梅雨季西南氣流特性對台灣降水分佈影響之理想模擬研究

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2017

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台灣位於東亞大陸和西北太平洋交界,氣候深受海陸分布差異、季節變化轉換之影響。每年5、6月的梅雨季,常受鋒面影響而出現連續性的降雨,而期間的豪大雨則往往與西南氣流密切相關。由於梅雨季期間的降水常同時與鋒面、西南氣流、不同大小尺度的大氣運動、水氣含量等均有關,其機制十分複雜,不易加以區分。因此,本研究希望單純探討西南氣流與台灣地形兩者間的效應,故選用2008年西南氣流密集觀測實驗(Southwest Monsoon Experiment,簡稱SoWMEX)在南海北部,台灣上游地區之收集之具有代表性探空觀測資料,簡化後獲得一個理想、均勻且穩定的環境背景氣流場,來進行本次西南氣流模擬實驗研究。 本研究選取東沙島與台灣西南方研究船共6個時間探空之平均,並根據其簡化(平滑)後之垂直結構,依地轉風關係建構理想之三維環境背景流場與其他參數。實驗共設計了4種不同低層水氣含量,當水氣含量950 hPa以下相對濕度為100%、85%、70%、55%,3種風向210∘、240∘、270∘,與3種風速10、15、20 ms-1,共36種組合搭配的背景流場下,研究受真實台灣地形影響下,所造成的台灣降水之分布特徵。同時,為更進一步定量探討降水分佈關係,依地理位置將台灣分成北部、中部、南部及東部四個區域,再依地勢高低分成平原(0~250 m)、山坡(250~1000 m)及山區(1000 m以上)等三種高度分區,來討論各區域之降水分佈。另外,本研究也比較上述各環境之夫如數(Froude Number,簡稱Fr),與氣流遭遇地形之反應,及降水分佈特徵之間的關係。 由模式分析可知,在水氣含量影響下,當相對濕度高時,CAPE高,大氣穩定度低。當相對濕度100%時,全台平均降水較高。其在偏南風時,主要降水區在東部和中部山區,隨著風向轉為偏西,主要降水則漸漸集中於中部和南部山區。而當相對濕度低時(70%、55%),CAPE低,而大氣穩定度相對較高,此時盛行風轉為偏南風時,主要降水在東部和北部山區,隨著盛行風轉向偏西時,降水逐漸改為集中在南部山區。 在風向改變情況下,當210∘偏南風時,由於Fr皆小於0.2 ,氣流繞山而在背風面輻合產生降水,在東部及北部有明顯降水。240∘吹西南風時,氣流直接迎面受中央山脈抬升影響,是故為平均降水最多的情況,Fr 約為 0.4時,氣流有爬山的趨勢,因受地形阻擋尤其中部和南部山區有劇烈降水。270∘偏西風時,其Fr約在0.4~0.5間,因氣流受地形抬升現象明顯,且直接垂直撞向中央山脈,主要降水在中部和南部,但降水分佈有從山坡往山區延伸的趨勢。 在風速改變的情況下,並非單純的風速越大降水就越多。在相對濕度100%時,有風速越強,降水量越多的趨勢,但在其餘相對濕度情況下,15 m s-1的降雨量反而最少。
Taiwan is located at the boundary between East Asia and northwestern Pacific ocean; therefore, its climate is heavily affected by the land-sea distribution and seasons. Fronts usually bring continuous rain to Taiwan during the rainy season in May and June. The torrential rain happening in this period is closely connected with the southwesterly flow. Because the rainfall during rainy seasons is usually associated with fronts, southwesterly flow, different scales of atmospheric motion and the moisture contents, it’s difficult to differentiate them. Accordingly, this research hopes to focus simply on the effects between the southwesterly flow and the terrain of Taiwan. We want to simulate the southwesterly flow by simplifying the representative data gained from SoWMEX in the north of South China Sea into an ideal and stable background airflow field. We average sounding data at six different time from research ships from Pratas Island and Taiwan and simplify them to reconstruct the vertical structure of the atmosphere. According to the reconstruction, we construct ideal three dimensional background airflow field and other parameters by geostrophic wind relationship. The experiment designs four moisture contents in the lower atmosphere. We have 36 kinds of background airflow field, combine with (1) the 100%, 85%, 70%, 55% relative moisture when the moisture content is below 950 hPa, (2) 3 wind directions: 210° , 240° , 270° and (3) three wind speeds: 10 m/s, 15m/s, 20 m/s, to research the raindfall distribution affected by the terrains of Taiwan. Besides, to further quantitatively explore the rainfall distribution, we divide Taiwan into four regions: the north, the central, the south, and the east part. Also, we divide them based on relief into plains (0-250m), mountain slopes (250-1000m) and mountain areas (above 1000m) to discuss the rainfall distribution in each region. Additionally, this research will compare the relationships of Froude Number in each environment, short for Fr, the reactions when airflows encounter the terrains, and the rainfall distribution. We can know from the model analysis that (1) when the relative moisture is high, the CAPE is high and the atmospheric stability is low. (2) when the relative moisture is 100%, the rainfall in the whole Taiwan is comparatively high on average. When the wind mainly comes from the south, the main precipitation area is in eastern and central mountain areas. As the wind direction changes to the west, the main precipitation area will gradually concentrate in central and southern mountain areas. (4) when the relative moisture is low (70%, 55%), the CAPE is low and the atmospheric stability is comparatively high. When the prevailing wind comes from south, the main precipitation area is in eastern and northern mountain areas. As the wind direction changes to west, the precipitation area is concentrated in southern mountain areas. When the wind direction is 210° close to the south, because of Fr is smaller than 0.2, the wind will bypass the mountains and converge at the lee-ward side to precipitate where eastern and northern regions will have obvious precipitation. When the wind direction is 240°, the airflow is uplifted at the wind-ward side by the Central Range. Thus, it’s the situation that has the most precipitation on average. When the Fr is about 0.4, the airflow will ascend the mountain slopes and will be blocked by the terrains, so the central and southern mountain regions will have torrential rain. When the wind direction is 270°, its Fr is about 0.4-0.5. Because of the obvious uplifting of the air and direct collision with the Central Range, the main precipitation area is at central and southern regions. However, the rainfall distribution has a trend stretching from mountain slopes to mountain areas. While the wind speed changes, the precipitation and the wind speed aren’t completely positively correlated. When the relative moisture is 100%, there is a trend that while the wind speed strengthens, the amount of precipitation increases. Nonetheless, under the other conditions of relative moisture, the wind speed of 15m/s has the least amount of precipitation.

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梅雨季, 地形, Froude Number

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