scholarly journals Parameterization of Cloud Drop Activation Based on Analytical Asymptotic Solutions to the Supersaturation Equation

2009 ◽  
Vol 66 (7) ◽  
pp. 1905-1925 ◽  
Author(s):  
Vitaly I. Khvorostyanov ◽  
Judith A. Curry
Keyword(s):  
Physicochemical Properties ◽  
Vertical Velocity ◽  
Climate Models ◽  
Numerical Solutions ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Power Laws ◽  
Size Spectrum ◽  
Model Parameters ◽  
Condensation Coefficient

Abstract Toward improving parameterization of cloud droplet activation in cloud and climate models, the integro–differential equation for supersaturation is solved analytically for the algebraic size spectrum of the cloud condensation nuclei (CCN) that is equivalent to the lognormal spectrum. The analytical solutions are obtained for four limiting cases that are combinations of two different values of the updraft vertical velocity (small and large) and two different values of the condensation coefficient that correspond to pure and polluted cloud drops. The characteristics of the CCN can vary within each limit. Thus, these four limits and interpolation among them cover the vast majority of cloudy conditions. Analytical expressions are obtained for the time of CCN activation, maximum supersaturation, and the concentration of activated droplets. For small updraft vertical velocities, these quantities are the products of the power laws by six variables: CCN concentration, mean radius, soluble fraction, vertical velocities, surface tension, and condensation coefficient. At large updraft vertical velocities, the activation time and maximum supersaturation are the products of the power laws of only two variables—CCN concentration and vertical velocity—and are independent of the CCN physicochemical properties. The first limit is a generalization of the Twomey power laws, with Twomey’s coefficient CT and index k expressed via CCN physicochemical properties; the other three limits are new. The accuracy and regions of validity of these limits are determined by comparison with the exact numerical solution to the supersaturation equation. These solutions can be used for parameterization of drop activation in cloud and climate models and for control of numerical solutions. An advantage of this method is that it does not require running parcel models, and the drop concentrations can be obtained from lookup tables or as simple interpolation among the limiting solutions for the instantaneous model parameters.

scholarly journals Broadening of Cloud Droplet Spectra through Eddy Hopping: Turbulent Entraining Parcel Simulations

2018 ◽  
Vol 75 (10) ◽  
pp. 3365-3379 ◽  
Author(s):  
Gustavo C. Abade ◽  
Wojciech W. Grabowski ◽  
Hanna Pawlowska
Keyword(s):  
Droplet Size ◽  
Turbulent Mixing ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Droplet Growth ◽  
Size Spectrum ◽  
Entrainment Rate ◽  
The Mean ◽  
The Impact

This paper discusses the effects of cloud turbulence, turbulent entrainment, and entrained cloud condensation nuclei (CCN) activation on the evolution of the cloud droplet size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to entrainment events modeled as a random process. Entrainment events, subsequent turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet activation and growth by condensation are simulated using a Monte Carlo scheme. Quantities characterizing the turbulence intensity, entrainment rate, CCN concentration, and the mean fraction of environmental air entrained in an event are all specified as independent external parameters. Cloud microphysics is described by applying Lagrangian particles, the so-called superdroplets. These are either unactivated CCN or cloud droplets that grow from activated CCN. The model accounts for the addition of environmental CCN into the cloud by entraining eddies at the cloud edge. Turbulent mixing of the entrained dry air with cloudy air is described using the classical linear relaxation to the mean model. We show that turbulence plays an important role in aiding entrained CCN to activate, and thus broadening the droplet size distribution. These findings are consistent with previous large-eddy simulations (LESs) that consider the impact of variable droplet growth histories on the droplet size spectra in small cumuli. The scheme developed in this work is ready to be used as a stochastic subgrid-scale scheme in LESs of natural clouds.

scholarly journals The aerosol-cyclone indirect effect in observations and high-resolution simulations

2017 ◽  
Author(s):  
Daniel T. McCoy ◽  
Paul R. Field ◽  
Anja Schmidt ◽  
Daniel P. Grosvenor ◽  
Frida A.-M. Bender ◽  
...  
Keyword(s):  
High Resolution ◽  
Indirect Effect ◽  
Climate Models ◽  
Climate Model ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
2014 Eruption ◽  
Cloud Droplet Number Concentration ◽  
Aerosol Cloud Interactions ◽  
First Time

