Abstract

This chapter reviews the history of the discovery of cloud nuclei and their impacts on cloud microphysics and the climate system. Pioneers including John Aitken, Sir John Mason, Hilding Köhler, Christian Junge, Sean Twomey, and Kenneth Whitby laid the foundations of the field. Through their contributions and those of many others, rapid progress has been made in the last 100 years in understanding the sources, evolution, and composition of the atmospheric aerosol, the interactions of particles with atmospheric water vapor, and cloud microphysical processes. Major breakthroughs in measurement capabilities and in theoretical understanding have elucidated the characteristics of cloud condensation nuclei and ice nucleating particles and the role these play in shaping cloud microphysical properties and the formation of precipitation. Despite these advances, not all their impacts on cloud formation and evolution have been resolved. The resulting radiative forcing on the climate system due to aerosol–cloud interactions remains an unacceptably large uncertainty in future climate projections. Process-level understanding of aerosol–cloud interactions remains insufficient to support technological mitigation strategies such as intentional weather modification or geoengineering to accelerating Earth-system-wide changes in temperature and weather patterns.

Abstract

A method is described to estimate the critical supersaturation of quasi-monodisperse, dry particles using measurements of hygroscopic growth at relative humidities below 100%. Köhler theory is used to derive two chemical composition–dependent parameters, with appropriate accounting for solution effects through a simplified model of the osmotic coefficient. The two unknown chemical parameters are determined by fitting the Köhler model to data obtained from humidified tandem differential mobility analyzer (HTDMA) measurements, and used to calculate the critical supersaturation for a given dry particle size. In this work the theory and methodology are presented, and sensitivity studies are performed, with respect to assumptions made and uncertainties in key input parameters to the Köhler model.

Results show that for particle diameters of 40 and 100 nm, the average error between critical supersaturations derived using the proposed method and theoretical values is −7.5% (1σ = 10%, n = 16). This error is similar to experimental uncertainties in critical supersaturations determined from laboratory studies on particles of known chemical composition (−0.6%, 1σ = 11%, n = 16).

Abstract

Laboratory studies are used to test the method proposed in Part I for estimating the critical supersaturation of quasi-monodisperse, dry particles from measurements of hygroscopic growth at relative humidities below 100%. An advantage of the proposed technique is that it directly links dry particle size to cloud condensation nuclei (CCN) activity and simultaneously provides some information on particle chemical composition. Studies have been conducted on particles composed of NaCl, (NH₄)₂SO₄, NH₄HSO₄, internally and externally mixed NaCl–(NH₄)₂SO₄, and on ambient particles of unknown chemical composition. A modified form of the Köhler equation is fit to measurements from a humidified tandem differential mobility analyzer to derive two chemical composition–dependent parameters and the critical supersaturation for a given dry particle size. A cloud condensation nucleus counter is used to simultaneously observe the critical supersaturation of the same dry particles.

Results show that for particles composed of single salts and for diameters between 32 and 57 nm, the average agreement between critical supersaturations derived from measurements of hygroscopic growth and theoretical values of S _crit is −13% (1 σ = 8.5%, n = 9). This agreement is similar to experimental uncertainties in critical supersaturations determined from laboratory studies on particles of known chemical composition. The agreement between values of S _crit predicted by the fit technique and CCN study-derived values is poorer (−6% to −65%) for ambient particles. This is likely due to both changes in ambient particle characteristics during the study and limitations in the modified Köhler model derived in this work.

