نوع مقاله : مقاله پژوهشی
نویسندگان
1 'گروه مهندسی آبیاری و آبادانی، دانشکده مهندسی و فناوری کشاورزی، دانشکده کشاورزی، دانشگاه تهران
2 استادیار/گروه مهندسی آبیاری و آبادانی، دانشکده مهندسی و فناوری کشاورزی، پردیس کشاورزی و منابع طبیعی دانشگاه تهران
3 گروه مهندسی آب، دانشکده کشاورزی، دانشگاه شیراز
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
This study investigates upper-level atmospheric circulation over Iran's North-Western region, centered on Tabriz, from 1948 to 2010. The core of the research is the creation of a Daily Cyclonicity Index (DCI) for the 500 hPa and 700 hPa levels, designed to quantify the influence of cyclonic troughs and anticyclonic ridges, utilizing 23011 daily geopotential height maps from the NCEP/NCAR Reanalysis. The study classifies circulation into five distinct types: Trough Line (TL), Trough Edge (TE), Geopotential Col (COL), Ridge Edge (RE), and Ridge Line (RL). The analysis of mean annual frequencies at 500 hPa reveals a pronounced dominance of high-pressure systems. Ridge patterns (RL and RE) collectively influence the East Azerbaijan area for 53% of the year, while trough patterns (TL and TE) occur only 22% of the time. This anticyclonic dominance is especially strong in the warm season, governed by the subtropical high-pressure belt. A more balanced distribution emerges in the cold season, with high, col, and low systems occurring 43%, 28%, and 29% of the time, respectively. The long-term trend assessment using the Mann-Kendall test identifies a significant shift in circulation patterns over the 63-year period. The study documents a statistically significant decrease in the frequency of low-pressure systems and a concurrent increase in high-pressure systems. This trend provides a clear signal of changing atmospheric dynamics in the region, with important implications for local climate, including potential impacts on precipitation amount and patterns and drought frequency in Iran's North-West.
کلیدواژهها [English]
Synoptic indices are criteria by which temporal changes in intensity and spatial changes in circulation patterns are measured. The cyclonicity index represents the degree of influence of a cyclonic or anticyclonic system in a region. This study investigates the upper-level atmospheric circulation over the northwestern region of Iran, centered on Tabriz (the Capital of East Azerbaijan province), from 1948 to 2010. The principal innovation of this research is the formulation of a Daily Cyclonicity Index (DCI) at the 500 hPa and 700 hPa levels. Given the availability of long-term climate statistics and daily synoptic pressure maps through climate databases, it is necessary to conduct sufficient studies on the classification of atmospheric circulation in Iran on a daily scale; to identify the types of prevailing pressure patterns over each region; and to extract their temporal and spatial distribution. The objectives of this research are as follows: 1) to create daily Cyclonicity indices for East Azerbaijan province at the 500 and 700 millibar levels during the period 1948-2010; 2) to calculate the frequency of pressure systems based on the number of days of dominance per year; 3) To determine the trend in the vorticity index over the study period.
This study utilizes 23,011 daily geopotential height maps from the NCEP/NCAR reanalysis to investigate the upper-level atmospheric circulation over the northwestern region of Iran, centered on Tabriz, from 1948 to 2010. One method of classifying atmospheric circulation is through the use of indices. For this purpose, a Daily Cyclonicity Index (DCI) centered around Tabriz was created based on the mean daily 500 hPa and 700 hPa level maps extracted from NCEP reanalysis data using GrADS software for the period 1948-2010, using the criterion of height level curvature. The advantage of these methods is that they have a physical basis and region. The method used in this research for classifying atmospheric circulation is objective, but the assignment of scores is based on a manual (visual observation) method. Furthermore, the Mann-Kendall non-parametric test was used to analyze data trends and their significance.
The mean DCI on an overall annual scale, for the cold half-year (October, November, December, January, February, March), and for the warm half-year (April, May, June, July, August, September) during the period 1948-2010 for the 500 mb (and 700 mb) level were -0.97, -0.19, and -0.90 (-0.27, -0.06, and -0.25), respectively. Furthermore, the frequency of the 500 hPa and 700 hPa geopotential height patterns, representing various types of atmospheric circulation around Tabriz, was calculated for the period 1948-2010. The frequency of high-pressure systems was 192 days per year (53%), compared to a frequency of 81 days per year (22%) for low-pressure systems.
According to the DCI, five types of atmospheric circulation were identified: trough line, trough edge, height col, ridge edge, and ridge line. Subsequently, the frequency of pressure systems passing over Tabriz was determined. Overall, the frequency of days with high-pressure systems, corresponding to the ridge pattern (192 days per year, equivalent to 53%), is greater than that of low-pressure systems, corresponding to the trough pattern (81 days per year, equivalent to 22%). On an intra-annual scale, during the warm half of the year, the number of low-pressure systems (30 days per year, equivalent to 8%) contrasts with that of high-pressure systems (114 days per year, equivalent to 31%). Therefore, the ridge pattern has absolute dominance, indicating the emergence of the subtropical high-pressure belt over the region. Consequently, Tabriz has a relatively warm and dry climate during the warm half of the year. The results of the Mann-Kendall trend test on an annual scale show a significant decrease in the frequency of low-pressure systems (trough pattern) with a slope of -0.79 and a significant increase in the frequency of high-pressure systems (ridge pattern) with a slope of +1.67 during the period 1948-2010. This signals a change in the regional atmospheric circulation climate of East Azerbaijan. Since the trend's intensity is greater in the spring and autumn seasons than in summer and winter, it can be inferred that the days at the end of spring and the beginning of autumn have merged with the summer pattern, and the period of dominance of the subtropical high-pressure belt has lengthened.
Data available on request from the author.
This research is derived from Project No. 99031950 of the Health, Environment, and Climate Change Working Group of the Iran National Science Foundation (INSF). The authors would like to express their special thanks to the vice chancellor for research affairs.
The authors avoided data fabrication, falsification, plagiarism, and misconduct.
All authors declare that they have no conflict of interest.
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