Abstract. Aerosol-cloud interactions are a major source of uncertainty in predicting 21st century climate change. Using high-resolution, convection-permitting global simulations we predict that increased cloud condensation nuclei (CCN) interacting with midlatitude cyclones will increase their cloud droplet number concentration (CDNC), liquid water (CLWP), and albedo. For the first time this effect is shown with 13 years of satellite observations. Causality between enhanced CCN and enhanced cyclone liquid content is supported by the 2014 eruption of Holuhraun. The change in midlatitude cyclone albedo due to enhanced CCN in a surrogate climate model is around 70 % of the change in a high-resolution convection-permitting model, indicating that climate models may underestimate this indirect effect.

scholarly journals Biomass burning aerosol as a modulator of the droplet number in the southeast Atlantic region

2020 ◽  
Vol 20 (5) ◽  
pp. 3029-3040 ◽  
Author(s):  
Mary Kacarab ◽  
K. Lee Thornhill ◽  
Amie Dobracki ◽  
Steven G. Howell ◽  
Joseph R. O'Brien ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Biomass Burning ◽  
Vertical Velocity ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Droplet Formation ◽  
Aerosol Concentration ◽  
Aerosol Size Distribution ◽  
Small Shift ◽  
Significant Difference

Abstract. The southeastern Atlantic (SEA) and its associated cloud deck, off the west coast of central Africa, is an area where aerosol–cloud interactions can have a strong radiative impact. Seasonally, extensive biomass burning (BB) aerosol plumes from southern Africa reach this area. The NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) study focused on quantitatively understanding these interactions and their importance. Here we present measurements of cloud condensation nuclei (CCN) concentration, aerosol size distribution, and characteristic vertical updraft velocity (w∗) in and around the marine boundary layer (MBL) collected by the NASA P-3B aircraft during the August 2017 ORACLES deployment. BB aerosol levels vary considerably but systematically with time; high aerosol concentrations were observed in the MBL (800–1000 cm−3) early on, decreasing midcampaign to concentrations between 500 and 800 cm−3. By late August and early September, relatively clean MBL conditions were sampled (<500 cm−3). These data then drive a state-of-the-art droplet formation parameterization from which the predicted cloud droplet number and its sensitivity to aerosol and dynamical parameters are derived. Droplet closure was achieved to within 20 %. Droplet formation sensitivity to aerosol concentration, w∗, and the hygroscopicity parameter, κ, vary and contribute to the total droplet response in the MBL clouds. When aerosol concentrations exceed ∼900 cm−3 and maximum supersaturation approaches 0.1 %, droplet formation in the MBL enters a velocity-limited droplet activation regime, where the cloud droplet number responds weakly to CCN concentration increases. Below ∼500 cm−3, in a clean MBL, droplet formation is much more sensitive to changes in aerosol concentration than to changes in vertical updraft. In the competitive regime, where the MBL has intermediate pollution (500–800 cm−3), droplet formation becomes much more sensitive to hygroscopicity (κ) variations than it does in clean and polluted conditions. Higher concentrations increase the sensitivity to vertical velocity by more than 10-fold. We also find that characteristic vertical velocity plays a very important role in driving droplet formation in a more polluted MBL regime, in which even a small shift in w∗ may make a significant difference in droplet concentrations. Identifying regimes where droplet number variability is driven primarily by updraft velocity and not by aerosol concentration is key for interpreting aerosol indirect effects, especially with remote sensing. The droplet number responds proportionally to changes in characteristic velocity, offering the possibility of remote sensing of w∗ under velocity-limited conditions.

scholarly journals Observations of clouds, aerosols, precipitation, and surface radiation over the Southern Ocean: An overview of CAPRICORN, MARCUS, MICRE and SOCRATES

2020 ◽  
pp. 1-92 ◽  
Author(s):  
Greg M. McFarquhar ◽  
Chris Bretherton ◽  
Roger Marchand ◽  
Alain Protat ◽  
Paul J. DeMott ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Climate Models ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Polar Front ◽  
Mixed Phase ◽  
Surface Radiation ◽  
Energy Budgets ◽  
Small Scale ◽  
Radiative Processes

AbstractWeather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation and radiative processes, and their interactions. Projects between 2016 and 2018 used in-situ probes, radar, lidar and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN) and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase cloudsnucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF/NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the RV Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons.Results show a largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multi-layered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

scholarly journals Reducing uncertainties in satellite estimates of aerosol-cloud interactions over the subtropical ocean by integrating vertically resolved aerosol observations