Abstract

On 11–12 September 2013, portions of northern Colorado experienced flash flooding as a result of high rain rates accumulating over 180 mm of rain in 6 h. From 0400 to 0700 UTC 12 September a mesovortex was observed traveling northwestward toward the city of Boulder, Colorado, with enhanced upslope flow on its north side and localized deep convection. Although the mesovortex was observed in an area common for lee vortex formation, namely that associated with the Denver Cyclone, it is shown via ARW model simulations that the mesovortex intensified through the release of latent heat, similar to the processes leading to mesoscale convective vortices, rather than by dry topographic-flow dynamics. High rates of cloud water condensation at relatively low altitudes led to a strong vertical gradient in latent heating, resulting in a near-surface positive potential vorticity anomaly. Reducing the contribution of cloud water condensation to latent heating by 50% resulted in no mesovortex development in the model and a substantial decrease in precipitation. On the other hand, removing the topographical forcing in the model did not inhibit the mesovortex formation, confirming the secondary role of topography. The mesovortex enhanced upslope winds and convection, and was thus a key feature in the generation of intense precipitation over Boulder. The ability to forecast the development of these mesovortices and their subsequent environmental and hydrological effects could be critical for decision-makers and the public, given their association with high rainfall rates.

Abstract

Ice formation in ammoniated sulfate and sulfuric acid aerosol particles under upper-tropospheric conditions was studied using a continuous flow thermal diffusion chamber. This technique allowed for particle exposure to controlled temperatures and relative humidities for known residence times. The phase states of (NH₄)₂SO₄ and NH₄HSO₄ particles were found to have important impacts on their ice formation capabilities. Dry (NH₄)₂SO₄ particles nucleated ice only at high relative humidity (RH ≥ 94%) with respect to water at temperatures between −40° and −60°C. This result suggested either an impedance or finite time dependence to deliquescence and subsequent homogeneous freezing nucleation. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze homogeneously at relative humidities that were 10% lower than where ice nucleated on initially dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH₄HSO₄ particles required higher relative humidities for ice nucleation than did initially liquid bisulfate particles. Liquid particles of size 0.2 μm composed of either ammonium sulfate or bisulfate froze at lower relative humidity at upper-tropospheric temperatures than did 0.05-μm sulfuric acid aerosol particles. Comparison of calculated homogeneous freezing point depressions suggest that size effects on freezing may be more important than the degree of ammoniation of the sulfate compound.

Abstract

A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between ∼3- and 10-μm diameter in 3–10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than ∼0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational procedures, and performance. Limitations of the technique are presented, and examples of measurements are shown.

Abstract

The impact of giant and ultragiant cloud condensation nuclei (>5-μm radius) on drizzle formation in stratocumuli is investigated within a number of modeling frameworks. These include a simple box model of collection, a trajectory ensemble model (comprising an ensemble of Lagrangian parcel models), a 2D eddy-resolving model, and a 3D large-eddy simulation model. Observed concentrations of giant cloud condensation nuclei (GCCN) over the ocean at ambient conditions indicate that 20-μm radius haze particles exist in concentrations of between 10⁻⁴ and 10⁻² cm⁻³, depending on ambient wind speed and seastate. It is shown that these concentrations are sufficient to move a nonprecipitating stratocumulus into a precipitating state at typical cloud condensation nucleus (CCN) concentrations of 50 to 250 cm⁻³, with higher concentrations of GCCN being required at higher CCN concentrations. However, at lower CCN concentrations, drizzle is often active anyway and the addition of GCCN has little impact. At high CCN concentrations, drizzle development is slow and GCCN have the greatest potential for enhancing the collection process. Thus, although drizzle production decreases with increasing CCN concentration, the relative impact of GCCN increases with increasing CCN concentration. It is also shown that in the absence of GCCN, a shift in the modal radius of the CCN distribution to larger sizes suppresses drizzle because larger modal radii enable the activation of larger droplet number concentrations. Finally, calculations of the impact of GCCN on cloud optical properties are performed over a range of parameter space. Results indicate that the presence of GCCN moderates the effect of CCN on optical properties quite significantly. In the absence of GCCN, an increase in CCN from 50 to 150 cm⁻³ results in a threefold increase in albedo; when GCCN exist at a concentration of 10⁻³ cm⁻³, the increase in albedo is only twofold. Thus the variable presence of GCCN represents yet another uncertainty in estimating the influence of anthropogenic activity on climate.

