Analyzing the relationship between Siberian high pressure and critical temperatures in the cold period of the year in central Iran

Dissertation for Master's Degree (M.Sc)

Field: Natural Geography - Hydrology and Synoptic Meteorology

Abstract

Damage caused by critical cold temperatures is increasing, especially in natural ecosystems and densely populated urban areas. Central Iran is a dry land prone to cold weather, especially during winter, and lacks resources to moderate temperature tensions, such as water resources and dense vegetation. In the present research, to determine the cold waves in central Iran, the daily maximum temperature data of eight cities including Shahrood, Tehran, Semnan, Kashan, Isfahan, Yazd, Kerman and Bam were prepared and their zero and subzero temperatures were extracted as cold waves. Based on this, 47 cold waves were identified in the statistical period (1980-2012) and in the second stage, 17 cases of critical cold were determined based on 2 temporal and spatial criteria, i.e. lasting for two days and the conflict of two stations and more. Opening the air alignment data files prepared from the NCEP/NCAR archives, converting them into text folders during the dates of critical colds and drawing daily and combined air maps using Surfer software were the three steps of the work that led to the design of synoptic patterns to determine the alignment and dominant side of Siberian cold air advection. The results showed that the cold advection towards central Iran occurred in 70% of the time at the lowest levels (sea level and 1000 hPa) and in the form of Siberian high pressure tongues, and only in 12% of the time the effect of the high pressure of Tibet and 17% of the effect of the migrating cold high pressure was revealed.

Analysis of wind and temperature vector patterns at the level of 1000 AD. For the test of the advection side of cold air during the days of critical cold events, it showed that in 70% of cases (12 out of 17 cases), the wind vectors came from the southeast as a sequence of easterly winds over Afghanistan and Pakistan, and sometimes from the northeast and east over Central Iran.

Keywords: Central Iran, critical cold, eastern advection.

Chapter First, the beginning

Change in the frequency of weather disasters such as cold waves is one of the most important aspects of climate change (Kankal et al.; 1999). Every year, many people die due to the spread and occurrence of heat waves or cold storms (Alijani, 2018). Meanwhile, very cold temperatures are defined as danger or crisis for humans. Basically, the element of temperature has been of interest to meteorologists for a long time (Asakara; 1388), its short-term and long-term changes can change the climate structure of any place. According to the report of the International Commission on Climate and Climate (IPPC, 2001), the temperature of the earth's surface has increased by about 0.6 degrees Celsius between 1861-2001 AD. This is while the behavior of the minimum and maximum temperature meter is different from each other, the minimum temperature has obviously increased. Despite the increase in maximum temperature, its rate has been lower than the rate of minimum temperature (Karl et al., 1993). Therefore, according to Shabani et al. (1392: 896), modeling the limit variables of air temperature and forecasting the maximum and minimum temperatures as one of the most important weather parameters with regard to climate changes, global warming, and droughts, definitely provide more opportunities to plan and provide the necessary preparations, especially in climates. provides dry and semi-dry to planners.

1-1).  Problem statement

Iran has special conditions in terms of temperature due to its large extent in latitude and longitude, complexity in the configuration of unevenness and the attack of air masses of various origins. Repetition of air masses that reach Iran by means of circulation and vorticity systems or the expansion of their tongues and determine the humidity and temperature conditions of the daily air; In the long term, it creates the climatic conditions of Iran (Alijani, 2013: 37).

Due to special geographical conditions, i.e. its location in relation to the general circulation of the atmosphere and its location in the middle latitudes, in different seasons of the year it is affected by anticyclones with different origins and different physical characteristics, among these anticyclones is the Siberian high pressure, which is the most important center of Eurasian atmospheric action during winter. and has a noticeable effect on the climate of Iran (Lashkari 1375: 4)..

Among the systems affecting the air mass entering Iran, the following can be mentioned;

n generally warm and dry air mass caused by the subtropical high pressure: The subtropical high pressure is a large dynamic (dynamic) system on a planetary scale that has a focus on the North Atlantic Ocean. During the hot period of the year, there is a band of high pressure near Iran over Iran. The territory of this tab extends from the level of 700 to 1000 AD and it causes hot and dry air rule over a large part of Iran (Shabankar and Jalebian, 2009: p. 48), especially in the lands far from the coast.  With the movement of high pressure subtropical monsoons and the accompanying monsoon to lower latitudes, from December and the entry of western winds into Iran in the cold period of the year (Imam Hadi, 2013: 36), we can witness the arrival of other air masses.

n Cold and dry air masses caused by Siberian high pressure: Siberian high pressure dominates Asia from mid-October to mid-April. This system plays an important temperature role in Iran and as previously researched by Barati and Alijani (1378: 55), its cold fronts penetrate into Iran from the north-northeast and sometimes the northwest during the spring season. Masoudian (2016: 22) also refers to the spread of this very cold and dry air over parts of the country.

n Mediterranean oceanic air masses caused by western winds: The Mediterranean Sea is in the path of western winds and its effects spread to Iran through these winds. In the cold period of the year, due to the establishment of the Mediterranean high, the western air pressure systems, including high-level waves and gyres on the ground, come to Iran. Therefore, the moisture of rainfall in the cold period of the year is provided by the migratory systems of the Mediterranean Sea (Alijani; 2013).

n Relatively warm and humid air mass caused by the entry of Sudanese low pressure zones: Sudanese low pressure which forms in North Africa and acts as a thermal low pressure in the summer season and has a dynamic behavior in the winter. Whenever the Sudanese low pressure and Mediterranean low pressure are connected together, A deep trough occurs in the eastern Mediterranean and causes the entry of relatively warm and humid masses and heavy rains in most parts of Iran (Joanmard et al., 2012).

1-1-1). The importance of critical colds according to researchers

Temperature is one of the most important elements of climate and its sudden changes, especially temperature drops below zero degrees, are the source of many physical, chemical and environmental changes. For this reason, the study of temperature trends in different temporal and spatial scales has been of interest to researchers and has occupied a large part of meteorology literature. In this context, we are going to express the importance of the issue of cold critical temperatures from three aspects: