Low Droplet Concentrations Research Articles

Recognition of Inter‐Cloud Versus Intra‐Cloud Controls on Droplet Dispersion With Applications to Microphysics Parameterization

AbstractAerosols in the atmosphere can have a large impact on weather and climate. One of its lesser‐known impacts is the aerosol‐dispersion effect, which refers to the feedback between the aerosol (or cloud droplet) concentration and the relative dispersion of the droplet size distribution. This study attempts to reconcile the inconsistent sign of the correlation between aerosol number and droplet dispersion found in past studies by distinguishing local correlation from cloud‐mean correlation. We find that while the cloud‐mean correlation between aerosol concentration and droplet size relative dispersion is largely nonexistent, in‐cloud regions with high relative dispersion are typically associated with low local droplet concentration. Even stronger correlation is found with respect to the local activation fraction (the ratio of droplet concentration to total droplet and aerosol concentration). Based on this local correlation, we propose a new equation to diagnose the shape parameter in marine stratocumulus clouds for double moment bulk schemes in cloud‐resolving or large‐eddy simulations. This new equation is more physically realistic than many existing ones that are based on the weak cloud‐mean correlation.

How Well Do Large‐Eddy Simulations and Global Climate Models Represent Observed Boundary Layer Structures and Low Clouds Over the Summertime Southern Ocean?

AbstractClimate models struggle to accurately represent the highly reflective boundary layer clouds overlying the remote and stormy Southern Ocean. We use in situ aircraft observations from the Southern Ocean Clouds, Radiation and Aerosol Transport Experimental Study (SOCRATES) to evaluate Southern Ocean clouds in a cloud‐resolving large‐eddy simulation (LES) and two coarse resolution global atmospheric models, the CESM Community Atmosphere Model (CAM6) and the GFDL Atmosphere Model (AM4), run in a nudged hindcast framework. We develop six case studies from SOCRATES data which span the range of observed cloud and boundary layer properties. For each case, the LES is run once forced purely using reanalysis data (fifth generation European Centre for Medium‐Range Weather Forecasts atmospheric reanalysis, “ERA5 based”) and once strongly nudged to an aircraft profile(“Obs based”). The ERA5‐based LES can be compared with the global models, which are also nudged to reanalysis data and are better for simulating cumulus. The Obs‐based LES closely matches an observed cloud profile and is useful for microphysical comparisons and sensitivity tests and simulating multilayer stratiform clouds. We use two‐moment Morrison microphysics in the LES and find that it simulates too few frozen particles in clouds occurring within the Hallett‐Mossop temperature range. We tweak the Hallett‐Mossop parameterization so that it activates within boundary layer clouds, and we achieve better agreement between observed and simulated microphysics. The nudged global climate models (GCMs) simulate liquid‐dominated mixed‐phase clouds in the stratiform cases but excessively glaciate cumulus clouds. Both GCMs struggle to represent two‐layer clouds, and CAM6 has low droplet concentrations in all cases and underpredicts stratiform cloud‐driven turbulence.

Low‐CCN concentration air masses over the eastern North Atlantic: Seasonality, meteorology, and drivers

AbstractA 20 month cloud condensation nucleus concentration (NCCN) data set from Graciosa Island (39°N, 28°W) in the remote North Atlantic is used to characterize air masses with low cloud condensation nuclei (CCN) concentrations. Low‐CCN events are defined as 6 h periods with mean NCCN<20 cm−3 (0.1% supersaturation). A total of 47 low‐CCN events are identified. Surface, satellite, and reanalysis data are used to explore the meteorological and cloud context for low‐CCN air masses. Low‐CCN events occur in all seasons, but their frequency was 3 times higher in December–May than during June–November. Composites show that many of the low‐CCN events had a common meteorological basis that involves southerly low‐level flow and rather low wind speeds at Graciosa. Anomalously low pressure is situated to the west of Graciosa during these events, but back trajectories and lagged SLP composites indicate that low‐CCN air masses often originate as cold air outbreaks to the north and west of Graciosa. Low‐CCN events were associated with low cloud droplet concentrations (Nd) at Graciosa, but liquid water path (LWP) during low‐CCN events was not systematically different from that at other times. Satellite Nd and LWP estimates from MODIS collocated with Lagrangian back trajectories show systematically lower Nd and higher LWP several days prior to arrival at Graciosa, consistent with the hypothesis that observed low‐CCN air masses are often formed by coalescence scavenging in thick warm clouds, often in cold air outbreaks.

