Aerosol nucleation over oceans and the role of galactic cosmic rays

International audienceWe investigate formation of sulfate aerosol in the marine troposphere from neutral and charged nucleation of H2SO4 and H2O. A box model of neutral and charged aerosol processes is run on a grid covering the oceans. Input data are taken from a model of galactic cosmic rays in the atmosphere, and from global chemistry and transport models. We find a weak aerosol production over the tropical oceans in the lower and middle troposphere, and a stronger production at higher latitudes, most notably downwind of industrial regions. The highest aerosol production, however, occurs in the upper troposphere, in particular in the tropics. This finding supports the proposition by which non-sea salt marine boundary layer aerosol in tropical regions does not form in situ, but nucleates in the upper troposphere from convectively lifted and cloud processed boundary layer air rich in aerosol precursor gases, from where it descends in subsiding air masses compensating convection. Convection of boundary layer air also appears to drive the formation of condensation nuclei in the tropical upper troposphere which maintains the stratospheric aerosol layer in the absence of volcanic activity. Neutral nucleation contributes only marginally to aerosol production in our simulations. This highlights the importance of charged binary and of ternary nucleation involving ammonia for aerosol formation. In clean marine regions however, ammonia concentrations seem too low to support ternary nucleation, making binary nucleation from ions a likely pathway for sulfate aerosol formation. On the other hand, our analysis indicates that the variation of ionization by galactic cosmic rays over the decadal solar cycle does not entail a response in aerosol production and cloud cover via the second indirect aerosol effect that would explain observed variations in global cloud cover. We estimate that the variation in radiative forcing resulting from a response of clouds to the change in galactic cosmic ray ionization and subsequent aerosol production over the decadal solar cycle is smaller than the concurrent variation of total solar irradiance

Is aerosol formation in cirrus clouds possible?

International audienceThe recent observation of ultrafine aerosol particles in cirrus clouds has raised the question whether aerosol formation within cirrus clouds is possible, and if so, what mechanisms are involved. We have developed an aerosol parcel model of neutral and charged H2SO4/H2O aerosol processes, including nucleation from the gas phase and loss onto cirrus ice particles. Laboratory thermodynamic data for sulfuric acid uptake and loss by small neutral and charged clusters are used, allowing for a reliable description of both neutral and charged nucleation down to the very low temperatures occurring in the upper troposphere and lower stratosphere. The model implements a first order scheme for resolving the aerosol size distribution within its geometric size sections, which efficiently suppresses numerical diffusion. We operate the model offline on trajectories generated with a detailed 1-D cirrus model which describes ice crystal nucleation, deposition growth, vertical advection of ice crystals and water vapor, and ice crystal sedimentation. In this paper we explore the possibility of aerosol formation within non-convective cirrus clouds and draw conclusions for aerosol formation in anvil cirrus. We find that sulfate aerosol formation within cirrus clouds can proceed even at high ice surface area concentrations, and depends strongly on the size of the cirrus ice crystals and on the surface area concentration of preexisting aerosol particles

A semi-analytical method for calculating rates of new sulfate aerosol formation from the gas phase

International audienceThe formation of new aerosol from the gas phase is commonly represented in atmospheric modeling with parameterizations of the steady state nucleation rate. Present parameterizations are based on classical nucleation theory or on nucleation rates calculated with a numerical aerosol model. These parameterizations reproduce aerosol nucleation rates calculated with a numerical aerosol model only imprecisely. Additional errors can arise when the nucleation rate is used as a surrogate for the production rate of particles of a given size. We discuss these errors and present a method which allows a more precise calculation of steady state sulfate aerosol formation rates. The method is based on the semi-analytical solution of an aerosol system in steady state and on parameterized rate coefficients for H2SO4 uptake and loss by sulfate aerosol particles, calculated from laboratory and theoretical thermodynamic data

Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine stratocumulus clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine stratocumulus clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning–afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in clouds with LWP ⪆ 50 g m−2, longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick stratocumulus clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed

Mesoscale organization, entrainment, and the properties of a closed-cell stratocumulus cloud

