant high bias for Advanced Very High Resolution Radiometer or MODIS retrievals that rely on the 0.63 and 3.7 µm radiance pair for an overlying aerosol layer of mineral dust. Analysis of 1 km resolution MODIS data for the SAFARI 2000 period suggests that the effective radius derived from the 0.86 and 1.63 µm radiance pair is, indeed, subject to a low bias in the presence of overlying biomass burning aerosol. These results show the difficulties associated with remote sensing retrievals, which must be kept in mind when attempting to assess any potential indirect effect.

Abstract. The VAMOS Ocean-Cloud-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from 15 October to 15 November 2008 in the South East Pacific (SEP) region to investigate interactions between land, sea and atmosphere in this unique tropical eastern ocean environment and to improve the skill of global and regional models in representing the region. This study synthesises selected aircraft, ship and surface site observations from VOCALSREx to statistically summarise and characterise the atmospheric composition and variability of the Marine Boundary Layer (MBL) and Free Troposphere (FT) along the 20 • S parallel between 70 • W and 85 • W. Significant zonal gradients in mean MBL sub-micron aerosol particle size and composition, carbon monoxide, sulphur dioxide and ozone were seen over the campaign, with a generally more variable and polluted coastal environment and a less variable, more pristine remote maritime regime. Gradients in aerosol and trace gas concentrations were observed to be associated with strong gradients in cloud droplet number. The FT wasCorrespondence to: G. Allen (grant.allen@manchester.ac.uk) often more polluted in terms of trace gases than the MBL in the mean; however increased variability in the FT composition suggests an episodic nature to elevated concentrations. This is consistent with a complex vertical interleaving of airmasses with diverse sources and hence pollutant concentrations as seen by generalised back trajectory analysis, which suggests contributions from both local and longrange sources. Furthermore, back trajectory analysis demonstrates that the observed zonal gradients both in the boundary layer and the free troposphere are characteristic of marked changes in airmass history with distance offshore -coastal boundary layer airmasses having been in recent contact with the local land surface and remote maritime airmasses having resided over ocean for in excess of ten days. Boundary layer composition to the east of 75 • W was observed to be dominated by coastal emissions from sources to the west of the Andes, with evidence for diurnal pumping of the Andean boundary layer above the height of the marine capping inversion. Analysis of intra-campaign variability in atmospheric composition was not found to be significantly correlated with observed low-frequency variability in the large scale flow pattern; campaign-average interquartile ranges of Published by Copernicus Publications on behalf of the European Geosciences Union. 5238 G. Allen et al.: South East Pacific composition during VOCALS-REx CO, SO 2 and O 3 concentrations at all longitudes were observed to dominate over much smaller differences in median concentrations calculated between periods of different flow regimes. The campaign climatology presented here aims to provide a valuable dataset to inform model simulation and future process studies, particularly in the context of aerosolcloud interaction and further evaluation of dynamical processes in the SEP region for conditions analogous to those during VOCALS-REx....