Low Droplet Concentrations Research Articles

Structure of w/o-microemulsion templated polymer networks

The droplet phase of a water-in-oil microemulsion is used as a matrix to create amphiphilic polymer network structures from a poly(oxyethylene)-poly(isoprene)-poly(oxyethylene) triblock copolymer bearing polymerizable methacrylate end groups. Conductivity measurements and freeze fracture electron microscopy are used to obtain information about the structure of these gels. The droplet structure of the underlying microemulsion is only preserved for low droplet and polymer concentration. For higher concentrations the crosslinking reaction can induce the formation of a bicontinuous structure.

An Example of Supercooled Drizzle Drops Formed through a Collision-Coalescence Process

Abstract The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of −7.5° and −12°C, respectively. The cloud was characterized by a low droplet concentration of 21 cm−3 and a large droplet median volume diameter of 29 µm, with a concentration of interstitial aerosol particles of less than 15 cm−3 (larger than 0. 13 µm in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 µm observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 L−1 for droplets larger than 40 µm, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0...

Precipitation from a maritime cloud layer with very low droplet concentrations

Precipitation from a maritime cloud layer with very low droplet concentrations

Cloud Microphysical Relationships in California Marine Stratus

Abstract Cloud microphysical measurements off the southern California coast are presented and compared with in situ airborne measurements of cloud condensation nuclei (CCN) spectra. Large-scale variations in cloud droplet concentrations were due to CCN variations, some medium-scale variations may be a result of the conversion of droplets to drops by coalescence, while small-scale variations were due to different proportions of the CCN spectra being activated because of variations in updraft velocity at cloud base. This latter internal mixing process produces an inverse relationship between droplet concentration and mean size and an increase in droplet spectral width with mean droplet size. Drizzle drop concentrations are strongly associated with lower droplet concentrations, larger droplets, and greater droplet spectral width.

Conditions Associated with Large-Drop Regions

In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops. This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and large drops were considered independent of each other, was rejected to a significance level of 0.01. From this it can be inferred that large drops and wind shear are related. The depth of the shear layer was usually small, less than 150 m. The validity of the condition of low droplet concentration is questioned since several cases of large drops were found in the presence of a high droplet concentration. These cases were marked by strong wind shear.

Dissipation of Marine Stratiform Clouds and Collapse of the Marine Boundary Layer Due to the Depletion of Cloud Condensation Nuclei by Clouds

When the production of cloud condensation nuclei in the stratocumulus-topped marine boundary layer is low enough, droplet collisions can reduce concentrations of cloud droplet numbers to extremely low values. At low droplet concentrations a cloud layer can become so optically thin that cloud-top radiative cooling cannot drive vertical mixing. Under these conditions, model simulations indicate that the stratocumulus-topped marine boundary layer collapses to a shallow fog layer. Through this mechanism, marine stratiform clouds may limit their own lifetimes.

Boundary layer and heat transfer in a two-phase gas-evaporating droplet medium

An asymptotic model of the flow in the laminar boundary layer of a gas-evaporating droplet mixture is constructed within the framework of the two-continuum approximation. The case of evaporation of the droplets into an atmosphere of their own vapor is examined in detail with reference to the example of longitudinal flow over a hot flat plate. Numerical and asymptotic solutions of the boundary layer equations constructed are found for a number of limiting situations (low droplet concentration, no droplet deposition, significant droplet deposition). The development of the flow with respect to the longitudinal coordinate is studied and it is shown that in the absence of droplet deposition a region of pure vapor may be formed near the surface. Similarity criteria are established and the mechanism of surface heat transfer enhancement is studied for a low evaporating droplet concentration in the boundary layer. In the inertial deposition regime the results of calculating the integral heat transfer coefficient are found to correspond with the experimental data [1].

Quinone-containing amphiphiles and bolaform amphiphiles that form redox-active lipid membranes

We have used a time-resolved fluorescence method to investigate the structure and dynamics of water-in-oil (W/O) and oil-in-water (O/W) microemulsions stabilized by the nonionic surfactant C 12 E 5 . The measured aggregation numbers are consistent with the formation of psherical droplets with a low degree of polydispersity. The droplet sizes are in agreement with light-scattering values determined for systems close to the solubilization phase boundaries and at low droplet concentrations

Aircraft Observations of the Radiative and Microphysical Properties of Stratocumulus and Cumulus Cloud Fields

Abstract This paper reports on a series of flights that were conducted off the east coast of Australia through and over stratocumulus and fair-weather cumulus cloud fields. The CSIRO Fokker F-27 research aircraft was used to obtain radiation and in situ cloud microphysical and thermodynamical measurements. Central to the analyses presented in this paper were the measurements obtained by a spectrally scanning visible near-infrared radiometer (SPERAD) which was designed specifically for the experiments reported herein. Analyses of the data obtained during the flights that are reported in this paper showed that the clouds sampled were warm and mainly maritime in character, with both low droplet concentrations and liquid water contents. The stratiform clouds were shallow, with optical depths of about 10. Despite the lack of cloud vertical development significant concentrations of large droplets were recorded by the Knollenberg 2D probe. Variance analyses of the cloud optical properties indicated that the samp...

Sensitivity Studies on the Continentality of a Numerically Simulated Cumulonimbus

Abstract The cloud model of Tripoli and Cotton was used to simulate a cumulonimbus cloud observed during the Cooperative Convective Precipitation Experiment (CCOPE). We tested the sensitivity of the precipitation pathways in the model to the initial concentration of cloud droplets above cloud base Nc (which is related to the concentration of cloud condensation nuclei). The results showed that for large Nc, Manton and Cotton's autoconversion parameterization properly suppressed supercooled rain formation via the “warm-rain” process in a cold-based, continental cloud, forcing ice processes (e.g., riming, aggregation and deposition of vapor) to produce graupel. With lower droplet concentrations, rain formed first through warm-rain processes, then graupel formed through freezing. The value of Nc, which determined the transition from graupel formation by freezing rain to graupel formation by ice processes was found to be sensitive to the parameter acm, which represents the critical mean radius at which collisi...

The Characteristics and Distribution of Cloud Water over the Mountains of Northern Colorado during Wintertime Storms. Part II: Spatial Distribution and Microphysical Characteristics

The Physical and microphysical structure of the supercooled water fields in wintertime storms over the Park Range of the northern Colorado Rocky Mountains is examined using aircraft and ground-based measurements. Cloud top, cloud base, and zones of strong orographic lift are identified as regions in stratiform systems where supercooled water production can occur. Cloud systems over Colorado's Park Range were found to have low droplet concentrations (≪300 cm−3). In clouds with the lowest droplet concentrations (<100 cm−3), broad droplet spectra were consistently observed. Significant numbers of large (<20 μm) droplets were present in these cases. The data presented here and in Part I are used to construct conceptual models of the structure and evolution of the liquid water fields in 1) shallow cloud systems with warm cloud tops, 2) deep stratiform clouds with cold tops, and 3) deep convective regions.