Tenerife, Spain LLWAS Site Visit Report

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Tenerife, Spain LLWAS Site Visit Report Larry Cornman National Center for Atmospheric Research Introduction A site visit to the Tenerife Reina Sofia (Tenerife-Sur) Airport was conducted on 9-10 February, 2005. Three issues regarding the LLWAS system were to be addressed: (1) Evaluate the sensor siting, (2) look into the over-alerting problem, and (3) hold discussions with airport meteorological and air traffic staff. Each of these will be discussed in detail the following sections. Sensor Siting Evaluation On 9 February, a visit was made to all of the LLWAS sites at the Tenerife-Sur airport. Figure 1 shows the island of Tenerife, and the location of the Tenerife-Sur airport. A brief discussion of each site will be given below, but as can be seen from Figure 2, the overall the sensors are laid out in a close to optimal fashion. This network should provide good wind shear protection for the runway and one nautical mile to either side. There is a single runway, oriented 08/26. Due to the prevailing winds, the normal airport operations are landing on runway 08 and take off on runway 26. On the larger-scale, the terrain is gently sloping from North to South, and hence sites 1-5 are above the runway height and sites 6-10 are below it. There are small terrain features (typically small ditches or terrain depressions) in close proximity to many of the sites; however due to the sensor heights, these should not present any problems in providing unbiased wind measurements. There are some buildings in close proximity to the south of site number 10; however the tower height for this site is adequate, and the prevailing winds (ENE) do not come from this direction. Figure 1. View of the island of Tenerife, indicating the Reina Sofia Airport in the South. Figure 2. Aerial view of Tenerife-Sur, with approximate locations of the sensors and runway ends. Sensor Site #1 This site is situated to the North-West of the West end of the runway, and approximately 20 meters higher. There are a number of small ditches in close proximity to Site 1, especially to the South-West. Since the tower is reasonably high (15m), the terrain indentations should not pose any problems in making wind measurements. Figure 3 shows a view East-South-East from this site and Figure 4 is looking to the North. One of the ditches can be seen on the left-hand side of this latter figure. Figure 3. Looking East-South-East from Site 1. Figure 4. Looking North from Site 1. Sensor Site #2 Site 2 is reasonably unobstructed; it is to the North of the runway and approximately 40 meters higher. There is small ditch to the North-East and small ridge to the North. Part of the ditch can be seen in Figure 5 and Figure 6. The 15 m tower is high enough, so that these terrain features should not pose any problems. Figure 5. Looking South from Site 2. Figure 6. Looking East from site 2. Sensor Site #3 Site 3 is just to the East of the main road heading into the airport, and just to the North of the main airport complex. This site is reasonably unobstructed, with small bushes and small trees nearby. The tower is 25 meters tall, so there should be no problem in getting accurate winds from this site. The location is approximately 60 meters above the runway height. Figure 7 is a view looking South-South-East from the sensor location. The control tower and the terminal building can be seen in the middle of the picture. Figure 8 is a view looking West-South-West from the site. The lighting fixtures and bushes and small trees along the airport road can be seen in this photo. Figure 7. Looking South-South-East from Site 3. Figure 8. Looking West-South-West from Site 3. Sensor Site #4 Site 4 is an unobstructed site on a 15 meter tower, to the North of the runway. It is approximately 60 meters above the runway height. There is a small ditch to the West of the site, but it should not pose any problem in getting accurate wind measurements. Figure 9 is a view looking South-South-East from this site, and Figure 10 is a view looking to the North. The small ditch to the West of the site can be seen in the middle-left of Figure 10. Figure 9. View south-south-east from site 4. Figure 10. Looking North from Site 4. Sensor Site #5 Site 5 is to the North-East of the East end of the runway, at approximately 40 meters above the runway height. The tower is 20 meters tall. There are some agricultural hot-houses in close proximity to the South-East, as can be seen in Figure 11. There is a large ditch to the West of the site, as can be seen in Figure 14. Neither of these items should pose any problems in getting good wind measurements from this site. Figure 11. Looking South-South-East from Site 5. Figure 12. Looking West from Site 4. Sensor Site #6 Site 6 is to the South-East of the East end of the runway, and is approximately 20 meters below the runway height. The tower is 15 meters tall. This is an unobstructed site. Figure 13 is a view looking to the South-East of the site, and Figure 14 is a view looking to the West. Figure 13. Looking South-East from Site 6. Figure 14. Looking West from Site 6. Sensor Site #7 Site 7 is South of the runway, approximately 15 meters below the runway height. The tower is 15 meters tall. This is an unobstructed site, with a small ditch to the East. This ditch can be seen in both Figure 15, which looks to the South-East, and Figure 16, which looks to the East. Figure 15. Looking South-East from Site 7. Figure 16. Looking East from Site 7. Sensor Site #8 Site 8 is to the South of the mid-point of the runway, at an elevation of approximately 15 meters below the runway height. The tower is 15 meters tall. The terrain slopes down and then up going to the East, as can be seen in Figure 17, and is relatively flat to the West. There are some agricultural hot-houses in close proximity to the South-East, as can be seen in Figure 18. Neither of these features should pose any