نوع مقاله : مقاله پژوهشی
عنوان مقاله English
نویسندگان English
ABSTRACT
This research identifies the structural nature of the systems that create warm extreme temperatures in the east and northeast of Iran. Two databases were applied: meteorological station data and data from the National Center for Environmental and Atmospheric Environmental Statistics (NCEP / NCAR). To isolate days exhibiting warm extreme temperatures, we analyzed geopotential height and sea-level pressure data to identify circulation patterns leading to this phenomenon. The results show six different patterns in the middle level of the atmosphere and the Earth's surface. One day was representative of a further understanding of the structure of heat-related systems in the region. The research findings show that the advance of the high-pressure cyclone to higher latitudes and its continuation for several days over the region causes the formation of convergence nuclei in the upper atmosphere and results in dynamic air descent. The subtropical high pressure in this month (June) has two cores of activity, one over the Sahara and the other over Arabia, with an average height between 5,900 and 5,910 geopotential meters. On June 13, it is 5,900 geopotential meters based on long-term averaging. Generally, Large-scale patterns of change, such as the displacement of high-pressure nuclei, also play a role in this phenomenon.
Introduction
The atmosphere's general circulation follows a specific pattern of time and location. However, significant changes in parameters can disrupt these patterns and impact temperature. Heat waves are one of the consequences of these changes. Heat waves occur when stable air with decreasing motion brings hot air into an area. The disruption of Western currents, caused by systems such as bundles during warm years, can create heat waves in specific areas. Identifying the governing patterns and synoptic systems variability during heat waves is crucial. Previous research on heat waves has been conducted globally and in Iran. For instance, Campetella and Rusticucci (1998) analyzed a severe heat wave in Argentina in March 1980 and identified the continuous action of anticyclones at all atmospheric levels, as well as the impact of warm air convection and high radiation-induced energy absorption, as the causative factors of the phenomenon. This research aims to identify the patterns of ground level and upper level of the atmosphere during the occurrence of extremely warm temperatures in eastern and northeastern Iran and examine the variability of related atmospheric systems.
Material and Methods
The study used daily maximum temperature statistics from 21 weather stations in Golestan, North Khorasan, Razavi, and South provinces to analyze the synoptic pattern during warm extreme temperatures. The data, covering the years 1986 to 2011, were obtained from the country's Meteorological Organization. The 95th percentile index was used to identify days associated with warm extreme temperatures. Additionally, data on surface pressure, geopotential heights, air temperature, relative humidity, wind components, flux, instability index, and landmarks were obtained from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) website.
Results and Discussion
The analysis of the 500-hPa level map shows the formation and deployment of a high ridge at 15 degrees with a height of 5850 geopotential meters over Iran. This has led to the eastern part of the country being under the influence of the system. The upper-level airflow and heating have created a stable atmosphere for the region. At the surface, the heating caused by the ridge system has created a low-pressure system in the northeast of Iran. In Pakistan, a high-pressure system was observed in the southeast of the country. The Outgoing Longwave Radiation (OLR) index has indicated the presence of high pressure at the upper level and air movement. In conclusion, the findings of this study indicate that the presence of tropical high pressure at higher latitudes, along with its persistence in the region for multiple days, leads to the formation of convergence areas in the upper troposphere.
Conclusion
This results in the dynamic descent of air and its subsequent heating. Due to the heating generated by the system, a low-pressure area with a central pressure of less than 1,000 HPAs is formed in eastern Iran, creating favorable conditions for the occurrence of extreme temperatures. However, due to the high altitude of the middle atmosphere, this circulation does not extend beyond the 850 to 700 hPa level. Another characteristic of this system is the rapid decrease in humidity and rapid increase in temperature, both vertically and horizontally. By examining the conditions and extent of the tropical high-pressure system, it was identified that its core activity was most pronounced in June. During this month, the tropical high pressure has two activity centers, one in the desert and one in Saudi Arabia, with an average height of 5900 to 5910 geopotential meters. On June 13, the long-term average height is 5900 geopotential meters, but there is a spatial variation with a representative core height of 20 geopotential meters. In these changes, the Saudi Arabian core completely retreats and merges with the West African core. On the other hand, over the Atlantic Ocean, a strong core with a height of 5920 geopotential meters is present. Therefore, it can be concluded that the occurrence of extremely warm temperatures in the east and northeast of Iran is closely related to synoptic and planetary patterns. The advancement of higher latitudes and the intensification of their activity, along with the warming of the upper atmosphere, contribute to the occurrence of these extreme temperatures during the warm period of the year.
کلیدواژهها English