The impact of ground‐based glaciogenic seeding on clouds and precipitation over mountains: A case study of a shallow orographic cloud with large supercooled droplets

AbstractThis paper examines the impact of ground‐based glaciogenic seeding on a shallow, lightly precipitating orographic cloud with rather large (~35 µm) supercooled droplets. The storm was observed on 22 February 2012 as part of the AgI (silver iodide) Seeding Cloud Impact Investigation experiment in Wyoming. The cloud base (top) temperature was about −5°C (−12°C). Vertical velocity data from an airborne Doppler W‐band (3 mm) profiling Wyoming Cloud Radar (WCR) indicate broad ascent due to the strong wind (20 m s‐1) impinging on the terrain and small pockets of intense updrafts. The large droplets, low droplet and ice particle concentrations, and strong updrafts lead to natural snow growth mainly by accretion (riming). The treated (seeded) period is compared with the preceding untreated period. The main target site, located on a mountain pass, was impacted by AgI seeding, according to a trace chemistry analysis of the falling snow. Data from three radar systems were used in the analysis of the impact of seeding on snow growth: the WCR, two Ka‐band (1.2 cm) profiling Micro Rain Radars , and an X‐band (3 cm) scanning polarization Doppler‐on‐Wheels radar. This case is complicated somewhat by a natural increase in cloud liquid water and in snow growth by riming, starting halfway during the seeding period, and continuing after seeding ended. Composite data from the centimeter‐wave radar systems indicate an increase in low‐level reflectivity during seeding, even after accounting for the natural trend observed in the upwind control region. A precipitation particle probe at the main target site shows an increase in concentration of both small and large hydrometeors.

A global process-based study of marine CCN trends and variability

Abstract. Low-level clouds have a strong climate-cooling effect in oceanic regions due to the much lower albedo of the underlying sea surface. Marine clouds typically have low droplet concentrations, making their radiative properties susceptible to changes in cloud condensation nucleus (CCN) concentrations. Here, we use the global aerosol model GLOMAP to investigate the processes that determine variations in marine CCN concentrations, and focus especially on the effects of previously identified wind speed trends in recent decades. Although earlier studies have found a link between linear wind speed trends and CCN concentration, we find that the effects of wind speed trends identified using a dynamic linear model in the Northern Equatorial Pacific (0.56 m s−1 per decade in the period 1990–2004) and the North Atlantic (−0.21 m s−1 per decade) are largely dampened by other processes controlling the CCN concentration, namely nucleation scavenging and transport of continental pollution. A CCN signal from wind speed change is seen only in the most pristine of the studied regions, i.e. over the Southern Ocean, where we simulate 3.4 cm−3 and 0.17 m s−1 increases over the 15-year period in the statistical mean levels of CCN and wind speed, respectively. Our results suggest that future changes in wind-speed-driven aerosol emissions from the oceans can probably have a climate feedback via clouds only in the most pristine regions. On the other hand, a feedback mechanism via changing precipitation patterns and intensities could take place over most oceanic regions, as we have shown that nucleation scavenging has by far the largest absolute effect on CCN concentrations.

Untangling dynamical and microphysical controls for the structure of stratocumulus

AbstractStratocumulus clouds can assume closed‐ or open‐cell structures with strikingly different abilities to reflect solar radiation. While open cells have been linked to the presence of precipitation and low droplet concentrations, complete understanding of processes leading to their formation is lacking. Here we show that the structure of stratocumulus can be linked to two time scales: an updraft time scale (tup) and a rain initiation time scale (tr). When sufficient drizzle forms within updrafts (tr ≤ tup), cloud water in the outflow is depleted enough that an overcast cloud cannot be sustained. Using a simple parcel model, we relate these time scales to three observable parameters (droplet number concentration, cloud depth, and updraft speed) and derive a functional representation for the transition from closed to open cells. Eight well‐documented observed and simulated cases of open‐ and closed‐cell stratocumulus fit well into the classification based on our model.

Modulation of food texture using controlled heteroaggregation of lipid droplets: Principles and applications

ABSTRACTThis article reviews the development and potential application of novel food‐grade materials based on heteroaggregation of oppositely charged particles. These particles typically consist of a hydrophobic lipid core and an electrically charged emulsifier shell. Heteroaggregation leads to the formation of emulsion‐based products that are highly viscous or gel‐like at much lower droplet concentrations than in nonaggregated systems. Heteroaggregation may therefore be useful for developing food products with novel textural characteristics or for creating reduced‐calorie versions of full fat foods. We give an overview of the principles of heteroaggregation, the relationship between emulsion microstructure and texture, and possible commercial applications of this method. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3833–3841, 2013

ICONE19-43772 DROPLET DEPOSITION RATE IN VERTICAL ANNULAR TWO-PHASE FLOW

Droplet entrainment and deposition phenomena plays important role in dynamics of annular two-phase flow. Modeling of these phenomena is essential for the estimation of dryout margins in light water reactors and boilers. In this study, the droplet entrainment and deposition rates are measured in adiabatic vertical annular two-phase flow. Two types of experiments are performed: air-water and organic fluid (Freon-113) experiments. Liquid film extraction technique is used for the measurement of entrainment fraction (i.e., amount of entrained droplets), droplet entrainment rate, and deposition rate. Additionally, in air-water experiments, liquid film thickness is measured using ring type conductance probes. This paper presents analysis of the deposition rate data. Several mechanistic and empirical correlations are available for prediction of droplet deposition rate; however, due to lack of complete understanding of the deposition phenomenon, the mechanistic correlations are still not as accurate as the empirical correlation. Assessment of the most recently published empirical deposition rate correlation (Okawa, et al., 2005) using the experimental data obtained in this study and the data available in literature shows that the correlation could not predict the data correspond to high gas velocity conditions and in a transition region between the low and high droplet concentrations. The experimental data indicates that the deposition rate approaches a maximum limiting value at high gas velocity; however, the correlation predicted continuously increasing or decreasing deposition rates. Detailed understanding of the deposition mechanism under high gas velocity is developed based on the study of droplet dynamics.

Southeast Pacific stratocumulus clouds, precipitation and boundary layer structure sampled along 20° S during VOCALS-REx

Abstract. Multiplatform airborne, ship-based, and land-based observations from 16 October–15 November 2008 during the VOCALS Regional Experiment (REx) are used to document the typical structure of the Southeast Pacific stratocumulus-topped boundary layer and lower free troposphere on a~transect along 20° S between the coast of Northern Chile and a buoy 1500 km offshore. Strong systematic gradients in clouds, precipitation and vertical structure are modulated by synoptically and diurnally-driven variability. The boundary layer is generally capped by a strong (10–12 K), sharp inversion. In the coastal zone, the boundary layer is typically 1 km deep, fairly well mixed, and topped by thin, nondrizzling stratocumulus with accumulation-mode aerosol and cloud droplet concentrations exceeding 200 cm−3. Far offshore, the boundary layer depth is typically deeper (1600 m) and more variable, and the vertical structure is usually decoupled. The offshore stratocumulus typically have strong mesoscale organization, much higher peak liquid water paths, extensive drizzle, and cloud droplet concentrations below 100 cm−3, sometimes with embedded pockets of open cells with lower droplet concentrations. The lack of drizzle near the coast is not just a microphysical response to high droplet concentrations; smaller cloud depth and liquid water path than further offshore appear comparably important. Moist boundary layer air is heated and mixed up along the Andean slopes, then advected out over the top of the boundary layer above adjacent coastal ocean regions. Well offshore, the lower free troposphere is typically much drier. This promotes strong cloud-top radiative cooling and stronger turbulence in the clouds offshore. In conjunction with a slightly cooler free troposphere, this may promote stronger entrainment that maintains the deeper boundary layer seen offshore. Winds from ECMWF and NCEP operational analyses have an rms difference of only 1 m s−1 from collocated airborne leg-mean observations in the boundary layer and 2 m s−1 above the boundary layer. This supports the use of trajectory analysis for interpreting REx observations. Two-day back-trajectories from the 20° S transect suggest that eastward of 75° W, boundary layer (and often free-tropospheric) air has usually been exposed to South American coastal aerosol sources, while at 85° W, neither boundary-layer or free-tropospheric air has typically had such contact.

