| 摘要: | 本研究將呈現沙塵事件的時空分佈與沙塵事件的區域趨勢，以及地表風速與降水對蒙古地區沙塵事件的影響。所用資料為2000年至2013年間蒙古國內(可大略區分為四大區域：森林、草原、戈壁沙漠與山區)的113個氣象站。本篇研究分為三部份：(1)蒙古沙塵事件的時空特性 (2)強風(風速超過6.5 m/s)與降水對沙塵事件的關係及其影響 (3)沙塵事件的衛星遙測。
根據Natsagdorj (2003)與本篇研究結果，近20年期間，蒙古地區年際的沙塵事件天數有些微減小的趨勢。年際沙塵事件天數少於5天的區域大多分佈於蒙古北部森林草原區域，而年際沙塵事件天數大於30天以上的區域大多分佈於蒙古東南及西部。本研究分析沙塵事件頻率較高之年份(2006, 2008, 2009)、沙塵事件頻率較低之年份(2003, 2011)、沙塵頻率較平均之年份(2000-2002, 2005, 2007, 2010, 2012, 2013)，其沙塵天數與強風及降水的關係。結果發現：沙塵頻率較高的年份，其沙塵天數與強風天數的相關性較高，且其間平均降水為10 mm/month，相對較少；而在沙塵較少的年份(2003, 2011)，戈壁沙漠東南區域於5月期間，平均降水為22 mm，降雨量相對較多，因此發生沙塵事件的機會被抑制。研究中，利用沙塵天數與強風、降水組成的沙塵潛勢指數(PDI)，可做為預測隔年春天沙塵事件多寡的指標。 本研究嘗試探討降水強弱對沙塵事件天數的影響。在戈壁沙漠5至6月期間，由於有31 mm-118 mm的降水，沙塵天數減少至12天；另外在草原及森林地區，由於有45 mm-178 mm的降水，沙塵天數減少至4天，然而在山區，其降水(117mm)與沙塵事件天數似乎沒有太大關係。
本研究亦利用衛星遙測觀測把蒙古沙塵事件分為沙塵暴(dust storm)與高吹塵(blowing dust)。因為粗顆粒與細顆粒不同的光學性質，沙塵粒子可被衛星辨別。利用MODIS( Moderate Resolution Imaging Spectroradiometer )衛星資料分析地表，利用光學厚度與地表能見度的關係，可比地面測站觀測更有效率地標定沙塵事件。研究結果指出：光學厚度(AOD)與地表能見度呈指數相關，相關性達到0.70，而此關係也可把沙塵做分類。此利用衛星反演資料之沙塵種類空間分佈情況與2004年蒙古氣象局的報告一致，代表利用衛星資料做沙塵分類是可行的。雖然在某些缺少AOD資料的區域，其事件天數與2004年報告仍有差異，但取AOD值為0.25來分類沙塵事件(沙塵暴、高吹塵)，就2004年5月期間而言，超過70%的測站有相當不錯的一致性。
;This study presents the spatiotemporal distribution and regional trend in dust event, and the impact of surface wind and precipitation on dust occurrences in Mongolia. We used data collected between 2000 and 2013 from 113 meteorological stations for natural zones of the forest steppe, steppe, the Gobi Desert and the mountains. Generally this thesis can be divided three parts as 1) spatiotemporal characterization of dust event in Mongolia; 2) we analyzed the relationship between dusty days, which is derived the sum of days with dust storm and/or drifting dust, and days with strong wind (at a threshold wind speed of a constant 6.5 m/s, hereafter, strong wind days) and precipitation and precipitation impacts on dust event; and 3) dust analyses with satellite remote sensing.
From the result of the previous study (Natsagdorj et al. 2003) and the present study, the annual dusty days have been a slight decreased over the last two decades. Annual distributions of dust storm days consisting of less than 5 days were found over the forest steppe zone in northern Mongolia, whereas areas with dust storms more than 30 days included southeast and western Mongolia. Dusty days, strong wind days and precipitation were compared among in dust-frequent years (2006, 2008 and 2009), dust-less years (2003, 2011) and dust-mean years (2000-2002, 2005, 2007, 2010, 2012 and 2013) in spatially and seasonally. The results found that dusty days in dust-frequent years were associated with strong wind days when precipitation is about the mean of 10 mm while dust occurrences were suppressed by large amounts of precipitation (approximately 22 mm) in dust-less years (2003, 2011) in May over the southeastern part of the Gobi Desert zone. We propose a potential dust index (PDI) based on the correlations among dusty days, strong winds and precipitation. The PDI performed as predicted in most areas of the country in the spring season. We attempted to present the impact of precipitation on dust events comparing between dusty days with less precipitation as a dry condition and dusty days with larger precipitation (more than the mean precipitation). Dusty days reduced by up to 12 days during March-June in the Gobi Desert due to 31-118mm precipitation and reduced up to 4 days with 45-175mm precipitation at some stations in the steppe and forest steppe zones whereas no relation found between increasing precipitation amounts (up to 117mm) and dust events in the mountains zone.
Here we categorizes dust events types (dust storm and blowing dust) by means of satellite remote sensing over Mongolia. Airborne dust particles can be identified by satellite remote sensing because of the different optical properties exhibited by coarse and fine particles (i.e. varying particle sizes). We used datasets consisting of collocated products from Moderate Resolution Imaging Spectroradiometer Aqua and surface measurements. Based on correlation between the retrieved aerosol optical properties and surface visibility, the intensity of dust occurrence can be more effectively and consistently discerned using satellite rather than surface observations. The results indicate an exponential relationship between the surface visibility and the satellite-retrieved aerosol optical depth (AOD), which is subsequently used to categorize the dust event and its correlation is above 0.70. The satellite-derived spatial frequency distributions in the dust phenomenon types are consistent with Mongolia’s weather station reports during April in 2004, indicating that dust phenomenon classification using satellite data is highly feasible. Although there were the discrepancies in the number of days where aerosol information was retrieved from MODIS and the ground-based dust event reports, which may be caused by the lack of satellite-observed AOD, they are a well consistent with the values more than 70% at many stations in April and May of 2004 when a criterion (AOD is 0.25) is used for dust phenomenon classification (dust storm and blowing dust). |
