海洋性低雲於不同環境下之特性

Characteristics of Marine Low Clouds Under Various Environmental Conditions

吳佳瑩(Jia-Ying Wu)

指導教授 ： 郭鴻基

共同指導教授 ： 吳健銘(Chien-Ming Wu)

國立臺灣大學/理學院/大氣科學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201700728

摘要

本研究旨在探討數值理想模式中，海洋性低雲由副熱帶往赤道方向移行時之雲型轉變現象。利用三維向量渦度方程雲解析模式（Vector Vorticity equation cloud-resolving Mode，VVM）模擬東北太平洋海域海洋性低雲由層雲型態轉變至對流胞型態。控制實驗中，海表面溫度增加與大尺度沉降減少等條件以拉格朗日（Lagrangian）軌跡所經過的環境來調整；另外，敏感度實驗則改變邊界層頂之上自由大氣與邊界層內的總水混和比差值（〖-∆q〗_t）及液態水位溫差值（〖∆θ〗_l），討論水氣含量差異及逆溫強度對於海洋性邊界層低雲雲型轉換期間特性的影響。
控制實驗之模式環境場初始設定為：-∆q_t=6.34 g kg-1；〖∆θ〗_l=11.16 K，發現在模式內雲型會發展為具有較強對流胞的淺積雲與在邊界層頂的層積雲共存的邊界層型態。根據液態水路徑（Liquid Water Path，LWP）的機率密度函數（Probability Density Function，PDF）分析，邊界層內層雲在模式模擬約45分鐘後破裂，並於約三個半小時後轉為層積雲底下積雲型態。當增加〖-∆q〗_t時，邊界層內的低雲會提前破裂，也越早發展出對流胞：若-∆q_t增加（減少）2.00 g kg-1，則從自由大氣向下逸入的空氣變得更為乾燥（潮濕），邊界層內層雲破裂提前（延後）約35分鐘（1小時20分鐘）；形成層積雲底下積雲型態提前（延後）約1小時50分鐘（6小時25分鐘）。
倘若降低∆θ_l，使得邊界層頂的逆溫減弱，邊界層內的海洋性低雲迅速發展為對流胞結構，在〖∆θ〗_l減小4.98 K的情況下，邊界層內的層雲均會提前破裂，並發展為層積雲底下積雲型態：若-∆q_t不變（減少2.00 g kg-1），邊界層內層雲破裂提前約50分鐘（1小時40分鐘）；形成層積雲底下積雲型態提前約1小時30分鐘（7小時20分鐘），均比有較大〖∆θ〗_l的環境快3.8倍，且邊界層頂的高度成長速率為較大〖∆θ〗_l環境的1.7倍。
我們發現，自由大氣與邊界層內的水氣含量差異及逆溫強度均對海洋性低雲在雲型轉換時期的雲層破裂速率、對流胞的生成速率有顯著影響，而邊界層頂的高度成長速率主要受逆溫強度影響。顯示海洋性低雲隨著氣流線往低緯度移行時，自由大氣狀態的重要性。

關鍵字

海洋性低雲 ； 雲型轉換 ； 層積雲底下積雲 ； 液態水路徑

並列摘要

This study aims to discuss the cloud structure transition of marine low clouds propagating equatorward from the subtropics. Using the three dimensional Vector Vorticity equation cloud-resolving Model (VVM), idealized experiments are performed to determine the timing of stratus cloud to cumulus-under-stratus transition. In the control experiment, sea surface temperature (SST) increases as the large-scale subsidence decreases following the observational track calculated with the Lagrangian method. Sensitivity experiments are performed by modifying the total water mixing ratio difference (〖-∆q〗_t) and liquid water potential temperature difference (∆θ_l) between the free atmosphere and the boundary layer to evaluate the timing of stratus cloud breakup and cumulus-under-stratocumulus cloud development.
The timing of the transition is determined by the liquid water path (LWP) probability density function (PDF) analyses. The results suggest that the stratus clouds breakup occurs around 44 minutes in the control run, and transits to cumulus-under-stratocumulus around 3 hours 28 minutes. While 〖-∆q〗_t increases (decreases) by 2.00 g kg-1, the timing of the stratus clouds breakup advances (postpones) 35 minutes (1 hour 20 minutes), and the timing of the cumulus-under-stratocumulus development advances (postpones) 1 hour 50 minutes (6 hours 25 minutes).
In the experiments when the ∆θ_l decreases 4.98 K, the timing of stratus cloud breakup and cumulus-under-stratocumulus development both advances. While 〖-∆q〗_t stays the same (decreases by 2.00 g kg-1), the timing of the stratus clouds breaking advances 50 minutes (1 hour 40 minutes), and the timing of the cumulus-under-stratocumulus development advances 1 hour 30 minutes (7 hours 20 minutes). The timing of the cumulus-under-stratocumulus development is 3.8 times faster as well as the boundary layer height raises 1.7 times faster than the experiments which have higher ∆θ_l.
The above experiments suggest that the transition of the marine boundary clouds are influenced by both 〖-∆q〗_t and ∆θ_l. On the other hand, the development of boundary layer depth is mainly influenced by ∆θ_l.

並列關鍵字

marine low cloud ； cloud structure transition ； cumulus-under-stratocumulus ； liquid water path

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