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2020年宜蘭劇烈降雨實驗期間降雨分布特徵與豪大雨形成機制的探討
http://ir.lib.ncu.edu.tw/handle/987654321/93355
title: 2020年宜蘭劇烈降雨實驗期間降雨分布特徵與豪大雨形成機制的探討 abstract: 本研究統計2011年至2021年01、10、11、12月(共計44個月)期間於宜蘭地區的平均風花圖,分析了蘭陽平原的南邊與西邊,以及白天與夜晚時段的風場特性,以瞭解宜蘭區域環流的變化。根據平均風花圖的結果,不論是白天或夜晚,北邊主要受東北季風影響。然而在白天受盛行風和海風影響下,南邊的西風比例仍然偏高。表示宜蘭的風場分布除了受區域環流影響外,還受到宜蘭特殊地形的影響。
為了深入瞭解宜蘭地區高時空解析度的三度空間環流,本研究使用2020年宜蘭劇烈降雨實驗(Yilan Experiment for Severe Rainfall, YESR2020)期間的觀測資料,根據日累積雨量分成為11月21日第一波強降雨個案、11月22日弱降雨個案以及11月23日第二波強降雨個案,探討地形效應下宜蘭局部環流的變化、降雨分布特徵以及豪大雨形成機制。在綜觀環境場中,11月22日弱降雨個案主要受弱東風影響,上游大氣環境相對穩定,水氣通量較少,因此白天時段宜蘭地區未出現降雨,並且有明顯的海陸風環流。相較之下,11月21和23日這兩波強降雨個案有不穩定且相當高的水氣通量,隨東北季風將水氣傳送到宜蘭地區,並在南邊山區受地形抬升而產生降雨。再者,台灣東部海上均有強對流隨東北季風往宜蘭地區移入,導致蘇澳沿海和南澳山區累積了大量降雨。由YESR2020的觀測資料發現,這兩波強降雨在宜蘭地區的地面風場有所不同,尤其是第二波強降雨個案中,宜蘭南邊山區前出現西風分量,且地面風場有風向不連續帶,而此現象在第一波強降雨個案中並未發現。此外,兩者的降雨強度也有所差別,第一波強降雨個案的降雨強度和降雨極值均較第二波大。
本研究使用WRF數值模式(Weather Research and Forecast Model)模擬11月21日第一波強降雨個案,探討這天宜蘭強降雨的生成與發展過程。透過模擬的結果,我們發現宜蘭當天降雨強度大的原因可歸納為以下幾點。首先,東北季風在南邊山區的迎風面受到地形抬升的影響,產生了降雨,同時中高層的南風分量抑制降水系統往下游移動,使得降水系統固定在此區域,進而造成南部山區發生持續性的降雨。其次,外海的雨帶隨東北季風移入宜蘭地區,增強了蘇澳沿海的降雨。最後,隨著綜觀環境風場發生變化,蘇澳外海出現了氣旋式環流的輻合雨帶,此環流與宜蘭的地形產生交互作用,延續了降水系統的生命期,再次造成南澳山區和沿海地區發生劇烈降水。由此可知,宜蘭的地形對此強降雨個案的形成和維持有著顯著的影響。
;This study conducted an analysis of wind roses in the Yilan region during the autumn and winter seasons from 2011 to 2021, covering a total of 44 months. The investigation focused on the variations in land and sea breezes in the southern and western areas of the Lan-Yang Plain (LYP) during both daytime and nighttime. The results of the wind rose analysis indicate that the northern part of the LYP is primarily influenced by the northeast monsoon, while the southern part exhibits a noticeable westerly wind component even during the daytime. This highlights that the wind field distribution in Yilan is influenced by the unique topography of the region.
