Abstract. This paper introduces and evaluates the second version of the global aerosol-climate model ECHAM-HAM. Major changes have been brought into the model, including new parameterizations for aerosol nucleation and water uptake, an explicit treatment of secondary organic aerosols, modified emission calculations for sea salt and mineral dust, the coupling of aerosol microphysics to a two-moment stratiform cloud microphysics scheme, and alternative wet scavenging parameterizations. These revisions extend the model's capability to represent details of the aerosol lifecycle and its interaction with climate. Nudged simulations of the year 2000 are carried out to compare the aerosol properties and global distribution in HAM1 and HAM2, and to evaluate them against various observations. Sensitivity experiments are performed to help identify the impact of each individual update in model formulation.Results indicate that from HAM1 to HAM2 there is a marked weakening of aerosol water uptake in the lower troposphere, reducing the total aerosol water burden from 75 Tg to 51 Tg. The main reason is the newly introduced κ-Köhler-theory-based water uptake scheme uses a lower value for the maximum relative humidity cutoff. Particulate organic matter loading in HAM2 is considerably higher in the upper troposphere, because the explicit treatment of secondary organic aerosols allows highly volatile oxidation products of the precursors to be vertically transported to regions of very low temperature and to form aerosols there. Sulfate, black carbon, particulate organic matter and mineral dust in HAM2 have longer lifetimes than in HAM1 because of weaker incloud scavenging, which is in turn related to lower autoconversion efficiency in the newly introduced two-moment cloud microphysics scheme. Modification in the sea salt emission scheme causes a significant increase in the ratio (from 1.6 to 7.7) between accumulation mode and coarse mode emission fluxes of aerosol number concentration. This leads to a general increase in the number concentration of smaller particles over the oceans in HAM2, as reflected by the higher Angström parameters.Evaluation against observation reveals that in terms of model performance, main improvements in HAM2 include a marked decrease of the systematic negative bias in the absorption aerosol optical depth, as well as smaller biases over the oceans inÅngström parameter and in the accumulation mode number concentration. The simulated geographical distribution of aerosol optical depth (AOD) is better correlated with the MODIS data, while the surface aerosol mass concentrations are very similar to those in the old version. The total aerosol water content in HAM2 is considerably Published by Copernicus Publications on behalf of the European Geosciences Union. K. Zhang et al.: Aerosol properties in ECHAM-HAM2closer to the multi-model average from Phase I of the AeroCom intercomparison project. Model deficiencies that require further efforts in the future include (i) positive biases in AOD over the ocean, (ii) n...

Nucleation from the gas phase is an important source of aerosol particles in the Earth's atmosphere, contributing to the number of cloud condensation nuclei, which form cloud droplets. We have implemented in the aerosol-climate model ECHAM5-HAM a new scheme for neutral and charged nucleation of sulfuric acid and water based on laboratory data, and nucleation of an organic compound and sulfuric acid using a parametrization of cluster activation based on field measurements. We give details of the implementation , compare results with observations, and investigate the role of the individual aerosol nucleation mechanisms for clouds and the Earth's radiative forcing. The results of our simulations are most consistent with observations when neutral and charged nucleation of sulfuric acid proceed throughout the troposphere and nucleation due to cluster activation is limited to the forested boundary layer. The globally averaged annual mean contributions of the individual nucleation processes to total absorbed solar shortwave radiation via the direct, semi-direct, indirect cloud-albedo and cloud-lifetime effects in our simulations are −1.15 W/m 2 for charged H 2 SO 4 /H 2 O nucleation, −0.235 W/m 2 for cluster activation, and −0.05 W/m 2 for neutral H 2 SO 4 /H 2 O nucle-ation. The overall effect of nucleation is −2.55 W/m 2 , which exceeds the sum of the individual terms due to feedbacks and interactions in the model. Aerosol nucleation contributes over the oceans with −2.18 W/m 2 to total absorbed solar shortwave radiation, compared to −0.37 W/m 2 over land. Correspondence to: J. Kazil (jan.kazil@noaa.gov) We explain the higher effect of aerosol nucleation on Earth's radiative forcing over the oceans with the larger area covered by ocean clouds, due to the larger contrast in albedo between clouds and the ocean surface compared to continents, and the larger susceptibility of pristine clouds owing to the saturation of effects. The large effect of charged nucleation in our simulations is not in contradiction with small effects seen in local measurements: over southern Finland, where cluster activation proceeds efficiently, we find that charged nucleation of sulfuric acid and water contributes on average less than 10% to ultrafine aerosol concentrations, in good agreement with observations.

