以動力凝結程序整合全球氣候模式之巨觀與微觀雲物理方案

A Kinetic Treatment in Condensation Process for the Unification of Cloud Macro- and Micro-physics Schemes in Global Climate Models

皮家容(Chia-Jung Pi)

指導教授 ： 陳正平

國立臺灣大學/理學院/大氣科學研究所/博士(2022年)

https://doi.org/10.6342/NTU202200361

摘要

氣候模式中有關層狀雲之處理分成巨觀與微物理兩個模組。巨觀物理過程主要處理雲量與水氣凝結成雲水的過程；微物理過程包含水氣、水、雨、冰、雪之間不同相態和粒子之轉換。受限於電腦計算資源的影響，氣候模式在處理網格點中的水氣含量時，一個積分時間步長約二十到四十分鐘，因此假設雲內的飽和度一直維持在剛好飽合的狀態，此方式被稱為飽和度調整。然而，該假設簡化許多和雲內過飽和度相關的過程，只能透過經驗式推估在不同的條件之下雲內的水氣含量。本研究提供由基本的物理理論所推導出動力凝結過程的方法（簡稱KCM），連結雲的巨觀與微觀物理模組。KCM可預報雲內的對水、對冰過飽和度或次飽和度，取代巨觀雲物理的飽和度調整假設，並透過質量成長方程式取代原本微觀雲物理中凝結水分配的診斷式，以合理計算冰、水共存時水氣相爭的白吉龍過程。KCM的計算上需要使用更精確的雲滴與冰晶的數量及粒徑，因此需要可以提供詳盡雲滴與冰晶粒子資訊的對流和雲微物理模組。而其所提供的雲內的飽和度，亦可提供用於診斷或預報雲滴的活化，或其他和雲內飽和度相關的過程，減少模式中受限於飽和度調整所產生的誤差。KCM將原本分別由不同參數化法所計算的物理過程整合至同一個簡單且具物理基礎的方法之中，做為巨觀物理模組和微物理模組的橋樑。
KCM被放入CESM地球系統模式中進行單點氣柱模擬以及全球模擬的測試。單點氣柱模擬結果顯示動力凝結方法對於雲內冰、水混合狀態有明顯的改善。以TWP–ICE個案為例，KCM雲內相對於水的過飽和度約為0.1%，相對於冰的過飽合度約為15%，且在適合的環境條件之下，在接近–40℃的高度有尚未結冰的過冷水。受到模式中水物和能量守恆的影響，氣柱模擬的結果增加對流降水的比例。
全球模式測試顯示，與觀測值相比，原始模式（簡稱CTRL）與KCM皆高估熱帶輻合帶和低估中緯度地區的雲量，總平均結果CTRL低估而KCM高估總雲量。KCM增加赤道與熱帶地區的高雲雲量，減少多數對流旺盛區域混合雲的雲量，增加熱帶海洋地區的低雲，總雲量高於觀測值；在模式未調校之前，雲量的估計較CTRL偏離觀測值。動力凝結過程因為改變了雲內的物理過程進而改變動力結構，透過部分減少對流降水或是增加層狀降水量，使得南、北緯30度以內的對流降水占總降水的比例，從原始模式的81.85%降低至75.49%，更接近平均觀測值54.20%；相反的，在南北半球溫帶地區，對流降水比例增加。但由於動力回饋過程而低估了好發於海洋東側、陸地西岸的低層海洋性層積雲。
初步測試結果顯示，針對KCM運用於全球模式的結果造成雲量高估以及液態水和冰光程量的不足，特別針對雲量參數法與降水效率係數進行調校。雲量參數法的部分，增加高層與減少低層的機率密度函數寬度，可有效的減少熱帶區域高雲過多的問題並增加低層雲量，讓模式結果較接近觀測值。針對降水效率，調降為0.1倍的對流及提高10倍的層狀雲水轉換成雨水的自動轉換係數的狀況之下，較多的液態水和冰存留在空中，大幅增加原本被低估的液態水和冰光程量。全球平均對流降水比例皆減少，其中熱帶地區原始模式與新發法的對流降水比例降至79.80%與72.79%。由於觀測與模擬結果的對流降水量相當，而模擬所得到的層狀降水量偏低，因此剩下的差異應從其他雲微物理過程著手改善。整體平均而言，全球平均觀測雲量為64.92%，原始模式與調校後的KCM平均雲量為66.83%和63.18%，經調校後的KCM模擬其對流降水比例和液態水和冰光程量更接近於觀測值。KCM在計算中受到粒子數量與半徑影響的特性，需要配合能提供此資訊的對流參數化法才能相得益彰，而KCM所提供雲內飽和度的資訊也可以利用在其他物理過程參數化的改良上。KCM為整合模式中的雲物理過程的目標踏出第一步。

關鍵字

雲巨觀物理 ； 雲微觀物理 ； 混合態雲 ； 飽和度 ； 白吉龍過程

並列摘要

A kinetic condensation method (KCM) was developed to remove the saturation adjustment assumption and resolve the condensation process in the grid-scale cloud macrophysics scheme to build an integrated stratiform cloud physics scheme for global climate models. By applying a saturation prediction equation with calculations based on cloud hydrometeor properties, supersaturation or undersaturation can be resolved within the macrophysics scheme. This treatment provides the basis for condensation calculation. It allows the Wegener–Bergeron–Findeisen (WBF) process to be resolved explicitly to render a realistic liquid–ice partition in mixed-phase clouds. KCM provides a theoretical-based way to determine the cloud liquid and ice partition rather than linearly depending on temperature. The cloud fraction scheme was modified based on physics principles to complement the condensation scheme.
The KCM scheme was examined in the Community Earth System Model (CESM) version 1.2.2 by incorporating it into the Community Atmosphere Model version 5 (CAM5) in single–column mode and global simulation. The results revealed that grid-scale cloud properties are sensitive to the new liquid–ice partition and condensation treatment. The dynamic condensation method has a significant effect on the mixed-phase cloud. The KCM increases the amount of high clouds in the equator and tropical areas, reduces the mixed-phase cloud fraction in the regions with vigorous convections, and increases the low-level clouds in the tropical ocean. It is defective that the marine stratocumulus is underestimated due to the change of the dynamic processes. The kinetic condensation method decreases the proportion of convective precipitation from 81.85% (CTRL run) to 75.49% between 30ºN to 30ºS, which is closer to the observations (54.20%). Through model tuning and decreasing the convective autoconversion coefficient to 0.1 times and stratiform autoconversion coefficient to 10 times, the proportion of convective precipitation from CTRL run decreased to 79.80% and 72.79% to the new method. Since the observation and simulation results are equivalent to the convective precipitation, and the stratiform precipitation obtained by the simulation is relatively low, the differences should be improved by other cloud microphysical processes. Overall, the global annual mean observed total cloud fraction is 64.92%, the result for the original model is 64.92%, and the results for KCM and KCM after tuning are 66.83% and 63.18%. The proportion of convective precipitation, liquid water path, and ice water path are closer to the observation than the CTRL when tuning the KCM.
Overall, the new parameterization scheme produced rather different liquid–ice partitions through condensation/deposition and consisted of the cloud macrophysics and microphysics scheme to provide a unified cloud module.

並列關鍵字

cloud macrophysics ； cloud microphysics ； mixed-phase cloud ； supersaturation ； Wegener–Bergeron–Findeisen process

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