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A Comparative Study of Sodar, Lidar Wind Measurements and Aircraft Derived Wind Observations

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A Comparative Study of Sodar, Lidar Wind Measurements and Aircraft Derived Wind Observations A COMPARATIVE STUDY OF SODAR, LIDAR WIND MEASUREMENTS AND AIRCRAFT DERIVED WIND OBSERVATIONS LIVIUS BUZDUGAN1,2, SABINA STEFAN1* 1 University of Bucharest, Faculty of Physics, P.O.BOX MG-11, Magurele, Bucharest, Romania 2 ROMATSA, 10 Ion Ionescu De la Brad Str., Bucharest, Romania E-mail: [email protected] * Corresponding author: [email protected] Received December 4, 2019 Abstract. The paper is focused on the comparison between Sodar and Lidar wind measurements and high resolution wind observations derived from Mode-S data sent by aircraft arriving at and departing from Bucharest Henri Coanda airport (LROP) to an air traffic control radar. Between 7–9 January 2018, the two prevailing wind directions – with their corresponding configurations of runways – manifested themselves at the airport as westerly winds veered to easterly, with colder air flowing in the Romanian Plain behind a cold front that swept up north. The results show that, for both prevailing wind directions, Sodar and Lidar wind profiles within the 40–600 m height domain are representative of the aerodrome and vicinity in non-convective conditions. Similarly, the “virtual” wind profiles obtained from wind data derived from Mode-S data of aircraft flying in the aerodrome area offer a possible alternate solution, where remote sensing instruments are not available. The aim of the study was to assess how the synergy of these instruments and data sources can service the detection of low level wind shear and monitoring of the wind field in the terminal area of the aerodrome. Key words: wind profiles collocation, Romania. 1. INTRODUCTION The potentially adverse effects on flight safety of wind shear to aviation relate to its effect on aircraft performance. Although it may be present at all levels in the atmosphere, the occurrence of wind shear in the lowest 500 m is of particular importance to aircraft landing and taking off. During the initial climb-out and approach phases, aircraft fly at low heights and near critically low airspeeds, therefore being especially exposed to the most adverse effect of wind shear: sharp variations of lift force [1]. Low-level wind shear (LLWS) at the airports or in their vicinity has been cited in a number of aircraft accidents/incidents and is considered by the aviation community be one of the major technical problems facing aviation [1]. Remote-sensing techniques such as Doppler radar, Lidar and, to some extent, Sodar, have been increasingly used, sometimes in integrated systems, at a number of airports worldwide for detecting LLWS and for providing information to pilots and air traffic controllers [1]. Among the variety of phenomena that can cause LLWS, thunderstorms and low-level jet streams (LLJ) are high ranking. Using Lidar observations, Weipert et al. Romanian Journal of Physics 65, 810 (2020) Article no. 810 L. Buzdugan, Sabina Stefan 2 (2014) [2] found that at Munich and Frankfurt airports most cases of LLWS were associated to LLJ. During the last two decades, LLJs were studied using both Lidar [3, 4] and Sodar [5, 6]. Studies related to the synoptic context of LLJ occurrence in the region of Bucharest airports were published by Balmez et al. [7, 8]. During the last decade, a new source of wind observations of high temporal and spatial resolution at heights ranging from the surface to the upper troposphere and lower stratosphere emerged: Mode-Select Enhanced Surveillance (Mode-S EHS) data, obtained from reports sent by aircraft in response to interrogations of Secondary Air Traffic Control Surveillance radars (SSR). These reports contain the aircrafts speed, direction, altitude and Mach number measured by sensors to determine the aircraft flight status [9]. Wind velocity and temperature can be derived from the reports, which can be obtained from the SSR [10]. The use of aircraft based observation data has continuously expanded during the last decades, their quality within the boundary layer being investigated in the case of Aircraft Meteorological Data Relay (AMDAR) – meteorological observations automatically made by commercial aircraft at predetermined times and relayed to the ground [10, 11, 12] and in [13, 14, 15] for Mode-S EHS. To reduce the risk of LLWS affecting operations at LROP by increasing its probability of detection, ROMATSA (the Romanian National Air Navigation Services provider) is currently deploying a wind monitoring and wind shear detection system based on the use of a scanning Doppler Lidar system and of a Doppler Sodar system. The aim of this paper is to assess how the synergy of these instruments and data sources can service the detection of low level wind shear and monitoring of the wind field in the terminal area of the aerodrome. The paper is structured as follows. The instruments, their locations and meteorological conditions are presented in the part 2.1 of Section 2 – Data and Methodology. In the same section are discussed, in the parts 2.2 and 2.3, data from Sodar and Lidar and how the wind observations are derived from Mode-S data, as well as the collocation algorithm used to generate comparable Lidar, Sodar and Mode-S derived wind data series. The Section 3 is dedicated to the results of the statistical analysis of the Lidar, Sodar and Mode-S derived wind data sets. The concluding remarks end the paper. 