海洋大陸對季內震盪產生的地形效應： 動力機制以及熱力與雲結構之研究

Topographic Effects on the MJO in the Maritime Continent: Aspects of Dynamics, Heating Profiles, and Cloud Distribution

吳政翰(Cheng-Han Wu)

指導教授 ： 許晃雄

國立臺灣大學/理學院/大氣科學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.03433

摘要

本研究發現，當季內震盪 (Madden-Julian Oscillation, MJO) 在北半球冬季東移經過海洋大陸時，它的環流結構、雲系統(深對流、強降水以及不同雲種)分佈、以及相應的熱力剖面等特性，都會被海洋大陸的高聳複雜地形所影響。這些地形效應可以總結如下：1) 大體上，面積愈狹長、地形愈高聳的島嶼，對於流場的阻擋作用愈顯著。2) 新幾內亞這樣一個又高又長的島嶼所造成的阻擋作用，引發的氣流分流現象從地面到500毫巴的高度都可見到。3) 狹長但高度較低的蘇門達臘與爪哇島鏈，除了導致MJO西風帶向南轉向之外，也造成氣流過山以及下風處發生垂直波動的現象。4) 地形較集中如蘇拉維西與婆羅洲等島嶼，只有引發局部性的阻擋作用，並也造成下風處發生垂直波動的現象。這樣的地形阻擋作用，在這些特定島嶼區域形成了顯著的渦度場與幅合場。地形的存在使得季內震盪的大尺度環流產生了局部舉昇與下沉現象，因而造成季內震盪的深對流系統在其東移通過海洋大陸的過程中，在這些主要地形附近發生停頓現象。與深對流系統相關的降水、紊流、以及在大氣層頂與地面輻射通量，也因而受到了影響。由此可知，正確解析地形的細部節構，對於季內震盪東移通過海洋大陸的正確模擬，有著重要的影響；而忽略地形影響的aqua-planet模擬，可能就不再適用於我們對MJO的進一步研究中。

關鍵字

海洋大陸 ； 季內震盪 ； 地形 ； 動力 ； 熱力 ； 雲結構

並列摘要

This study demonstrates that during the passage of the MJO through the Maritime Continent in the boreal winter, the corresponding circulation structure, cloud systems, as well as consequential heating profiles, are redistributed via complex topographic effects from mountainous islands. The effects could be summarized as follows. 1) Overall, more spatially-elongated and higher mountainous islands exert stronger blocking effect on the incoming flow. 2) The blocking effect of the elongated and high-rising New Guinea is so strong that it causes a complete flow bifurcation from the surface to above 500-hPa. 3) The long but low island chain of Sumatra and Java not only results in the southward deflection of the incoming westerly anomaly, but also allows the westerly anomaly to flow over and creates vertical wave-like perturbation in the downstream. 4) Less spatially-extended islands such as Sulawesi and Borneo exert only a localized blocking effect, causing significant downstream wave-like perturbation in the vertical. Distinctive vorticity and convergence distributions are therefore generated in this specific domain. The existence of topography seems to create extra lifting and sinking within the large-scale circulation and thus the convective system exhibits quasi-stationary features near the major topography during the MJO passage through the Maritime Continent. Associated precipitation, turbulent and radiative flux anomalies at TOA and surface are therefore affected. It is suggested that resolving the detailed topographic effects may play a key role in simulating realistic characteristics of the MJO in the Maritime Continent, while ignoring influence from tropical topography with an aqua-planet model may not be a proper approach for our further understanding of MJO.

並列關鍵字

topographic effects ； MJO ； Maritime Continent ； dynamics ； heating profiles ； cloud

參考文獻

Adler, R. F., G. J. Huffman, D. T. Bolvin, S. Curtis, and E. J. Nelkin, 2000: Tropical rainfall distributions determined using TRMM combined with other satellite and rain gauge information. J. Appl. Meteor., 39, 2007–2023.

Chepfer, H., Goloub, P., Spinhirne, J. et al., (2000) Cirrus cloud properties derived from POLDER-1/ADEOS polarized radiances: first validation using a ground-based Lidar network, J. Appl. Meteor.,39, 154-168.

Cho, H. -R. and D. Pendlebury, 1997: Wave CISK of equatorial waves and the vertical distribution of cumulus heating. J. Atmos. Sci., 54, 2429-2440.

Chou, M.-D., P.-K. Chan, and M. M.-H. Yan, 2001: A sea surface radiation data set for climate applications in the tropical western Pacific and South China Sea. J. Geophys. Res., 106(D7), 7219–7228.

Chou, S.-H., E. Nelkin, J. Ardizzone, R. Atlas, and C.-L. Shie, 2003: Surface turbulent heat and momentum fluxes over global oceans based on the Goddard satellite retrievals, version 2 (GSSTF2). J. Climate, 16, 3256-3273.