一個對流過程參數法對季內振盪模擬之影響

The effect of parameterized convective processes on the simulation of intraseasonal oscillations

陳奕安(Yi-an Chen) ； 陳柏孚(Buo-Fu Chen) ； 侯書宇(Shu-yu Hou) ； 隋中興(Chung-Hsiung Sui)

《大氣科學》 47卷2期 (2019/06) Pp. 139-172

https://doi.org/10.3966/025400022019064702002

摘要

本研究目的為探討積雲參數法和雲微物理參數法對於對流模擬的影響，實驗包含單向預測實驗和Model for Prediction Across Scales （MPAS）全球模式模擬兩種方式，著重於分析Tiedtke積雲參數法對於季內振盪對流模擬之特性。單向預測實驗結果顯示，需同時使用積雲參數法與雲微物理參數法，方能產生和觀測一致的降雨量。積雲參數法中淺雲扮演將邊界層水氣向上傳送的角色，被積雲參數法溼化的大氣有利後續雲微物理參數法產生更多降雨；而深對流雲對大氣之效應則為加熱和變乾，使雲微物理參數法產生較少的降雨。且積雲參數法能合理模擬出淺雲及深雲在季內振盪乾濕相位中的轉換過程。本研究亦利用MPAS全球模式，以預設Tiedtke積雲參數法、逸入率乘上1.8倍、及改變逸出率三組實驗進行模擬。結果顯示，預設Tiedtke積雲參數法所模擬之對流較零散且降雨強度小於觀測，而增加逸入率和改變逸出率兩種方式皆能讓對流在更溼的大氣中組織發展。逸出率實驗中判斷當積雲參數法的深對流雲中界定次網格內雲和環境的差異小於特定程度時，將雲內水氣逸出改用大尺度動力和雲微物理參數法進行調整，讓對流調整在垂直分佈上具有變化。在逸入和逸出實驗中，調整後之積雲參數法能加強雲微物理降雨的產生，進而使降雨增加到與觀測相同量值、可降水量最大值增加、模式的大尺度上升速度增強，故積雲參數法與雲微物理參數法的相互配合對熱帶對流系統之發展亦扮演著重要角色。

關鍵字

積雲參數化 ； Tiedtke積雲參數法 ； 逸入率 ； 逸出率 ； 雲微物理參數化 ； 對流組織

並列摘要

This study focuses on the influence of entrainment and detrainment rates on the interaction between the cumulus scheme and the cloud microphysics scheme. Offline analysis and the Model for Prediction Across Scales (MPAS) model simulations were conducted to examine the role of Tiedtke cumulus scheme on the simulation of convection associated with intraseasonal oscillation. The offline analysis shows that shallow convection transports moisture from the boundary layer to lower troposphere and provides a favorable environment for deep convection. After adjusting atmosphere temperature and moisture profiles by the cumulus scheme, the cloud microphysics scheme generates more precipitation in shallow convective region and generates less precipitation in the deep-convective region. Total precipitation in deep convection region is consistent with TRMM observation. Cumulus parameterization could simulate reasonable shallow-to-deep cloud transition in different phases of intraseaonal oscillation. We also conducted three sets of MPAS experiments: default Tiedtke scheme (control experiment), Tiedtke scheme with 1.8 times entrainment rate, and Tiedtke scheme with modified detrainment rate. The higher-entrainment-rate experiment simulates more organized convection than that in the control experiment because convections are inhibited in the dry region and tend to aggregate in the moist region. Traditional cumulus parameterization in the framework of mass-flux is usually difficult to simulate convection development and to produce high cloud top. In the detrainment experiment, the model with a modified cumulus scheme detrains the precipitation and switches to cloud microphysics scheme to produce rainfall when the difference between subgrid-cloud and subgrid-environment is smaller than a certain threshold. In both entrainment and detrainment experiments, column water vapor and large-scale vertical velocity could be increased with a cloud microphysics scheme that produces enhanced rainfall comparable with observation. A cloud microphysics scheme interacting with cumulus parameterization seems to be important to convective organization and development.

並列關鍵字

cumulus parameterization ； Tiedtke scheme ； entrainment rate ； detrainment rate ； cloud microphysics ； convective organization