Growth Of Cloud Droplets Research Articles

Droplet Growth in Warm Water Clouds Observed by the A-Train. Part I: Sensitivity Analysis of the MODIS-Derived Cloud Droplet Sizes

Abstract This study examines the sensitivity of the retrieved cloud droplet radii (CDR) to the vertical inhomogeneity of droplet radii, including the existence of a drizzle mode in clouds. The focus of this study is warm water-phase clouds. Radiative transfer simulations of three near-infrared Moderate Resolution Imaging Spectroradiometer (MODIS) channels centered on wavelengths of 1.6, 2.1, and 3.7 μm reveal that the retrieved CDR are strongly influenced by the vertical inhomogeneity of droplet size including (i) the existence of small cloud droplets at the cloud top and (ii) the existence of the drizzle mode. The influence of smaller droplets at cloud top affects the 3.7-μm channel most, whereas the presence of drizzle influences radiances of both the 2.1- and 1.6-μm channels more than the 3.7-μm channel. Differences in the CDR obtained from MODIS 1.6-, 2.1-, and 3.7-μm channels that appear in global analysis of MODIS retrievals and the CDR derived from data collected during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) intensive observation period in 1987 can be explained by the results obtained from the sensitivity experiments of this study.

Investigation of cloud condensation nuclei properties and droplet growth kinetics of the water‐soluble aerosol fraction in Mexico City

We present hygroscopic and cloud condensation nuclei (CCN) relevant properties of the water‐soluble fraction of Mexico City aerosol collected upon filters during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign. Application of κ‐Köhler theory to the observed CCN activity gave a fairly constant hygroscopicity parameter (κ = 0.28 ± 0.06) regardless of location and organic fraction. Köhler theory analysis was used to understand this invariance by separating the molar volume and surfactant contributions to the CCN activity. Organics were found to depress surface tension (10–15%) from that of pure water. Daytime samples exhibited lower molar mass (∼200 amu) and surface tension depression than nighttime samples (∼400 amu); this is consistent with fresh hygroscopic secondary organic aerosol (SOA) condensing onto particles during peak photochemical hours, subsequently aging during nighttime periods of high relative humidity. Changes in surface tension partially compensate for shifts in average molar volume to give the constant hygroscopicity observed, which implies the amount (volume fraction) of soluble material in the parent aerosol is the key composition parameter required for CCN predictions. This finding, if applicable elsewhere, may explain why CCN predictions are often found to be insensitive to assumptions of chemical composition and provides a very simple way to parameterize organic hygroscopicity in atmospheric models (i.e., κorg = 0.28ɛWSOC). Special care should be given, however, to surface tension depression from organic surfactants, as its nonlinear dependence with organic fraction may introduce biases in observed (and predicted) hygroscopicity. Finally, threshold droplet growth analysis suggests the water‐soluble organics do not affect activation kinetics.

A Hybrid Bulk–Bin Approach to Model Warm-Rain Processes

Abstract This paper presents a hybrid approach to model warm-rain processes, merging the diverse schemes of bulk and detailed (bin) microphysics. In the bulk scheme, the key assumption is that the exact saturation is maintained inside a cloud. In contrast, the supersaturation inside a cloud is predicted in the bin scheme and is applied to calculate the diffusional growth of cloud droplets. Predicting the supersaturation is numerically cumbersome, however, and typically requires spatial and temporal resolutions that are significantly higher than those that can be applied in the bulk scheme. At the same time, supersaturations inside clouds are small, and the condensate amounts in bulk and bin schemes differ insignificantly. This critical observation forms a starting point for the hybrid bulk–bin approach. In this approach, when the cloud water first appears, the activation scheme inserts cloud droplets at the low end of the bin representation. Subsequent diffusional and eventually accretional growth shift the spectrum toward larger sizes so that the saturation inside a cloud is maintained. Details of the hybrid approach are discussed in this paper, and the validation against the traditional bin scheme in a framework of the adiabatic rising parcel is presented. Before the scheme can be applied to the multidimensional cloud model, a 1D advection–condensation problem of Grabowski and Smolarkiewicz is used to address the issue of the numerical difficulties that finite-difference schemes experience near cloud edges. In the bulk case, these are in the form of condensation rate overshoots and undershoots; and this aspect requires special attention in the hybrid scheme. A novel approach is developed that provides a physically consistent solution near cloud edges using the hybrid bulk–bin scheme. The key is to allow grid boxes near the edges to be partly cloudy and to include spectral changes of cloud droplets that take this into account. Application of the hybrid scheme to an idealized 2D problem of moist thermal rising from rest and producing rain illustrates the application of the scheme to practical problems of cloud dynamics and warm-rain microphysics.

