نقش رودباد در چرخندزايي ميانه مديترانه

On the Role of the Jet Stream in the Central Mediterranean Cyclogenesis

منطقه ميانه مديترانه يكي از مناطق چرخندزايي در حوضه مديترانه مي‌باشد كه تغييرات رخ داده در رودباد و واگرايي سطوح بالا عامل مهمي درچرخندزايي اين منطقه مي‌باشد. به منظور بررسي اثر رودباد داده­هاي سرعت مداري و نصف النهاري باد، سرعت قائم، ارتفاع ژيوپتانسيل از داده‌هاي بازيابي شده در تارنماي NCEP/NCAR  استخراج شدند. سپس بر اساس آن ميدان واگرايي و فرارفت مثبت تاوايي نسبي نيز محاسبه شدند. در ابتدا الگوهاي تركيبي حاصل از چند سامانه ميانه مديترانه ترسيم شده و در انتها نيز به يك مورد خاص در فوريه 1974 اشاره مي‌شود. در الگوهاي تركيبي هر دو رودباد جنب حاره و قطبي مي‌توانند در شكل گيري سامانه هاي ميانه مديترانه موثر باشند. ميدان واگرايي در سمت چپ خروجي رودباد قطبي شمالي جنوبي عاملي در جهت شكل گيري همگرايي سطحي، سرعت قائم بالاسو و توسعه ناوه وارون فشاري به سمت ميانه مديترانه مي باشد. اما باتغيير ميدان واگرايي و باد در سطوح بالاي جو، سامانه به سمت چپ خروجي رودباد جنب حاره انتقال يافته و درعين حال تقويت نيز مي شود. به اين ترتيب رودباد جنب حاره در طي فرآيند توسعه موثر است. اما در برخي موارد فرآيند توسعه سامانه با حضور رودباد قطبي غربيشرقي بر روي درياي سياه، با برهم كنش خروجي رودباد جنب حاره و ورودي شكل مي­گيرد. در سامانه فوريه 1974 در روز 21 فوريه حضور رودباد جنب حاره منجر به گسترش ناوه وارون فشاري به سمت ميانه مديترانه مي شود و در اين سامانه خاص رودباد قطبي شمالي جنوبي نقشي در شكل گيري سامانه ندارد. اما در طي 24 ساعت بعد بيشينه تندي باد در مركز رودباد جنب حاره به72 متر بر ثانيه رسيده و همچنين خميدگي چرخندي و چينش افقي باد در سمت چپ رودباد و در بالاي منطقه ميانه مديترانه تا شمال ليبي تشديد مي شود كه منجر به تقويت كم فشارسطحي مي شود. بنابراين مي توان نتيجه گرفت كه فرآيند توسعه كم فشار سطحي تماماً تحت تاثير رودباد جنب حاره بوده وروباد قطبي غربي شرقي و يا شمالي – جنوبي در اين چرخندزايي نقشي ندارد

Introduction The central Mediterranean Sea is well known as a cyclogenesis area. The Jet streams associated with divergence, are important factors in cyclogenesis in the area. The Subtropical and polar jet streams are flowing normally in the upper layer in 300 200 hPa. Although the subtropical or polar jet is associated with cyclogenesis, sometimes the cyclones develop in lower latitudes with respect to the interaction region of the jet streams.   Method and Material The meridional and zonal wind, vertical motion, geopotentail height (gpm) and mean sea level pressure (mslp) from the reanalysis data of NCEP/NCAR were derived. The data are being archived from 1948 with a 2.5 degree horizontal resolution at 17 levels. Then, the divergence and positive vorticity advections were calculated by the wind data. Finally, the composite structure of several Mediterranean cyclonic systems was plotted and the figures for the case study (21 and 23 Feb 1974) were obtained.      Results and Discussions The wind speed of 45 with a  divergence in the exit region of the subtropical jet in 25 to 27.5 No latitudes is demonstrated in the composite Figures for the formed cyclonic event. In addition there is a northsouth polar jet in Western Europe of its left exit with weak divergence. In the cases with the process of cyclone development, normally the wind speed increase to 50 and more and an intensive horizontal wind shear forms in the left side of the Subtropical jet. Intensified convergence in 850 hPa and upward motion in 500 hPa are favorable factors for cyclogenesis development and changes in the related wind filed cause an increase in divergence up to. In the Central Mediterranean, cyclones are developed normally by presence of the subtropical jet stream near the North of Libya. In the case study, of 21 Feb 1974, the maximum wind speed of 62 were observed in the exist region of the jet over North Libya with a northeast to southwest direction in 200 hPa.  The inverted pressure trough was formed in lower troposphere under the upper level jet and weak divergence field. The positive vorticity advection coincided with maximum upward motion in east of the upper trough was above the midlevel inverted trough. the upper level wind speed and divergence field extended from  north Libya to central Mediterranean and west Turkey were developed and led to increase of  convergence in 850 hPa in east of the cyclone.  In middle level, the trough lification is associated with enhancement of upward motion and positive vorticity advection. Thus the cyclone was developed due to forcings from lower and upper levels and midlevel forcings support cyclogensis. Conclusion Although the subtropical and polar jets can affect central Mediterranean cyclogenesis by a composite structure, so that cyclone can be formed in the polar jet left exit and developed in subtropical jet left exit. Thus, cyclones develop only by subtropical jet and form by both subtropical and polar jet. Review of weather system shows expansion of the inverted pressure trough   on 21 Feb 1974 to central Mediterranean from Africa, affected by Subtropical jet associated divergence filed. The polar jet was not observed on the day.  Within the next 24 hours ending to 23 Feb, wind speed in 200 hpa reaches to 72 m/s with intensified cyclonic curvature and caused upper level divergence and favorable conditions for cyclogenesis. So, some central Mediterranean cyclones can be formed and developed only by subtropical jet without interaction by polar jet.