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Remote Sensing of Low Visibility Over Otopeni Airport EPJ Web of Conferences 176, 11001 (2018) https://doi.org/10.1051/epjconf/201817611001 ILRC 28 REMOTE SENSING OF LOW VISIBILITY OVER OTOPENI AIRPORT Livius Buzdugan1,2 , Denisa Urlea1,2, Paul Bugeac1,2, Sabina Stefan1 1 University of Bucharest, Faculty of Physics, P.O.BOX: MG-11, Magurele, Bucharest, Romania ([email protected]) 2 Romanian Air Traffic Services- ROMATSA, 10 Ion Ionescu De la Brad Str., Bucharest, Romania ABSTRACT wind direction and speed are investigated and The paper is focused on the study of atmospheric results are presented in Section3. Section 4 is conditions determining low vertical visibility over reserved to conclusions. Henri Coanda airport. A network of ceilometers and a Sodar were used to detect fog and low level 2. METHODOLOGY cloud layers. In our study, vertical visibility from 2.1 Instruments and Measuring Sites ceilometers and acoustic reflectivity from Sodar for November 2016 were used to estimate fog The geographical and climatological depth and top of fog layers, respectively. The characteristics of the site arise from Henri Coanda correlation between fog and low cloud occurrence airport being situated nearby the city of Bucharest, and the wind direction and speed is also the largest in Romania with strong urban and plain investigated. climatology. 1. INTRODUCTION The boundary layer conditions accompanying fog and low cloud are studied using 4 ceilometers and The prediction of low visibility conditions is of a Sodar installed at Bucharest Henri Coanda paramount importance for operational weather airport. The ceilometers are used operationally forecast services. Adverse visibility conditions only to report cloud base height and vertical can strongly reduce the efficiency of a terminal visibility [3] and to detect and evaluate the area traffic flow. The application of Low fractional extent of cloud layers [3, 5]. They are Visibility Procedures (LVP) reduces airport integrated in the local automated weather efficiency for takeoffs and landings by a factor of observing system (AWOS). two. Costly delays and flight cancellations ensue The data used are gathered from the ceilometer [1]. and Sodar which are co-located at At Bucharest Henri Coanda (OTP) international approximatively 1 km east of the eastern threshold airport, LVP are applied when visibility is less of one of the runways of Bucharest Henri Coanda than 600 m (2,000 feet) or the ceiling is below 60 airport (44.58N, 26.13E, 92m AMSL). The m (200 feet). Ceilometer is installed on the roof of a building The goal of this work is to test how the use of formerly hosting a radio-navigation facility, at 4m Sodar and CL31 VÄÏSÄLA Ceilometer height and owned and operated by ROMATSA measurements can help the detection of the top of (the Romanian Air Traffic Services fog layers which could in turn be used for the Administration). The Sodar is also operated by initialisation of a high resolution numerical model ROMATSA, but owned by INOE (National dedicated to fog forecasting. Institute of Opto-Electronics). The Sodar and Ceilometers are operated at The ceilometer is VÄÏSÄLA CL31 on single-lens Otopeni Airport close to Bucharest, capital of technology, 910nm (near-infrared) wavelength, Romania. operated with a 10m vertical resolution and 15s The site, meteorological conditions and the temporal resolution. Its laser diodes are pulsed instruments are presented in Section 2. with a repetition rate of 10 kHz [3]. The The capabilities of instruments to detect fog and ceilometer has been set up for a maximum range the link between fog and low cloud occurrence the of 7700m AMSL. It is able to detect three cloud © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 176, 11001 (2018) https://doi.org/10.1051/epjconf/201817611001 ILRC 28 layers simultaneously. If the cloud base is The change in frequency produced by a scatterer obscured due to precipitation or ground-based fog, (Doppler shift) is proportional to the rate of CL31 reports vertical visibility (VV) [3]. change of the distance between receiver and By assuming a linear relationship between scatterer and the initial frequency, so for a known backscatter and extinction coefficient and by transmitted frequency, a radial velocity - assuming that the Lidar ratio (k) is constant over corresponding to motions along the beam - can be the observation range, it is possible to invert the calculated by measuring the received backscatter profile, basically obtaining an backscattered wave frequency. Thus the extinction coefficient profile which would determination of the three-dimensional wind produce the measured backscatter profile [3]. vector requires at least 3 beams with different orientations [1,4]. It has been discovered that in many cases, k can The METEK PCS 2000-24 Sodar uses a phased- be assumed to equal 0.03, tending to be lower array antenna consisting of an array of 24 (down to to 0.02) in high humidity, and higher (up