層積雲動力系統之分歧現象

Abstract

本研究目的展示層積雲系統的雙穩態(bi-stable)行為以及海溫和大尺度沉降對層積雲雲量的遲滯(hysteresis)效應。透過以渦度方程為基礎的三維雲解析模式VVM (Vector Vorticity equation Model)，理想化模擬建立在南加州外海海洋層積雲典型的環境條件。長時間模擬呈現一數學架構，在暖海溫和弱大尺度沉降環境條件下，不同自由大氣濕度初始條件的層積雲動力系統演化趨向兩吸引子(attractors)：一是深厚層積雲，另一是淺積雲。分岐點被辨識在1至2天的中間熱力調整時間尺度。層積雲的演變因此被區分成在6至24小時快速逸入時間尺度上關係逸入與LWP負反饋的快動力以及在2天後多天緩慢逆溫層調整時間尺度上關係常數逸入率的慢動力。層積雲系統的破裂就發生在當上部邊界層開始變得乾，也就是，中間熱力調整時間尺度與緩慢逆溫層調整時間尺度的交界。另外，短時間模擬顯示大尺度沉降作用為立即性抑制逆溫層高度抗衡海溫，在乾自由大氣下立即性控制逆溫層上下濕度差異絕對值與LWP正反饋。海溫的作用是緩慢的，卻扮演協助乾自由大氣情境、控制濕自由大氣情境至關重要的角色。這份工作提出系統性且按照自然法則合理的框架，有助於層積雲轉換成淺積雲的現有知識。

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.