Low-level clouds forming in the warm marine boundary layer continue to be a leading source of uncertainty in global climate models (i.e., Bony & Dufresne, 2005;Boucher et al., 2013;Zelinka et al., 2016). Challenges associated with the study of these clouds include resolving the internal cloud processes at a fine scale while maintaining an accurate representation of the meteorology in which the clouds occur. The time scale and seasonality are also important, as summertime and wintertime shallow cumuli observed from the same oceanic regions may exhibit different characteristics (

Abstract. New particle formation (NPF), which generates nucleation mode aerosol, was observed in the tropical Upper Troposphere (UT) and Tropical Tropopause Layer (TTL) by in situ airborne measurements over South America (JanuaryMarch 2005), Australia (November-December 2005), West Africa (August 2006) and Central America (2004. Particularly intense NPF was found at the bottom of the TTL. Measurements with a set of condensation particle counters (CPCs) with different d p50 (50 % lower size detection efficiency diameter or "cut-off diameter") were conducted on board the M-55 Geophysica in the altitude range of 12.0-20.5 km and on board the DLR Falcon-20 at up to 11.5 km altitude. On board the NASA WB-57F size distributions were measured over Central America in the 4 to 1000 nm diameter range with a system of nucleation mode aerosol specCorrespondence to: R. Weigel (weigelr@uni-mainz.de) trometers. Nucleation mode particle concentrations (N NM ) were derived from these measurements which allow for identifying many NPF events with N NM in the range of thousands of particles per cm 3 . Over Australia and West Africa, we identified NPF in the outflow of tropical convection, in particular of a Mesoscale Convective System (MCS). Newly formed particles with N NM > 1000 cm −3 were found to coexist with ice cloud particles (d p > 2 µm) as long as cloud particle concentrations remained below 2 cm −3 . The occurrence of NPF within the upper troposphere and the TTL was generally confined within 340 K to 380 K potential temperature, but NPF was of particular strength between 350 K and 370 K (i.e. ∼1-4 km below the cold point tropopause). Analyses of the aerosol volatility (at 250 • C) show that in the TTL on average 75-90 % of the particles were volatile, compared to typically only 50 % in the extra-tropical UT, indicative for the particles to mainly consist of H 2 SO 4 -H 2 O and possibly organic compounds. Along two flight segments over Central and South America (24 February 2005 and 7 August Published by Copernicus Publications on behalf of the European Geosciences Union. 2006, at 12.5 km altitude) in cloud free air, above thin cirrus, particularly high N NM were observed. Recent lifting had influenced the probed air masses, and N NM reached up to 16 000 particles cm −3 (ambient concentration). A sensitivity study using an aerosol model, which includes neutral and ion induced nucleation processes, simulates N NM in reasonable agreement with the in situ observations of clear-air NPF. Based on new, stringent multi-CPC criteria, our measurements corroborate the hypothesis that the tropical UT and the TTL are regions supplying freshly nucleated particles. Our findings narrow the altitude of the main source region to the bottom TTL, i.e. to the level of main tropical convection outflow, and, by means of measurements of carbon monoxide, they indicate the importance of anthropogenic emissions in NPF. After growth and/or coalescence the nucleation mode particles may act as cloud condensation nuclei in the tropical UT, or, upon ...

Abstract. Microphysical and meteorological controls on the formation of open and closed cellular structures in the Southeast Pacific are explored using model simulations based on aircraft observations during the VAMOS Ocean-CloudAtmosphere-Land Study Regional Experiment (VOCALSREx). The effectiveness of factors such as boundary-layer moisture and temperature perturbations, surface heat and moisture fluxes, large-scale vertical motion and solar heating in promoting drizzle and open cell formation for prescribed aerosol number concentrations is explored. For the case considered, drizzle and subsequent open cell formation over a broad region are more sensitive to the observed boundarylayer moisture and temperature perturbations (+0.9 g kg −1 ; −1 K) than to a five-fold decrease in aerosol number concentration (150 vs. 30 mg −1 ). When embedding the perturbations in closed cells, local drizzle and pockets of open cell (POC) formation respond faster to the aerosol reduction than to the moisture increase, but the latter generates stronger and more persistent drizzle. A local negative perturbation in temperature drives a mesoscale circulation that prevents local drizzle formation but promotes it in a remote area where lower-level horizontal transport of moisture is blocked and converges to enhance liquid water path. This represents a potential mechanism for POC formation in the Southeast Pacific stratocumulus region whereby the circulation is triggered by strong precipitation in adjacent broad regions of open cells. A simulation that attempts to mimic the influence of a coastally induced upsidence wave results in an increaseCorrespondence to: H. Wang (hailong.wang@pnl.gov) in cloud water but this alone is insufficient to initiate drizzle. An increase of surface sensible heat flux is also effective in triggering local drizzle and POC formation.Both open and closed cells simulated with observed initial conditions exhibit distinct diurnal variations in cloud properties. A stratocumulus deck that breaks up due solely to solar heating can recover at night. Precipitation in the opencell cases depletes the aerosol to the extent that cloud formation is significantly suppressed within one diurnal cycle. A replenishment rate of cloud condensation nuclei of order 1 mg −1 h −1 is sufficient to maintain clouds and prevent the boundary layer from collapsing the following day, suggesting that some local and/or remote aerosol sources is necessary for POCs to be able to last for days.

