مدل‌سازی عددی و آشکارسازی اتوماتیک پیچک‌های زیرمیان‌مقیاس (Submesoscale) در خلیج‌فارس با استفاده از یک الگوریتم هندسه برداری

نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشجوی دکتری، گروه فیزیک دریا، دانشکده علوم دریایی و اقیانوسی، دانشگاه علوم و فنون دریایی خرمشهر، خرمشهر، ایران

2 استادیار، گروه فیزیک دریا، دانشکده علوم دریایی و اقیانوسی، دانشگاه علوم و فنون دریایی خرمشهر، خرمشهر، ایران

3 دانشیار، گروه فیزیک دریا، دانشکده علوم دریایی، دانشگاه مازندران، بابلسر، ایران

چکیده

پیچک­ها یکی از پدیده­های معمول دریایی هستند که باعث انتقال انرژی و جرم در دریا می­شوند. شناسایی و استخراج پیچک­ها یکی از جنبه­های مهم اقیانوس­شناسی فیزیکی است و الگوریتم­های اتوماتیک شناسایی پیچک­ها از اساسی­ترین ابزارها برای آشکارسازی و تحلیل پیچک­ها هستند. از نظر ابعاد و مدت­دوام، پیچک­ها به دو نوع میان­مقیاس و زیرمیان­مقیاس تقسیم می­شوند که پیچک­های زیرمیان­مقیاس دارای مقیاس طولی 100 متر تا 10 کیلومتر و مقیاس زمانی 1 تا 10 روز هستند. در این مطالعه از الگوریتم هندسه برداری برای شناسایی اتوماتیک پیچک­ها در خلیج­فارس استفاده شده است که این الگوریتم بر مبنای چرخش بردار سرعت جریان عمل می‌کند. داده­های اصلی استفاده­شده برای آشکارسازی پیچک­ها، خروجی­های مدل عددی شامل مؤلفه‌های سرعت هستند. این خروجی­ها حاصل مدل‌سازی عددی با درنظرگرفتن واداشت‌های گرما-شوری و تنش باد هستند که پس از پایداری و صحت­سنجی مدل عددی جهت استخراج پیچک­ها به‌عنوان ورودی الگوریتم در نظر گرفته شده­اند. در مجموع 4308 پیچک چرخندی و 2860 پیچک واچرخندی در لایه­ی سطحی و 617 پیچک چرخندی و 329 پیچک واچرخندی در پایین­ترین لایه یعنی عمق 50 متری به‌ازای داده‌های روزانه طی یک سال خروجی مدل شناسایی شد. تعداد پیچک­ها در فصل زمستان بیشترین و در فصل تابستان کمترین است و شعاع میانگین پیچک­های واچرخندی در فصل زمستان و چرخندی در فصل تابستان بیشینه است. بیشترین شعاع پیچک­ها در بازه بین 5-10 کیلومتر و مدت دوام بیشتر آنها نیز بین 3-6 روز است. همچنین هرچه طول عمر پیچک‌ها و شعاع آنها بیشتر باشد، می‌توانند در عمق بیشتری نفوذ کنند.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Numerical Modelling and Automatic Detection of submesoscale eddies in Persian Gulf Using aVector Geometry Algorithm

نویسندگان [English]

  • Omid Mahpeykar 1
  • Amir Ashtari Larki 2
  • Mohammad Akbarinasab 3
1 Ph.D. Student, Department of physical oceanography, Faculty of marine science and oceanography, Khorramshahr University of Marine Science and Technology, Khorramshahr, Iran
2 Assistant Professor, Department of physical oceanography, Faculty of marine science and oceanography, Khorramshahr University of Marine Science and Technology, Khorramshahr, Iran
3 Associate Professor, Department of Marine Physic, Faculty of Marine and Oceanic Sciences, University of Mazandaran, Babolsar, Iran
چکیده [English]

Nowadays, marine data containing both observational and measured values as well as the output of numerical models are largely available; But analyzing and processing this data is time consuming and tedious due to the heavy volume of information.Identifying and extracting eddies is one of the most important aspects of physical oceanography, and automatic detection algorithms of eddies are one of the most basic tools for analysing eddies. The general circulation of the Persian Gulf is a cyclonic circulation that is affected by tide, wind stress, and thermohaline forces. In this study, using the Mike model based on the three-dimensional solution of the Navier Stokes equations, assumption of incompressibility, Boussinesq approximation and hydrostatic pressure, the circulation in the Persian Gulf was modeled. Then a vector geometry algorithm has been used for detection of eddies in this region. Four constraints were derived in conformance with the eddy velocity field definition and characteristics in this algorithm. Eddy centers are determined at the points where all of the constraints are satisfied. The four constraints follow: (i) Along an east–west (EW) section, v has to reverse in sign across the eddy center, and its magnitude has to increase away from it; (ii) Along a north–south (NS) section, u has to reverse in sign across the eddy center, and its magnitude has to increase away from it: the sense of rotation has to be the same as for v; (iii) Velocity magnitude has a local minimum at the eddy center; and (iv) Around the eddy center, the directions of the velocity vectors have to change with a constant sense of rotation. The constraints require two parameters to be specified: one for the first, second, and fourth constraints and one for the third one. The first parameter, a, defines how many grids points away the increases in the magnitude of v along the EW axes and u along the NS axes are checked. It also defines the curve around the eddy center along which the change in direction of the velocity vectors is inspected. The second parameter, b, defines the dimension (in grid points) of the area used to define the local minimum of velocity. The main data used to detect eddies are numerical model outputs, including velocity components. These outputs are the result of numerical modeling with thermohaline and wind stress forces. In total, 4308 cyclonic and 2860 anticyclonic eddies are detected at the surface and 617 cyclonic and 329 anticyclonic eddies are found in the deepest layer, depth of 50 meters, for daily data during one year. The number of eddies is highest in winter, and the lowest in summer and the average radius of anticyclonic eddies is maximum in winter and minimum in summer for cyclonic eddies. Most eddies have a radius of 5-10 km and lifespan of 3-6 days. Also, as the lifespan of eddies increases, they penetrate deeper into the water. The percentage of eddy penetration or the ratio of the number of eddies of the deepest layer to the surface layer is 15% for cyclonic eddies and 10% for anticyclonic eddies. This indicates that the energy loss in the cyclonic eddies is less than in the anticyclonic eddies and is probably due to the alignment of the rotating eddy with the overall circulation of the Persian Gulf.

کلیدواژه‌ها [English]

  • Persian Gulf
  • Eddy
  • Submesoscale
  • Vector Geometry Algorithm
  • Eddy Radius
  • Eddy Life time
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