problems in making good wind measurements. Figure 17. Looking East from site 8. Figure 18. Looking south-east from site 8 Sensor Site #9 Site 9 is located to the South of the runway, towards its West end. It is situated approximately 16 meters below the runway height. The tower is 20 meters tall. This is an unobstructed site with relatively flat ground surrounding it. Figure 19 is a view looking to the South of the site, and Figure 20 looks to the West-North-West. The dirt mound that can be seen in Figure 20 is due to some road work, and does not pose any problems with the site. Figure 19. Looking South from site 9. Figure 20. Looking West-North-West from site 9. Sensor Site #10 Site 10 is to the South-West of the West end of the runway. It is approximately 30 meters below the runway height. The tower is 25 meters tall. There is a new apartment complex with three storey buildings just to the South of the site – as can be seen in Figure 21. The only potential problem would be with winds from the south, but as the sensor site is slightly above the first floor of the buildings, and with height of the tower, there should not be any significant degradation in the wind measurements. Furthermore, the prevailing winds are from the Easterly directions. Figure 22 is a view looking to the West-North- West of the site, showing the unobstructed terrain in that direction. Figure 21. Looking South from site 10. Figure 22. Looking West-North-West from site 10. The Over-alerting Problem One of the items that UCAR was asked to look at during the Tenerife site visit was a persistent over-alerting with the LLWAS system. It was quite clear on inspection that the problem was not of a meteorological nature, but rather, something to do with the system hardware or software. Almos subsequently discovered a database error in the Airport Configuration File (ACF). Almos then generated and loaded a new ACF that should solve the problem. Another problem that was observed during the visit was intermittency in some of the sensor readings. Telvent personnel believed that the problem was due to a lack of solar battery power at some of the sites, which in turn was due to a lack of maintenance at the sites. Discussions with Airport Meteorological and Air Traffic Staff On 10 February, a meeting was held with airport meteorological staff and air traffic controllers. One interesting wind shear condition was discussed: a persistent summertime condition wherein pilots lose airspeed when descending through 1000 feet on landings from the West. Air traffic controllers indicated that airspeed loses on the order of 30 knots had been encountered. These are significant values, and have resulted in go-around procedures on occasion. Without further investigation, it is unclear what the specific mechanisms are that could be generating this vertical shear of the horizontal wind phenomenon. Nevertheless, a few potential causes can be postulated. From Figure 23, it is clear that the island of Tenerife is dominated by the Pico de Teide, a 3718 meter volcanic peak. Vortices shed off of this large terrain feature are one potential cause. It is well-known that verticallyaligned vortices, known as a von Karman street, can be shed by oragraphic features such as the Teide. However, there is also a secondary terrain feature which is located in close proximity to the approach path for Runway 08. This is a 430 meter hill, (indicted by the arrow on Figure 23), approximately 10 km to the West-North-West of the airport. Orographic wind effects from this terrain feature, by themselves, or in combination with the larger scale vortices could also be the source. A brief discussion of these two mechanisms is presented below, along with a discussion of measurement devices that could be used to investigate the phenomenon – or even be used as part of an operational warning system. Figure 23. Topographic map of the island of Tenerife. A brief discussion of the synoptic trade wind patterns over the Canary Islands can be found in Varela et al.1 (see also the references sited in that paper). Figure 24 from that reference illustrates a typical synoptic wind pattern: cooler maritime air flowing from the North-East at the surface, with cooler dryer air from the North-West aloft. A thermal inversion layer forms between 1000-1500 meters. Figure 24. Synoptic trade wind behavior in the Canary Islands. (from Varela et al.) von Karman vortex street It is well-known from fluid mechanics that as a fluid flows around an obstacle, such as a cylinder, the boundary layers separate from each side of the cylinder surface and form two shear layers that trail aft in the flow and bound the wake. Since the innermost portion of the shear layers, which is in contact with the cylinder, moves much more slowly than the outermost portion of the shear layers, which is in contact with the free flow, the shear layers roll into the near wake, where they fold on each other and coalesce into discrete swirling vortices. A regular pattern of vortices, called a von Karman vortex street, trails aft in the wake. Figure 25 illustrates this phenomenon. For a vertically aligned obstacle, in this case a cylinder, the vortices are aligned vertically. Figure 25. A view from above of a von Karman vortex street forming behind a cylinder. 