Windshear and Turbulence Alerting Services Hong Kong Observatory December 2010
Frequency distribution by Significant windshear: 1 in 500 flights Significant turbulence: month 2nd peak 1 in 2000 flights 1st peak
Typical weather scenarios for windshear and turbulence Winds blowing across terrain (terrain-induced) induced) Sea breeze Gust fronts Microbursts Low-level jets decreasing frequency of occurrence
Monitoring network Anemometer TDWR Weather buoy Wind profiler LIDAR
Automatic detection algorithms HKO develops a number of windshear detection algorithms: Anemometer-based Windshear Alerting Rules Enhanced (AWARE) LIDAR Windshear Alerting System (LIWAS) WTWS also shows alerts based on TDWR data and hilltop algorithm.
AWARE Based on anemometers (ground-truth data) Skillful in detecting sea breeze, gust front and shear line Weather buoys Tai Mo To Weather buoys
Sea breeze case on 3 November 2005 10 windshear reports (+15 to +20 knots) over 07LA between 0515 and 0539 UTC
LIWAS Based on glide- path scan of the LIDAR Operational since Dec 2005 For arrival corridors only
Scanner Graphical display Transceiver Signal processing computer RASP (Real-time Accessed Signal Processor) Inside the LIDAR shelter
LIDAR Principle
LIDAR versus TDWR LIDAR works best in fine weather TDWR works best in rainy weather
Major Parameters of LIDAR Peak pulse energy 2 mj Wavelength 2 μm Range resolution ~100 m Maximum range ~10 km (weather dependent) Unambiguous velocity range -20 m/s to 20 m/s (extendable to -40 m/s to 40 m/s with reduction in range)
Safety Issues Classified as Class 1M under IEC 60825-1 (IEC 2001) and Class 3b under ANSI Z136.1-2000 1 Could be harmful if viewed with optical aids Safety measures Sector blanking Scan rate interlock Laser Safety Officer appointed
Schematic diagram of dual LIDAR operation (after relocation) laser beams N glide path
Glide-path scan 3 deg for arrival corridors 6 deg for departure corridors
Windshear ramp Sustained change of headwind: Ramps prioritized headwind change from one state according to a to another metric Headwind ΔV 3 S = ( ) / 1/3 ΔV H V app H Distance/time
LIWAS algorithm Measure headwind data along the glide path Generate windshear alerts to WTWS Generate headwind profile Identify windshear ramp
What LIWAS sees in 30 August 2004 case
Windshear event on 30 March 2005 Windshear reports : 0005 07RA -15 KT 4NM 0029 07RA -30 KT 1NM 0032 07RA 35 KT 1NM 0045 07RA 25 KT 1NM 0049 07LA -20 KT 2NM
24KT Wind direction, speed and headwind derived from on-board data from aircraft arriving at 0049 UTC. LIWAS headwind profile
Winds Blowing Across Terrain (Terrain-induced) induced) d) Easterly to southwesterly winds (e.g. in spring time and in tropical cyclone situation)
Winds Blowing Across Terrain (Terrain-induced) induced) d) Northwesterly to northeasterly winds (e.g. in northeast monsoon in winter time)
Sea breeze Develops under sunny weather Sea breeze converges with the prevailing wind Sea breeze front Background flow Sea Breeze Usually headwind gain
Sea breeze Flying through a sea breeze front could experience headwind loss as well
Gust front Leading edge of cool air spreading out from the downdraft of intense storms Headwind gain
Characteristics of windshear and turbulence at HKIA Most are terrain-induced induced Transient and sporadic Gain and loss events can co-exist Generally more significant on south runway due to closeness to hills
Windshear is transient and sporadic Small-scale disturbances over RWY 25L 30 August 2004
Windshear is transient and sporadic Arrows show the movement of the windshear Arrows show the movement of the windshear features in the next 4 minutes
Windshear is transient and sporadic 6000 30 path (ft) 6-degree glide Height of 5000 4000 3000 2000 1000 Headwind profile @ 14:41 UTC Headwind profile @ 14:43 UTC 6-degree glide path 25 20 15 10 5 LIDAR R's radial veloc city (knots) 0 2 1 Distance from runway end (NM) 0 Runway corridor 25 LD -1 0 Sequence of headwind changes reversed in 2 minutes!
