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.

Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds

Science ◽  
2019 ◽  
Vol 363 (6427) ◽  
pp. eaav0566 ◽  
Author(s):  
Daniel Rosenfeld ◽  
Yannian Zhu ◽  
Minghuai Wang ◽  
Youtong Zheng ◽  
Tom Goren ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
Cloud Cover ◽  
Climate Models ◽  
Cloud Condensation Nuclei ◽  
Climate Forcing ◽  
Present Climate ◽  
Cloud Radiative Forcing ◽  
Cloud Properties ◽  
Aerosol Effects ◽  
Meteorological Effects

A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds.

The Role of Small Soluble Aerosols in the Microphysics of Deep Maritime Clouds

2012 ◽  
Vol 69 (9) ◽  
pp. 2787-2807 ◽  
Author(s):  
A. P. Khain ◽  
V. Phillips ◽  
N. Benmoshe ◽  
A. Pokrovsky
Keyword(s):  
Vertical Velocity ◽  
Cloud Condensation Nuclei ◽  
Synergetic Effect ◽  
Supercooled Water ◽  
Ice Crystals ◽  
Cloud Base ◽  
Droplet Concentration ◽  
High Concentrations ◽  
Drop Size Distributions

Abstract Some observational evidence—such as bimodal drop size distributions, comparatively high concentrations of supercooled drops at upper levels, high concentrations of small ice crystals in cloud anvils leading to high optical depth, and lightning in the eyewalls of hurricanes—indicates that the traditional view of the microphysics of deep tropical maritime clouds requires, possibly, some revisions. In the present study it is shown that the observed phenomena listed above can be attributed to the presence of small cloud condensation nuclei (CCN) with diameters less than about 0.05 μm. An increase in vertical velocity above cloud base can lead to an increase in supersaturation and to activation of the smallest CCN, resulting in production of new droplets several kilometers above the cloud base. A significant increase in supersaturation can be also caused by a decrease in droplet concentration during intense warm rain formation accompanied by an intense vertical velocity. This increase in supersaturation also can trigger in-cloud nucleation and formation of small droplets. Another reason for an increase in supersaturation and in-cloud nucleation can be riming, resulting in a decrease in droplet concentration. It has been shown that successive growth of new nucleated droplets increases supercooled water content and leads to significant ice crystal concentrations aloft. The analysis of the synergetic effect of the smallest CCN and giant CCN on production of supercooled water and ice crystals in cloud anvils allows reconsideration of the role of giant CCN. Significant effects of small aerosols on precipitation and cloud updrafts have been found. The possible role of these small aerosols as well as small aerosols with combination of giant CCN in creating conditions favorable for lightning in deep maritime clouds is discussed.

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 Relationship between Shortwave Cloud Radiative Forcing and Local Meteorological Variables Compared in Observations and Several Global Climate Models

2006 ◽  
Vol 19 (17) ◽  
pp. 4344-4359 ◽  
Author(s):  
Markus Stowasser ◽  
Kevin Hamilton
Keyword(s):  
Relative Humidity ◽  
Vertical Velocity ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Grid Point ◽  
Global Climate Models ◽  
Coupled Models ◽  
Cloud Radiative Forcing ◽  
Cloud Forcing

Abstract The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolved-scale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability. Overall there is considerable variation among the results for the various models, and all models show some substantial differences from the comparable observed results. The most notable deficiency is a weak representation of the cloud radiative response to variations in vertical velocity in cases of strong ascending or strong descending motions. While the models generally perform better in regimes with only modest upward or downward motions, even in these regimes there is considerable variation among the models in the dependence of SWCRF on vertical velocity. The largest differences between models and observations when SWCRF values are stratified by relative humidity are found in either very moist or very dry regimes. Thus, the largest errors in the model simulations of cloud forcing are prone to be in the western Pacific warm pool area, which is characterized by very moist strong upward currents, and in the rather dry regions where the flow is dominated by descending mean motions.

scholarly journals The effects of aerosols on precipitation and dimensions of subtropical clouds: a sensitivity study using a numerical cloud model

2006 ◽  
Vol 6 (1) ◽  
pp. 67-80 ◽  
Author(s):  
A. Teller ◽  
Z. Levin
Keyword(s):  
Water Vapor ◽  
Climate Models ◽  
Cloud Model ◽  
Cloud Condensation Nuclei ◽  
Sensitivity Study ◽  
Meteorological Conditions ◽  
Dust Particles ◽  
Noticeable Effect ◽  
Convective Clouds ◽  
Ice Nuclei

Abstract. Numerical experiments were carried out using the Tel-Aviv University 2-D cloud model to investigate the effects of increased concentrations of Cloud Condensation Nuclei (CCN), giant CCN (GCCN) and Ice Nuclei (IN) on the development of precipitation and cloud structure in mixed-phase sub-tropical convective clouds. In order to differentiate between the contribution of the aerosols and the meteorology, all simulations were conducted with the same meteorological conditions. The results show that under the same meteorological conditions, polluted clouds (with high CCN concentrations) produce less precipitation than clean clouds (with low CCN concentrations), the initiation of precipitation is delayed and the lifetimes of the clouds are longer. GCCN enhance the total precipitation on the ground in polluted clouds but they have no noticeable effect on cleaner clouds. The increased rainfall due to GCCN is mainly a result of the increased graupel mass in the cloud, but it only partially offsets the decrease in rainfall due to pollution (increased CCN). The addition of more effective IN, such as mineral dust particles, reduces the total amount of precipitation on the ground. This reduction is more pronounced in clean clouds than in polluted ones. Polluted clouds reach higher altitudes and are wider than clean clouds and both produce wider clouds (anvils) when more IN are introduced. Since under the same vertical sounding the polluted clouds produce less rain, more water vapor is left aloft after the rain stops. In our simulations about 3.5 times more water evaporates after the rain stops from the polluted cloud as compared to the clean cloud. The implication is that much more water vapor is transported from lower levels to the mid troposphere under polluted conditions, something that should be considered in climate models.

