| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 103 === In this study, we attempt to demonstrate the bi-stable behaviour of stratocumulus systems and the hysteresis effect forced by SST and large-scale subsidence on marine stratocumulus cloud amount. To address this aim, the idealized simulations are carried out with a 3D vector vorticity equation based cloud resolving model (VVM). The typical stratocumulus conditions over Southern California coast are considered as initial conditions and large-scale forcings. Simulations of long-term (8 days) responses to the warmer sea surface temperature and the weaker large-scale subsidence are conducted in order to clearly recognize the evolution of stratocumulus and to detect an occurrence of the stratocumulus to shallow cumulus cloud transition. Two experiments with different initial free atmospheric humidity under the fixed large-scale forcings are used to represent a mathematical framework for the stratocumulus dynamical systems. By analyzing characteristic time scales and instability, the results of this research support the idea that the initial free atmospheric humidity yields two attractors. During a critical transition when a bifurcation occurs, a decoupled and thick stratocumulus cloud boundary layer is evolved into either a deep and thick stratocumulus cloud boundary layer or a shallow and scattered cumulus cloud boundary layer. A critical transition is recognized on the intermediate thermodynamic adjustment time scale of 1 to 2 days. The evolution of stratocumulus is thus distinguished into fast dynamics concerning the entrainment-LWP negative feedback on the fast entrainment time scale of 6 to 24 hours and slow dynamics regarding constant entrainment rate on the slow inversion adjustment time scale of multiple days starting after the 2nd day. A breakdown of stratocumulus systems occurs just when the upper boundary layer becomes dry, that is, the border between the intermediate thermodynamic adjustment time scale and the slow inversion adjustment time scale. Simulations of short-term (6 hours) responses to the various perturbed large-scale environments lead to a better understanding of the sensitivities of strarocumulus properties to large-scale controlling variables such as SST, subsidence, and free tropospheric humidity. The large-scale subsidence functions as an immediate restraint on the inversion height against SST and on the positive feedback between absolute value of moisture jumps across the inversion and LWP against the dry upper troposphere. The effect of SST is tardy, but plays a crucial role to assist dry upper tropospheric scenarios and to control wet ones. This work contributes to existing knowledge about the stratocumulus to shallow cumulus cloud transition by proposing a systematic and physically reasonable framework.
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