透過雲解析模式VVM探討複雜地形下日夜對流的新分析觀點

Abstract

本研究提供了一個概念框架用以了解在複雜地形下由局地環流主導的日夜對流現象，從對流的深層入流混合（deep-inflow mixing）角度進行分析，重點關注來自上游高濕靜能（MSE）傳送的非局部動力效應以及相關的對流強度。我們使用渦度向量方程雲解析模式（VVM）對臺灣附近典型弱綜觀條件下在山地島嶼上的日夜對流進行了模擬，通過修改中對流層相對濕度（RH）和雲凝結核（CCN）濃度進行兩組敏感性實驗，以研究環境變異度的影響。在所有模擬中，日夜降水在時間上呈現多個降水峰值並且降水熱點受到地形鎖定於山區。前兩波降水與海陸山谷風的演進密切相關，降水強度與對流可用位能（CAPE）呈現負相關，並且展示出深層入流混合的特徵：在高度約6公里以下的上升氣流強度隨高度呈線性增加，表明存在一個深厚的橫向入流層，傳送環境的MSE進入對流，貢獻了對流浮力所需的能量。這個過程受到局地環流驅動1公里以下的上游MSE傳送所主導，其主導程度取決於背景環境條件和局地環流結構的完整性。較早發生的對流調整了環境的能量和水氣，增加了低層上游MSE輸送估計對流強度的能力。此外，隨著局地環流的進一步發展，更強的低層入流增強了上游MSE的傳送，進而增加了降水強度。修改中對流層RH的敏感性實驗表明，在較乾燥的模擬中，相同的低層MSE傳送可以貢獻較強的對流強度；而修改CCN濃度的敏感性實驗則表明，增加CCN濃度，低層MSE傳送對對流強度的貢獻程度並不受影響，但可以增加低層的MSE傳送。未來，我們將重點分析冷池的強度，以進一步研究第一和第二波降水之間的相互作用。

This study provides a conceptual framework to understand the diurnal convection dominated by local circulation over complex topography. The results are analyzed from the perspective of deep-inflow mixing of convection, focusing on the non-local dynamic in entraining high upstream moist static energy (MSE) and its related convective strength. We use the Vector Vorticity equation cloud resolving Model (VVM) to simulate diurnal convection over an idealized mountainous island under typical weak synoptic conditions near Taiwan. Two sets of sensitivity experiments are conducted with idealized environmental conditions by modifying the free atmosphere relative humidity (RH) and cloud condensation nuclei (CCN) concentration to investigate the effects of environmental variabilities. In all simulations, the diurnal precipitation exhibits multiple peaks in time and its hotspots are orographically locked in the mountain areas. The first two peaks, in which the developments are closely related to the evolution of sea-valley breeze circulations, show negative relationships between precipitation intensity and convective available potential energy (CAPE). Both peaks demonstrate deep-inflow mixing features. The linear increase of updrafts below about 6 km denotes the presence of a deep layer of lateral inflow, with entrained MSE contributing to convective buoyancy. This process is dominated by the local circulation-driven upstream MSE transport below 1 km, with its contribution to convective strength dependent on the background environmental conditions and the scale of the established local circulation. Moreover, the earlier precipitation peaks modulate the environmental moisture, magnifying the influence of low-level upstream MSE transport on estimating convection strength. Furthermore, as the local circulation develops further, stronger inflow enhances upstream MSE transport, resulting in increased precipitation intensity. The sensitivity experiment with modifying the free atmosphere RH indicates that the same low-level MSE transport in drier simulation contributes to stronger convective strength. On the other hand, the sensitivity experiment with increasing CCN concentration shows that while higher CCN concentration does not affect the contribution of low-level MSE transport to convective strength, it enhances the MSE transport in the lower atmosphere. In the future, we will focus on analyzing cold pool strength to gain further insights into the interaction between the first two precipitation peaks.