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J Hatami, S Sabetghadam, F Ahmadi-Givi,
Volume 6, Issue 1 (5-2019)
Volume 6, Issue 1 (5-2019)
Abstract
Investigation of the daily minimum visibility meteorological conditions using RVR data at IKA airport during 2013-2014
Hatami, J. 1, Sabetghadam, S. 2*, Ahmadi-Givi, F. 3
1M.Sc. Student, Institute of Geophysics, University of Tehran
2Assistant Professor, Institute of Geophysics, University of Tehran
3Associate Professor, Institute of Geophysics, University of Tehran
Abstract
Atmospheric visibility is defined as the greatest distance at which an observer can see a black object viewed against the horizon sky, which is usually known as visual range. It shows the degree to which the atmosphere is transparent to visible light, therefore its impairment results from light scattering and absorption that can originate from natural or anthropogenic sources. Visibility is an important atmospheric parameter in landing and takeoff of an aircraft. Reduced visibility due to snow, rain, fog, and haze is an important consideration in the landing and takeoff of aircraft. Visibility and the related quantity Runway Visible Range (RVR) are meteorological parameters that are crucial for the operations at an airport. The Runway Visible Range is defined as the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface marking or lights delineating the runway or identifying its centre line. A large number of aviation accidents are happened cause many passengers to die. Today, safety is very important in aviation. In fact, it is a competitive factor among aviation companies. Measuring the exact visual range is one of the most important factors in flight security. According to the international standards, whenever the visual range is less than a certain threshold for runways, take-off and landing will not be authorized, and pilots will be ordered land on an alternative airport that costs airlines a lot of expenses.
One of the methods in determining the runway visual range is to use instruments such as transmisometer and forward scaterometer to measure the amount of scattering and absorption of light by the atmosphere. A transmissometer measures the extinction of light over an atmospheric path between an emitter and a receiver and it is directly related to the extinction. A forward scatter meter measures the amount of light scattered by a small measurement volume. RVR instruments usually locate at three places across each runway that is mandatory for operation in international airports.
For the first time in Iran, data obtained from the RVR system from Imam Khomeini International Airport are applied in this study to examine the circumstances under which the runway visual range reached its minimum during two years 2013 and 2014. The high accuracy of these devices is a valuable factor for researchers to get more precise results. The data used include visibility range, temperature, dew point temperature, humidity, wind speed and wind direction, which are measured using the RVR system. The main part of this study concentrates on fast decreases of RVR, meaning a decrease of visibility to below 1500 m which takes more than 10 minutes. Therefore some cases of RVR data have been investigated in more detail utilizing one-minute observations are presented. For these cases, some meteorological parameters are investigated before and after this fast decrease of RVR occurred. These parameters as well as RVR are plot to find out what happened before and during each specific event.
Results show that the critical low visibilities were mainly occurred in May and March and no cases of low visibility were seen between July to September. This can be due to the impact of more atmospheric systems and variable weather conditions in the relatively cold months. The highest visibilities were mostly occurred in July-September, due to the weakness of atmospheric systems and their less frequency of occurrences. Low visibility days were usually accompanied by dust, fog, mist and precipitation events.During 2013 and 2014, categorizing the weather events that may lead to the decrease of visibility to less than 1500 meter, shows that the 45 percent of the cases with the low visibilities caused by by dust, 35 percent by haze, 15 percent by fog and 5 percent caused by haze.
For the critical cases, case studies show that the high relative humidity and the change of wind direction were also favored in the occurrence of low visual range. Case studies of the events suggest that these factors differ from one another based on how they are formed. After the fast decreases of RVR, the relative percentage of RVR events show an increasing in relative humidity especially during fog and precipitation.
Keywords: runway visual range, scattering and absorption of light, low visibility.
Parisa Jaberi, Samaneh Sabetghadam, Sarmad Ghader,
Volume 7, Issue 3 (11-2020)
Volume 7, Issue 3 (11-2020)
Abstract
Visibility is one of the most important optical characteristics of the atmosphere. Prediction of visibility is essential for air pollution, air traffic, flight safety, road traffic and shipping. Visibility reduction may be caused by different reasons. Fog is one of the most common reasons of visibility reduction, i.e. the droplets of water suspended in the atmosphere reduce the visibility to less than 1 km. Precipitation may also reduce visibility. Prediction of visibility in NWP models is usually accomplished by using the relationship between visibility and liquid water content, temperature, relative humidity. Purpose of the present work is to predict visibility during fog and precipitation over Tehran area in January 11th, 2014 and March 7th, 2013. Different algorithms including UPP1, AFWA, FSL and SW99 have been experimented to predict visibility.. Predicted visibility has been compared to observations, including Synoptic and METAR data in Imam Khomeini and Mehrabad airport. The WRF version 3.8.1 has been used to simulate precipitation and fog. In this simulation model configuration defined in Lambert uniform space. The model consist three nested domains. First domain was a 27-km grid model (83×65), surrounding a 9-km grid model (112×94) which was surrounding a 3-km grid model (112×97). Center of all domains was at longitude 51° and 44' and latitude 36° and 5' which is located almost at center of Tehran. All domains had 40 vertical layers and model top was located at 100hPa. The out puts of 3-km domain is used for visibility estimation. Initial and boundary conditions were set by using FNL data which is 1°×1° degree grid data. This data is available every 6 hours. Simulations were in 36 hours and first 12 hours was the spin up time. Results show that most of these algorithms can partly predict visibility reduction. The FSL algorithm works better than the other methods in fog situation and SW99 works better in snow situation. Comparing results shows that the visibility reduction during snow is more reliable than during fog. There were some errors in model predictions some of them were due to visibility algorithms, because the coefficients of these algorithms were driven in other parts of earth. The other errors were systematic errors of WRF. Predictions of temperature had warm bias and also there were positive bias in prediction of relative humidity.
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