Analyzing the synoptic pattern of dust storms in the southern region of the Caspian Basin

Dissertation for Master's degree (M.A)

In the field of natural geography, climate risk trend

Abstract

Dust storms are destructive weather phenomena that cause irreparable damage to urban areas, fields, facilities, roads, etc. every year. This phenomenon is affected by special weather conditions, and if the mechanism of its development and expansion is known, it is possible to reduce or deal with the many damages they cause to the environment. Due to the geographical location of Iran in the arid and semi-arid belt of the world, this country is frequently exposed to local and synoptic dust systems. According to the direction of movement of the systems and the location of low pressures and landings, the most important sources of dust entering Iran are the north of the Arabian Peninsula, the north of the Great African Sahara, the eastern regions of Syria and the northwest of Iraq. Due to the location of the southern Caspian region, between the Alborz mountain range and the Caspian sea, although these two natural phenomena are considered to be among the factors preventing the occurrence of dust, nevertheless, we still witness days with dust in the mentioned region. In this research, the simultaneous factors affecting the occurrence of this atmospheric phenomenon in the southern Caspian basin were investigated. Also, the daily data of dust, wind speed and synoptic maps in the stations of the mentioned region prepared by the Meteorological Organization have been used. Analyzing the synoptic conditions that cause dust storms in the region can also be effective in predicting dust storms in the northern regions of the country. It is expected that the results of this research will be useful in identifying the meteorological and synoptic conditions effective in spreading dust to the northern regions of the country and will be used to increase the accuracy of the prediction of the dust phenomenon in these regions. The results of the present research showed that the amount of dust has a difference both in terms of time and place, and in some places more dusty days occur in relation to their geographical location. Also, different systems play a role in the occurrence of dust in the region.

Chapter one: Research design

1-1 Introduction

From the point of view of synoptic meteorology, a storm is a unique destructive phenomenon on synoptic maps that includes a combination of pressure, cloud, rainfall, etc. phenomena and can be detected by radar. In arid and semi-arid areas, the phenomenon of dust occurs frequently in different forms, such as days with dust or dust storms and tornadoes. In deserts and dry areas, the rapid change in air temperature causes pressure gradients in different places and the formation of strong and permanent winds. In addition, due to the lack of moisture and vegetation in these areas, the adhesion of particles to each other is reduced and the wind can separate particles with a diameter of less than 2 mm from the soil surface and carry them with it. Many of these particles have a diameter of less than 10 microns and their fall speed cannot be measured under the influence of the earth's gravity.  Therefore, these particles remain suspended in the atmosphere, for this reason, in desert areas, the air may remain dusty for a long time. Therefore, storms appear in the scales of tornadoes, lightning storms, dust storms, and the like.

One of the most important conditions for creating dust next to unstable air is the presence or absence of moisture, so that if the unstable air has enough moisture, rain and thunderstorms; And if it lacks moisture, it creates a dust storm. The type and vegetation also play an effective role in the severity of dust occurrence. In fact, the creation of dust can be a kind of reaction to the change of the earth's vegetation, in this regard, the role of human activities should be considered along with the natural conditions of geographical environments. For example, in a region like the north of Iran, which is not naturally favorable for dust because it has both high humidity and dense vegetation, but due to human activities, today we see days with dust in this region. The most important of these effects is the reduction of vision and causing health problems such as breathing problems and cancellation of flights as a result of reduced vision.. The dust phenomenon in recent years is different and much more than the past dust storms in terms of the concentration and launch of suspended particles, its continuity, extent and time, and this has caused a lot of concern. In these years, continuous droughts, reduction of rainfall and relative humidity of the environment along with the aggravation of environmental factors by humans, such as excessive use of water resources and loss of vegetation, have caused a strong spread of dust. One of the main causes of this phenomenon is relatively strong winds blowing on lands with favorable conditions for creating dust. These factors, together with the upward movement of air caused by atmospheric systems, provide the vertical transfer of suspended dust particles to higher levels of the atmosphere. Suspended particles, according to their diameter, are placed in layers from the bottom to the top, and then move with the air flow and cover large areas. Most of the pervasive and widespread dust that is observed is extra-local and originates from other distant and nearby areas, but it is possible that it is caused by local factors and particles are formed from the same place where the dust is seen. Dust and sand storms have different definitions in different regions, because this phenomenon occurs in different places and under different conditions. According to the agreement of the World Meteorological Organization, whenever the wind speed exceeds 15 meters per second at a station and the horizontal visibility is less than one kilometer due to dust, a dust storm is reported. A sand storm refers to a wind that can move particles with a diameter of 0.15 to 0.30 mm up to a height of 15 meters. In this case, it is a sand storm, but in a dust storm, the suspended particles are smaller (Dehghanpour, 2014). Its occurrence causes many damages in the fields of environment, agriculture, transportation and health. Dust storms are mostly formed in arid and semi-arid regions due to the high speed of wind blowing on the surface of the soil without vegetation cover, and in the most severe case of dust storms, the concentration of suspended particles reaches 6000 mg per cubic meter of air (Song et al. [1], 2007: 112).  The results of the investigations have shown that environmental and human factors are effective in creating or intensifying dust storms. Geographical and climatic factors such as improper time and place distribution of rainfall and the occurrence of drought are natural factors and excessive livestock grazing, drying up of lakes and wetlands, population increase, changing the course of rivers and dam constructions are human factors that aggravate dust storms (Khushel Dastjerdi et al., 2013). In recent years, due to human intervention in nature and unprincipled exploitation of it, the phenomenon of dust has increased and thus the problem of dust has become more important than before. The increase in dust affects the formation of clouds and precipitation (Engelstadler et al.[2], 2006), the increase of heavy metals in the atmosphere (Vina et al.[3], 2008), the pollution of drinking water and the exacerbation of digestive diseases (Kruger et al.[4], 2004). For every 10 micrograms of dust per cubic meter of air, cardiovascular diseases increase by 12% and lung cancer by 14% (Shahsoni et al., 2010).

Due to its location in the dry and semi-arid belt of the world, Iran is continuously exposed to numerous local and synoptic dust systems. The origin of the dust affecting Iran is the deserts of North Africa, the Arabian Peninsula, Iraq and Syria, whose climate is very dry and the average rainfall is low. These dusts are transported to Iran due to the warming of the earth's air and the increase of wind speed. Along with the upward movement of the air caused by atmospheric systems, the fine particles go to the higher levels of the atmosphere and there, depending on the size of the diameter, they fit in the layers of the air. Then, with the air flow in the levels, they move and cover large areas (Alpert and Ganor[5], 1993; Dayan et al.[6], 1991). Dust systems in Iran lead to the cancellation of flights, closure of schools, offices and factories, the occurrence of respiratory diseases such as asthma (Hosseinzadeh, 1376) and also causes the reduction of agricultural products by 20% (Shahsoni et al., 1389). In recent years, due to the occurrence of droughts, it seems that there has been an increase in the occurrence of this dangerous weather phenomenon, which has caused problems in some regions of Iran (Rosoli et al. 2019).