ASSESSMENT OF LOWLEVEL WIND CHARACTERISTICS AND ITS EFFECT ON AIRCRAFT OPERATIONS IN NAIROBI USING NCEP REANALYSIS

Transcription

1 ASSESSMENT OF LOWLEVEL WIND CHARACTERISTICS AND ITS EFFECT ON AIRCRAFT OPERATIONS IN NAIROBI USING NCEP REANALYSIS By Mercy Wanjiru Mwangi I56/81597/2015

2 1.0 INTRODUCTION Wind shear is the sustained change in the wind direction and or speed, resulting in a change in the headwind or tail wind encountered by an aircraft (Hong Kong observatory, 2010). Winds blow more frequently from one direction than any other at many locations-prevailing wind. Major runways at airports must be aligned with the prevailing wind in order to assist in taking off and landing (Ahrens, 2009). However gust fronts, funnel cloud and microbursts which are thunderstorm related, mountain waves, strong surface winds coupled with local topography, urbanization-manmade structures, sea breeze fronts and surface heating can occur suddenly leading to wind changes.

3 Its significance to aviation lies in its effect on aircraft performance and hence potentially adverse effects on flight safety. It is particularly important during final approach and landing phase respectively take-off run and climb-out. Its height is at near critical values therefore rendering the aircraft susceptible to the adverse effects of wind. Also the aircraft is operating at relatively low speeds In cases of turbulence, it causes abrupt changes in attitude of an aircraft leading to temporary loss of control as well as bumps and jolts that affect the intended flight path. (Hong Kong Observatory, 2010). Terminal operations are also impacted since the change to either a crosswind or a tailwind impede landings and require changing the runway configuration. When occurring in thunderstorm it can lead to airport closures, degrade airport capacities for acceptance and departure and hinder or stop ground operations.

4 To improve air traffic efficiency, safety and management, the wind changes must be timely and accurately forecasted Operational weather nowcasting is therefore essential. Forecasts timing of significant changes are supposed to be accurate to within 1 hour. Huang et al (2012) describes nowcasting as short period weather forecasting concerned with current weather conditions and the changes over the next few minutes to the ensuing 6 hours. The spatial scale is no more than a few kilometers with frequent updates. It is highly location specific and requires data of very high spatial and temporal resolution to produce accurate forecasts.

5 1.1 PROBLEM STATEMENT The erratic nature of wind shear causes difficulty in the accurate alerting of the phenomena yet it is vital to provide accurate wind forecasts. Tailwinds on landing can increase landing distances especially on wet conditions while crosswinds increase the risk of veering off the runway when landing. Runways are designed to benefit from prevailing winds, thus a wind from a less climatologically preferred direction may result in the air traffic controllers having to rearrange air traffic to depart or land in a different direction. Jomo Kenyatta International Airport does not have cross runways to mitigate against excessive crosswind. The airport has only one runway. This means that depending on the strength of the crosswind or tailwind, the air traffic management makes decisions on which direction planes will land or whether they will land at all.

6 1.2 OBJECTIVE The main objective of the study is to determine and assess the wind and shear characteristics within Nairobi Airspace SPECIFIC OBJECTIVES To determine the wind and shear characteristics at various levels. To determine monthly and diurnal periods when wind shear is prevalent. To establish the relationship between the incidences and surface observations captured by the NCEP reanalysis and causes of the shear

7 1.3 JUSTIFICATION Detection and prediction of wind shear occurring at low levels in the surrounding of airports enables efficient planning in making runway switches using the best possible information about the timing of a wind shift. For the airlines, right action is taken on time avoiding damages likely to be incurred which could remove an aircraft from operations and result in both lost revenues and excess maintenance costs.

8 1.4 AREA OF STUDY JKIA is located 18km from Nairobi city. It is at an elevation of (5330 ft) meters above sea level, the airport has a single asphalt runway alignment in direction and measuring 4,117m in length.

9 2.0 LITERATURE REVIEW Motions are always present and can cause substantial wind direction variation. (Anfossi et al. 2005). The presence of shear is often visible to an observer. It can be in the form of cloud layers moving in different directions, smoke plumes sheared and moving in different directions at different heights and trees bending in all directions in response to sudden gusts from a squall line among others (ICAO, 2005). The spectrum of wind shears and eddy motions associated with different synoptic, mesoscale and microscale weather regimes is quite large and their effects on aircraft performance are complicated in nature(arkell, 2000) Most important types of wind shear for aviation are commonly referred to as low level wind shear (LLWS) and low level turbulence.

