以雲解析模式探討對流集結特徵

Aggregation or No Aggregation and Beyond: from a Cloud-Resolving Model Perspective

陳彥婷(Yan-Ting Chen)

指導教授 ： 吳健銘

國立臺灣大學/理學院/大氣科學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802385

摘要

本研究使用雲解析模式(VVM)與淺薄海洋探討對流-輻射平衡環境之對流集結機制，並以海溫梯度驅動初始熱力環流。以大氣觀點，輻射、邊界層焓通量與溼淨能輻合在早期對流集結過程，皆為正貢獻，且海溫變化更加強邊界層焓的效應。當環境乾溼差異擴大，溼淨能轉為負貢獻，但潛熱釋放足以維持對流集結狀態。前述機制在乾區特別強。以大氣-海洋耦合觀之，長波冷卻特別重要。儘管實驗設計中，海表為不均勻邊界條件，但並非對流集結的關鍵。輻射-對流-海溫交互作用在集結過程中特別有效。

對流集結產生的關鍵在於低層溼淨能由低區輸送至高區的逆向輸送。低層水氣梯度建立後，藉乾溼空氣密度差異產生風場，進一步加強此逆向傳輸。估計逆溫強度(EIS)能代表低層穩定度，並進一步決定溼淨能平流狀態。

若無對流集結調整，潮濕環境因強烈溫室效應，海溫不斷提高，在100天內提升6度。從海洋能量收支分析，不論對流集結與否，海氣交互作用皆將海水混和層能量調整至平衡態，但對流集結為更有效的調整機制。

關鍵字

對流集結 ； 淺薄海洋 ； 輻射-對流平衡 ； 雲解析模式

並列摘要

In this study, a slab ocean is employed to a vector vorticity equation cloud-resolving model (VVM) to investigate the development of convective aggregation in radiative-convective equilibrium. The model is initialized by a weak sea surface temperature gradient to generate a thermal-direct circulation. From the perspective of the atmosphere, the radiative heating, surface enthalpy fluxes and moist static energy (MSE) convergence induce positive feedback in the early stage of convective aggregation. In particular, the interactive sea surface temperature (SST) intensifies surface enthalpy flux variance by enhanced heating at high MSE region and suppressed flux at low MSE area. As the difference of dry and moist area enlarges, wind convergence homogenizes MSE variance by pumping out MSE from the wet to dry, while the convective heating terms are strong enough to sustain the convective aggregation. All of the above processes exhibit the strongest signal in the dry zone during the convective aggregation stage.

From the view of the ocean-atmosphere coupling system, the longwave radiative cooling is particularly important for the convective aggregation. Although the simulation is conducted with the uneven surface boundary condition, the spatial inhomogeneity becomes less important within 5 days of integration and the result suggests that the radiative-convection-SST coupling is very strong after the convective aggregation is triggered.

The key mechanism for convective aggregation is the low-level MSE up-gradient transport. As the low-level moisture gradient is established, resulting in the development of virtual temperature gradient near surface which drives the low-level wind and promotes the convective aggregation. We found that the estimated inversion strength (EIS) appears to be related to wind stress and better indicates the low-level stability that further decides the direction of MSE transport.

For the non-aggregation simulation, the climate falls into a runaway state in which the domain average SST increases from initial 300K to 306K in 100 days. The mixed layer budget shows that the air-sea interaction evolves to an equilibrium state that the net flux in the mixed layer is close to zero whether there is aggregation or not. Even so, the convective aggregation is a much more efficient adjustment that promptly saves the climate from a runaway state.

並列關鍵字

aggregation ； convective organization ； slab ocean ； radiative-convective equilibrium ； cloud-resolving model

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