積雲參數化中對流質量通量之人為就地補償之效應

Abstract

本研究旨在對積雲參數化中的對流質量通量，比較人為就地補償與模式動力補償的異同。本研究使用天氣研究與預報模式(the Weather Research and Forecasting Model, WRF)對以下兩個方面比較，一為對水平解析度的依賴性，二為在熱帶氣旋模擬中的效應。使用模式動力補償的積雲參數化方案稱為混成質量通量積雲參數化方案(hybrid mass flux cumulus schemes, HYMACSs)，它們將次網格質量通量輻合或輻散視同參數化質量源或匯來處理。只要在非靜力平衡且完全可壓縮模式中加入這些質量源或匯，模式動力就能解析質量補償運動。質量補償實驗的結果證實，對3公里到27公里的水平解析度來說，就地補償比動力補償更敏感。『背著跑』實驗的結果證明，在9公里解析度使用Kain-Fritsch方案(KF)模擬熱帶氣旋，就地補償導致暖化與低層乾化並可使海平面氣壓深化與降水強化。『各自跑』實驗的結果暗示混成質量通量KF可以減少在9公里解析度使用KF模擬高估熱帶氣旋強度的傾向，不過此結果有氣旋尺度與積雲尺度過程之非線性交互作用所造成的不確定性。我們得出此結論：在非靜力平衡且完全可壓縮模式中創制積雲參數化時，動力補償比就地補償更符合自然。另外討論消除就地補償對整合性積雲參數化的重要性。 • 在非靜力平衡且完全可壓縮模式中創制積雲參數化時，動力補償比就地補償更符合自然。 • 對水平解析度來說，就地補償比動力補償更敏感。 • 就地補償導致暖化與低層乾化並可使海平面氣壓深化與降水強化。

In this study, cumulus parameterization schemes with artificial local compensation of convective mass flux are compared with hybrid mass flux cumulus schemes (HYMACSs), implementing dynamic compensation of that, with the Weather Research and Forecasting Model (WRF) in terms of the dependence on the horizontal resolution and the effects in tropical cyclone simulations. HYMACSs treat subgrid-scale mass flux convergence or divergence as subgrid-scale mass sources or sinks. When the mass sources or sinks are introduced to the mass continuity equation in a nonhydrostatic fully compressible model, the model dynamics would resolve the mass-compensating motion. The results of the mass compensation experiment corroborate that the local compensation is more sensitive than the dynamic compensation to the horizontal resolution between 3 and 27 km. The results of the piggyback experiment substantiate that the local compensation in the Kain-Fritsch scheme (KF) causes warming and low-level drying and could lead to sea level pressure deepening and precipitation strengthening at 9 km resolution in tropical cyclone simulations. With uncertainty due to the nonlinear interaction between cyclone-scale and cumulus-scale processes, the results of the fully coupled experiment imply that the hybrid mass flux KF (HY) could reduce the tendency of KF to overestimate the intensity of simulated tropical cyclones at 9 km resolution. We conclude that the dynamic compensation is more realistic than the local compensation when formulating cumulus schemes in nonhydrostatic fully compressible models. In addition, the importance of the elimination of the local compensation for the unified cumulus parameterization problem is discussed. • The dynamic compensation is more realistic than the local compensation when formulating cumulus schemes in nonhydrostatic fully compressible models • The local compensation is more sensitive than the dynamic compensation to the horizontal resolution • The local compensation causes warming and low-level drying and could lead to sea level pressure deepening and precipitation strengthening