| 摘要: | 氣膠可作為雲凝結核，其數量濃度的增加會導致雲滴有效半徑減小，進而抑制與延遲降水事件發生，使雲的生命週期變長，並對後續降水發生頻率及強度造成影響。然而由於氣膠-雲-降水交互作用的複雜性，加上氣象條件會同時改變氣膠與雲的特性，使得目前科學界對氣膠影響降水系統之瞭解仍有很大的不確定性及挑戰。夏季降水提供臺灣中部平原區主要水資源來源，約佔全年總降水量的60%，降水的多寡與時空分布變化會對當地社會經濟、農業活動產生重大衝擊。隨著快速的都市化與人口增長，加上大氣環境的影響，使中部平原比起鄰近區域有較嚴重的空氣污染情形，增加氣膠效應影響雲與降水的可能性。因此本研究結合2008-2019年6-8月夏季期間，地面與衛星觀測、再分析資料，對氣膠、雲與降水特性進行長期統計分析，由PM2.5資料定義乾淨與污染條件日數，試圖探討氣膠對夏季降水日變化以及雲參數之影響，並提出可能機制進行說明。
研究結果顯示，乾淨條件具有較多的水氣量，風場由夏季季風所主導；污染條件的環境較乾燥，以海陸風環流為主，有利於污染物的累積。考慮相似的環境風場(盛行風向120°-240°範圍)情境下，進行乾淨與污染條件下降水日變化之比較，結果顯示，在14時至19時，污染條件下的降水頻率以及降水強度比乾淨條件大，小時降水發生頻率最高值發生在18時，量值約為12%；降水強度最大值發生在15時，量值約為8 mm hr -1，暗示在高氣膠濃度的環境下，降水傾向發生在一天中較晚的時期，且降水強度較乾淨條件大1-4 mm hr -1。除此之外，檢視所有的午後降水個案後，亦可發現污染條件有較大午後降水強度之特徵。衛星Aqua MODIS (Moderate-Resolution Imaging Spectroradiometer)雲參數的分析中顯示，污染環境比起乾淨環境的暖雲，呈現雲滴有效半徑減小0.99 μm；雲量增加0.09、雲光學厚度增加1.49、雲水光程增加12.97 g m-2；雲頂溫度下降0.91 K之情形。
最後統合觀測分析結果，吾人提出一套氣膠影響降水日變化可能機制。臺灣中部平原夏季所產生的氣膠進入暖雲系統內，使雲滴有效半徑減小，降低碰撞合併效率，改變雲的生命週期，進而抑制上午降水之發生，使雲不易消散且持續發展至更高高度，從而使更多的雲水聚積於雲內，並在午後不穩定的環境下，產生高頻率且高強度之降水。本研究透過長期地面、衛星觀測與再分析資料進行分析，得出氣膠濃度與夏季降水日變化之明確統計關係，未來建議用模式進行個案模擬，以更好瞭解雲內微物理與動力過程，與進行氣膠效應影響之定量。此初步成果可應用於未來中部平原區的儀器部署，以及氣膠-雲-降水交互作用之觀測實驗規劃。;Aerosols can serve as cloud condensation nuclei (CCN), an increase in CCN number concentration results in the decrease of cloud effective radius, thereby suppressing precipitation, delaying raindrop initiation and prolonging cloud lifetimes. Further changes the frequency and intensity of precipitation. Moreover, due to the complex in aerosol-cloud-precipitation interactions (ACPIs) and the coincident impact of meteorological factors on both aerosol and cloud properties. There still remain many challenges and uncertainties for the scientific community about aerosol effects on precipitation system.
Summertime precipitation plays a crucial role in central plain of Taiwan’s water resources, it contributes about 60% to the annual precipitation. Changes in spatio-temporal precipitation amounts will cause influences to local socioeconomics and agricultural activities. With the rapid urbanization coupled with population growth, central plain of Taiwan is subjected to severe air pollution, the likelihood of aerosols impact on clouds and precipitation. This study aims to investigate the ACPIs over central plain of Taiwan with a statistical analysis of long-term observation data from ground-based measurement, satellite-based observation, and reanalysis data. We segregate PM2.5 data into clean and polluted day conditions to study aerosol effects on the diurnal variations of summertime precipitation. Finally, the plausible mechanisms are proposed based on our results.
The results indicate that the high moisture and the dominance of summer monsoon are characterized for clean conditions. The influence of land-sea breeze favors the accumulation of pollutants under polluted conditions. Considering the similar wind field environment scenario (i.e. prevailing wind from 120° to 240°), the diurnal variations of precipitation under clean and polluted conditions are compared. The results show that the frequency and intensity of precipitation under polluted conditions are higher than under clean conditions between 14:00 LST and 19:00 LST. The highest hourly precipitation frequency occurs at 18:00 LST, occupying about 12% of the total and the maximum value of precipitation intensity occurs at 15:00 LST, about 8 mm hr -1. Such a result implies precipitation occurs more frequently later in the day under high aerosol loading environments, and the precipitation intensity is 1-4 mm hr -1 greater than clean conditions. In addition, after examining all the afternoon precipitation cases, the characteristic of afternoon precipitation tends to be greater in intensity under polluted conditions. The results from MODIS/Aqua cloud properties show that when comparing with clean environments, in the polluted environments, cloud effective radius decreased by 0.99 μm, cloud top temperature decreased by 0.91 K, while cloud fraction increased by 0.09, cloud optical depth increased by 1.49, and cloud water path increased by 12.97 g m-2.
Finally, based on the observation analysis results, we proposed a plausible mechanism for aerosol effects on diurnal variations of precipitation. Aerosols originating from central plain of Taiwan in summer will modify cloud droplets with smaller effective radius and then reduce the efficiency of the collision-coalescence process in clouds. As a result, it will change the cloud lifetime and then suppress the occurrence of morning precipitation, which allows more cloud water to be accumulated in the cloud. This lead to more frequent and high intensity precipitation under an unstable environment in the afternoon. In this study, we integrate surface observations, satellite measurement and reanalysis data to conduct the analysis, and obtain a clear statistical relationship between aerosols and diurnal variations of summertime precipitation. Numerical model simulations are needed to examine the detailed mechanisms in our future study. In order to better understand the microphysics and dynamic processes in clouds and quantify the aerosol effects. These preliminary results can be applied to field deployment over central plain of Taiwan and to strategy planning in the observation of ACPIs in the future. |
