Combined Laboratory and Modeling Studies of Ice Vapor Growth

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. Previous laboratory measurements suggest that the deposition coefficient, a measure of growth efficiency, is very small for small ice crystals. In addition, it now appears that aspect ratio prediction may be critical for the simulation of ice-containing clouds; however competing hypotheses exist regarding mass distribution on the crystal faces during growth. Modeling studies show that simulated ice concentrations and supersaturation in cirrus clouds, as well as the rates of glaciation of mixed-phase clouds, depend sensitively on ice growth rates.

Intellectual merit. This integrated study will focus on ice growth from the vapor phase with the intention to reduce uncertainties in past measurements and to test hypotheses regarding molecular incorporation and aspect ratio evolution. 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 aspect ratio. These data will be used to test competing growth hypotheses and constrain the parameterizations 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 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 research and laboratory can also be demonstrated to diverse audiences, including K-12 students.