碩士 / 國立臺灣大學 / 大氣科學研究所 / 90 / This thesis seeks to improve the current understanding on the processes linking cloud condensation nuclei (CCN) number concentration to cloud droplet number by analyzing the stability of CCN number concentration (NCCN) in the stratocumulus-topped marine boundary layer (STMBL). Under the mixed-layer assumption, NCCN is determined by CCN production, Brownian coagulation, self-collection of cloud droplets, accretion, and scavenging. Microphysics is incorporated with a new set of parameterization schemes, so as to achieve high precision and save computation time.
From a purely microphysical point of view, the present analyses affirm the existence of two separate equilibrium states in NCCN: the lower equilibrium corresponds to particle production being counter-balanced by self-collection and accretion; the higher equilibrium represents a balance between particle production and Brownian coagulation. These two equilibrium states (stable nodes) can exist independently or coexist in a limited range of particle production rate. The equilibrium NCCN obtained by sweeping through a range of CCN production rate (S) from high to low (with high initial NCCN) will follow a different path than that of sweeping S from low to high (with low initial NCCN), thus producing a sort of hysteretic curve. With respect to the first indirect effect of aerosols, these findings imply that in a pristine cloud, the pollution rate will have to reach a critical strength before cloud microphysics is significantly affected; on the other hand, once its microphysical properties are altered, the cloud may persist at the high NCCN and high albedo regime.
The timescale for NCCN to reach its equilibrium states often exceeds one day, leaving room for other processes to further regulate cloud lifetime and coverage, including dynamics, aerosol physics and chemistry, radiation, and other microphysics. In particular, the removal of cloud water content (qc) by precipitation proves to be an important factor toward the realization of stable nodes. At high NCCN precipitation is shut down and qc retains, which may in turn lead to prolongation of cloud lifetime or extension of cloud cover; these findings qualitatively confirm the second indirect effect of aerosols.
| Identifer | oai:union.ndltd.org:TW/090NTU00022014 |
| Date | January 2002 |
| Creators | Tzung-May Fu, 傅宗玫 |
| Contributors | Jen-Ping Chen, 陳正平 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 141 |
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