To understand the three-dimensional circulation in the Yilan region, this study analyzed observational data from the 2020 Yilan Experiment for Severe Rainfall (YESR2020). Based on daily accumulated rainfall, the study categorized three cases: the first intense rainfall event on November 21, a weak rainfall event on November 22, and the second intense rainfall event on November 23. The research explored variations in local circulation under the influence of topography, characteristics of rainfall distribution, and the mechanisms leading to heavy rainfall. In the synoptic environmental context, the weak rainfall event is primarily influenced by a weak east wind. The upstream atmospheric environment is relatively stable with lower moisture flux, resulting in no daytime rainfall in the Yilan. Additionally, there is a noticeable sea-land breeze circulation. In contrast, the two intense rainfall events exhibit unstable and significantly high moisture flux. The northeast monsoon transports moisture to the Yilan, leading to rainfall in the southern mountainous region of the LYP. In addition, there is also a convective system off the eastern coast of Taiwan that moves into Yilan with the northeast monsoon, causing Yilan to receive heavy rainfall. According to the observation data of YESR2020, it is found that the surface wind fields of these two heavy rainfall are different in Yilan. Especially in the second heavy rainfall cases, there is a westerly component in front of the mountainous area in the south, and the surface wind field has a wind direction discontinuity zone. This phenomenon was not found in the first heavy rainfall cases. In addition, the rainfall intensity between the two is also different. The rainfall intensity and extreme rainfall values of the first heavy rainfall cases are greater than those of the second heavy rainfall cases.
This study uses the Weather Research and Forecast Model (WRF) to simulate the first heavy rainfall case on November 21 to understand the generation and development process of heavy rainfall in Yilan. Through the simulation results, we found that the reasons for the heavy rainfall intensity in Yilan that day can be summarized as the following points. First, the northeast monsoon produces rainfall due to orographic lifting on the windward side of the southern mountainous areas. At the same time, the southerly winds of the upper atmosphere stationary the precipitation system in this area, causing continuous rainfall in the southern mountainous areas. Secondly, the rainbands from the eastern sea of Taiwan move into the Yilan with the northeast monsoon, intensifying the rainfall along the Suaou coast. Finally, as the overall environmental wind field changed, a convergent rainband of cyclonic circulation appeared in the eastern sea of Taiwan. This circulation interacted with Yilan′s topography, again causing precipitation in the southern mountainous and coastal areas. It can be seen that the topography of Yilan has a significant impact on the formation and maintenance of this heavy rainfall case.
<br>The extreme weather climate in northern Vietnam during northern summer;The extreme weather climate in northern Vietnam during northern summer
http://ir.lib.ncu.edu.tw/handle/987654321/93354
title: The extreme weather climate in northern Vietnam during northern summer;The extreme weather climate in northern Vietnam during northern summer abstract: 本研究使用越南北部氣象測站雨量及溫度資料,做為描繪此區域的夏季降雨和酷熱天氣的極端特徵。依據測站月雨量觀測資料統計顯示,越南紅河三角洲主要降雨發生在晚夏的7 – 9月間,並具有明顯的年際變化情形。在1983 – 2015年間,以季節總雨量的 ± 0.8個標準差門檻得以分別篩選出7個相對濕年及6個乾年。除強降雨日貢獻了70.5%季節總雨量外,濕年及乾年強降雨累積雨量的顯著差異,基本上已建造了兩個極端濕、乾年的特徵。進一步分析降雨的變異度發現,主要係受到熱帶氣旋、7–24- 和30–60-天季內振盪影響;特別在濕(乾)年時,影響紅河三角洲的熱帶氣旋數量及所引致的雨量增加(減少),且季內振盪的振幅也明顯的增強(減弱)。多數強降雨日係由熱帶氣旋與季內振盪合成共伴效應影響,但仍有些強降雨事件則僅由季內振盪所主導引起的。水氣收支分析得知在濕(乾)年時,一距平氣旋(反氣旋)盤據在中南半島上空,促使多(少)量水氣被傳輸到紅河三角洲,再藉由水氣通量的輻合(輻散)環流得以維持此區域過多(不足)的降雨。然而,聖嬰南方振盪對於紅河三角洲雨量的年際變化則僅有微量的影響。