Abstract. Chemical and aerosol processes in the transition from closed-to open-cell circulation in the remote, cloudy marine boundary layer are explored. It has previously been shown that precipitation can initiate a transition from the closed-to the open-cellular state, but that the boundary layer cannot maintain this open-cell state without a resupply of cloud condensation nuclei (CCN). Potential sources of CCN include wind-driven production of sea salt from the ocean, nucleation from the gas phase, and entrainment from the free troposphere. In order to investigate CCN sources in the marine boundary layer and their role in supplying new particles, we have coupled in detail chemical, aerosol, and cloud processes in the WRF/Chem model, and added state-of-the-art representations of sea salt emissions and aerosol nucleation. We conduct numerical simulations of the marine boundary layer in the transition from a closed-to an open-cell state. Results are compared with observations in the Southeast Pacific boundary layer during the VAMOS Ocean-CloudAtmosphere-Land Study Regional Experiment (VOCALSREx). The transition from the closed-to the open-cell state generates conditions that are conducive to nucleation by forming a cloud-scavenged, ultra-clean layer below the inversion base. Open cell updrafts loft dimethyl sulfide from the ocean surface into the ultra-clean layer, where it is oxidized during daytime to SO 2 and subsequently to H 2 SO 4 . Low H 2 SO 4 condensation sink values in the ultra-clean layer allow H 2 SO 4 to rise to concentrations at which aerosol nuCorrespondence to: J. Kazil (jan.kazil@noaa.gov) cleation produces new aerosol in significant numbers. The existence of the ultra-clean layer is confirmed by observations. We find that the observed DMS flux from the ocean in the VOCALS-REx region can support a nucleation source of aerosol in open cells that exceeds sea salt emissions in terms of the number of particles produced. The freshly nucleated, nanometer-sized aerosol particles need, however, time to grow to sizes large enough to act as CCN. In contrast, mechanical production of particles from the ocean surface by near-surface winds provides a steady source of larger particles that are effective CCN at a rate exceeding a threshold for maintenance of open-cell circulation. Entrainment of aerosol from the free troposphere contributes significantly to boundary layer aerosol for the considered VOCALS-REx case, but less than sea salt aerosol emissions.

The stratocumulus to cumulus transition (SCT) is typically considered to be a slow, multiday process, caused primarily by dry air entrainment associated with overshooting cumulus, with minor influence of drizzle. This study revisits the role of drizzle in the SCT with large eddy simulations coupled with a two‐moment bulk microphysics scheme that includes a budget on aerosol (Na) and cloud droplet number concentrations (Nc). We show a strong precipitation‐induced modulation of the SCT by drizzle initiated in penetrative cumulus under stratocumulus. Lagrangian SCT simulations are initiated with various, moderate Na (100–250 cm−3), which produce little to no drizzle from the stratocumulus. As expected, drizzle formation in cumuli is regulated by cloud depth and Nc, with stronger dependence on cloud depth, so that, for the current case, drizzle is generated in all simulations once cumulus clouds become sufficiently deep. The drizzle generated in the cumuli washes out stratocumulus cloud water and much of the aerosol, and a cumulus state appears for approximately 10 h. With additional simulations with a fixed Nc (100 cm−3), we show that prediction of Nc is necessary for this fast SCT since it is a result of a positive feedback of collision‐coalescence‐induced aerosol depletion that enhances drizzle formation. A fixed Nc does not permit this feedback, and thus results in weak influence of drizzle on the SCT. Simulations with fixed droplet concentrations that bracket the time varying aerosol/drop concentrations are therefore not representative of the role of drizzle in the SCT.

The transition from stratocumulus to cumulus clouds in the presence of elevated light‐absorbing smoke layers is investigated with idealized large‐eddy simulations. A smoke layer is placed 1 km above stratocumulus top and evolves with the cloud fields over the course of a 3 day simulation. The simulations presented vary the smoke‐generated heating and the moisture content of the smoke layer. A control case without smoke is simulated for comparison. On day 2 of the transition, when still above cloud, smoke generates a more broken cloud field than the control case, depending weakly on the strength of the aerosol heating but strongly on the water vapor content in the smoke layer. Following nighttime recovery and contact with the stratocumulus, smoke hinders the transition by strengthening the inversion, limiting boundary layer deepening and reducing precipitation‐related breakup. This modulation delays the transition, which may extend the stratocumulus deck westward, with concomitant implications for climate forcing.