John C. Carstens

Condensation Coefficient Measurement for Water in the UMR Cloud Simulation Chamber

Abstract A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.

Analysis of the Heat and Vapor Propagation from the Walls of the Nolan, Pollak and Gardner Type Condensation Nucleus Counters

Abstract The measurement of the temperature drop in a dry Pollak condensation nucleus counter by Israel and Nix has been interpreted as an indication that the expansion process does not yield as high a supersaturation as predicted by thermodynamics. An analysis of the heat and vapor diffusion from the walls of the chamber indicates that the counter does indeed develop the supersaturations predicted by thermodynamics in the absence of dropwise condensation and that the natural sensitive time is of the order of 0.3 sec. The measurements of Israel and Nix can be explained in terms of a thermal analysis of the thermocouple itself. The seemingly rapid response of the thermocouple is an indication of the attainment of the steady-state heat flow from the thermocouple and is not an indication that the thermocouple is reading the temperature of the ambient gas accurately. Moreover, the anomalous temperature drop observed about 1 sec after the expansion marks the point at which convection currents sweep away the he...

Theory of Droplet Growth in Clouds: II. Diffusional Interaction Between Two Growing Droplets

Abstract The interaction of two growing droplets in a supersaturated atmosphere has been examined, and the temperature and vapor density profiles have been determined. It is found that the smaller droplet tends to “catch up” with the larger at a slower rate than predicted by conventional diffusion theory. Consideration of droplet fallspeeds leads to the conclusion that, under atmospheric conditions, growth interaction becomes significant only for droplet “pairs” having equal or nearly equal radii. The number of such pairs is generally small enough so that the effect on the size distribution is quite small. Of a much greater importance is the possibility of a resulting attractive diffusio-phoretic force between two growing drops which, in turn, gives rise to a net velocity of one drop toward the other. If this diffusion force of attraction becomes sufficiently strong to overcome the hydrodynamic and thermo-phoretic forces acting in the opposite direction, both collision efficiencies and coagulation of smal...

Measurement of Growth Rate to Determine Condensation Coefficients for Water Drops Grown on Natural Cloud Nuclei

Abstract Growth rate measurements were made for water drops grown on nuclei in atmospheric air samples taken in Rolla, Missouri. Rolla, having a population of 15,000 and very little industry, is relatively free of urban pollutants. The measurements were made in a vertical flow thermal diffusion chamber at supersaturations of 0.5 and 1%. The time to grow from near dry radius to the final radius (6 to 7.5 µm) was measured. If one assumes the thermal accommodation coefficient is unity, the measurements indicate an average value of 0.026 for the condensation coefficient. The temperature ranged from 22.5 to 25.7°C.

Drop growth in the atmosphere by condensation: Application to cloud physics

Abstract A practical theory of drop growth by condensation is discussed with a view toward its meteorological (mainly cloud physics) applications; the size range contemplated extends to about thirty microns. The working equation is derived in some detail and presented as an interpolation between the continuum and free molecular regimes of the the Knudsen number. A number of important consequences of the theory are identified. Its immediate application to cloud growth is discussed.

On the Analysis of the Condensational Growth of a Stationary Cloud Droplet in the Vicinity of Activation

Abstract The theory of the growth (or evaporation) of a stationary drop has been cast into a form which reveals explicitly the role of both condensation and thermal accommodation coefficients. The equations for growth are integrated under constant ambient conditions. An application to cloud physics is discussed.

University of Missouri–Rolla cloud simulation facility: Proto II chamber

The Graduate Center for Cloud Physics Research at UMR has developed a cloud simulation facility to study phenomena occurring in terrestrial clouds and fogs. The facility consists of a pair of precision cooled‐wall expansion chambers along with extensive supporting equipment. The smaller of these chambers, described in this article, is fully operational, and is capable of simulating a broad range of in‐cloud thermodynamic conditions. It is currently being used to study water drop growth and evaporation for drops nucleated (activated) on well‐characterized aerosol particles. Measurements have been made not only for continuous expansions (simulated updraft) but also for cyclic conditions, i.e., sequences of expansion‐compression cycles resulting in alternating drop growth and evaporation. The larger of the two cloud chambers is nearing completion and will provide a broader range of conditions than the smaller chamber. The facility is supported by a fully implemented aerosol laboratory which routinely produce...

Theory of droplet growth in clouds. I. The transient stage of the boundary-coupled simultaneous heat and mass transport in cloud formation☆

Abstract Two solutions to the system of equations describing the simultaneous heat and mass transport involved in the condensational growth of a droplet in a supersaturated atmosphere are presented. The first, valid for very short times, describes the transient stage of such growth; the second, valid for longer times, presupposes the establishment of a steady-state condition. The two are shown to be complementary for the cases examined. The equations examined satisfy the usual boundary conditions imposed on a drop in a concentric sphere as required by the cellular model for cloud formation. Hence our results can be immediately extended to the treatment of the growth rate of drops in assemblage.

Numerical Simulation of a Widely Used Cloud Nucleus Counter

Abstract The performance of the conventional horizontal plate thermal diffusion chamber as a cloud nucleus counter was studied. Numerical calculations were performed in order to follow the simultaneous drop growth and sedimentation in the spatially and temporally nonuniform supersaturation field. The effect of nuclei distribution and smallest detectable drop size were investigated. The results indicate order of magnitude uncertainty in the count at 0.1% supersaturation, and a factor of 2 uncertainty at 1.0% supersaturation.

Some Remarks on Modeling of the Early Stage of Cloud Formation in a Simulation Chamber

Abstract A numerical model has been developed to describe the early stage of cloud formation in a relatively small simulation chamber. The results for adiabatic expansion show a tendency for the cloud droplet spectrum to narrow, similar to the results obtained by other authors. The influence of updraft fluctuations is not as important as the fluctuation of temperature which depends upon the amplitude and frequency of the fluctuations, and the expansion rate of volume. Simple models of the sedimentation of the drops show that sedimentation might be a factor limiting the rate of expansion. Nuclei suddenly introduced into the cloud parcel indicate that the seeding effect depends significantly on the concentration and size of predominantly larger nuclei.