تحلیل جانمایی سامانه‌های فشار در پهنه آفریا (Afria) طی روزهای غباری پنجاه ساله اخیر ایران

نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه جغرافیای طبیعی، دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران

2 پژوهشگاه هواشناسی و علوم جو، تهران، ایران

چکیده

مقدمه
سامانه‌های فشار شامل کم­فشارها و پرفشارها از مهمترین عوامل شکل­گیری گردش عمومی جو روی کره زمین هستند. این سامانه‌ها، گردش فصلی هوا و الگوی بادها را شکل می‌دهند و بسته به شرایط پهنه‌های آبی و خشکی که از روی آنها می‌وزند، نوع ذرات جابجا شونده بویژه ذرات شناور (در این پژوهش غبار) را تعیین می‌کنند.
منطقه مورد مطالعه
سرزمین پژوهش این تحقیق در رویکرد محیطی، کشور ایران را در جنوب باختری آسیا و با مساحت 1.65 میلیون کیلومتر مربع در برمی‌گیرد و با رویکرد همدید، پهنه‌ای را در عرض جغرافیایی از استوا تا 70 درجه شمالی و در طول جغرافیایی از 20 درجه باختری تا 100 درجه خاوری شامل است.
مواد و روش­ها
در پژوهش کنونی، کدهای ساعتی «06» به مفهوم کلی «افق دید بر حسب متر»، از مجموعه کدهای غبار برای 38 شهر از ایران طی نیم سده (1970 تا 2020) و مربوط به ساعت 15 به گاه محلی از سازمان هواشناسی کشور تهیه شد. در این تحقیق، روز غباری، روزی تعریف شد که دست کم دارای یک گزارش ساعتی از گرد و غبار باشد. گزینش ایستگاه‌های هواشناسی به شیوه‌ای انجام شد که سراسر ایران را فراگیرد. با چینش مقادیر افق دید در جداول روزانه و دستیابی به فراوانی ایستگاه‌های دارای ریزگرد (به شیوه کوته‌نوشت: اِدِر)، 612 نمودار از افت و خیزهای فراوانی ادرها (برای هر ماه یک نمودار) ترسیم شد و با کاربست دو شناسه «رخداد غبار بر اساس مسافت افق دید دست کم در 3 روز پیاپی» و «گزارش غبار دست کم در دو ایستگاه کنار هم»؛ جمعا 561 موج غبار شناسایی شد. از این رو هر موج غبار دارای سه عنصر آشکار، به ترتیب شامل یک دامنه افزایشی، یک روز اوج و سپس یک دامنه کاهشی از فراوانی ادرها بود. هدف از شیوه انتخاب پیوسته روزها برای بازیابی موج‌های غبار، آزمودن گمان تاثیر سامانه‌های فشار بر ترتیب و دوام عناصر یاد شده در قالب الگوهای جانمایی این سامانه‌ها بود. در بخش همدید، بارگیری نقشه‌های روزانه هوا در تراز 1000 هکتوپاسکال از پایگاه بازکاوی داده‌های جو و طراحی الگوهای جانمایی مراکز فشار اعم از کم فشار و پرفشار طی روزهای اوج غبار در دو نَهَنج زمانی انجام شد:
بار نخست «به تفکیک دوره‌های گرم و سرد سال» و بار دوم «به تفکیک دو دوره 25 ساله». برای این کار، ماه‌های «ژوئن، جولای،  اوت و سپتامبر» نماینده دوره گرم سال و ماه‌های «دسامبر، ژانویه، فوریه، مارس» نماینده دوره سرد سال در نظر گرفته شد.
نتایج و بحث
نخستین نتایج نشان داد طی نیم سده اخیر از میان 38 شهر برگزیده ایران، سه شهر بندرعباس، بندر گناوه و دزفول با تفاوتی آشکار از دیگر شهرها؛ غباری­ترین شهرهای ایران بوده‌اند. همچنین مشخص شد از 561 موج غبار بازیابی شده در ایران، 189 موج در دوره گرم سال و 207 موج در دوره سرد سال رخ داده‌اند. روندیابی فراوانی موج‌های غبار گویای افزایش مخاطره ریزگرد طی نیم سده اخیر بویژه در دوره گرم سال است. در بخش همدید، نتایج گویای پیشروی مراکز پرفشار جنب‌حاره طی 25 سال اخیر به سمت شمال بویژه در دوره سرد سال بود. به گونه‌ای که کمربند بیابانی در این بخش از کره زمین، به پهنه‌ای حایل میان سامانه‌های عموما کم­فشار در جنوب و سامانه‌های عموما پرفشار در شمال تبدیل شده است. این شرایط با شدتی کمتر در دوره گرم سال هم دیده می‌شود. در پژوهش کنونی، این پهنه تهی شده از سامانه‌های فشار طی روزهای اوج توفان‌های دوره سرد سال که از بیابان‌های آفریقا تا بیابان‌های آسیا امتداد یافته است؛ «آفریا» یعنی پهنه‌ای غبارخیز و مشترک میان «آفریقا و آسیا» نامیده شد تا گویای راستای مداری آن روی سرزمین‌های بیابانی این دو خشکی شامل ساهارا در خشکی افریقا و بیابان‌های حجاز، ایران، توران، تاریم و گبی در خشکی آسیا باشد. بیابان‌های این پهنه؛ وزشگاه جریان‌های هوا یا بادهای بسامان از یاخته هدلی است.
نتیجه­گیری
مقایسه جابجایی‌های شمالسوی رخ داده در جانمای کانون‌های کم­فشار و پرفشار یاد شده؛ بویژه در گستره سه پهنه «بیابان افریقا»، «آب‌های مدیترانه جنوبی» و «حجاز»، گمان گسترش یاخته هدلی را مطرح می‌کند. چنانچه در آینده و با داده‌های کافی بتوان این گسترش را علاوه بر سه پهنه یاد شده، به سرزمین ایران هم تعمیم داد؛ می‌توان گفت از میان چشمه‌های غبار که از پیرامون، کشور ما را نشانه رفته‌اند، در آینده باید شاهد تقویت بیش از پیش چشمه توران (آسیای میانه) به سوی چاهه سیستان و بلوچستان در خاور و جنوب خاوری ایران و در نتیجه روزهایی تاریک‌تر از غبار باشیم. البته مرتبط با این هشدار، در الگوی همدید دوره گرم سال نیز به شکل محسوس، تقویت و جنوبی‌ترشدن پرفشارهای آسیای میانه مشاهده شد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Analysis of the positioning of pressure systems in the Afria Zone during the last fifty years of dust days in Iran