2. DATA AND METHODOLOGY The comparative study between the Sodar and Lidar – measured wind data and Mode-S derived wind data was conducted during January 2018, as part of the initial operational assessment of the Sodar system. Thus, between 7–9 January 2018, our study captured two wind direction regimes and consequent runway operational setups. Initial westerly winds veering to easterly, starting from the eastern part of the area, as colder air flowed in the Romanian Plain behind a cold front that swept north of the area. This led to the occurrence of "type III" LLJ, according to [8] (Fig. 1). 3 A comparative study between the Sodar/Lidar measurements wind Article no. 810 Fig. 1 – (Color online) left: Windgram time-height display of wind speed and wind vectors, 1000–925 hPa layer, Bucharest, 07.01.18/06z–09.01.18/06z, generated using Copernicus Climate Change Service Information [2019]); right: Sodargram of u wind component, 40–600 m height, 07.01.18/06z–09.01.18/06z generated using METEK Graphics. The study used 3 wind data series – of Sodar and Lidar measurements and collocated Mode-S derived wind observations, obtained by the methods described in 2.2.1 and 2.3. 2.1. INSTRUMENTS AND MEASURING SITES The remote sensing instruments used in this study were a METEK PCS.2000- 64 Doppler monostatic Sodar and a Halo Photonics Stream Line Doppler, located at 44.57N, 26.13E (elevation: 95m), 1 km east of one of the runways of LROP. A Sodar is a ground-based remote-sensing instrument for measuring wind based on turbulence in the lower atmosphere. A monostatic Sodar emits short acoustic pulses into the atmosphere and receives backscattered sound generated by small-scale density fluctuations associated only with thermal inhomogeneity of the air [16], generated by turbulence in the presence of a temperature gradient. The change in the frequency produced by a scatter is proportional to the rate of change of the distance between receiver and scatterer and the initial frequency. This frequency change is measured and the motion of the scatter relative to the transmitter-receiver can be calculated. The transmitted signals can be phase shifted to point the sound beam in different directions. The ranging of the measurement volume is determined from the propagation time of the acoustic wave and the estimated acoustic velocity. By measuring the Doppler shift for different beam directions, the full 3-dimensional wind at specific heights can be determined, with the assumption of a horizontally homogeneous flow in the measurement volume [17]. The Sodar was operated with a vertical resolution of 30 m, a first range gate (minimum height) of 40 m and a potential maximum range of 610 m. The zenith angle of the sound beams was set to 17° and the integration time to 10 min. The Sodar is particularly appropriate for monitoring the development of the LLJ, due to its operability in low cloud or low visibility conditions that may accompany low-level temperature inversion and frontal related LLJ, thereby Article no. 810 L. Buzdugan, Sabina Stefan 4 limiting the line-of-sight Lidar measurements. A scanning Doppler Lidar was also temporarily installed at the location, for test purposes. The Stream Line Doppler Lidar operates at a wavelength of 1.5 μm using the heterodyne technique to detect the Doppler shift of the signal backscattered by atmospheric aerosol. The pulse repetition frequency was set to 15 kHz and the pulse length to 140 ns, hence a line-of-sight effective resolution of 21 m. The Lidar was operated in a velocity azimuth display mode, with 6 azimuth steps of 60° and a zenith angle of 15°. Thus, the Lidar measurement volume is comparable to that of the Sodar measurement. Also, the Lidar integration time was set to 10 min. The “overlapping mode” was activated, i.e. consecutive height ranges were shifted only by 3 m, reducing the potential range to 900 m. At some times during the study, the detection range decreased below this value because of fog or low clouds. Liquid clouds are excellent Lidar targets, but strongly attenuate the Lidar signal, thereby usually limiting the detection range to their bases [18]. Similarly, the Sodar detection range can be reduced by increased attenuation and inhibited turbulence in thermal inversion layers and during increased background noise that can be caused by strong wind, heavy rain and hail. The SSR used in this study is sited near LROP and operated by ROMATSA.
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