Headwind gain and loss could co-exist Lifting +20Kt Sinking -11Kt 07LA Sinking -17Kt Lifting +12Kt
Terrain-induced induced windshear and turbulence points to note Small-sized and evolving rapidly Windshear and turbulence as experienced by an aircraft may at times differ from the conditions reported by the preceding and the ensuing aircraft, and from the alerts provided. Time Aircraft Type Intensity Location 3:50 - None reported - 4:00 A330 15 kt loss 3MF 4:02 B747-400 25-29 kt gain 2MF 4:04 B747-400 None reported - 4:10 B767 15 kt gain 2MF A day in March 2000 Landings on RWY 07L
Gentle windshear ramp 07LA null report of 07LA, null report of windshear at 10:00 UTC, 16 April 2005
But sometimes there could be unexpected reports (*) 07LA, report of 20-knot headwind gain at 600 feet at 03:53 UTC, 29 August 2005
Gentle windshear ramps at departure corridor
TDWR To detect microburst and WS associated with convective storms ~12 km NE of HKIA Radar antenna ~60 m AMSL Runways approx. aligned with TDWR radial TDWR
TDWR is purpose-built Radar to detect Hong Kong International Airport microburst and windshear Chek Lap Kok Airport associated with convective storms unobstructed view of airport Raytheon Hardware Supplier Lincoln Lab Algorithms Same model as US FAA s 45 TDWRs operational at US airports easily accessible by transport TDWR 60m above PD; reduce clutters from sea and land
583 12 km from airport; roughly align with runway direction 300 465 721 751 934 869 747 766
Beams align well with runway Radial winds roughly measuring headwinds 8º 10º 14º 18º
Special features of TDWR Narrow antenna beamwidth 0.5 deg. Good sidelobe performance Excellent target differentiation Highly stable Klystron-based transmitter Able to detect reflectivity below -20 dbz (can detect some returns even in rainless conditions)
Special features of TDWR (Cont.) Advanced velocity dealiasing algorithm Employs dual PRFs in the lowest scan to uniquely determine unambiguous velocity data Velocity dealiasing at higher elevations based on assumption of vertical continuity of velocity Unambiguous velocity range -80 m/s to 80 m/s
Special features of TDWR (Cont.) Range dealiasing algorithm Low PRF scan at lowest elevation to provide initial view of radar echo returns from the longest range This information is then used to predict the expected degree of obscuration in higher h level l scans s
Special features of TDWR (Cont.) Powerful clutter removal mechanism A 55 dbz filter to low velocity data (-2 to 2 m/s) to remove stationary clutter A point-target target removal algorithm to remove moving objects (aircraft, birds and ships) User-defined 30 dbz clutter polygons to remove areas contaminated with moving clutters (traffic, trees)
Scanning strategy
Change of mode - Specifically, only one of the following within the Hazardous Scan Sector is needed to switch the TDWR from Monitor to Hazardous Weather Mode: a region of 30 dbz (level 2 precipitation) a gust front detection, or a wind-shear or microburst detection. - The TDWR switches from Hazardous Weather to Monitor Mode when the above Hazardous Weather Mode constraints are absent for half an hour.
Winds Blowing Across Terrain Another example: (Terrain-induced) induced) d) Severe tropical storm Severe tropical storm Hagupit (11 September 2002)
Microburst Most violent form of downdraft from thunderstorms Typical horizontal extent: a few km Headwind increase -> Downdraft -> Tailwind increase Symmetric microburst
Some notes on microburst Microburst can be asymmetric. Downdraft column can hit the ground at an angle. Strong terrain-induced induced windshear (loss of 30 knots or greater), coupled with rain, may cause TDWR to issue microburst alerts. Don t expect the typical sequence of events (gain preceding downdraft followed by loss)!