2019 ◽  
Author(s):  
David Painemal ◽  
Fu-Lung Chang ◽  
Richard Ferrare ◽  
Sharon Burton ◽  
Zhujun Li ◽  
...  
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Global Climate Models ◽  
Orthogonal Polarization ◽  
Near Surface ◽  
Aerosol Cloud ◽  
Error Characterization ◽  
Aerosol Cloud Interactions

Abstract. Satellite quantification of aerosol effects on clouds relies on aerosol optical depth (AOD) as a proxy for aerosol concentration or cloud condensation nuclei (CCN). However, the lack of error characterization of satellite-based results hampers their use for the evaluation and improvement of global climate models. We show that the use of AOD for assessing aerosol-cloud interactions (ACI) is inadequate over vast oceanic areas in the subtropics. Instead, we postulate that a more physical approach that consists of matching vertically resolved aerosol data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite at the cloud-layer height with Aqua Moderate-resolution Imaging Spectroradiometer (MODIS) cloud retrievals reduces uncertainties in satellite-based ACI estimates. Combined aerosol extinction coefficients (σ) below cloud-top (σBC) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and cloud droplet number concentrations (Nd) from Aqua-MODIS yield high correlations across a broad range of σBC values, with σBC quartile correlations > 0.78. In contrast, CALIOP-based AOD yields correlations with MODIS Nd of less than 0.62 for the two lower AOD quartiles. Moreover, σBC explains 41 % of the spatial variance in MODIS Nd, whereas AOD only explains 17 %, primarily caused by the lack of spatial covariability in the eastern Pacific. Compared with σBC, near-surface σ weakly correlates in space with MODIS Nd, accounting for a 16 % variance. It is concluded that the linear regression calculated from ln(Nd)−ln(σBC) (the standard method for quantifying ACI) is more physically meaningful than that derived from the Nd−AOD pair.

scholarly journals On the drivers of droplet variability in Alpine mixed-phase clouds

2020 ◽  
Author(s):  
Paraskevi Georgakaki ◽  
Aikaterini Bougiatioti ◽  
Jörg Wieder ◽  
Claudia Mignani ◽  
Fabiola Ramelli ◽  
...  
Keyword(s):  
Vertical Velocity ◽  
Anthropogenic Activities ◽  
Cloud Condensation Nuclei ◽  
Ultrafine Particle ◽  
Cloud Droplet ◽  
Atmospheric Model ◽  
Droplet Formation ◽  
Mixed Phase ◽  
Velocity Variations ◽  
Mixed Phase Clouds

Abstract. Droplet formation provides a direct microphysical link between aerosols and clouds (liquid or mixed phase), and its adequate description poses a major challenge for any atmospheric model. Observations are critical for evaluating and constraining the process. Towards this, aerosol size distributions, cloud condensation nuclei, hygroscopicity and lidar-derived vertical velocities were observed in Alpine mixed-phase clouds during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign in the Davos, Switzerland region during February and March 2019. Data from the mountain-top site of Weissfluhjoch (WFJ) and the valley site of Davos Wolfgang are studied. These observations are coupled with a state-of-the art droplet activation parameterization to investigate the aerosol-cloud droplet link in mixed-phase clouds. The mean CCN-derived hygroscopicity parameter, κ, at WFJ ranges between 0.2–0.3, consistent with expectations for continental aerosol. κ tends to decrease with size, possibly from an enrichment in organic material associated with the vertical transport of fresh ultrafine particle emissions (likely from biomass burning) from the valley floor in Davos. The parameterization provides droplet number that agrees with observations to within ~25 %. We also find that the susceptibility of droplet formation to aerosol concentration and vertical velocity variations can be appropriately described as a function of the standard deviation of the distribution of updraft velocities, σw, as the droplet number never exceeds a characteristic limit, termed limiting droplet number, of ~150–550 cm−3, which depends solely on σw. We also show that high aerosol levels in the valley, most likely from anthropogenic activities, increase cloud droplet number, reduce cloud supersaturation (<0.1 %) and shift the clouds to a state that is less susceptible to aerosol and become very sensitive to vertical velocity variations. The transition from aerosol to velocity-limited regime depends on the ratio of cloud droplet number to the limiting droplet number, as droplet formation becomes velocity-limited when this ratio exceeds 0.5. Under such conditions, droplet size tends to be minimal, reducing the likelihood that large drops are present that promote glaciation through rime splintering and droplet shattering. Identifying regimes where droplet number variability is dominated by dynamical – rather than aerosol – changes is key for interpreting and constraining when and which types of aerosol effects on clouds are active.

