Deposition Coefficient, Habit and Ventilation Influences on Cirriform Cloud Properties
Open Access
- Author:
- Sheridan, Lindsay Marie
- Graduate Program:
- Meteorology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 20, 2008
- Committee Members:
- Jerry Y Harrington, Thesis Advisor/Co-Advisor
- Keywords:
- cirrus
ventilation
crystal
habit
deposition coefficient - Abstract:
- Cirriform clouds are atmospheric phenomena of importance, due to their role in global radiative and water vapor budgets. Despite their importance, many of the important influences that define their structure and evolution are still unknown, due to their limited accessibility. We examine the evolution of cirriform cloud properties as they respond to variations three such factors: deposition coefficient, crystal habit, and ventilation. In an effort to examine surface--kinetic effects on depositional growth of cirrus particles, a parameterization of the deposition coefficient, varying in response to changes in the growth mechanism (2D nucleation or spiral dislocations) and crystal and ambient properties, is incorporated into a microphysical model for spherical crystals. Feedbacks between ice surface and larger--scale properties, such as crystal size and ambient supersaturation, are developed and placed in the context of distinct cirriform cloud types. Following the analysis on spherical crystals, the effects of crystal habit are explored for their influences on crystal growth. Spheroidal approximations to the primary habits of plates and columns are made, and links between crystal shape and depositional growth rates are determined. An interesting relationship between habit and the evolution of the crystal size distribution is discovered, in which the smallest nucleated crystals are seen to have the most deviation from a spherical shape. This pronouncement of the aspect ratio is found to enhance the growth rates of the crystals to the point where they actually grow larger than the other crystals, altering the structure of the size distribution. Concluding the analysis of cirriform cloud properties is a study of the effects of ventilation on crystal sublimation in subsaturated air. Two methods for computing ventilation effects, one based on experimental work and the other on theoretical analysis, are incorporated into the model and compared. The fall distances of cirrus particles are analyzed for the two methods of ventilation computation and and the two surface--kinetic growth methods to determine which physical representations lead to the greatest survival of the particles. Crystals growing under 2D nucleation constraints are found to persist longest, and the applicability of one of the ventilation computations is called into question.