Cloud Droplet Growth by Condensation in Homogeneous Isotropic Turbulence

AbstractThe growth of cloud droplets by diffusion of water vapor in a three-dimensional homogeneous isotropic turbulent flow is considered. Within a simple model of advection and condensation, the dynamics and growth of millions of droplets are integrated. A droplet-size spectra broadening is obtained and it is shown to increase with the Reynolds number of turbulence by means of two series of direct numerical simulations at increasing resolution. This is a key point toward a proper evaluation of the effects of turbulence for condensation in warm clouds, where the Reynolds numbers typically achieve extreme values. The obtained droplet spectral broadening as a function of the Reynolds number is shown to be consistent with dimensional arguments. A generalization of this expectation to Reynolds numbers not accessible by direct numerical simulation (DNS) is proposed, yielding upper and lower bounds to the actual size spectra broadening. It is argued that the lower bound is the relevant limit at high Reynolds numbers. A further DNS matching the large scales of the system suggests consistency of the picture drawn. The assumptions underlying the model are expected to hold up to spatial scales on the order of 100 m; no direct comparison with in situ measures is possible. Additional effort is needed to evaluate the impact of this effect for condensation in more realistic cloud conditions.

Cloud condensation nuclei activity, closure, and droplet growth kinetics of Houston aerosol during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

In situ cloud condensation nuclei (CCN) measurements were obtained in the boundary layer over Houston, Texas, during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) campaign onboard the CIRPAS Twin Otter. Polluted air masses in and out of cloudy regions were sampled for a total of 22 flights, with CCN measurements obtained for 17 of these flights. In this paper, we focus on CCN closure during two flights, within and downwind of the Houston regional plume and over the Houston Ship Channel. During both flights, air was sampled with particle concentrations exceeding 25,000 cm−3 and CCN concentrations exceeding 10,000 cm−3. CCN closure is evaluated by comparing measured concentrations with those predicted on the basis of measured aerosol size distributions and aerosol mass spectrometer particle composition. Different assumptions concerning the internally mixed chemical composition result in average CCN overprediction ranging from 3% to 36% (based on a linear fit). It is hypothesized that the externally mixed fraction of the aerosol contributes much of the CCN closure scatter, while the internally mixed fraction largely controls the overprediction bias. On the basis of the droplet sizes of activated CCN, organics do not seem to impact, on average, the CCN activation kinetics.

Diffusional and accretional growth of water drops in a rising adiabatic parcel: effects of the turbulent collision kernel

Abstract. A large set of rising adiabatic parcel simulations is executed to investigate the combined diffusional and accretional growth of cloud droplets in maritime and continental conditions, and to assess the impact of enhanced droplet collisions due to small-scale cloud turbulence. The microphysical model applies the droplet number density function to represent spectral evolution of cloud and rain/drizzle drops, and various numbers of bins in the numerical implementation, ranging from 40 to 320. Simulations are performed applying two traditional gravitational collection kernels and two kernels representing collisions of cloud droplets in the turbulent environment, with turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3. The overall result is that the rain initiation time significantly depends on the number of bins used, with earlier initiation of rain when the number of bins is low. This is explained as a combination of the increase of the width of activated droplet spectrum and enhanced numerical spreading of the spectrum during diffusional and collisional growth when the number of model bins is low. Simulations applying around 300 bins seem to produce rain at times which no longer depend on the number of bins, but the activation spectra are unrealistically narrow. These results call for an improved representation of droplet activation in numerical models of the type used in this study. Despite the numerical effects that impact the rain initiation time in different simulations, the turbulent speedup factor, the ratio of the rain initiation time for the turbulent collection kernel and the corresponding time for the gravitational kernel, is approximately independent of aerosol characteristics, parcel vertical velocity, and the number of bins used in the numerical model. The turbulent speedup factor is in the range 0.75–0.85 and 0.60–0.75 for the turbulent kinetic energy dissipation rates of 100 and 400 cm2 s−3, respectively.