scholarly journals Kinetics of Cloud Drop Formation and Its Parameterization for Cloud and Climate Models

2008 ◽  
Vol 65 (9) ◽  
pp. 2784-2802 ◽  
Author(s):  
Vitaly I. Khvorostyanov ◽  
Judith A. Curry
Keyword(s):  
Power Law ◽  
Vertical Velocity ◽  
Climate Models ◽  
Cloud Condensation Nuclei ◽  
Activation Process ◽  
Algebraic Form ◽  
Droplet Concentration ◽  
Activity Spectrum ◽  
Aerosol Effects ◽  
Kinetics Of

Abstract To study the kinetics of drop nucleation in clouds, the integro–differential equation for integral water supersaturation in cloud is derived and analyzed. Solving the supersaturation equation with an algebraic form of the cloud condensation nuclei (CCN) activity spectrum, analytical expressions are obtained for the time tm of CCN activation process, the maximum supersaturation sm, and droplet concentration Ndr(sm), limited by the total aerosol concentration at high supersaturations. All three quantities are expressed as functions of vertical velocity and characteristics of the CCN size spectra: mean geometric radius, dispersion, and parameters of solubility. A generalized power law for the drop activation, Ndr(sm) = C(sm)sk(sm)m, is formulated that is similar in form to the Twomey power law, but both the coefficient C(sm) and index k(sm) are functions of supersaturation expressed analytically in terms of vertical velocities and CCN microphysical parameters. A simple and economical numerical solution was developed that describes all of these characteristics without conducting numerous simulations using parcel models. An extended series of numerical experiments was performed, in which the dependencies of tm, sm, Ndr(sm), C(sm), k(sm), and several other important characteristics of activation process were studied as functions of vertical velocity and physicochemical properties of the aerosol. In particular, it is shown that a decrease in the condensation coefficient αc leads to slower CCN activation and higher maximum supersaturation and droplet concentration. Uncertainties in αc may prevent correct estimates of the direct and indirect aerosol effects on climate. The solutions and expressions for the parameters presented here can be used for parameterization of the drop activation process in cloud and climate models.

scholarly journals Large-Eddy Simulations of Trade Wind Cumuli Using Particle-Based Microphysics with Monte Carlo Coalescence

2013 ◽  
Vol 70 (9) ◽  
pp. 2768-2777 ◽  
Author(s):  
Sylwester Arabas ◽  
Shin-ichiro Shima
Keyword(s):  
Monte Carlo ◽  
Droplet Size ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Large Eddy Simulations ◽  
Trade Wind ◽  
Type Model ◽  
Size Spectrum ◽  
Large Eddy ◽  
Condensational Growth

Abstract A series of simulations employing the superdroplet method (SDM) for representing aerosol, cloud, and rain microphysics in large-eddy simulations (LES) is discussed. The particle-based formulation treats all particles in the same way, subjecting them to condensational growth and evaporation, transport of the particles by the flow, gravitational settling, and collisional growth. SDM features a Monte Carlo–type numerical scheme for representing the collision and coalescence process. All processes combined cover representation of cloud condensation nuclei (CCN) activation, drizzle formation by autoconversion, accretion of cloud droplets, self-collection of raindrops, and precipitation, including aerosol wet deposition. The model setup used in the study is based on observations from the Rain in Cumulus over the Ocean (RICO) field project. Cloud and rain droplet size spectra obtained in the simulations are discussed in context of previously published analyses of aircraft observations carried out during RICO. The analysis covers height-resolved statistics of simulated cloud microphysical parameters such as droplet number concentration, effective radius, and parameters describing the width of the cloud droplet size spectrum. A reasonable agreement with measurements is found for several of the discussed parameters. The sensitivity of the results to the grid resolution of the LES, as well as to the sampling density of the probabilistic Monte Carlo–type model, is explored.

scholarly journals Reducing uncertainties in satellite estimates of aerosol–cloud interactions over the subtropical ocean by integrating vertically resolved aerosol observations

2020 ◽  
Vol 20 (12) ◽  
pp. 7167-7177 ◽  
Author(s):  
David Painemal ◽  
Fu-Lung Chang ◽  
Richard Ferrare ◽  
Sharon Burton ◽  
Zhujun Li ◽  
...  
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Global Climate Models ◽  
Orthogonal Polarization ◽  
Near Surface ◽  
Aerosol Cloud ◽  
Error Characterization ◽  
Aerosol Cloud Interactions

