邊界層物理過程對平流霧之影響

Abstract

本研究探討邊界層之環境特徵和物理過程對平流霧維持之影響。初始條件由金門平流霧觀測實驗資料簡化而來，呈現近飽合的潮濕邊界層和逆溫結構，可影響垂直混合與霧頂長波輻射冷卻作用。並使用渦度方程建立的雲解析模式以4公尺高解析度進行一系列理想化的大渦流實驗。在透過開關特定物理過程來探討物理過程的模擬結果中，地表通量在霧已形成後具有較小的影響力且作用僅限於近地層;長波輻射冷卻能有效的增加霧水，但在此個案的環境中無法有效產生紊流動能;太陽輻射是使霧消散的重要機制，其加熱霧層並減少霧水;垂直風切加強霧層內的混合作用並擴大霧層的垂直分布，且在此個案中的紊流動能貢獻分布上，風切的貢獻遠重要於浮力的貢獻。在探討環境條件的模擬結果中，垂直風切強度可由臨界理查森數(Ri=0.25)分為強、弱垂直風切兩類，強垂直風切會將穩定層化邊界層調整成混合邊界層，並使霧層上移而變成層雲，但在弱垂直風切時，霧層則能夠穩定地維持。最後探討風切和海溫交互作用的影響，發現在弱垂直風切時(Ri > 0.25)，冷海溫的冷卻作用能加強霧層的穩定度並明顯的增加霧水;而高海溫的加熱作用則會破壞逆溫結構，將邊界層調整成混合邊界層並使霧層消散;而在強垂直風切時(Ri < 0.25)，冷海溫無法有效地透過冷卻作用建立逆溫結構，雖層雲中液態水含量增多，但仍無法使層雲下降而成霧;而暖海溫的加熱作用則更加增厚混合邊界層的厚度並減少了層雲中的雲水含量。海溫作用所需的反應時間較紊流混合作用長，因此海溫作用調整邊界層結構的效果並不如風切的作用。本研究指出環境近地面風場和與紊流傳送作用對平流霧結構有最明顯的影響，因此在使用區域模式模擬平流霧時，所選取的邊界層和紊流參數化以及垂直解析度配置能否正確呈現穩定層化與混合邊界層之間的轉換，是決定模擬結果的關鍵因素。

This study investigates the influences of boundary layer (BL) processes and the environmental conditions on the maintenance of advection fog through idealized large-eddy simulations. The initial conditions were simplified from the in-situ sounding data during the fog episode at Kinmen Island. A near saturated moist layer with temperature inversion is throughout the domain, which represents the characteristics of advection fog. This kind of vertical profile of moisture can influence the effects of turbulent mixing and longwave radiative cooling at fog top. A series of idealized large-eddy simulations were carried out using the Vector Vorticity equation cloud-resolving model at a high resolution of 4 m. By turning on/off selected physical processes, the results show that surface heat and moisture fluxes have little influence on liquid water content and are restricted to surface layer when the fog layer already exists. Longwave radiative cooling is the dominant process to generate fog water but in this case cannot effectively generate turbulent kinetic energy (TKE) through buoyancy flux due to the specific environmental moisture condition. Solar radiation heats and evaporates the liquid water content within the fog layer. Vertical wind shear enhances the turbulent mixing and extents the vertical distribution of fog layer. Vertical wind shear also drives the shear instability, and the averaged magnitude of shear production of TKE is much larger than that of buoyancy production of TKE in this case. As to evaluate the impact of environmental wind profiles on the maintenance of the fog layer, Richardson number (Ri) is identified as the critical parameter. The stronger vertical wind shear (Ri < 0.25) modifies the boundary layer from a stable boundary layer to a well-mixed boundary layer. As a result, the fog layer no longer maintains but shifts upward into stratus cloud. In contrast, the fog layer can maintain under the condition of weaker vertical wind shear (Ri > 0.25). Lastly, the sensitivity of the fog layer structure to both the environmental vertical wind shear and sea surface temperature (SST) is investigated. Results show that the combination of weaker vertical wind shear (Ri > 0.25) and colder SST is favorable to the maintenance of fog. However, warmer SST dissipates the fog layer in the condition of weaker vertical wind shear (Ri > 0.25). On the other hand, fog layer cannot exist in the condition of stronger vertical wind shear (Ri < 0.25) whether the SST is colder or warmer. This study suggests that the vertical profile of wind speed and the effect of turbulent transport have the most significant influence on the structure of advection fog. Therefore, when simulating the advection fog by regional models, the results would highly depend on whether the chosen BL and turbulence parameterization schemes and vertical resolution can accurately represent the transition between a stable and well-mixed BL.