چكيده فارسي :
نظر به اهميت كاهش ديد افقي ناشي از وقوع مه براي عمليات فرود هواپيما در يك فرودگاه ، در اين مطالعه كوشش شده است ضمن بررسي آماري باد، ارتفاع كف ابر و ديد افقي كمتر از حداقل ديد فرودگاه در موارد فرودگاه هاي مهم و پرترافيك كشور شامل مهرآباد تهران، مشهد، رشت، تبريز، اهواز، كرمانشاه و بندر عباس، الگوي ميانگين جوي شاخص هاي فشار تراز دريا، باد و مقدار ابر تعيين و با استفاده از الگوريتم شناسايي مه، انواع مه ايجاد شده در فرودگاه هاي منتخب و در دوره ي 1995-2014 (20 سال) شناسايي شوند. مهمترين نتايج اين بررسي نشان مي دهد در ماه هاي سرد سال (دسامبر، ژانويه و فوريه) بيشترين رخداد مه در كل كشور گزارش شده است. غالب گزارش ها قبل از طلوع و بعد از غروب خورشيد و به ترتيب در ساعت هاي 03، 00 و UTC21 ديدباني شده اند. الگوي ميانگين سمت وسرعت باد در ارتفاع 10 متري نشان دهنده ي هواي آرام در غرب كشور و بادهاي غربي با سرعت ميانگين 5 تا 10 نات در نواحي مركزي و شرق كشور ديده مي شوند كه در حاشيه شمال شرق ايران گردش چرخندي كوچكي را نشان مي دهند. بيشترين آمار رخداد مه در فرودگاه هاي اصفهان، كرمانشاه و رشت مربوط به مه تابشي نوع دوم ( وزش باد ملايم و ارتفاع كف ابر 300 پا يا كمتر) مي باشد. مه تابشي نوع يك (وزش ملايم باد و آسمان بدون ابر) بيشترين هواي مه آلود را در فرودگاه هاي به خود اختصاص داده است. در فرودگاه هاي مهرآباد تهران و تبريز ضمن اينكه تعداد رخداد ها كمتر از ساير فرودگاه ها مي باشد، بيشترين رخداد به مه ناشي از پايين آمدن ارتفاع كف ابر برمي گردد. اين امر مي تواند در اثر وقوع بارش جبهه اي ايجاد شود. مه فرارفتي تنها در فرودگاه مشهد قابل بررسي و توجه مي باشد. به نظر مي رسد گردش چرخندي در الگوهاي ميانگين باد موجب شده شكل گيري مه فرارفتي كه نيازمند وزش بادهاي حداقل 6 ناتي مي باشد، در فرودگاه مشهد بيشتر از ديگر فرودگاه ها باشد كه نيازمند بررسي هاي موردي ساير عوامل در اين فرودگاه مي باشد.
چكيده لاتين :
Introduction
Fog is among the most important weather hazards from the aviation perspective. This phenomenon can lead to horizontal visibility reduction. Therefore, accurate prediction is essential for flight safety and easing air traffic. Fog consists of a weather condition in which water drops and ice crystals reduce the horizontal visibility to less than 1000 meters. Various methods are suggested for fog forecasting. Numerical and statistical methods, experimental approaches, and very short range fog forecasting are some of the most common methods. Experimental methods are commonly used for first guess in forecasting centers. Saunders technique is one of the forecasting methods for radiation fogs using radio sounds data. Although this technique goes back to many years ago, it is being used in many parts of the world, including UK Met Office, and is recommended by World Meteorological Organization.
Material and method
Present study tries to evaluate the performance of two experimental methods using real data after studying synoptic condition of fog occurrence in two selected airports. The validity of them is measured then with the real occurrence in a number of case studies of fog occurrence for the selected airports using the bias technique in order to choose the more appropriate method. In the next step, the more appropriate method is administered using the numerical prediction model output and is again evaluated with the bias technique. In both these methods, an index called fog point temperature has been used, and the fog occurrence has been determined by calculating this temperature and comparing it with the minimum temperature. The selected airports are Mehrabad Airport, Tehran and Shahid Hasheminezhad Airport, Mashhad, which have been chosen because of high flight traffic (in Tehran) and high fog occurrence (in Mashhad). Experimental methods examined in this study are Saunders and Prichars-Crodack techniques, which 25 case studies in selected Airports tried to offer the best results for first guess of fog occurrence. The accuracy of these relations was evaluated comparing real conditions using Bias technique. After choosing the more appropriate method, a similar process has been gone through using numerical prediction model of WRF for the next 12 hours.
Results and discussion
Results of synoptic evaluations show that high-pressure systems are a major factor in creating coldness in lower levels of the atmosphere. Evaluation of pressure field in this study doesn't show figures below 1020 hPa. Specific humidity values were 6-8 g/kg and 4-6 g/kg for 1000 and 925 hPa levels respectively. Winds are frequently northern or eastern and cold weather advection is seen in selected stations.
In Saunders technique, using radio sound data of 1200 UTC in 25 case studies for mentioned airports, the fog point is calculated. This temperature is then compared with next day's minimum temperature and if the difference is less than -2°C, fog occurrence would be ruled out. Saunders considers this method mostly useful for radiation fog. In Crodack-Prichars technique, which is performed by creating a regression association between temperature and dew point temperature, the fog point temperature is determined. Here again, fog is not formed If the temperature difference is less than -2°C.
After calculating fog point temperature using Saunders technique and comparing it with actual observation, it was found that among 25 cases, 15 fog observations were consistent with Saunders technique calculations. In the five cases of fog nonoccurrence, the results of this method were consistent with reality. So, Bias evaluation technique shows 75% for probability of detection.
The same process has been gone through for Crodack-Prichars technique. In this method, a linear relationship exists between temperature, due point temperature, and fog point temperature. Wind condition and cloudiness are also presented experimentally in the form of a table. For different amounts of these two factors, a numerical amount of 1.5 to -1.5 is added to the fog point temperature. Fog occurrence is determined by calculating fog point temperature using Crodack-Prichars technique and comparing it with the minimum temperature according to table 2. This evaluation showed that in 13 of 20 fog occurrence cases, the right answer were obtained, and 5 cases of fog nonoccurrence, were consistent with reality.
Conclusion
Therefore, POD index was reduced to 65%. Based on the results, Saunders technique has been considered as the more appropriate method for initial guessing in fog forecasting in the airports under study. Now, the amounts of temperature and due point temperature were determined for the next 12 hours using WRF numerical prediction model, and Saunders technique was used again for predicting fog (using predicted data). The results of this evaluation were also investigated using Bias evaluation technique, which were not so agreeable, so that it was consistent with reality in 50% of cases. Hence, it seems that careful consideration of numerical prediction models output is needed.
