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University in Taiwan (2016)

A Study on Aeolian Dust Event in Mongolia

Amgalan Ganbat

Titre : A Study on Aeolian Dust Event in Mongolia

Auteur : Amgalan Ganbat

Université de soutenance : University in Taiwan

Grade : Doctorat 2016

Résumé
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).

Mots clés : Dusty Day、Mongolia、Precipitation、Potential Dust Index、Nature Zone

Présentation (NDLTD)

Page publiée le 17 avril 2018