Dust storms over Saudi Arabia: temporal and spatial characteristics, climatology and synoptic case studies

摘要

In the past few decades, the climate has undergone significant change, and through the 1990s the world has experienced a major El-Ni o. Such change is likely to affect dust storm activity in dust prone parts of the world. A significant dust prone area is Saudi Arabia. To determine whether the ENSO cycle has had any influence on synoptic conditions over Saudi Arabia and hence dust activity, dust frequency has been examined over the period covering the past three decades. In this thesis, dust events have been classified into Haze, Local Dust Events (LDEs) and Dust Storms (DSs). Dust activity for 25 cities across Saudi Arabia was analysed for frequency of each type of dust event. It was found that in general, the cities in the south and south-east experienced the most number of dust events, with Haze being the most prevalent dust event. However, there were cities, which as a result of either their altitude or surrounding topography experienced much fewer dust events than their neighbours in the same region. Spring was shown to be the most dust prone season in the northern parts whilst the southern parts experience more dust events in summer. Potential dust sources were identified using back trajectories generated by the HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) modelling program, in conjunction with wind roses, during the days of dust in the most active dust season, spring. In the northern region, the An-Nafud Desert appears to be the main local source, while the North African deserts (Sahara), the Syrian Desert and Iraqi deserts are the main remote and neighbouring sources. In the western region, the eastern North African deserts appear to be the main sources, although dust can come from the Rakbah Plains to the east. In the central region, the Iraqi desert area is the major source dust. Gassim in the north-central region is also affected by dust from the Sahara, while Riyadh being closer to the Rub al Khali, is affected more by dust from Rub al Khali. There is also some contribution from the Ad-Dahna Desert. For the eastern region, the Iraqi desert areas on the border in the northern region are the main source of dust, while the Rub al Khali and the Ad-Dahna Desert are local sources. For the southern region, the Rub al Khali is a major local source and the eastern North African deserts are remote sources. There is also a measurable contribution from the arid areas of Yemen for Najran and Sharurah. For most cities, maximum wind speeds were always higher on dust-days (averaging 20 ms-1) than on dust-free days (averaging about 10 ms-1). The study period was classified into three time periods based on the frequency of total dust events: 1985-1994, 1995-2009 and 2010-2013. The frequency of all three types of dust events was determined for eight of the most dust prone cities in Saudi Arabia. Except for Riyadh, which has had a statistically significant increase in Haze, nearly all cities had a statistically significant decrease in dust activity for all three types of dust events. An examination of the mean sea level pressure patterns over the same period shows some subtle changes in the intensity and position of the dominant pressure systems, particularly in spring and summer. This change may be reflected in the change in dust event frequency. However, the relationship between the Southern Oscillation Index (SOI) and dust activity was found to be quite weak. The strongest correlation was found between annual DS (dust storm) frequency and the SOI, and between annual Haze and LDE frequency and rainfall over the Rub al Khali. There was also a moderate correlation between annual Haze frequency and total rainfall over Saudi Arabia. A number of case studies were examined for both Saudi Arabia and Australia, which showed similarities in preceding synoptic conditions. In all cases, there was the existence of a zone of baroclinity, which causes atmospheric instability, a major trigger for dust storms. This research will contribute to the knowledge of dust research in Saudi Arabia. The contribution to research in Saudi Arabia is based on comprehensive analysis of temporal and spatial distribution of different types of dust over the last three decades. It has found significant behavioural change in dust activity, especially in Haze and LDE related to the changing phase of the ENSO cycle and also to rainfall variation over the Rub al Khali. The airflow pathways of dust and dust-free days were identified using HYSPLIT for spring, enabling potential dust sources to be pinpointed.