The Effect of Water Evaporation on Photoacoustic Signals in Transition and Molecular Flow

Evaporation of water changes the response of photoacoustic instruments to light-absorbing particles. Previous calculations of this effect are valid for particles much larger than the mean free path of air. These calculations are extended here to include transition and molecular flow as well as various mass accommodation coefficients for water. For commonly encountered conditions, evaporation can significantly reduce the photoacoustic signal if the mass accommodation coefficient of water on aerosol particles is larger than about 0.01. Unlike the growth of cloud droplets, the photoacoustic signal is very sensitive to changes in the accommodation coefficient between 0.1 and 1. This may provide a way to measure large accommodation coefficients. For a given accommodation coefficient, the change in the photoacoustic signal depends more on absolute than relative humidity. To minimize the effects of evaporation it is better to remove water from the air rather than reduce relative humidity with heating.

Direct and indirect impacts of Saharan dust acting as cloud condensation nuclei on tropical cyclone eyewall development

The mechanisms by which Saharan dust acting as cloud condensation nuclei (CCN) impact tropical cyclone (TC) evolution were examined by conducting numerical simulations of a mature TC with CCN added from lateral boundaries. CCN can affect eyewall development directly through release of latent heat when activated and subsequent growth of cloud droplets and indirectly through modulating rainband development. Convection in the rainbands was negatively correlated with that in the eyewall in all simulations. The development of rainbands tended to promote latent heat release away from the eyewall, block the surface inflow and enhance cold pools. The maximum impact of rainbands on the eyewall (or vice versa) occurred with a time lag of 3.5 to 5.5 hr. The convection in the eyewall and rainbands did not show a monotonic relationship to input CCN due to the non‐linear feedback of heating from a myriad of microphysical processes on storm dynamics.

Relating CCN activity, volatility, and droplet growth kinetics of β-caryophyllene secondary organic aerosol

Abstract. This study investigates the droplet formation characteristics of secondary organic aerosol (SOA) formed during the ozonolysis of sesquiterpene β-caryophyllene (with and without hydroxyl radicals present). Emphasis is placed on understanding the role of semi-volatile material on Cloud Condensation Nucleus (CCN) activity and droplet growth kinetics. Aging of β-caryophyllene SOA significantly affects all CCN-relevant properties measured throughout the experiments. Using a thermodenuder and two CCN instruments, we find that CCN activity is a strong function of temperature (activation diameter at ~0.6% supersaturation: 100±10 nm at 20°C and 130±10 nm at 35°C), suggesting that the hygroscopic fraction of the SOA is volatile. The water-soluble organic carbon (WSOC) is extracted from the SOA and characterized with Köhler Theory Analysis (KTA); the results suggest that the WSOC is composed of low molecular weight (<200 g mol−1) slightly surface-active material that constitute 5–15% of the SOA mass. These properties are similar to the water-soluble fraction of monoterpene SOA, suggesting that predictive understanding of SOA CCN activity requires knowledge of the WSOC fraction but not its exact speciation. Droplet growth kinetics of the CCN are found to be strongly anticorrelated with WSOC fraction, suggesting that the insoluble material in the SOA forms a kinetic barrier that delays droplet growth. Overall, volatilization effects can increase activation diameters by 30%, and depress droplet growth rate by a factor of two; these results may have important implications for the droplet formation characteristics of SOA, and the atmospheric relevance of CCN measurements carried out at temperatures different from ambient.