Abstract

Simulations of two leading-line, trailing-stratiform mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) have been used to understand the relative microphysical impacts of lower- versus midtropospheric aerosol particles (APs) on MCS precipitation. For each MCS event, four simulations were conducted in which the initial vertical location and concentrations of cloud droplet nucleating APs were varied. These simulations were used to determine the precipitation response to AP vertical location. Importantly, the total integrated number and mass of the initial aerosol profiles used in the sensitivity simulations remained constant, such that differences in the simulations could be directly attributable to changes in the vertical location of cloud droplet nucleating APs. These simulations demonstrate that lower-tropospheric APs largely influenced the precipitation response directly rearward of the leading cold pool boundary. However, farther rearward in the MCS, the relative impact of lower- versus midtropospheric APs largely depended on the MCS structure, which varied between the two events because of differences in line-normal wind shear. Midtropospheric APs were able to activate new cloud droplets in the midtropospheric levels of convective updrafts and to enhance mixed-phase precipitation through increased cloud riming, and this microphysical pathway had a more significant impact on mixed-phase precipitation in weaker line-normal wind shear conditions. This result exposes the importance of properly representing midtropospheric APs when assessing aerosol effects on clouds. This study also demonstrates the utility of assessing aerosol effects within the different regions of MCSs.

Mixed-phase stratus clouds are ubiquitous in the Arctic and play an important role in climate in this region. However, climate and regional models have generally proven unsuccessful at simulating Arctic cloudiness, particularly during the colder months. Specifically, models tend to underpredict the amount of liquid water in mixed-phase clouds. The Mixed-Phase Arctic Cloud Experiments (M-PACE), conducted from late September through October 2004 in the vicinity of the Department of Energy's Atmospheric Radiation Measurement (ARM) North Slope of Alaska field site, focused on characterizing low-level Arctic stratus clouds. Ice nuclei (IN) measurements were made using a continuous-flow ice thermal diffusion chamber aboard the University of North Dakota's Citation II aircraft. These measurements indicated IN concentrations that were significantly lower than those used in many models. Using the Regional Atmospheric Modeling System (RAMS), we show that these low IN concentrations, as well as inadequate parameterizations of the depletion of IN through nucleation scavenging, may be partially responsible for the poor model predictions. Moreover, we show that this can lead to errors in the modeled surface radiative energy budget of 10–100 Wm⁻2. Finally, using the measured IN concentrations as input to RAMS and comparing to a mixed-phase cloud observed during M-PACE, we show excellent agreement between modeled and observed liquid water content and net infrared surface flux.

Abstract

The abundance of atmospheric ice nucleating particles (INPs) is a source of uncertainty for numerical representation of ice-phase transitions in mixed-phase clouds. While sea spray aerosol (SSA) exhibits less ice nucleating (IN) ability than terrestrial aerosol, marine INP emissions are linked to oceanic biological activity and are potentially an important source of INPs over remote oceans. Inadequate knowledge of marine INP identity limits the ability to parameterize this complex INP source. A previous manuscript described abundances of marine INPs in relation to several aerosol composition and ocean biology observations during two laboratory mesocosm experiments. In this study, the abundances and chemical and physical properties of INPs found during the same mesocosm experiments were directly probed in SSA, seawater, and surface microlayer samples. Two unique marine INP populations were found: 1) dissolved organic carbon INPs are suggested to be composed of IN-active molecules, and 2) particulate organic carbon INPs are attributed as intact cells or IN-active microbe fragments. Both marine INP types are likely to be emitted into SSA following decay of phytoplankton biomass when 1) the surface microlayer is significantly enriched with exudates and cellular detritus and SSA particles are preferentially coated with IN-active molecules or 2) diatom fragments and bacteria are relatively abundant in seawater and therefore more likely transferred into SSA. These findings inform future efforts for incorporating marine INP emissions into numerical models and motivate future studies to quantify specific marine molecules and isolate phytoplankton, bacteria, and other species that contribute to these marine INP types.