Atmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.

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.

Abstract. Multiplatform airborne, ship-based, and landbased observations from 16 October-15 November 2008 during the VOCALS Regional Experiment (REx) are used to document the typical structure of the Southeast Pacific stratocumulus-topped boundary layer and lower free troposphere on a transect along 20 • S between the coast of Northern Chile and a buoy 1500 km offshore. Strong systematic gradients in clouds, precipitation and vertical structure are modulated by synoptically and diurnally-driven variability. The boundary layer is generally capped by a strong (10-12 K), sharp inversion. In the coastal zone, the boundary layer is typically 1 km deep, fairly well mixed, and topped by thin, nondrizzling stratocumulus with accumulationmode aerosol and cloud droplet concentrations exceeding 200 cm −3 . Far offshore, the boundary layer depth is typically deeper (1600 m) and more variable, and the vertical structure is usually decoupled. The offshore stratocumulus typically have strong mesoscale organization, much higher peak liquid water paths, extensive drizzle, and cloud droplet concentrations below 100 cm −3 , sometimes with embedded pockets of open cells with lower droplet concentrations. The lack of drizzle near the coast is not just a microphysical response to high droplet concentrations; smaller cloud depth and liquid water path than further offshore appear comparably important.Moist boundary layer air is heated and mixed up along the Andean slopes, then advected out over the top of the boundary layer above adjacent coastal ocean regions. Well offshore, the lower free troposphere is typically muchCorrespondence to: C. S. Bretherton (breth@washington.edu) drier. This promotes strong cloud-top radiative cooling and stronger turbulence in the clouds offshore. In conjunction with a slightly cooler free troposphere, this may promote stronger entrainment that maintains the deeper boundary layer seen offshore.Winds from ECMWF and NCEP operational analyses have an rms difference of only 1 m s −1 from collocated airborne leg-mean observations in the boundary layer and 2 m s −1 above the boundary layer. This supports the use of trajectory analysis for interpreting REx observations. Two-day back-trajectories from the 20 • S transect suggest that eastward of 75 • W, boundary layer (and often free-tropospheric) air has usually been exposed to South American coastal aerosol sources, while at 85 • W, neither boundary-layer or free-tropospheric air has typically had such contact.

Abstract. Aircraft measurements are presented from the 27/28 October 2008 case study of the VOCALS Regional Experiment (REx) over the remote subtropical southeast Pacific (18 • S, 80 • W). Data from two aircraft that took measurements approximately twelve hours apart but in the same advected airmass are used to document a remarkably sharp spatial transition in marine boundary layer (MBL), cloud, and aerosol structure across the boundary between a well-mixed MBL containing overcast closed mesoscale cellular stratocumulus, and a pocket of open cells (POC) with significantly lower cloud cover. Long (∼190-250 km) straight and level flight legs at three levels in the marine boundary layer and one level in the lower free troposphere permit sampling of the closed cells, the POC, and a 20-30 km wide transition zone with distinctly different structure from the two airmasses on either side. The POC region consists of intermittent active and strongly precipitating cumulus clouds rising and detraining into patches of drizzling but quiescent stratiform cloud which is optically thin especially toward its edges.Mean cloud-base precipitation rates inside the POC are several mm d −1 , but rates in the closed cell region are not greatly lower than this. This latter finding suggests that precipitation is not a sufficient condition for POC formation from overcast stratocumulus. Despite similar cloud-base precipitation rates in the POC and overcast region, much of the precipitation (>90%) evaporates below cloud in the overcast region, while there is significant surface precipitation inside the POC. In the POC and transition region, although the majority of the condensate is in the form of drizzle, the inteCorrespondence to: R. Wood (robwood@atmos.washington.edu) grated liquid water path is remarkably close to that expected for a moist adiabatic parcel rising from cloud base to top.The transition zone between the POC and the closed cells often consists of thick "boundary cell" clouds producing mean surface precipitation rates of 10-20 mm d −1 , a divergent quasi-permanent cold/moist pool below cloud, a convergent inflow region at mid-levels in the MBL, and a divergent outflow near the top of the MBL.The stratiform clouds in the POC exist within an ultraclean layer that is some 200-300 m thick. Aerosol concentrations (N a ) measured by a PCASP in the diameter range 0.12-3.12 µm in the center of the ultra-clean layer are as low as 0.1-1 cm −3 . This suggests that coalescence scavenging and sedimentation is extremely efficient, since N a in the subcloud layer, and droplet concentration N d in the active cumuli are typically 20-60 cm −3 . The droplet concentrations in the quiescent stratiform clouds are extremely low (typically 1-10 cm −3 ), and most of their liquid water is in the form of drizzle, which mainly evaporates before reaching the surface. The cloud droplet concentration in the overcast region decreases strongly as the transition region is approached, as do subcloud accumulation mode aerosol concentrations, suggesting that coa...