روابط ناوه تراز مياني جو با رخداد شديدترين يخبندان‌هاي ايران

Midtropospheric Trough and Severe Frosts in Iran

هر ساله رخداد يخبندان‌ هاي شديد اعم از پاييزه، زمستانه و بهاره ، باعث بروز زيان‌هاي فراوان‌ در بخش كشاورزي، دامداري، صنعت و ترابري ايران مي‌شود. در اين تحقيق،با تهيه آمار ميانگين دماي روزانه‌ي صفر و زير صفر از 50 ايستگاه داده سنجي جوي از نوع همديد در سراسر ايران، 75 موج يخبندان طي دهه اخير يعني از سال 1994 تا سال 2003 شناسايي شد. بر پايه سنجه‌­هاي كمي تعريف شده شامل فراواني موج‌هاي يخبندان، تاريخ‌هاي آغاز و پايان هر موج، شدت و تداوم زماني آن‌ها، 31 موج شاخص برگزيده شد و ويژگي‌هاي همديد آن‌ها بررسي گرديد. نتايج تحقيق نشان داد همه ‌ساله شديدترين يخبندان‌ها در سطح سرزمين ايران طي زمستان رخ داده‌اند بجز سال 1373 كه در پاييز بوده است. در ميان ماه‌هاي سال، ماه بهمن رتبه‌ي پريخبندان ترين و ماه دي رتبه‌ي سردترين ماه را كسب كردند. در حاليكه از پيش نيز انتظار مي‌رفت، خسارت باري يخبندان‌هاي بهاره بيشتر باشد؛ نتايج نشان داد در عوض فراواني يخبندان هاي پاييزه بيشتر بوده است. در اين حال بر پايه الگو هاي همديد تهيه شده، محل بيشينه ناوه‌ هاي غربي در ايران 10 درجه جغرافيايي پايين‌تر از محل بيشينه آن‌ها در نيمكره جنوبي مشاهده شد و شدت يخبندان‌ها تا 58 درصد با عمق ناوه‌هاي غربي رابطه معني‌دار داشت. الگوي طراحي شده براي محورهاي ناوه‌ طي روزهاي اوج، جاگيري عموم محورها را در نيمه‌ي خاوري ايران با امتداد شمال شرقي – جنوب غربي نشان داد. اين جاگيري به تسهيل شارش جريان‌هاي بسيار سرد از عرض‌هاي بالاي جغرافيايي تا جنوبي‌ترين مناطق ايران انجاميد. ناوه­هاي مهاجر عرض‌هاي بالاتر يخبندان‌هاي شديدتر و ناوه­هاي مهاجر عرض‌هاي پايين‌تر يخبندان هاي فراگيرتر در پي داشتند. در اين راستا راهكار بنيادين براي كاهش زيان‌هاي كشاورزان بويژه باغداران در مناطق پرآسيب، تغيير الگوي كشت به لحاظ مكاني و استفاده از بذرها و گونه‌ هاي مقاوم و ديربازده به لحاظ زماني است.

Sever frosts during autumn, winter and spring seasons cause damages in the agriculture, industry and transport parts of Iran every year. These frosts are including radiation and frontal types. The location of Iran in midlatitudes and the spreading of westerlies over much of Iran especially during cold halfyear provide the happening and strengthening of frontal frosts. The cA airmasses from north (Arctic basin), cP and mP ones respectively from northeast (Siberian basin) and (Europe and Black Sea basin) causes frosts in Iran. Our study area within 25° 40° north latitude and 44° 63° east longitude is at the risk of cold air attacks during cold halfyear. Thus we selected 50 air stations from throughout Iran in the southwest of Asia. In first step, we determined 75 frost waves based on mean daily temperatures including the zero and under zero degrees Celsius during recent decade from 1994 to 2003 years. In second step, we could determine 31 cold air waves based on the daily distribution of low temperatures and so their beginning, peak and end (based on date), their severity (based on air temperature) and duration (based on day). We measured the depth of every trough during the peak day of every cold air waves for statistical analysis including plot and regression. Results showed the severe frosts have happened during winter except the frost wave of 1994 that happened during autumn. We found February as the most frosty month (based on the frequency of frosty days) and January as the coldest one. While it was accepted that spring frosts cause more damages, instead it our results showed that autumn ones has more frequency. It was determined that the location of western troughs at most in Iran is 10 degrees lower than middle location in southern hemisphere and frosts severity had significant relationship with troughs depth (58 %). Synoptic patterns showed that all axes were located over Eastern Iran and they are oriented northeasternsouthwestern during peak days. This orientation facilitates very cold air flows from high latitudes toward southern areas. Migrant troughs from higher latitudes caused sever frosts and migrant troughs from low latitudes cause widespread ones. The irregular occurrence of continued and low temperatures over vast areas as frost waves is an expected characteristic for Iran climate. We were losing resources and products in North, Central and sometimes South of Iran because of these cold air waves. Although the safety of the frosts over Southern Iran is better than other parts but the water and soil resources of southern areas for agriculture are so limited. It means, we have many fertile plains and permanent rivers over Western and Northern Iran and so frost risk can not take up agriculture activities. It looks; reducing damage to growers in these damageprone areas require changes in cropping patterns, usage of resistant and late seed and varieties as basic strategies.