10 In aviation, low-level is that wind shear occurring on the approach path or take-off path or during circling approach between runway level and 500m (1600ft) above that level and aircraft on the runway during landing roll or take-off run. If due to local topography, wind shears are significant at heights in excess of 500m above runway level, then 500m shall not be considered restrictive. (WMO Annex 3, 2013). LL wind shear is noticeable below 600m where frictional drag on the air closest to the earth s surface causes changes in both wind speed and direction with height. This layer is generally referred to as the friction layer, which can be further subdivided as follows into: a). surface boundary layer : earth s surface- 100 m (330 ft) - air motion is controlled by friction with the earth s surface; and

11 b). The Ekman layer :100 m (330 ft) up to at least 600 m (2 000 ft) -the effect of friction diminishes with height, - controlling factors, like coriolis force and horizontal pressure gradient force, become increasingly important. The wind direction tends to remain constant with height in the surface boundary layer but to veer (back) with height in the northern (southern) hemisphere throughout the Ekman layer.

12 CAUSES OF LLWS AND TURBULENCE It can be convective or non-convective. Non-convective LLWS evolves slowly over the course of a few hours for example, in the case of nocturnal inversion. Convective LLWS changes more quickly resulting from the discontinuity between the ambient environmental winds and gust front and from discontinuities within the outflow region behind the gust front Turbulent shear on the other hand changes quite rapidly in time.

13 Surface heating and instability causes turbulence. As the earth surface heats, thermals rise and convection cells form. The resulting vertical motion creates thermal turbulence. Thermal and mechanical turbulence occur together in the atmosphere. In stable air with weak winds, eddies are non-existent or small. As surface heating increases and wind speed increases, instability develops and the eddy becomes large and extends through a greater depth. The rising side of the eddy becomes larger and extends through a greater depth. It carries slow moving surface air upward causing a frictional drag on the faster flow of air aloft. Some of the faster moving air is brought down with the descending part of the eddy, producing a momentary gust of wind.

14 The circulating eddies in unstable air lead to strong, gusty surface winds. Greater instability also leads to a greater exchange of faster moving air from upper levels with slower moving air at lower levels. This exchange increases the average wind speed near the surface. This is why surface winds are usually stronger in the afternoon. A small increase in wind speed can greatly increase the wind force of an object. Some of the mechanisms also cause LLWS showing the inter-related nature of the two phenomena. Strong LLWS and low-level turbulence is seen in thunderstorm environments making it hard to distinguish the two. During the early morning, when the air is most stable, thermal turbulence is normally at a minimum. (Arkell,2000)

15 Ground topography and large buildings such as man-made structures like hangars to large natural obstructions such as hill, mountains and canyons cause obstructions on the ground affecting the flow of wind. They break up the flow of the wind and create wind gusts that change rapidly in direction and speed. The intensity of the turbulence depends on the size of the obstacle and the primary velocity of the wind. On a weekly perhaps daily basis, wind speed and direction will be influenced by the passage of cyclones or anticyclones. On an hourly basis, localized influences such as the surrounding topography, sea/land breeze, mountain/valley breeze and urbanization will further alter the wind. This makes it difficult to detect wind shear onset and is thus a silent danger to aviation.

16 The pilots response to shear conditions determines how well the aircraft will respond to the rapid changes (Arkell, 2000). This response can be to change the aircrafts power settings and or its angle of attack. These changes will inturn alter the aircrafts indicated airspeed (IAS) and/or its climb or descent rate. An aircraft size, weight and speed are also critical factors in determining its response to a given shear environment. For a non-convective LLWS ; If the shear is an increasing tailwind, the pilot can increase power to increase IAS. This will prevent decreased lift to the wings which diminishes the response of control surfaces and drops the aircraft below the desired flight path or glide slope or may even stall the aircraft. (ICAO, 1987).

17 If the shear is an increased headwind, the pilot will usually cut back on power otherwise the aircraft will rise above the desired flight path. If the pilot is unsure about the direction of the shear he or she will usually increase power to be on the safe side. Studies on wind shear accidents have shown that pilots will only have 5 to 15 seconds of time to react correctly, to safely negotiate the hazards. The hazards are rapidly changing headwind and tailwind, strong side gusts and variable lift on the wings, all during a time when an aircraft is most vulnerable (Minor, 2000). Operationally non-convective LLWS usually presents the pilot with one wind shift or vector wind change while convective LLWS often presents the pilot with multiple wind shifts along the flight path thus requiring different response from the pilot.