Bandaid shapes microbursts detected by TDWR TDWR microburst algorithm searches for significant divergence radial shear Wind loss (Velocity difference) across divergence region is computed, mptd if loss >= 15 kts AND reflectivity >= 30 dbz -> declare as microburst shape Microburst bandaid shapes showing 20 kts loss 0.6 deg PPI Velocity GSD actually showing 0.6 deg PPI Reflectivity
TDWR Alphanumeric Alerts Shear integration of windshear across microburst shape to reduce false alarms (probability only)! Alerts rounded to nearest 5 kts MBA is alerted when loss >= 30 kts WSA is alerted when loss < 30 kts 40 kts loss shown on shape 07L 25R 1NM 2NM 3NM Alerts shown on GSD: 07LD WSA 25K- 2MD 25RA WSA 25K- 2MF D Departure F Final
Low-level jet Narrow band of strong winds Usually affects aircraft on departure (steeper glide path) Rl Relatively l infrequent at HKIA
Microwave radiometer
Observations in a windshear event
100 90 WTWS + Forecaster 80 Percentage of Detection 70 60 50 40 30 20 10 WTWS GLYGA (1 min) GLYGA (5 min) G and R (1 min) G and R (5 min) GorR(1min) G or R (5 min) Radiometer 0 0 10 20 30 40 50 60 70 80 90 100 Percentage of Time on Alert
Short-range range LIDAR
+ WS reports
Observations by the existing LIDAR
Summary Surface-based anemometers LIDAR TDWR Wind profiler Other new instruments, e.g. microwave radiometer and short-range range LIDAR
User education Windshear booklet Windshear posters Briefing Bifi at the airlines ili
Major topics of discussion Nature of windshear at Hong Kong International Airport Detection methods Discrepancy between detected and experienced windshear Different practices of windshear reporting by pilots
Trend arrow
Trend arrow
User group meetings Windshear and Turbulence Warning System Working Group (WTWS WG) Low Level Wind Study Working Group (LLWS WG)
Participants Met. service Air traffic control Airport standard d Flight standard Airline representative Pilot representative
WTWS WG Review of windshear service (objective vs. subjective) New instrument developments Issues of instrument maintenance New procedural developments Windshear phraseology
LLWS WG Wind study of effect of buildings/structures on the airflow Windshear warning criteria Deployment of instruments for detecting low level l wind effects
Publication HKO homepage: www.weather.gov.hk HKO email address: outreach@hko.gov.hk IFALPA homepage: www.ifalpa.org GAPAN homepage: www.gapan.org
Automatic Alerts Issued by WTWS (Windshear and Turbulence Warning System) covering 3 NM from runway thresholds Nominal update rate 1 min.
Automatic ti WS Alerts Microburst Alert (MBA) Generated by TDWR (Terminal Doppler Weather Radar) and integrated into WTWS Runway-oriented wind speed loss >= 30 kt and accompanied id by precipitation iitti Windshear Alert (WSA) Runway-oriented wind speed loss or gain >= 15 kt
Automatic ti TURB Alerts Moderate or severe in intensity i With reference to heavy category aircraft
Warnings Provided d on ATIS Issued by Aviation i Met. Forecasters Based on pilot reports, forecasting rules developed from case studies, past 30-min TDWR (windshear and microburst), past 30- min WTWS (turbulence), real-time conditions as revealed by TDWR, LIDAR, etc.
Alert Phraseology Very common to have multiple occurrences of windshear on same corridor in HK WTWS consolidates multiple events into ONE integrated alert TDWR alerts higher priority than WTWS alerts Sinking shear (loss events) higher priority than lifting shear (gain events) Max intensity i given in alerts
E.g. APP to RWY 25L Alert Phraseology RWY 25L -30 +15 Caution Microburst Minus 30 knots on final approach (instead of specific location as in US) Note: The max intensity (-30 kt) can occur anywhere along the corridor First encounter (+15 kt @ 3NM) not the worst encounter (-30 kt @ 1NM)
E.g. APP to RWY 25L Alert Phraseology RWY 25L -15 +20 Caution Windshear Minus 15 knots on final approach Note: Some may report -15 kt, while others +20 kt (especially those who conducted a missed APP on encounter of the lf lifting shear)