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 Machine learning uncovers aerosol size information from chemistry and meteorology to quantify potential cloud-forming particles

2021 ◽  
Author(s):  
Arshad Nair ◽  
Fangqun Yu ◽  
Pedro Campuzano Jost ◽  
Paul DeMott ◽  
Ezra Levin ◽  
...  
Keyword(s):  
Climate Change ◽  
Machine Learning ◽  
Radiative Forcing ◽  
Climate Models ◽  
Cloud Condensation Nuclei ◽  
Global Climate Models ◽  
Aerosol Composition ◽  
Airborne Measurements ◽  
Aerosol Cloud ◽  
Aerosol Cloud Interactions

Abstract Cloud condensation nuclei (CCN) are mediators of aerosol–cloud interactions, which contribute to the largest uncertainty in climate change prediction. Here, we present a machine learning/artificial intelligence model that quantifies CCN from variables of aerosol composition, atmospheric trace gases, and meteorology. Comprehensive multi-campaign airborne measurements, covering varied physicochemical regimes in the troposphere, confirm the validity of and help probe the inner workings of this machine learning model: revealing for the first time that different ranges of atmospheric aerosol composition and mass correspond to distinct aerosol number size distributions. Machine learning extracts this information, important for accurate quantification of CCN, additionally from both chemistry and meteorology. This can provide a physicochemically explainable, computationally efficient, robust machine learning pathway in global climate models that only resolve aerosol composition; potentially mitigating the uncertainty of effective radiative forcing due to aerosol–cloud interactions (ERFaci) and improving confidence in assessment of anthropogenic contributions and climate change projections.

scholarly journals Supersaturation calculation in large eddy simulation models for prediction of the droplet number concentration

2012 ◽  
Vol 5 (3) ◽  
pp. 761-772 ◽  
Author(s):  
O. Thouron ◽  
J.-L. Brenguier ◽  
F. Burnet
Keyword(s):  
Cloud Condensation Nuclei ◽  
Simulation Models ◽  
Number Concentration ◽  
Eddy Simulation ◽  
Droplet Concentration ◽  
Large Eddy ◽  
Condensed Water ◽  
Les Model ◽  
Adjustment Scheme ◽  
Cloud Core

Abstract. A new parameterization scheme is described for calculation of supersaturation in LES models that specifically aims at the simulation of cloud condensation nuclei (CCN) activation and prediction of the droplet number concentration. The scheme is tested against current parameterizations in the framework of the Meso-NH LES model. It is shown that the saturation adjustment scheme, based on parameterizations of CCN activation in a convective updraft, overestimates the droplet concentration in the cloud core, while it cannot simulate cloud top supersaturation production due to mixing between cloudy and clear air. A supersaturation diagnostic scheme mitigates these artefacts by accounting for the presence of already condensed water in the cloud core, but it is too sensitive to supersaturation fluctuations at cloud top and produces spurious CCN activation during cloud top mixing. The proposed pseudo-prognostic scheme shows performance similar to the diagnostic one in the cloud core but significantly mitigates CCN activation at cloud top.

scholarly journals Revisiting particle dry deposition and its role in radiative effect estimates

2020 ◽  
Vol 117 (42) ◽  
pp. 26076-26082
Author(s):  
Ethan W. Emerson ◽  
Anna L. Hodshire ◽  
Holly M. DeBolt ◽  
Kelsey R. Bilsback ◽  
Jeffrey R. Pierce ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Radiative Forcing ◽  
Particle Deposition ◽  
Climate Models ◽  
Transport Model ◽  
Cloud Condensation Nuclei ◽  
Measurement Techniques ◽  
Chemical Transport ◽  
Radiative Effect ◽  
Accumulation Mode

Wet and dry deposition remove aerosols from the atmosphere, and these processes control aerosol lifetime and thus impact climate and air quality. Dry deposition is a significant source of aerosol uncertainty in global chemical transport and climate models. Dry deposition parameterizations in most global models were developed when few particle deposition measurements were available. However, new measurement techniques have enabled more size-resolved particle flux observations. We combined literature measurements with data that we collected over a grassland in Oklahoma and a pine forest in Colorado to develop a dry deposition parameterization. We find that relative to observations, previous parameterizations overestimated deposition of the accumulation and Aitken mode particles, and underestimated in the coarse mode. These systematic differences in observed and modeled accumulation mode particle deposition velocities are as large as an order of magnitude over terrestrial ecosystems. As accumulation mode particles form most of the cloud condensation nuclei (CCN) that influence the indirect radiative effect, this model-measurement discrepancy in dry deposition alters modeled CCN and radiative forcing. We present a revised observationally driven parameterization for regional and global aerosol models. Using this revised dry deposition scheme in the Goddard Earth Observing System (GEOS)-Chem chemical transport model, we find that global surface accumulation-mode number concentrations increase by 62% and enhance the global combined anthropogenic and natural aerosol indirect effect by −0.63 W m−2. Our observationally constrained approach should reduce the uncertainty of particle dry deposition in global chemical transport models.

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