The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?

Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM). The strength of the second aerosol indirect effect simulated by the two model types agrees within 50% for cases in which the LES-simulated boundary layer remains well mixed, if the MLM entrainment closure includes the effects of cloud droplet sedimentation. To achieve this agreement, parameters in the MLM entrainment closure and the drizzle parameterization must be retuned to match the LES. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a comparable decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and parameterized microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates the central assumption of an MLM, substantially limiting the range of conditions that MLM-predicted sensitivities to droplet concentration are meaningful.

An Investigation of the Influence of Droplet Number Concentration and Giant Aerosol Particles upon Supercooled Large Drop Formation in Wintertime Stratiform Clouds

Abstract Supercooled large drops (SLD) can be a significant hazard for aviation. Past studies have shown that warm-rain processes are prevalent, or even dominant, in stratiform clouds containing SLD, but the primary factors that control SLD production are still not well understood. Giant aerosol particles have been shown to accelerate the formation of the first drizzle drops in some clouds and thus are a viable source of SLD, but observational support for testing their effectiveness in supercooled stratiform clouds has been lacking. In this study, new observations collected during six research flights from the Alliance Icing Research Study II (AIRS II) are analyzed to assess the factors that may be relevant to SLD formation, with a particular emphasis on the importance of giant aerosol particles. An initial comparison of observed giant aerosol particle number concentrations with the observed SLD suggests that they were present in sufficient numbers to be the source of the SLD. However, microphysical calculations within an adiabatic parcel model, initialized with the observed aerosol distributions and cloud properties, suggest that the giant aerosol particles were only a limited source of SLD. More SLD was produced in the modeled clouds with low droplet concentrations, simply by an efficient warm-rain process acting at temperatures below 0°C. For cases in which the warm-rain process is limited by a higher droplet concentration and small cloud depth/liquid water content, the giant aerosol particles were then the only source of SLD. The modeling results are consistent with the observed trends in SLD across the six AIRS II cases.

An airborne icing characterization probe: nephelometer prototype

The aeronautical industry uses airborne probes to characterize icing conditions for flightcertification purposes by counting and sizing cloud droplets. Existing probes havebeen developed for meteorologists in order to study cloud microphysics. Theyare used on specific aircraft, instrumented for this type of study, but are notadapted for an industrial flight test environment. The development by Airbus ofa new probe giving a real time response for particle sizes between 10 and500 µm, adapted to operational requirements, is in progress. An optical principle bycoherent shadowgraphy with a low coherency point source is used for the application.The size of the droplets is measured from their shadows on a CCD. A pulsedlaser coupled to a fast camera freezes the movement. Usually, image processingrejects out-of-focus objects. Here, particles far from the focal plane can be sizedbecause of the large depth of field due to the point source. The technique usedincreases the depth of field and the sampled volume is enough to build a histogrameven for low droplet concentrations. Image processing is done in real time andresults are provided to the flight test engineer. All data and images are recorded inorder to allow on-ground complementary analysis if necessary. A non-telescopicprototype has been tested in a wind tunnel and in flight. The definitive probebeing retractable is designed to be easily installed through a dummy window.Retracted, it will allow the aircraft to fly at VMO (maximum operating limitspeed).