loudspeakers for sequential monostatic soundings to 0.05) in low humidity conditions. However, in of the three-dimensional wind field. The system precipitation conditions, k can have a wider range software controls the operation of each beam. of values [3]. Electronic phase shifters apply a different phase An estimate of vertical visibility can easily be shift to each antenna element to steer the beam. calculated from the extinction coefficient profile The shifted phase of each element causes the because of the straightforward extinction waves to interfere constructively, giving the coefficient-to-visibility relationship, assuming a maximum gain in the desired direction. Five constant contrast threshold. Visibility is simply beams are achieved, allowing cross checking the height where the integral of the extinction among the wind components. [4]. coefficient profile, starting from the ground, The Sodar was parameterized to a vertical equals the natural logarithm of the contrast resolution of 20 m and a first range gate (bottom threshold, sign disregarded [3]. altitude) of 40 m. The temporal resolution was set Tests and research have, however, shown that the to 10 min (integration time). 5% contrast threshold widely used in meteorological visibility measurements is 2.2. Data and methods unsuitable for vertical visibility measurement [3]. In order to find the days with fog and low cloud, ceilometer CL31 uses a contrast threshold value the ceilometers measurements for November 2016 which has been found to give vertical visibility have been investigated simultaneously with values closest to those reported by ground-based METAR reports from Bucharest Henri Coanda human observers. A safety margin is obtained airport, stored by the local AWOS system. The with regard to pilots looking down in the same month of November 2016 was investigated, as several fog situations occurred on the 4th, 5th, 12th, conditions since the contrast between objects, th th especially runway lights, is much more distinct on 19 and 26 . the ground [3]. The cloud base heights and vertical visibility from For the purpose of this study, the vertical visibility ceilometers measurements were stored and reported by the ceilometers has been used only to retrieved using the software running on the identify the cases with dense, ground-based fog. AWOS operated by ROMATSA. The stored data consist of a text file for each day and parameter The Sodar is a Doppler monostatic METEK PCS (cloud base-heights and vertical visibility) with 2000-24. temporal resolution of 15s. These data were For a Sodar the source of the received signal is the processed with a software to determine time series scattering eddy – essentially a density fluctuation of measurements with cloud base within specific - moving along with the air. It can be shown that intervals and with vertical visibility reported. in monostatic mode the backscattering is due to It was noticed that dense fog or precipitation is random temperature fluctuations only [2]. required for the ceilometer to output vertical visibility values instead of cloud base heights. 2 EPJ Web of Conferences 176, 11001 (2018) https://doi.org/10.1051/epjconf/201817611001 ILRC 28 layers simultaneously. If the cloud base is The change in frequency produced by a scatterer Dense fog conditions were declared on site at a monthly means at heights above 140 meters. This obscured due to precipitation or ground-based fog, (Doppler shift) is proportional to the rate of time frame when vertical visibility was reported can be explained by the fact that dense fog CL31 reports vertical visibility (VV) [3]. change of the distance between receiver and by the ceilometer, only if confirmed by METAR situations were observed mostly during nighttime By assuming a linear relationship between scatterer and the initial frequency, so for a known (SPECI) reports or by values of MOR below 1000 hours, with surface temperature inversions backscatter and extinction coefficient and by transmitted frequency, a radial velocity - meters measured by a nearby transmissometer at generating a significant amount of temperature assuming that the Lidar ratio (k) is constant over corresponding to motions along the beam - can be 2,5m above the ground. fluctuations in the inversion layer even at low the observation range, it is possible to invert the calculated by measuring the received The Sodar reflectivity and wind (direction and wind speeds. In Figure 2, the relatively low wind backscatter profile, basically obtaining an backscattered wave frequency. Thus the speed) profiles were retrieved using the METEK speeds characteristic to the fog events emphasize extinction coefficient profile which would determination of the three-dimensional wind Graphics software, as text files for the whole the decisive role of the temperature produce the measured backscatter profile [3]. vector requires at least 3 beams with different month, with a temporal resolution of 10 minutes.
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