Closed-cell mesoscale organization and its relationship to entrainment and the properties of a low, nonprecipitating stratocumulus cloud is investigated. Large eddy simulations were run over 10 periodic diurnal cycles during which mesoscale organization could fully develop and approach a quasi-steady state on five domains sized from 2.4 km × 2.4 km to 38.4 km × 38.4 km. The four smaller domains hosted a single cell with an aspect ratio that increased with domain size. On the largest domain, mesoscale organization consisted of a cell population that evolved over the course of the diurnal cycle. It is found that with increasing cell aspect ratio, entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled. This causes an increase in cloud water path and cloud radiative effect up to a cell aspect ratio of 16. With further increase in cell aspect ratio, circulation on the cell scale becomes less effective in supplying moisture to the cloud and in producing turbulent kinetic energy (TKE). This mechanism can explain scale saturation in closed-cell mesoscale organization. The simulations support a maximum stable aspect ratio of closed-cell mesoscale organization between 32 and 64, consistent with the observational limit of [asymp]40. The simulations show furthermore that entrainment does not, in general, scale with buoyant production of TKE. Instead, entrainment correlates with the vertical component of TKE. This implies vertical motion as a driver of entrainment, and a convective velocity scale based on the vertical component of TKE rather than on buoyant production of TKE. Key Points Entrainment weakens and the boundary layer becomes shallower, cooler, moister, and more decoupled with increasing closed cell aspect ratio Reduced cell scale moisture transport and TKE production at large aspect ratios can explain closed cell scale saturation Entrainment is driven by the vertical component of TKE rather than by TKE productio

Stratocumulus to cumulus transition by drizzle

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&nbsp;(100&ndash;250 cm-3), which produce little to no drizzle from the stratocumulus. As expected, drizzle formation in cumuli is regulated by cloud depth and N, 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&nbsp;(100 cm-3), we show that prediction of Nc&nbsp;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&nbsp;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.</p

Temperature dependence of the rates of reaction of trifluoroacetic acid with Criegee intermediates

The rate coefficients for gas-phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH2OO + CF3COOH)=(3.4±0.3)×10−10 cm3 s−1 and k((CH3)2COO + CF3COOH)=(6.1±0.2)×10−10 cm3 s−1 at 294 K exceed estimates for collision-limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre-reactive complex. Fits to a model incorporating this complex formation give k [cm3 s−1]=(3.8±2.6)×10−18 T2 exp((1620±180)/T) + 2.5×10−10 and k [cm3 s−1]=(4.9±4.1)×10−18 T2 exp((1620±230)/T) + 5.2×10−10 for the CH2OO + CF3COOH and (CH3)2COO + CF3COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates

From Sugar to Flowers: A Transition of Shallow Cumulus Organization During ATOMIC

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place in January&ndash;February, 2020. It was designed to understand the relationship between shallow convection and the large-scale environment in the trade-wind regime. A Lagrangian large eddy simulation, following the trajectory of a boundary-layer airmass, can reproduce a transition of trade cumulus organization from &ldquo;sugar&rdquo; to &ldquo;flower&rdquo; clouds with cold pools, observed on February 2&ndash;3. The simulation is driven with reanalysis large-scale meteorology, and in-situ aerosol data from ATOMIC and its joint field study EUREC4A. During the transition, large-scale upward motion deepens the cloud layer. The total water path and optical depth increase, especially in the moist regions where flowers aggregate. This is due to mesoscale circulation that renders a net convergence of total water in the already moist and cloudy regions, strengthening the organization. An additional simulation shows that stronger large scale upward motion reinforces the mesoscale circulation and accelerates the organization process by strengthening the cloud-layer mesoscale buoyant turbulence kinetic energy production. &nbsp;</p

Realism of Lagrangian Large Eddy Simulations Driven by Renalysis Meteorology: Tracking a Pocket of Open Cells Under a Biomass Burning Aerosol Layer

An approach to drive Lagrangian large eddy simulation (LES) of boundary layer clouds with reanalysis data is presented and evaluated using satellite (Spinning Enhanced Visible and Infrared Imager, SEVIRI) and aircraft (Cloud‐Aerosol‐Radiation Interactions and Forcing, CLARIFY) measurements. The simulations follow trajectories of the boundary layer flow. They track the formation and evolution of a pocket of open cells (POC) underneath a biomass burning aerosol layer in the free troposphere. The simulations reproduce the evolution of observed stratocumulus cloud morphology, cloud optical depth, and cloud drop effective radius, and capture the timing of the cloud state transition from closed to open cells seen in the satellite imagery on the three considered trajectories. They reproduce a biomass burning aerosol layer identified by the in‐situ aircraft measurements above the inversion of the POC. Entrainment of aerosol from the biomass burning layer into the POC is limited to the extent of having no impact on cloud‐ or boundary layer properties, in agreement with the CLARIFY observations. The two‐moment bin microphysics scheme used in the simulations reproduces the in‐situ cloud microphysical properties reasonably well. A two‐moment bulk microphysics scheme reproduces the satellite observations in the non‐precipitating closed‐cell state, but overestimates liquid water path and cloud optical depth in the precipitating open‐cell state due to insufficient surface precipitation. A boundary layer cold and dry bias occurring in LES can be counteracted by reducing the grid aspect ratio and by tightening the large scale wind speed nudging towards the surface. </div