Modeling of Droplet Separation Kinetics in Granular Bed Filters Based on MRI Measurements

AbstractIn droplet separation by granular bed filters, the transient loading regime plays an important role because, for gases with low droplet concentrations, steady state will be reached only after a long time. A mathematical model describing this transient loading regime as well as steady state was developed. It is based on differential balances for the dispersed droplets and for the separated liquid. The time‐dependent and spatially resolved liquid loading is calculated and compared with data obtained by magnetic resonance imaging (MRI), a noninvasive measuring technique. The good agreement between simulated and measured loading curves proves the general applicability of the model.

Droplet Spectra Measurements with the FSSP-100. Part I: Low Droplet Concentration Measurements

Abstract Entrainment-mixing processes such as droplet spectrum broadening and its impact on the onset of precipitation are, among many others, physical phenomena whose experimental study relies on in-cloud airborne measurements. For more than 30 yr, droplet sizing has been performed mostly with the Forward Scattering Spectrometer Probe (FSSP-100). The probe performance was extensively studied and probabilistic procedures were developed for characterizing the transfer function of the probe and correcting instrumental artifacts in the measurement of both the total droplet concentration and the droplet spectral shape. These procedures are revised here with a focus on low concentration measurements and the spectral distortion that may be generated by the real-time pulse processing, when the actual droplet spectrum is broad. The transfer matrix method is extended to account for this instrumental artifact.

Emulsiones tipo crema preparadas a base de leche de soja 3: Efecto de ciclos de temperatura

In this work, the effect of temperature cycling on emulsions formulated with reconstituted soy milk, sunflower oil and high melting point milk fat was analysed. The emulsions were prepared at 60 oC , stored at 4 oC (3 h), warmed at 40 oC or 50 oC (30 min) and afterwards recooled at 4 oC (2h). Although solid fat content of emulsions did not raised after tempering at 40 oC , increase of G' and G'' was observed only in those formulated with 40 % lipid phase. This phenomenon was attributed to droplet aggregation due to partial melting, recrystallization and reordering of fat crystals in oil droplets. The main droplet destabilization mechanism was the partial coalescence; it was corroborated by nuclear magnetic resonance, differential scanning calorimetry and droplet size distribution studies. On the other hand, the consistency of 20 % of lipid phase emulsion did not increased. Low droplet concentration disfavoured the partial coalescence of droplets. Similar assays on cream at 40 oC and cream-like soy milk emulsions at 50 oC , did not show rheological modifications, due to almost total melting of milk fat during tempering.

Correlations for the mass transfer rate of droplets in vertical upward annular flow

New correlations for the deposition rate and entrainment rate of droplets in vertical upward annular flow were developed from simple models and available experimental data. In the correlation for the deposition rate, the superficial gas velocity was used as the parameter of primary importance at low droplet concentration while the droplet concentration itself at high droplet concentration. In correlating the rate of droplet entrainment, the ratio of interfacial shear force to the surface tension force acting on the surface of liquid film was the appropriate scaling parameter to correlate the experimental data measured in varied conditions. The experimental data for air–water annular flow were used in the development of the present correlations since extensive databases were available. It was however confirmed that the present model provides satisfactory agreements with the experimental data for high-pressure steam–water annular flow.

Enhancement of cloud cover and suppression of nocturnal drizzle in stratocumulus polluted by haze

Recent satellite observations indicate a significant decrease of cloud water in ship tracks, in contrast to an ensemble of in situ ship‐track measurements showing no average change in cloud water relative to the surrounding clouds, and contrary to the expectation of cloud water increasing in polluted clouds. We find through large‐eddy simulations of stratocumulus that the trend in the satellite data is likely an artifact of sampling only overcast clouds. The simulations instead show cloud cover increasing with droplet concentrations. The simulations also show that increases in cloud water from suppressing drizzle by increasing droplet concentrations are favored at night or at extremely low droplet concentrations.

