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University of Tsukuba (2004)

A study on aeolian dust outbreak in East Asia

Kurosaki, Yasunori

Titre : A study on aeolian dust outbreak in East Asia

Auteur : Kurosaki, Yasunori

Grade : Ph. D. in Science 2004

Université de soutenance : University of Tsukuba

Résumé partiel
The impact of aeolian dust on climate has been recognized to be large due to radiative effect. Intergovernmental Panel on Climate Change (IPCC) [2001] also treats the aeolian dust as an important factor of climate change. However, the uncertainty of the effect of the aeolian dust is very large and the level of scientific understanding about the aeolian dust is classified as very low in IPCC [2001]. One of the major reasons of insufficient understanding is the inhomogeneous distribution of aeolian dust owing to their short lifetime. They are removed by rain typically within a week. As a result, the concentration of aeolian dust rises to its highest near its source just after its outbreak. The uncertainty still remains due to the difficulty of understanding the distribution of aeolian dust sources and the timing of aeolian dust outbreak. Aeolian dusts are generated by strong surface winds, while land surface conditions largely affect aeolian dust outbreaks because the threshold wind velocity of aeolian dust outbreak has various values according to land surface conditions. Therefore, it is important to recognize the distribution of aeolian dust sources, land cover types (e.g.,desert, grassland, forest, cultivation) around aeolian dust sources and threshold wind velocities according to land surface conditions. This study focuses on the aeolian dust outbreak in East Asia, hich is one of the major aeolian dust sources. The following three geographical characteristics are found to be essential for the aeolian dust outbreak in East Asia : (1) frequent synoptic disturbances in spring, (2) complicated distribution of land cover types, and (3) frequent snow covers in early spring. With regard to the first characteristic, it is well known that strong winds cause aeolian dust outbreaks when a synoptic disturbance passes over East Asia. With regard to the second characteristic, land cover types complicatedly distribute in East Asia in comparison with other aeolian dust sources such as Sahara Desert. This suggests that aeolian dust sources also complicatedly distribute in East Asia. With regard to the third characteristic, snow cover in early spring often prevents aeolian dust outbreaks in East Asia because arid and/or semiarid regions distribute in relatively high latitude. These geographical characteristics make it difficult to understand the spatial and temporal distributions of aeolian dust outbreaks in East Asia. The purposes in this study are to clarify (i) the relation between aeolian dust sources and land cover types, (ii) which largely control aeolian dust outbreaks, surface winds or land surface conditions, (iii) the spatial distribution of threshold wind velocities of aeolian dust outbreak, and (iv) the effect of snow cover on aeolian dust outbreak. For these purposes, this study conducts statistical analyses by use of surface meteorological data, land cover type data and snow cover data in East Asia for the period from March 1988 to June 2003. Conclusions are summarized as follows : 1. Aeolian dust sources in East Asia distribute in regions of Bare Desert, Semi Desert Shrubs, Grass/Shrub and Cultivation. The northern boundary of dust sources almost corresponds to the southern boundary of the Forest regions. The southern boundary of dust sources distribute around the southern boundaries of Bare Desert and Semi Desert Shrubs regions in middle and upper reaches of the Huang He River. 2. Aeolian dust outbreaks frequently occur at months of frequent strong winds around the Gobi Desert and in the Taklimakan Desert. The months of frequent dust outbreaks are limited in March, April and May around the Gobi Desert, while dust outbreaks frequently occur from March to July and/or August in the Taklimakan Desert. According to the correlation between strong wind frequency and dust outbreak frequency, the surface wind primarily controls dust outbreaks in March and April. This tendency is strong especially in the Taklimakan Desert. On the other hand, land surface conditions largely affect aeolian dust outbreaks in May around the Gobi Desert.

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