Ice-containing clouds can exist anywhere within the lower atmosphere. Cirrus clouds in the upper troposphere are a common example, being composed entirely or mostly of ice. Low-level clouds may contain both liquid and ice while mid-level clouds contain some liquid at most latitudes. Ice crystals take on a variety of complex shapes and can grow large by vapor diffusion alone. The presence of ice complicates the links between cloud microphysics, dynamics, and radiation, making accurate cloud simulations difficult. Ice growth from the vapor phase proves to be a key but perplexing link in this chain. Recent laboratory measurements suggest that the deposition coefficient, a measure of growth efficiency, is small for small ice crystals and that it depends on the supersaturation. Modeling studies show that simulated ice concentrations and supersaturations in cirrus clouds, as well as the rates of glaciation of mixed-phase clouds, depend sensitively on ice growth rates. The work herein seeks to advance understanding of ice vapor growth in cold atmospheric clouds.
Intellectual merit: This integrated study will produce a synergy between laboratory and modeling research. The work will focus on ice grown from the vapor phase with the intention being to reduce uncertainties in past measurements of the deposition coefficient. The laboratory methods make use of electrodynamic levitation to isolate ice particles from system walls and permit particle growth to be followed under precisely controlled conditions. New measurements of vapor growth rates will be obtained as functions of size, supersaturation, temperature, and pressure. These data can be used to explore the dependence of the deposition coefficient on environmental conditions similar to those ice crystals experience in the atmosphere. Moreover, these data can also be used to critique new and commonly used vapor growth methods, and constrain the parameterizations used in cloud models. Ice crystal growth theories and numerical models will provide guidance to the laboratory work, help interpret experimental findings, and provide a framework for extending lab results to cloud systems. The synergism afforded by this laboratory-modeling study will help shed new light on poorly understood ice processes that are currently limiting our ability to accurately predict cloud evolution.
Broader impacts: This research has potentially broad impacts on the atmospheric sciences and society. Large uncertainties exist in simulations of ice-containing clouds at all modeling scales indicating that the consequences of improving ice vapor growth parameterizations in models could be scientifically far-reaching. The research will train graduate students and give advanced undergraduate students exposure to modern research. The work can also be demonstrated to diverse audiences including K-12 students. Moreover, continue development of parcel microphysical models as classroom teaching tools using College resources is planned.