Microphysical relationships in warm clouds

Relationships among cloud microphysical parameters from six aircraft projects carried out over different parts of the globe in different seasons are presented. Comparisons of project averages showed that projects with higher droplet (2–50 μm diameter) concentrations generally had smaller droplets and less drizzle (diameter >50 μm). Hence, when all projects were combined there were negative correlations between droplet concentrations and sizes. However, this was not the case for individual cloud penetrations where only 24% had negative droplet concentration–size correlations. For the stratocumulus clouds, these negative correlations were more likely to be found in clouds with greater droplet spectral broadening and drizzle. The small cumulus clouds in this study were unique in that 95% of the droplet concentration–size correlations were positive regardless of spectral broadening or drizzle. Thus, for the majority of clouds, droplets were smaller in parcels with lower droplet concentrations. Entrainment in these clouds was then consistent with homogeneous mixing that occurs locally and intermittently. The dearth of observations of larger droplets in diluted cloud parcels suggested that mixing of entrained air did not promote droplet growth.

Microstructures of low and middle‐level clouds over the Beaufort Sea

AbstractAirborne measurements in low and middle‐level clouds over the Beaufort Sea in April 1992 and June 1995 show that these clouds often have low droplet concentrations (<100 cm3) and relatively large effective droplet radii. The highest average droplet concentrations overall were measured in altocumulus clouds that formed in airflows from the south that passed either over the North American continent or were from Asia. Droplet concentrations in low clouds tended to be higher in April than in June. The low clouds in June occasionally contained drops as large as 35 μm diameter; in these clouds the collision‐coalescence process was active and produced regions of extensive drizzle. Cloud‐top droplet concentrations were significantly correlated with aerosols beneath their bases, but appeared to be relatively unaffected by aerosols above their tops. Anthropogenic sources around Deadhorse, Alaska, increased local cloud droplet concentrations.Ice particle concentrations were generally low in April, but high ice particle concentrations were encountered in June when cloud‐top temperatures were considerably higher. On two days in June, tens per litre of columnar and needle ice crystals were measured in stratocumulus with top temperatures between −4 and −9 °C. Ice particle concentrations were poorly correlated with temperature (r = 0.39) but, for the two data sets as a whole, the concentrations of ice particles tended to increase with increasing temperature from −30 to −4.5 °C. Ice particle concentrations correlated better with the size of the largest droplets (r = 0.61).The most common mixed‐phased cloud structure encountered was a cloud topped by liquid water that precipitated ice. Liquid‐water topped clouds were observed down to temperatures of −31 °C. They are likely common in the Arctic, and may play an important role in the radiation balance of the region.Temperature lapse rates in the clouds were generally complex, reflecting either layering, due to differential advection, or radiational effects. In these cases, vertical profiles of liquid‐water content and effective cloud droplet radius did not vary systematically with height above cloud base, as they do in well‐mixed clouds. However, when the temperature in a cloud decreased with height at or very near the pseudo‐adiabatic lapse rate, the cloud liquid‐water content (and various measures of the broadness of the cloud droplet size distribution) generally increased monotonically with height until very close to cloud top.For clouds consisting entirely of droplets, or droplets and ice crystals, cloud coverage increased by about 10% when the definition of a cloud was changed from 10 to 5 droplets per cubic centimetre. For clouds containing ice particles, cloud coverage and/or cloud depth increased by about 40% when the definition of a cloud was changed from 1 to 0.1 ice particles per litre.The significance of these observations with respect to the effects of clouds on the radiation budget of the Arctic, and the potential for modification of arctic clouds by pollution, are discussed.

Preferential Concentration of Cloud Droplets by Turbulence: Effects on the Early Evolution of Cumulus Cloud Droplet Spectra

Abstract A mechanism is presented, based on the inherent turbulent nature of cumulus clouds, for the broadening of cloud droplet spectra during condensational growth. This mechanism operates independent of entrainment and, therefore, can operate in adiabatic cloud cores. Cloud droplets of sufficient size are not randomly dispersed in a cloud but are preferentially concentrated in regions of low vorticity in the turbulent flow field. Regions of high vorticity (low droplet concentration) develop higher supersaturation than regions of low vorticity (high droplet concentration). Therefore, on small spatial scales cloud droplets are growing in a strongly fluctuating supersaturation field. These fluctuations in supersaturation exist independent of large-scale vertical velocity fluctuations. Droplets growing in regions of high vorticity will experience enhanced growth rates, allowing some droplets to grow larger than predicted by the classic theory of condensational growth. This mechanism helps to account for tw...