越南北部除了強降雨外,夏季也是酷熱的季節。在31個夏季(1985–2015)中共計331酷熱天數,其中102酷熱天與熱帶氣旋有關連。統計分析顯示,在夏季前半時期5 – 6月中酷熱天的發生沒有受到熱帶氣旋的影響,而在晚夏的7 – 8 期間則明顯受到熱帶氣旋活動的影響。因此,夏季季風發展及熱帶氣旋活動成為引發北越夏季酷熱天的兩個主要天氣系統。根據水氣收支分析得知,由於東亞夏季季風的梅雨帶在夏季前半期持續往北遷徙,促使北越地區由豐沛降雨轉為相對偏乾環境,遂引致酷熱現象發生;而活躍的熱帶氣旋東北風分量穿越北越山嶺促成過山的焚風效應,以致北越成為偏乾且熱的環境。經局部位溫趨勢分析,不論在5 – 6月或7 – 8月的酷熱天都是藉由沿著背風下沉的乾空氣透過乾絕熱加壓過程增溫;由於熱帶氣旋的過山下沉氣流相對微弱些,遂而致使晚下地面增溫稍微緩和。另,在5 – 6 月的酷熱天數,則濕年比乾年顯著的多。
;The rainfall and maximum temperature datasets at operational stations in northern Vietnam are used to depict the characteristics of summer rainfall and hot days in this region. Based on station monthly observational statistics, the major rainfall in the Red River Delta (RRD) of Vietnam occurs in late summer (July–September) with conspicuous year-to-year variation. Using ± 0.8 of the seasonal total rainfall standard deviation as a criterion, seven wet and six dry years were identified over the period 1983–2015. In addition to the 70.5% of the seasonal total rainfall contributed by heavy rainfall days, the distinct difference in heavy rainfall accumulation between wet and dry years seems to fundamentally establish these two separate extreme wet and dry groups. As revealed by further analyses, the large variability in rainfall is attributed to the influence of tropical cyclones (TCs) and 7–24- and 30–60-day intraseasonal oscillations (ISOs); in particular, the number of TCs affecting the RRD and rainfall produced by TCs increase (decrease) during wet (dry) years, and the amplitudes of ISOs also increase (decrease). In many cases, heavy rainfall days are induced by the combined effect of both ISOs and TCs, while some heavy rainfall events are triggered mainly by ISOs. Water vapor budget analyses reveal that an anomalous cyclone (anticyclone) dominates the Indochina Peninsula in wet (dry) years, resulting in more (less) water vapor being transported to the RRD, whereas the anomalous convergence (divergence) of water vapor flux leads to the maintenance of excessive (insufficient) rainfall across the RRD. However, the El Niño–Southern Oscillation (ENSO) forcing has minor effects on the interannual variation in rainfall in the RRD.
In addition to heavy rainfall, summer is also a hot day season in northern Vietnam. There are 331 hot days found during 31 summers (1985–2015), 102 of which are related to tropical cyclones. Based on the analyzed statistics, hot days mainly occur without tropical cyclone effects during the first half of the summer (May and June), whereas hot days are strongly related to typhoon activities in the late summer (July and August). Summer monsoon development and tropical cyclone activities are the two major weather systems that induce hot days in northern Vietnam during summer. According to the water vapor transport analysis, the hot days without typhoon cases demonstrate that the East Asian rainband, rather the Mei-Yu rainband, shifts further northward, allowing the originally abundant rainfall area relatively to dry, including in northern Vietnam, to induce hot day phenomena during the first half of the summer. In late summer, the northeasterly winds from active tropical cyclones flowing through the mountain range north of northern Vietnam induce the Foehn effect on the leeward side of the mountain range, causing a dry and hot situation in northern Vietnam. Furthermore, based on the local potential temperature tendency analysis, the descending dry air along the leeside downslope essentially induces surface warming via an adiabatic process during the May–June and July–August hot days, except for the relatively weaker descending motions under the typhoon effect, which led to mild temperature warming in late summer. Regarding the extreme weather climate in northern Vietnam, the significant statistics indicate that more hot days may occur during May–June for dry years than for wet years.