Turbulent collision efficiency of heavy particles relevant to cloud droplets

The collision efficiency of sedimenting cloud droplets in a turbulent air flow is a key input parameter in predicting the growth of cloud droplets by collision–coalescence. In this study, turbulent collision efficiency was directly computed, using a hybrid direct numerical simulation (HDNS) approach (Ayala et al 2007 J. Comput. Phys. 225 51–73). The HDNS results show that air turbulence enhances the collision efficiency partly due to the fact that aerodynamic interactions (AIs) become less effective in reducing the relative motion of droplets in the presence of background air turbulence. The level of increase in the collision efficiency depends on the flow dissipation rate and the droplet size ratio. For example, the collision efficiency between droplets of 18 and 20 μm in radii is increased by air turbulence (relative to the stagnant air case) by a factor of 4 and 1.6 at dissipation rates of 400 and 100 cm2 s−3, respectively. The collision efficiency for self-collisions in a bidisperse turbulent suspension can be larger than one. Such an increase in self-collisions is related to the far-field many-body AI and depends on the volumetric concentration of droplets. The total turbulent enhancement agrees qualitatively with previous results, but differs on a quantitative level. In the case of cross-size collisions between 18 and 20 μm droplets, the total turbulent enhancement can be a factor of 7 and 2 at dissipation rates of 400 and 100 cm2 s−3, respectively. For intermediate size ratios (0.2<a2/a1<0.8), the overall enhancement typically falls below 2. Scaling arguments show that the overall enhancement factor tends to peak at the two limiting cases of a2/a1→1 and a2/a1→0.

Growth of Cloud Droplets by Turbulent Collision–Coalescence

Abstract An open question in cloud physics is how rain forms in warm cumulus as rapidly as it is sometimes observed. In particular, the growth of cloud droplets across the size gap from 10 to 50 μm in radius has not been fully explained. In this paper, the authors investigate the growth of cloud droplets by collision–coalescence, taking into account both the gravitational mechanism and several enhancements of the collision–coalescence rate due to air turbulence. The kinetic collection equation (KCE) is solved with an accurate bin integral method and a newly developed parameterization of turbulent collection kernel derived from direct numerical simulation of droplet-laden turbulent flows. Three other formulations of the turbulent collection kernel are also considered so as to assess the dependence of the rain initiation time on the nature of the collection kernel. The results are compared to the base case using the Hall hydrodynamical–gravitational collection kernel. Under liquid water content and eddy dissipation rate values typical of small cumulus clouds, it is found that air turbulence has a significant impact on the collection kernel and thus on the time required to form drizzle drops. With the most realistic turbulent kernel, the air turbulence can shorten the time for the formation of drizzle drops by about 40% relative to the base case, applying measures based on either the radar reflectivity or the mass-weighted drop size. A methodology is also developed to unambiguously identify the three phases of droplet growth, namely, the autoconversion phase, the accretion phase, and the larger hydrometeor self-collection phase. The important observation is that even a moderate enhancement of collection kernel by turbulence can have a significant impact on the autoconversion phase of the growth.

Effect of solute dissolution kinetics on cloud droplet formation: Extended Köhler theory

This study focuses on the importance of solute dissolution kinetics for cloud droplet formation. To comprehensively account for the kinetics, a numerical model of the process was developed. Simulations of cloud droplet growth were performed for solute diffusivity, droplet growth rates, dry particle and droplet diameters relevant for ambient conditions. Simulations suggest that high ambient supersaturations and low solute diffusivity are major contributors to significant decreases in effective solute surface concentrations during droplet growth. The numerical simulations were incorporated into Köhler theory to assess the impact of dissolution kinetics on the droplet equilibrium vapor pressure. The modified Köhler theory implies that only CCN with slowly dissolving solute could have a “dynamical” equilibrium saturation ratio that is appreciably different from that obtained using thermodynamic equilibrium arguments alone.

Surfactant properties of atmospheric and model humic‐like substances (HULIS)

Surface‐active organics such as humic‐like substances (HULIS) are abundant in aerosol particles and can lower the surface tension of cloud droplets forming on secondary organic and biomass burning aerosols. How fast is the diffusion of these species, relative to the time scale of cloud droplet growth? Here we report surface tension measurements of solutions containing HULIS extracted from smoke and pollution aerosol particles as well those of molecular weight‐fractionated aquatic fulvic acids. Diffusion coefficients are estimated based on the Gibbs adsorption isotherms. The results suggest that HULIS diffusion to the surface of forming droplets is typically more rapid than the time scale of droplet growth so that cloud microphysical properties are affected.