1 A.M. Varela, et al. 2004: Non-correlation between atmospheric extinction coefficient and TOMS aerosol index at the Canarian Observatories. Remote Sensing of Clouds and the Atmosphere IX, ed. Schafer et al., Proceedings of the SPIE Vol 5571, pp. 105-115. This vortex shedding behavior is often observed with isolated mountains. A satellite view of this phenomenon associated with the Canary Islands is shown in Figure 26. Tenerife is indicated by the arrow. The lack of clouds just downwind of Tenerife is due to subsidence of the air mass as it flows down the slopes of the Teide. This does not mean that the vortices are absent, rather there are no clouds there to mark them. Figure 26. von Karman street vortices formed downstream of the Canary Islands. The island of Tenerife is indicated by the arrow. Figure 27 and Figure 28 illustrate results from a numerical modeling simulation of the wind field in the wake of the Hawaiian of Kauai, performed by NCAR for NASA. As can be seen from Figure 27, the island of Kauai is very similar in structure to Tenerife. Figure 28 illustrates the simulated wind field at 1000 m, with the left-hand image showing contours of horizontal wind velocities (turbulence is indicated by the blue shading), and the right-hand image showing the horizontal wind vectors. In this case, the flow pattern was reasonably consistent in the vertical, so that the winds at 1000 feet are probably not too different than those shown here. Notice the sharp gradients in horizontal velocities as the air flows around the island (left-hand image). These simulations also captured von Karman vortices in the downstream flow. This flow pattern close to the island, as well as downstream is probably similar to patterns that would be encountered at Tenerife. The persistent trade wind field and the flow around and downstream of Tenerife, as indicated above, is certainly a potential source of the vertical wind shear that is experienced by pilots approaching Tenerife-Sur from the West. Figure 27. Model domain used for wind flow simulation around the Hawaiian island of Kauai. The contour lines indicate the terrain. Figure 28. Fine-scale model winds at 1000 meter altitude from Kauai simulation. Left is a contour of the wind field with turbulence indicated with the blue shading. On the right are the wind vectors. Smaller-scale orographic effects. As mentioned above, (Figure 23), there is a 430m terrain feature approximately 10 km to the West-North-West of the airport. Localized flow around and over this hill could also affect aircraft as they approach the airport from the West. Figure 29 shows another situation of von Karman vortices formed downstream of the Canary Islands. Figure 30 is a blow-up of the region surrounding Tenerife. The box in this figure is approximately centered over the 430m terrain feature. Note the sharp discontinuity of the (presumed) low-level clouds (translucent grey) along the South-West corner of the island. Note also, the small set of convective clouds that lie along the discontinuity, just downstream of the terrain feature. It is possible that these convective clouds are being formed by air being lifted by the terrain feature. Figure 29. Satellite image of the Canary Islands, showing von Karman vortices downstream. Figure 30. Blow-up of Figure 29, showing Tenerife. The box is centered approximately over the 430 terrain feature. Sensors for investigating/detecting the wind shear phenomenon. The discussion presented above, is not intended to answer the question as to the source of the vertical wind shear phenomenon that was mentioned by the Tenerife-Sur air traffic controllers. Rather, it was intended to indicate some potential factors that could be related to the condition. In order to further investigate the situation, and to perhaps be used in a warning system, there are two sensors that could be of use. The first is a scanning eyesafe Doppler lidar and the second is a vertically pointing Doppler radar. The Doppler lidars provides very accurate radial velocities by reflecting off of aerosols in the atmosphere. Depending on the level of aerosols, the range of these devices can reach 15 km, with a range resolution of 60-100 meters. Such a device, placed on the airport property and scanning in a vertical plane above and below the approach glide path of Runway 08, would most likely be able to see the wind shear phenomenon discussed above. Automated algorithms could be developed to detect the wind shear and provide alerts to the air traffic controllers. On of the downsides to these devices is their high cost (on the order of $1 million USD). The vertically pointing Doppler radars, also known as wind profilers, can provide a vertical profile of the horizontal wind above the device. So-called boundary-layer wind profilers can generate accurate winds up to a height of 1-2 km, with a 60-100 meter range resolution. These devices operate at microwave frequencies, and measure radial velocities by scattering off of index of refraction variations as well as Rayleigh scattering off of hydrometeors. If the wind shear phenomenon is relatively homogeneous in space, one of these devices placed on the airport property could detect the wind shear that is affecting the aircraft. These devices are far less expensive than the Doppler lidars (on the order of $2-300 K USD), on the other hand, there are more data quality control issues with Doppler wind profilers. UCAR/NCAR has a great deal of experience with both of these devices. Furthermore, UCAR/NCAR has developed wind shear detection algorithms similar to those just mentioned. A data collection campaign could be performed using either or both of these devices, to investigate their capabilities in detecting the wind shear and providing warnings. A decision could then be made as to whether such devices and warning algorithms should be deployed. It should be noted that addressing the abovementioned vertical wind shear problem does not mean that the LLWAS system is not needed. The LLWAS system is designed for, and does an excellent job of detecting low-level wind shear due to microbursts and gust fronts. Therefore, the use of other sensors to detect the vertical wind shear condition is viewed as complimentary to the LLWAS system. Furthermore, if other sensors and warning algorithms are deployed for the vertical wind shear problem, the alerts should be integrated with the LLWAS alerts. UCAR/NCAR would work with Almos, Telvent and INM as needed to assist in this process.

brightjm 发表于 2011-1-23 21:22:44

thank   you.

ingooooo 发表于 2018-9-24 18:18:40

谢谢分享，学习一下。

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Tenerife, Spain LLWAS Site Visit Report