Laboratory Measurements of Small Ice Crystal Growth Rates at Low Temperature

Restricted (Penn State Only)
- Author:
- Pokrifka, Gwenore Flora
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 08, 2024
- Committee Members:
- Jerry Harrington, Professor in Charge/Director of Graduate Studies
Jerry Harrington, Chair & Dissertation Advisor
William Brune, Major Field Member
Matthew Kumjian, Major Field Member
Miriam Freedman, Outside Unit & Field Member - Keywords:
- ice
cirrus
ice growth
clouds
laboratory
measurements
attachment kinetics
low temperature
microphysics - Abstract:
- Two thermal gradient diffusion chambers were used to measure the growth of ice crystals from vapor at temperatures (T) of -67 to -40◦C and ambient pressure (∼ 970 hPa), with supersaturation over ice (si) up to liquid equilibrium. One chamber levitated small frozen droplets (initial radii of 8 − 26μm) electrodynamically to provide mass growth rate data. Growth limited by attachment kinetics was frequently measured at low si, and high si resulted in growth rates that were enhanced compared to capacitance growth theory. The enhanced growth was likely due to the development of complex habit features (e.g., branching, hollowing). Crystal growth with complex shapes is often modeled with a sphere of some effective density (ρeff ) that is less than bulk ice density, though typically only for crystals with maximum dimensions greater than 200 μm. Fits to the data showed that ρeff for small particles ranges from 100 − 920 kg m−3, and the results are consistent a geometric model of budding rosette crystals. We used the distribution of the measured growth rates over si to develop two parameterization methods for ρeff of small ice particles that are suitable for current cloud microphysical models. On the other hand, growth limited by surface attachment kinetics is treated with a deposition coefficient (α) to represent the efficiency of molecular incorporation into the crystal. However α is generally assumed to be a constant near unity in many cloud models. The data showed that α ≪ 1 (kinetics-limited growth) occurs near ice equilibrium and that α → 1 (increasingly diffusion-limited growth) as si rises. This result is consistent with the theoretical si-dependencies of α, wherein crystals transition from growth by dislocations to step-nucleation as the supersaturation rises. A supersaturation-dependent parameterization of α was developed from the distribution of the measured growth rates. The α-parameterization represents the shift from dislocation to step-nucleation growth as si increases. Including this parameterization in a bulk mircophysics model with a gamma size-distribution produced, at low si, average mass growth rates that were reduced by up to 50% compared to the common modeling assumption that α = 1. The second diffusion chamber is a new device that was built to measure the dimensional growth rates of larger ice crystals (maximum dimension of ∼ 10 − 500μm) at low temperatures (-67 to -40◦C). The crystals grew from a fine fiberglass substrate that terminates at the center of the chamber, reducing vapor competition effects compared to earlier devices. High-resolution images of growing crystals were taken from two angles at regular intervals. Mainly hollowed columns and polycrystals were observed. Preliminary analysis showed that the crystal aspect ratio tends to increase with decreasing temperature, and it approaches a constant value in time. The growth rates of the primary dimensions also approach asymptotic values, but only the minor-dimension appears to be correlated to temperature. Consequently, the ratio of the dimensional growth rates was highly correlated with the aspect ratio. The results were agreement with similar prior measurements (Bailey and Hallett, 2004) and faceted growth theory with step-nucleation near crystal edges (Nelson and Baker, 1996). The data do not support either the capacitance theory or the theory of Chen and Lamb (1994). The results also provided circumstantial evidence that the crystals grown in the levitation chamber under similar conditions were indeed columnar polycrystals, potentially with hollowing.