Simultaneous Measurement of Condensation and Thermal Accommodation Coefficients for Cloud Droplet Growth in Due Consideration of a New Moving Surface-Boundary Effect

Abstract A droplet growth theory that describes a new effect of vapor and temperature field shift due to the growth-based movement of droplet surface boundary (moving boundary effect) was derived and found to enhance the growth rate as a function of supersaturation (S − 1) and droplet radius (a) in second order. The theory plays the role of a missing link and resolves the gap that exists among the measured data; the high (S − 1) used was found to overestimate thermal accommodation and condensation coefficients, α and β. The theory provided the basis for bettering the measurement and correction for data analyzed without the moving boundary effect, and suggested the broadening effect in the droplet size spectrum by accelerating the growth on the larger end together with the growth slowdown effect of α, β on the smaller. Measurements employing a horizontal-flow thermal diffusion chamber and the Mie scattering method using a He–Ne laser for droplet size estimation at temperature T = 277 K, (S − 1) = 0.32%, a ≈ 2 μm under two pressures of modulation, p = 100 and 860 hPa, with correction of the moving boundary effect for nuclei of NaCl and (NH4)2SO4, resulted in the simultaneous determination of α = 0.81 ± 0.07 and β = 0.043 ± 0.016 on average. For paper smoke, α = 0.68 and β = 0.022. Comparison with other data of the low moving boundary effect suggests a slight β increase with temperature lowering. Based on the result of the present measurement and other theoretical and experimental works, the mechanism of β being much smaller than α was suggested mostly as a kinetic process controlled by the removal rate of the released latent heat at the moment of molecular impact on the surface and vice versa for evaporation.

Temperature Dependence of Evaporation Coefficient for Water Measured in Droplets in Nitrogen under Atmospheric Pressure

Abstract The evaporation and the thermal accommodation coefficients for water in nitrogen were investigated by means of the analysis of evaporation of pure water droplet as a function of temperature. The droplet was levitated in an electrodynamic trap placed in a climatic chamber. The levitation time was in the range of seconds, which corresponds to the characteristic time scales of cloud droplet growth. Droplet radius evolution and evaporation dynamics were studied as a function of temperature, by analyzing the angle-resolved light scattering Mie interference patterns. A model of droplet evolution, accounting for the kinetic effects near the droplet surface, was applied. The evaporation coefficient for the temperature range from 273.6 to 298.3 K was found to be between 0.054 and 0.12 with a minimum of 0.036 ± 0.015 seemingly coinciding with water maximum density at 277.1 K. The average value of thermal accommodation coefficient over the temperature range from 277 to 289 K was found to be 0.7 ± 0.2.

Take a glass of water [1]... – concepts from physical chemistry used in describing the behaviour of aerosol and cloud droplets –

This is the text of a lesson explaining the concepts of kinetic gas theory, diffusion theory and thermodynamics used in describing the formation and growth of aerosol and cloud droplets in the atmosphere. The classical Koehler theory and its extensions are derived in a step-wise approach, starting from clean laboratory systems and ending with the polluted atmosphere. The meaning of the Koehler theory is analysed. Some comments follow regarding the central role the theory still plays in atmospheric research today; it indeed forms a formal framework to link air pollution and climate change.

Condensation of water vapor: Experimental determination of mass and thermal accommodation coefficients