نویسندگان [English]

  • Gholamreza Barati 1
  • Mohammad Moradi 2
  • Mersad Jafari Gharehchi 1
1 Department of Physical Geography, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
2 Research Institute of Meteorology and Atmospheric Science (RIMAS), Tehran, Iran
چکیده [English]

Introduction
Pressure systems, including low and high pressures, are among the most important factors in shaping the general circulation of the atmosphere on Earth. These systems influence seasonal air circulation and wind patterns, and depending on the conditions of the water and land areas over which they blow, they determine the type of movable particles, especially floating particles (in this research, dust).
Study area
The land of research for this study with an environmental approach, encompasses the country of Iran in the southwest of Asia, covering an area of 1.65 million square kilometers. With a synoptic approach, it includes a region extending from the equator to 70 degrees north latitude and from 20 degrees west to 100 degrees east longitude.
 
Materials and Methods
In the current research, the hourly codes "06," representing the general concept of "visibility in meters," were collected from the dust code set for 38 cities in Iran over half a century (1970 to 2020) and related to the local time of 15:00 from the National Meteorological Organization. In this study, a dusty day was defined as a day with at least one hourly dust report. The selection of meteorological stations was conducted in a way that covers all over Iran. By arranging visibility values in daily tables and obtaining the frequency of stations with dust (abbreviated as "EDR"), 612 charts of the fluctuations in EDR frequency (one chart for each month) were drawn, and by applying two identifiers of "dust events based on visibility distance for at least during 3 consecutive days" and "dust reports from at least two adjacent stations," a total of 561 dust waves were identified. Therefore, each dust wave has three distinct elements, which include an increasing amplitude, a peak day, and then a decreasing amplitude of dust frequency. The aim of the continuous selection method of days for retrieving dust waves was to test the hypothesis of the impact of pressure systems on the arrangement and duration of the elements mentioned above in the form of positioning patterns of these systems. In the synoptic section, the loading of daily air-maps at the level of 1000 hPa from NCEP/ENCAR and the design of positioning patterns of pressure centers, both low and high pressure, during peak dust days was conducted in two time frames: the first time "distinguished by warm and cold periods of the year" and the second time "distinguished by two 25-year periods." For this purpose, the months of "June, July, August, and September" were considered representative of the warm period of the year, while the months of "December, January, February, and March" were considered representative of the cold period of the year.
Results and Discussion
The initial results showed that over the past half-century, among the 38 selected cities in Iran, three cities -Bandar Abbas, Bandar Genaveh, and Dezful—have been distinctly the dustiest in Iran. It was also determined that out of 561 dust wave events recorded in Iran, 189 occurred during the warm and 207 during the cold season. The trend in the frequency of dust waves indicates an increase in this dust hazard over the past half-century, especially during the warm season. In the synoptic section, the results indicated the northward advance of subtropical high-pressure centers over the past 25 years, particularly during the cold season. As a result, the desert belt in this part of the Earth has transformed into a buffer zone between generally low-pressure systems in the south and generally high-pressure systems in the north. This condition is also observed, albeit with less intensity, during the warm season. In this study, this supercontinent devoid of pressure systems during peak storm days of the cold season, extending from the deserts of Africa to the deserts of Asia, was referred to as "Afria" to indicate its orbital direction over the desert lands of these two continents, including the Sahara in Africa and the deserts of Hijaz, Iran, Turan, Tarim and Gobi in Asia. The deserts of this zone are the source of air currents or well-organized winds from the Hadley cell.
 
Conclusion
Comparison of the northerly positioning of subtropical high-pressure foci suggested the expansion of the Hadley cell in three zones of "African Desert", "Southern Mediterranean Waters" and "Hejaz". If it is determined in the future and with sufficient data that this expansion has also included the land of Iran, we should be more concerned than ever about the arrival of dust from the dusty deserts of Turkmenistan (Central Asia) to Sistan and Baloochestan in the east and southeast of Iran, and as a result, days darker than dust.

کلیدواژه‌ها [English]

  • pressure systems
  • Afria zone
  • dust days
  • Iran

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پژوهشهای دانش زمین
دوره 16، شماره 2 - شماره پیاپی 62
پژوهشی
تیر 1404
صفحه 131-144

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