18 Figure 2.1: Effect of Head (Tail) wind shear on aircraft. ICAO 2005

19 As an aircraft flies into a region with horizontal wind drop and/or downdraft, lift will decrease, causing the aircraft to fly below the original flight-path (Li P, 2006). This is called sinking shear scenario. Conversely, when an aircraft encounters horizontal wind increase and/or updraft, the lift will increase, causing the aircraft to fly above the original flight path. This is called a headwind gain or lifting shear scenario. The headwind gain scenario could still cause trouble to the pilot during an aircrafts landing as the aircraft could be caused to fly above the approaching glide-path, hence missing the touch down zone and have to make a go-around The decreased headwind in a sinking shear case, reduces the lift and subsequently the aircraft would fly below its intended flight path.

20 3.0 DATA AND METHODOLOGY 3.1 DATA Metar wind data: Hourly wind speed and direction data will be extracted from METARs. Data will be collected for 5 years (2010 to 2015). The wind information will be for JKIA, Wilson airport, Ngong, and Moi airbase stations. NCEP reanalysis wind data: 6 hourly wind data at heights upto 700mb will be obtained from the 40-year reanalysis project between NCEP and NCAR (Kalnay et al, 1996). WIND SPEED DATA: The power law will be used to extrapolate wind speed at higher heights from stations with observed wind data (ICAO, 2005). This will cover smaller spatial scales not accommodated by the NCEP/NCAR data. Wind shear speed from a known height (usually 10 m) will be extrapolated as follows:

21 Power law equation: The power law is generally used under adiabatic conditions with strong wind speeds for the layer from 10 m to 200 m. Where: V= velocity to be calculated at height h h= height above ground level for velocity v V o =known velocity at height h o h o =reference height where V o is known α = wind shear exponent. a parameter that depends on the stability, surface roughness and height with a value between 0 and +1 determined empirically. For neutral stability conditions in open lands, α is approximately 1/7=0.143 (Masters, 2005)

22 wind shear incidences data: the wind shear occurrences data,when they occurred and at what level it was experienced will be retrieved from KCAA. The incidences are from what was reported by the pilots based to a large extent on their subjective assessment of the intensity of the wind shear encountered as recognized in Annex 3, Appendix 6, 6.2.4, Note 2.

23 3.2 METHODOLOGY DATA QUALITY CONTROL ESTIMATING MISSING DATA The arithmetic mean ratio method will be used given by; = Where X i Missing value for station A Y i -Corresponding value for station B

24 HOMOGENEITY TEST Single mass curve will be used to check for homogeneity of wind values. They will be plotted against time. A near straight line indicates good quality of data TIME SERIES The wind shear incidences will be plotted against time to determine periods when wind shear is prevalent WIND CHARACTERISTICS Windroses from Surface observations will be plotted to establish diurnal behavior of wind Diurnal vertical profile will be plotted to establish the magnitude of shear and relate the reported shear to the wind Causes of the shear will then be determined.

25 4.1 EXPECTED RESULTS The results are expected to show the behaviour of wind speeds and direction at various heights at small temporal and spatial scale so as to improve alertness of windshear affecting JKIA. Seasons when windshear is more prevalent will also be documented.

26 3.3 BUDGET ACTIVITY KSHs 1. Data stick and stationaries Data Acquisition and Analysis Typing Services Internet Services, Printing and Photocopying Compilation, Report writing and Binding Miscellaneous 3000 TOTAL 39,000

27 3.4 WORKPLAN ACTIVITY May Jun-Sept Oct-Dec January March April Proposal Writing Proposal Presentation Data Collection Data Organization and Analysis Progress Report Presentation Report Writing Final Presentation

28 5.0. REFERENCES Ahrens C. D, (2009): Meteorology today; An Introduction to weather climate and the environment, 9 th edition, Brooks/Cole CENGAGE Learning Arkell R. E, (2000): Differentiating between types of wind shear in aviation forecasting. Volume 24, Number 3, National Weather Service. EASA (2011): Authority, Organisation and Operations Requirements for Aerodromes. NPA (B.III). International Civil Aviation Organization, (2005). Manual on Low-Level Wind Shear. Doc 9817, International Civil Aviation Organization, 1 st Edition Kalnay et al (1996). The NCEP/NCAR 40 year reanalysis project, Bull. American Meteorological Society., 77, Kenya Airport Authority (July, 2015), Aeronautical Information Publication, Kenya Airport Authority. Huang et al (2012): Integrating NWP Forecasts and Observation Data to Improve Nowcasting Accuracy, Weather and Forecasting, Volume 27, DOI: /WAF-D Li, P.W., Windshear Its Detection and Alerting. In Symposium of Science in Public Service, Science Museum.