Experimental determinations of mass and thermal accommodation coefficients αm and αt for condensation of water vapor in air have been conducted covering a temperature range from about 250 to 290 K. For the first time, both coefficients have been determined directly and simultaneously. To this end, growth of water droplets in air has been observed at different total gas pressures ranging from about 1000 down to 100 hPa. Monodispersed seed particles have been used as condensation nuclei. After addition of water vapor with well‐defined partial vapor pressure, supersaturation was achieved by adiabatic expansion in an expansion chamber. Most experiments reported in the present paper were performed at vapor saturation ratios ranging from 1.30 to 1.50. Monodispersed Ag particles with a diameter of 9 nm have been used as condensation nuclei, and for humidification a diffusion humidifier was applied. One experiment was performed at the saturation ratio of 1.02, which resembles conditions observed in the Earth's lower atmosphere. In this experiment, monodispersed DEHS particles with a diameter of 80 nm were used as condensation nuclei, and water vapor was generated by quantitative evaporation of a liquid jet. Droplet growth was monitored using the CAMS method. For determination of the accommodation coefficients, experimental droplet growth curves were compared to corresponding theoretical curves. Quantitative comparison was performed by varying the respective accommodation coefficient and the starting time of droplet growth in a two‐parameter best fit procedure. Considering the uncertainty with respect to the starting time of droplet growth and the uncertainties of the experimental water vapor supersaturation, corresponding maximum errors have been determined. From the results obtained it can be stated that αt is larger than 0.85 over the whole considered temperature range. For 250–270 K, values of αm below 0.8 are excluded, and for higher temperatures up to 290 K we can exclude values of αm below 0.4. Both coefficients are likely to be unity for all studied conditions. The results of this study enable accurate predictions of the formation and growth of cloud droplets required to parameterize cloud light scattering/absorption and precipitation properties in climate models.

Effects of stochastic coalescence and air turbulence on the size distribution of cloud droplets

An open question in warm rain process and precipitation formation is how rain forms in warm cumulus as rapidly as it has sometimes been observed. In general, the rapid growth of cloud droplets across the size gap from 10 to 50 μm in radius has not been fully explained. Three aspects related to the air turbulence and stochastic coalescence are considered here in an attempt to resolve this open question. The first is the enhanced geometric collision rates caused by air turbulence. The second is the effect of air turbulence on collision efficiencies. The third is stochastic fluctuations and correlations in the collision–coalescence process. Rigorous approaches are developed to address these issues. Preliminary results indicate that turbulence could shorten the time for drizzle formation to about a half of the time needed for the same growth process based on hydrodynamic–gravitational mechanism alone. To address the effect of stochastic correlations, we derive and validate a true stochastic coalescence equation. It is hoped that this new mean field equation will be useful in the future to improve the deterministic kinetic collection equation.

Brownian drift‐diffusion model for evolution of droplet size distributions in turbulent clouds

Effects from turbulence‐induced fluctuations in water vapor saturation on cloud droplet growth are examined using a Brownian diffusion model. The model predicts diffusive broadening of the droplet size distribution, tempered by enhanced evaporation‐induced drift of droplets to smaller size from vapor depletion, and approach to a stationary condition (Weibull distribution) determined by the balance between size‐space diffusion and drift. Monte Carlo simulations of the approach to the stationary limit and of the distribution itself are presented and compared favorably with observation. A key turbulence parameter required by the kinetic potential theory of drizzle formation is estimated using the new results.

On phase transition in compressible flows: modelling and validation

A physical model for compressible flows with phase transition is described, in which all the processes of phase transition, i.e. nucleation, droplet growth, droplet evaporation and de-nucleation, are incorporated. The model is focused on dilute mixtures of vapour and droplets in a carrier gas with typical maximum liquid mass fraction smaller than 0.02. The new model is based on a reinterpretation of Hill's method of moments of the droplet size distribution function. Starting from the general dynamic equation, it is emphasized that nucleation or de-nucleation correspond to the rates at which droplets enter or leave droplet size space, respectively. Nucleation and de-nucleation have to be treated differently in agreement with their differences in physical nature. Attention is given to the droplet growth model that takes into account Knudsen effects and temperature differences between droplets and gas. The new phase transition model is then combined with the Euler equations and results in a new numerical method: ASCE2D. The numerical method is first applied to the problem of shock/expansion wave formation in a closed shock tube with humid nitrogen as a driver gas. Nucleation and droplet growth are induced by the expansion wave, and in turn affect the structure of the expansion wave. When the main shock, reflected from the end wall of the low-pressure section, passes the condensation zone, evaporation and de-nucleation occur. As a second example, the problem of the flow of humid nitrogen in a pulse-expansion wave tube, designed to study nucleation and droplet growth in monodisperse clouds, is investigated experimentally and numerically.