One of the confounding links in the chain of microphysical processes occurring in atmospheric clouds at temperatures below 0 degress C is the growth of ice particles from the vapor phase. Recent laboratory measurements suggest that the mass accommodation coefficient, a measure of growth efficiency, is very small for small ice crystals. Numerical modeling studies have shown that the simulated concentrations of ice particles and supersaturations in cirrus clouds, as well as the rates of glaciation of mixed-phase clouds, all depend sensitively on the assumed mass accommodation coefficient. If the mass accommodation coefficient is low, then growing ice particles do not deplete the supersaturation of water vapor as effectively as they would if the coefficient was higher. Then supersaturations reach higher levels in ascending cloud parcels, and more ice particles are nucleated on suitable aerosol particles due to the higher supersaturations. The increase in concentration of smaller growing ice particles eventually depletes the supersaturated vapor and the cloud reaches a mature state with higher concentrations of smaller particles than would be the case if the mass accommodation coefficient were higher.
This integrated laboratory-modeling study is focused on the early growth of ice from the vapor phase in order to reduce uncertainties in past measurements and simulations, and to test hypotheses regarding the molecular mechanisms of vapor deposition. Both laboratory techniques and numerical modeling capabilities have matured greatly in recent years, now permitting significant new progress to be made. The laboratory methods make use of electrodynamic levitation to isolate individual ice particles from system walls and permit particle growth to be followed indefinitely under precisely controlled environmental conditions. New data on ice growth rates as functions of size and supersaturation will help constrain the mathematical representation of ice growth in cloud models. A suite of numerical models will be used in conjunction with the laboratory data to guide the laboratory work, interpret the experimental findings in terms of mechanisms, and provide a means for extrapolating our laboratory results to cloud-scale systems. The synergism afforded by this laboratory-modeling study will allow new light to be shed on ice processes that currently are limiting capability to simulate cold-cloud evolution accurately.
This research has potentially broad impacts on the atmospheric sciences and society. Improved understanding of microphysical evolution of cold clouds will enhance understanding of the roles played by clouds in weather and climate processes. The research will train graduate students and give advanced undergraduate students exposure to modern research methods in modeling and experimentation. Past successes in demonstrating cloud processes to diverse audiences will be continued in collaboration with the College museum, helping to relate these processes to new generations of K-12 students. Moreover, parcel microphysical models will be developed into web-based college classroom teaching tools using College resources.