Abstract. Satellite quantification of aerosol effects on clouds relies on aerosol optical depth (AOD) as a proxy for aerosol concentration or cloud condensation nuclei (CCN). However, the lack of error characterization of satellite-based results hampers their use for the evaluation and improvement of global climate models. We show that the use of AOD for assessing aerosol–cloud interactions (ACIs) is inadequate over vast oceanic areas in the subtropics. Instead, we postulate that a more physical approach that consists of matching vertically resolved aerosol data from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite at the cloud-layer height with Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua cloud retrievals reduces uncertainties in satellite-based ACI estimates. Combined aerosol extinction coefficients (σ) below cloud top (σBC) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and cloud droplet number concentrations (Nd) from MODIS Aqua yield high correlations across a broad range of σBC values, with σBC quartile correlations ≥0.78. In contrast, CALIOP-based AOD yields correlations with MODIS Nd of 0.54–0.62 for the two lower AOD quartiles. Moreover, σBC explains 41 % of the spatial variance in MODIS Nd, whereas AOD only explains 17 %, primarily caused by the lack of spatial covariability in the eastern Pacific. Compared with σBC, near-surface σ weakly correlates in space with MODIS Nd, accounting for a 16 % variance. It is concluded that the linear regression calculated from ln(Nd)–ln(σBC) (the standard method for quantifying ACIs) is more physically meaningful than that derived from the Nd–AOD pair.

scholarly journals On the drivers of droplet variability in alpine mixed-phase clouds

2021 ◽  
Vol 21 (14) ◽  
pp. 10993-11012
Author(s):  
Paraskevi Georgakaki ◽  
Aikaterini Bougiatioti ◽  
Jörg Wieder ◽  
Claudia Mignani ◽  
Fabiola Ramelli ◽  
...  
Keyword(s):  
Vertical Velocity ◽  
Anthropogenic Activities ◽  
Cloud Condensation Nuclei ◽  
Ultrafine Particle ◽  
Cloud Droplet ◽  
Atmospheric Model ◽  
Droplet Formation ◽  
Mixed Phase ◽  
Velocity Variations ◽  
Mixed Phase Clouds

Abstract. Droplet formation provides a direct microphysical link between aerosols and clouds (liquid or mixed-phase), and its adequate description poses a major challenge for any atmospheric model. Observations are critical for evaluating and constraining the process. To this end, aerosol size distributions, cloud condensation nuclei (CCN), hygroscopicity, and lidar-derived vertical velocities were observed in alpine mixed-phase clouds during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign in the Davos, Switzerland, region during February and March 2019. Data from the mountain-top site of Weissfluhjoch (WFJ) and the valley site of Davos Wolfgang are studied. These observations are coupled with a state-of-the-art droplet activation parameterization to investigate the aerosol–cloud droplet link in mixed-phase clouds. The mean CCN-derived hygroscopicity parameter, κ, at WFJ ranges between 0.2–0.3, consistent with expectations for continental aerosols. κ tends to decrease with size, possibly from an enrichment in organic material associated with the vertical transport of fresh ultrafine particle emissions (likely from biomass burning) from the valley floor in Davos. The parameterization provides a droplet number that agrees with observations to within ∼ 25 %. We also find that the susceptibility of droplet formation to aerosol concentration and vertical velocity variations can be appropriately described as a function of the standard deviation of the distribution of updraft velocities, σw, as the droplet number never exceeds a characteristic limit, termed the “limiting droplet number”, of ∼ 150–550 cm−3, which depends solely on σw. We also show that high aerosol levels in the valley, most likely from anthropogenic activities, increase the cloud droplet number, reduce cloud supersaturation (< 0.1 %), and shift the clouds to a state that is less susceptible to changes in aerosol concentrations and very sensitive to vertical velocity variations. The transition from an aerosol to velocity-limited regime depends on the ratio of cloud droplet number to the limiting droplet number, as droplet formation becomes velocity limited when this ratio exceeds 0.65. Under such conditions, droplet size tends to be minimal, reducing the likelihood that large drops are present that would otherwise promote glaciation through rime splintering and droplet shattering. Identifying regimes where droplet number variability is dominated by dynamical – rather than aerosol – changes is key for interpreting and constraining when and which types of aerosol effects on clouds are active.

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