不均勻地表情況下淺對流的大渦流模擬研究

Large-Eddy Simulation of Shallow convection over a heterogeneous surface

侯昭平(Jou-Ping Hou)

指導教授 ： 許武榮

共同指導教授 ： 吳清吉(Ching-Chi Wu)

國立臺灣大學/理學院/大氣科學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.01053

摘要

本文使用台大-普度三維非靜力模式進行海面上淺對流和陸地上淺對流模擬。模擬中，冷空氣通過不均勻海面時，引發海面上的熱對流，對流邊界層因此順著環境風向下游抬升；上游風切較強的區域，出現雲街，並因紊流混合，使垂直風減弱，雲街到下游時，轉換為中尺度對流胞。考慮科氏力時，雲街順著風向向右偏移，對流胞區則是出現明顯的中尺度對流胞。雲型的變化，改變了熱通量的大小；冷空氣的變性過程，同時存在大、中、小不同尺度，彼此交互作用明顯。
二維海風模擬，結果顯示受科氏效應影響，海風通過時間越長，風向偏轉的角度越大，風速也越強；而其上的海風迴流風速較弱，偏轉效應較不明顯。科氏效應除了使海風偏向之外，也減弱海風向內陸延伸的距離和強度。在陸地上的對流邊界層裡，受紊流影響較大，其對流時間尺度短，因此對單一測站的科氏效應較不明顯。由初始背景環境風場敏感測試顯示，初始風場為向岸風時，會增加海風向內陸穿透的距離，並將地表傳送至大氣的熱量，迅速的向下游傳送；初始風場為離岸風的結果則相反。但離岸風能夠縮短海風鋒面合併對流胞的時間，因此海風向內陸推進過程中，海風鋒面的強弱變化變得更為明顯。初始風場為靜風時，海風鋒面合併對流胞所需時間，明顯大於初始風場為離岸風的情況，海風鋒面雖也呈現明顯的強弱變化，但變化週期因此變長。
三維海風模擬中，海風鋒面南北移速不均，彎曲明顯；海風鋒面和對流胞合併時，會受對流胞上積雲影響，改變移動速度；積雲和對流胞本身造成的下沈區，能夠減弱海風鋒面的強度，而海風鋒面和對流胞交互作用時，強弱的改變，能夠成為激發深對流的機制。

關鍵字

對流邊界層 ； 雲街 ； 科氏力 ； 海風鋒面 ； 深對流

並列摘要

Shallow convections are a distinctive system of atmosphere which has 0.5 to 2 km deep and a horizontal length scale of a few to a few tens of kilometers. There are large volumes of the atmosphere occupied by those. Sea-breeze circulation is one of the shallow convections over a heterogeneous surface. Once the sea-breeze circulation is established, the front which induced by the convective boundary layer (CBL) over land during daytime usually propagates onshore and sweeps through the convection cells inside the CBL over land near coastal area. The intensity of the front may change from time to time as it merges with those convection cells, while the front also modifies the basic characteristics of the CBL (such as surface fluxes, cloud thickness, etc). The interaction between the front and the CBL can be very complicated, and the situation differs from cases to cases with different environmental conditions.
Extremely cold air outbreak over warm oceans will result in another type of shallow convections. Once the cold air leaves the land surface, it is modified by vertical transfers of heat, momentum and moisture from warm oceans. The resultant transformation of the air mass eventually leads to the formation of clouds which frequently take the form of cloud streets, roughly oriented along the winds in the outbreak. Farther downwind in the outbreak, the cloud streets transform into three-dimensional cells, occasionally of meander form, sometimes as closed cells but most frequently as open cells. However, rolls exist only when the vertical wind shear was bigger than 7 m s-1 km-1, cells exist when the shear was less than 5 m s-1 km-1 (Tsuchiya and Fujita, 1967). The CBL quickly deepens away from the coastline with increasing fetch length and increasing sea surface temperature. As the depth of the CBL changes, the embedded roll vortices (cloud streets) grow in size.
A NOAA/Environmental Technology Laboratory Doppler lidar measured the life cycle of the land- and sea-breeze system at Montery Bay, California, in 1987, during the Land-Sea Breeze Experiment (LASBEX). Fine-scale lidar measurements showed the reversal from offshore to onshore flow near the coast, its gradual vertical and horizontal expansion, and a dual structure to the sea-breeze flow. Complicating factors include the effects of inland topography; for example, inland mountain ranges generate their own thermally forced slope flows, which interact with the sea breeze. Along the coast of central California the diurnal behavior is driven by the land-sea contrast and two ranges of mountains. A local-scale temperature contrast at the shoreline drives the earlier, shallow sea breeze, whereas a long coastline with two parallel ranges of heated mountains will produce thermally forced onshore flow at length and depth scales(Darby, 2002).
Once the sea-breeze circulation is established, the front which induced by the convective boundary layer over land during daytime usually propagates onshore and sweeps through the convection cells inside the CBL over land near coastal area. The intensity of the front may change from time to time as it merges with those convection cells, while the front also modifies the basic characteristics of the CBL (such as surface fluxes, cloud thickness, etc). The interaction between the front and the CBL can be very complicated, and the situation differs from cases to cases with different environmental conditions. With an extensive observation network, the 1991 Convection and Precipitation/Electrification Experiment (CaPE) have documented the phenomenon for several sea-breeze events in southern Florida, USA. Convection and subsequent precipitation induced by the sea breeze circulations are often observed in Florida peninsula during summer.
This study use the NTU/Purdue 3D nonhydrostatic numerical model to simulate shallow convections over a heterogeneous ocean and local circulation and its interaction with CBL over land. The model solves a fully compressible, nonhydrostatic system of equations explicitly with a two-stage forward-backward time integration scheme. Since the numerical procedure is neutral with respect to both sound waves and internal gravity waves, there is no need to impose any time-smoother in the model. Thus, the model results can be very accurate and numerically stable. In addition, the explicit algorithm is particularly suited for parallel computation. Our computer program is efficiently parallelized and it is suited for this very demanding problem in terms of computer resources.

並列關鍵字

convective boundary layer ； cloudstreet ； coriolis force ； sea-breeze front ； deep convection

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