<br>臺灣懸浮微粒之生命週期與年際變化;The Lifecycle and the Interannual Variation of Particulate Matter in Taiwan
http://ir.lib.ncu.edu.tw/handle/987654321/93353
title: 臺灣懸浮微粒之生命週期與年際變化;The Lifecycle and the Interannual Variation of Particulate Matter in Taiwan abstract: 近年來,對人體健康造成嚴重威脅的懸浮微粒(Particulate matter,簡稱PM),已在全球成了相當重要的議題。然而,我們對於其在氣候統計上的年週期變化以及年際變化依舊沒有充分的了解與估量。
在本研究中,我們發現東亞季風環流的垂直運動能影響邊界層高度的發展,進而調節臺灣PM濃度的年週期變化。藉由了解PM長期氣候的年週期變化,我們可以定義臺灣的污染季起於每年的10月並延續到隔年的4月。接著,我們進一步劃分污染季生命週期的5個相位,分別為:肇始(PM10 肇始日PM10 onset date,簡稱PMOD)、活躍(11月至隔年1月November to January,簡稱NDJ)、中斷(農曆新年期間,約1月底至2月初)、 復甦(2月至4月February to April,簡稱FMA)以及衰退(PM10 衰退日PM10 retreat date,簡稱PMRD),此結果正好與已知的東亞夏季季風生命週期相似。
有了PM污染季生命週期的明確定義後,我們得以更深入地針對其年際變化進行探討。結果顯示,在聖嬰時期(El Niño)的PM肇始日(PMODs)與衰退日(PMRDs)都較反聖嬰時期(La Niña)提早,尤其肇始日,在兩種不同的聖嬰-南方震盪(El Niño-Southern Oscillation,簡稱ENSO)事件下,有相當顯著的20天差異。在污染季的活躍期,不同的ENSO情境並未對PM濃度變化造成明顯的影響,反而是在復甦期有著顯著的差異。總結來說,氣候統計上PM污染季在深冬時期受到年循環的季節變化所主導,但在季節轉換時期的10月以及3月,ENSO則對PM濃度變化有著相當顯著的作用。此外,我們也測試了準兩年振盪(Quasi-Biennial Oscillation;QBO)是否同樣對PMOD/PMRD造成影響,結果顯示QBO與PMOD/PMRD皆未有顯著的關係。
;Particulate matter (PM), which causes severe problems in human health, has become an important global issue in recent years. However, the climatology of annual variations and the interannual variations in PM concentrations have still not been fully evaluated.
In our research, we found that the vertical motion of the East Asian monsoon system can affect the development of the boundary layer height and subsequently regulate the annual variation in PM over Taiwan. By climatologically understanding the annual variation in PM, the PM pollution season in Taiwan, from October to the following April, can be delineated. Then, we further defined five phases of the PM pollution lifecycle that are similar to the well-defined East Asian summer monsoon lifecycle: onset (PM10 onset date, PMOD), active (November to January, NDJ), break (between the end of January and early February), revival (February to April, FMA) and retreat (PM10 retreat date, PMRD).
After the PM pollution lifecycle was precisely defined, the interannual variation in the PM concentration became clearer. The starting (PMODs) and ending (PMRDs) dates of the PM pollution seasons are earlier during El Niño episodes than during La Niña episodes; in particular, there is a significant 20-day difference between their starting dates. For the active phase (NDJ), climatological PM pollution development does not show distinct features under the two different El Niño-Southern Oscillation (ENSO) episodes. On the other hand, the influence of the El Niño and La Niña forcings on PM pollution during the revival phase (FMA) is significant. In summary, the climatology of PM pollution in winter is dominated by the annual seasonal cycle, but during the seasonal transition periods, October and March are significantly modulated by ENSO. In addition, our study examined the relationship between PMOD/PMRD and quasi-biennial oscillation (QBO). The results show that the QBO has no significant influence on either the PMOD or the PMRD.
