西北太平洋雲街氣膠間接效應之模擬研究

Simulation of the Aerosol Indirect Effect on Cloud Streets over the Northwestern Pacific Ocean

吳鍾愷(Chung-Kai Wu)

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(2021年)

https://doi.org/10.6342/NTU202102881

摘要

本研究以WRF模式模擬西北太平洋面因冷空氣潰流而形成之雲街所呈現的氣膠間接效應。搭配了對氣膠敏感的雙矩量雲微物理參數法，在一公里的水平解析度下，雲街結構被合理地呈現。當海表面溫度比近地面空氣高3 K時，雲街現象即可產生；當海氣溫差擴大到9 K以上時，層積雲將過渡成較具對流性的雲種。相較太平洋東側的海洋性層積雲，雲街仰賴海面供給能量與水氣，對雲頂輻射冷的驅動依賴較低，因而使日夜變化不顯著。強海面可感熱通量能配合風切在低層初步產生渦流使邊界層混合，當雲形成於舉升凝結高度之上，潛熱加熱上部邊界層將進一步幫助雲層與邊界層的發展。
本研究設計一組氣膠濃度涵蓋乾淨背景到高度污染大氣狀況的數值實驗，檢測雲街系統對於氣膠在微觀與巨觀尺度的反應。由於雲街系統處於過渡狀態上，對氣膠的反應相當敏感；其反應雖符合Twomey效應，但與Albrecht效應有部分不符合的情形，特別在較為乾淨的環境中。與Albrecht效應相違來自於氣膠減弱了毛毛雨的形成，因而抑制毛毛雨在雲下蒸發所造成的邊界層穩定化，進而造成的動力上影響。當氣膠濃度上升，降水的機制式微，邊界層不穩度提升使對流加強，因而提高雲頂和上方自由大氣的交互作用使乾空氣逸入增強，個別的雲塊中逸入所消耗的雲水將與來自下方海面的水氣補充競爭。雖然雲塊內雲水含量上升，但使個別雲塊與整體雲覆量縮小。雖單一雲塊內的雲水量提升如Albrecht效應預期，使雲反照率提升，加強Twomey效應；但雲覆量隨氣膠粒子數量濃度降低而降低，在全天空反照率的計算上抵銷些許Twomey效應。

關鍵字

雲街 ； 氣膠間接效應 ； 層積雲 ； 冷空氣潰流 ； 海洋性邊界層

並列摘要

Aerosol indirect effects on cloud streets during cold-air outbreak over the Northeastern Pacific were simulated using the Weather Research and Forecast (WRF) Model coupled with an aerosol-interactive double-moment cloud microphysical scheme. Under a series of 1-km model resolutions, the cloud streets’ structure was reasonably resolved. The cloud streets were found to form when the sea surface temperature is at least 3 K higher than the surface air temperature. When the air-sea temperature difference is greater than 9 K, the stratiform cloud streets tended to transit to convective structure. The cloud streets depended more on the surface forcing than the cloud-top radiative cooling and thus the diurnal cycle was insignificant compared to the marine stratiform cloud over the Eastern Pacific. The stronger surface sensible heating combined with the wind shear to trigger the primary vortexes near the surface, and the latent heating further helped develop the cloud layer and reshaped the planetary boundary layer.
The micro- and macro-scale responses to aerosols were examined under a wide range of aerosol concentrations, covering pristine to heavily polluted conditions. The cloud streets generally remain in a transient state, indicating that the aerosol particles may significantly affect the cloud system’s development. Aerosol sensitivity tests showed that cloud streets are subject to the Twomey effect but not all of the Albrecht effects, especially under pristine conditions. The reversed Albrecht effects stem mainly from the dynamic response to changes in boundary-layer stability associated with drizzle evaporation. As the aerosol concentration increases, the drizzle mechanisms weaken, causing the boundary layer to become more convective experience stronger dry-air entrainment from the free troposphere above. These mechanisms lead to a reduction in cloud coverage. In individual clouds, the effect of dry entrainment on cloud water content may be compensated by more substantial vertical vapor flux and reduced precipitation. However, the reduction in cloud coverage offsets the albedo susceptibility, even though the cloud liquid water path enriches the cloud albedo as described by the Albrecht effects.

並列關鍵字

cloud street ； aerosol indirect effect ； stratocumulus cloud ； cold-air outbreak ； marine planet boundary layer

參考文獻

Torrence, C., and G. P. Compo (1998), A Practical Guide to Wavelet Analysis, Bulletin of the American Meteorological Society, 79(1), 61-78, doi:10.1175/1520-0477(1998)079<0061:Apgtwa>2.0.Co;2.

Ackerman, A. S., M. P. Kirkpatrick, D. E. Stevens, and O. B. Toon (2004), The impact of humidity above stratiform clouds on indirect aerosol climate forcing, Nature, 432(7020), 1014-1017, doi:10.1038/nature03174.

Ackerman, A. S., O. B. Toon, D. E. Stevens, A. J. Heymsfield, V. V. Ramanathan, and E. J. Welton (2000), Reduction of tropical cloudiness by soot, Science, 288(5468), 1042-1047, doi:10.1126/science.288.5468.1042.

Agee, E. M. (1987), Mesoscale cellular convection over the oceans, Dynam Atmos Oceans, 10(4), 317-341, doi:10.1016/0377-0265(87)90023-6.

Agee, E. M., and R. P. Howley (1977), Latent and Sensible Heat Flux Calculations at the Air-Sea Interface During AMTEX 74, Journal of Applied Meteorology, 16(4), 443-447, doi:10.1175/1520-0450(1977)016<0443:LASHFC>2.0.CO;2.