<br>利用衛星資料探討西北太平洋地區颱風生成之雲微物理特性及降水特徵
http://ir.lib.ncu.edu.tw/handle/987654321/93352
title: 利用衛星資料探討西北太平洋地區颱風生成之雲微物理特性及降水特徵 abstract: 西北太平洋(WNP)地區全年會形成數個熱帶低氣壓(TD),其中一些會增強為熱帶風暴(TS),而有些最終會消散。因此,本研究旨在瞭解其發展過程中的雲微物理和降水特性。我們使用了向日葵八號(Himawari-8)衛星和全球降水測量任務(GPM)衛星的資料,對2015年7月7日至2021年12月31日期間的194個熱帶氣旋進行統計分析,並分為TD消散組和TS發展組。通過選擇從風暴中心延伸六倍最大風半徑(RMW)的區域來研究熱帶氣旋發展的環境條件。
向日葵八號衛星資料分析顯示兩組在臨界時間前48小時內有較顯著差異。具體而言,雲光學厚度(COT)、雲有效半徑(CER)和雲頂高度(CTH)在TS發展組均高於TD消散組,顯示TS發展組有更多水相粒子抬升凝結成冰相粒子,增加的COT表示雲發展得更高且更厚,伴隨強烈上升運動,使較大CER粒子能夠保持在一定高度。隨著臨界時間接近,TS發展組在風暴中心的對流活動更強烈,COT和CTH更高,伴隨較大降雨率,降雨範圍更廣。另外,我們將TS發展組依生成位置分為南海海盆區和西北太平洋海盆區進行區域分析。根據雲微物理特性的結果推測西北太平洋海盆區可能具有較大的雲凝結核,伴隨更強的上升氣流;動力環境場的結果則顯示南海海盆區存在較強的上升速度和低層渦度,更有利颱風發展。此外,西北太平洋海盆區具有較高的降雨率。;Several tropical depressions (TDs) form in the Western North Pacific (WNP) region throughout the year. While some of them intensify into tropical storms (TSs), others eventually dissipate. Therefore, the objective of this study is to analyze the characteristics associated with the development of TDs by investigating cloud microphysics and precipitation properties. The analysis is based on data obtained from the Himawari-8 and Global Precipitation Measurement (GPM) mission satellites. A total of 194 tropical cyclones (TCs) occurring between July 7, 2015, and December 31, 2021, are statistically analyzed in the study. Meanwhile, TCs are classified into two types based on their development: TD-dissipating cases and TS-developing cases. Environmental conditions for TC development are investigated by selecting an area extending six times the radius of maximum wind (RMW) from the storm center.
Analysis of Himawari-8 cloud properties reveals significant distinctions between the cases within 48 hours prior to the critical time. Within 24 hours prior to the critical time, TS-developing cases maintain their intensity, while TD-dissipating cases exhibit a decreasing trend. As the critical time approaches, the cloud optical thickness (COT), the cloud effective radius (CER), and the cloud top height (CTH) increase within 6 RMW, indicating thicker cloud layers and larger cloud droplet size throughout the entire region, with stronger updraft. While approaching the critical time, TS-developing cases display more vigorous convective activity with higher COT and CTH near the storm center, along with higher rain rate and wider precipitation range. To examine regional disparities in TS-developing cases, we divided the WNP region into the South China Sea Basin region and the Western North Pacific Basin region. Our analysis based on Himawari-8 cloud properties suggest that the Western North Pacific Basin region may have larger cloud condensation nuclei, along with more intense upward motion; the results of the dynamic fields reveal a more favorable environment for typhoon development in the South China Sea Basin region, which differs from the previous conclusions. In addition, the Western North Pacific Basin region has higher rainfall rates.
<br>