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The ront November-DDecember, 2002 National Service Central Region Volume 1 Number 6

Technology at work for your safety In this issue: Conceived and deployed as stand alone systems for , weather and systems now share information to enhance safety and efficiency in the National Airspace System. ITWS - Integrated Jim Roets, Lead Forecaster help the flow of air traffic and promote air Terminal safety. One of those modernization com- Weather System The National Airspace System ponents is the Automated Surface (NAS) is a complex integration of many Observing System (ASOS). technologies. Besides the aircraft that fly There are two direct uses for ASOS, you and your family to vacation resorts, and the FAA’s Automated Weather or business meetings, many other tech- Observing System (AWOS). They are: nologies are at work - unseen, but critical Integrated Terminal Weather System MIAWS - Medium to . The Federal Aviation (ITWS), and the Medium Intensity Intensity Administration (FAA) is undertaking a Airport Weather System (MIAWS). The Weather System modernization of the NAS. One of the technologies that make up ITWS, shown modernization efforts is seeking to blend in Figure 1, expand the reach of the many weather and aircraft sensors, sur- observing site from the terminal to the en veillance radar, and computer model route environment. Their primary focus weather output into presentations that will is to reduce delays caused by weather, Gust fronts - Evolution and Detection

Weather radar displays NWS - Doppler FAA - ITWS

ASOS - It’s not just for airport observations anymore

Mission Statement

To enhance aviation safety by Source: MIT Lincoln Labs increasing the pilots’ knowledge of weather systems and processes Figure 1. ASOS is an important component in the FAA’s Integrated Terminal Weather and System. ITWS is a weather monitoring and short range prediction system designed to products and services. aid air traffic controllers and pilots for the safe and efficient movement of air traffic. 1 minal Doppler (TDWR), an ASR-9 Weather System Pro-cessor (WSP), or ITWS, but could benefit from the higher level of safety that a MIAWS system would provide. MIAWS seeks to provide weather coverage to these air- ports by using existing, nearby sensors. MIAWS, an enhancement of NEXRAD, has an initial objective of pro- viding real-time display of posi- tions and storm tracks using NEXRAD product data as the input to a weather ser- vice processor product generation and display system. speed and wind gust information is provided through ASOS/AWOS input to the LLWAS - relo- cation/sustainment network. The NEXRAD reflectivity data will be displayed in the NWS 6-level thresholds. Storm position and motion vectors show- ing direction and speed of storm cells, will provide extrapolation lines with 10 and 20 minute leading edge positions. Source: MIT Lincoln Labs The system will also show airport wind vectors depicting wind speed and direc- Figure 2. ITWS blends information from several technologies to give controllers an tion, and any current airport integrated assessment of storm impact. impact alerts. especially at the busiest airports. The Alert System (FAA) The main users of this information, integration of these many airport and NLDN - National as with ITWS, will be controllers, pilots, weather systems and sensors into a cohe- Detection Network and traffic managers. Two operational sive and easily understandable display MDCRS, ACARS - Aircraft Reports prototype systems have been providing will greatly enhance the safety of flight RVR - Runway Visual Range controllers with weather alerts at the operations within the usually busy termi- Sensors (later). Jackson International Airport, MS, and nal environment. Developed by MIT Lincoln The information is disseminated via Memphis, TN, since September 2000. color displays to pilots, controllers, and Two new MIAWS prototypes were sched- Integrated Terminal Weather traffic managers. uled to be installed at Little Rock, AR, System: According to Lincoln Laboratories, and Springfield, MO airports in the Laboratories, ITWS provides weather ITWS installation began at major airports Spring of 2002. products for air traffic automation sys- in 2001. Figure 3 shows end state deploy- tems by using data from FAA and ment goals. Lincoln Laboratory is contin- National Weather Service (NWS) sensors, uing to operate ITWS functional proto- aircraft reports, and NWS numerical types in Memphis, Orlando and Dallas/Ft. weather prediction to provide short-term Worth in order to increase the ITWS oper- forecasts (out to 30 minutes) for the ter- ational and meteorological database and minal area. See Figure 2. The FAA oper- to provide operational user benefits. ational users access the ITWS products The MIAWS program was estab- through a color situation display. ITWS Medium Intensity Airport Figure 3. ITWS will serve: provides information on microbursts, gust Weather System: fronts, tornadoes, storm location and 49 ATCTs, motion, , and at the airport ter- lished to develop a weather processing 37 TRACONs minal area. Besides ASOS and AWOS, and display system for airports with a 18 ARTCCs ITWS uses the following sensors: medium level of operations, but which 3 support facilities The areas served have the most season- TDWR -Terminal Doppler Weather lack the abundance of dedicated weather sensors found at larger airports. Its goal is wide and Radar (FAA) that affect air traffic and air safety. NEXRAD - Next-Generation to provide near-term, low-cost weather Weather Radar (NWS) information to these airports. According ASR-9 - Airport Surveillance Radar to the FAA, airports of this size have too (Sources: FAA and MIT Lincoln (FAA) few operations to justify the cost of a ded- Laboratories) For more information, visit www1.faa.gov/aua/ipt_prod/weather/ LLWAS - Low Level icated weather system such as the Ter- itws//menusys.htm 2 through the warm environment and lifts Wind shear detection by NEXRAD, the air to promote new along the boundary. To a pilot with a ITWS and MIAWS very local view, things seem pretty Gust fronts and microbursts may show obvious, visual signs of benign, but what may well be depicted on their presence, but in the less obvious situations, ITWS and an ITWS or MIAWS terminal is a devel- oping wind shear event. MIAWS are enabling controllers to see the hidden dangers. In a line of thunderstorms composed If you think the season as shown in Figure 4. of multiple cells, any number of ended, think again. The record thunder- These monsters are not the storms could be dissipating and producing out- storm event of November 10-11, 2002 you read about in wind shear accidents. flow boundaries. Where these boundaries spread destruction across a large portion Rather, storms of intensity level 3 have meet, new convection fires up thanks to of the southern and eastern U.S. Much of led to tragic accidents in years past the combined lift of the intersecting it occurred in the area where ITWS and because weak wind shear was not boundaries. MIAWS are being deployed. Judgement observed or detected by instruments on Radar reflectivity imagery, the type on this deployment appears rock solid. the field or by weather radar. shown most often on TV and on NWS Traditional pilot training inundates Decaying thunderstorms can leave web sites, may show an the students with ominous images of the sky mostly clear and the air cooler. as a thin curved line. See Figure 6. Note approaching thunderstorms. Gust fronts One would justifiably believe that the that the reflectivity diplay shows no are often marked with roll clouds or shelf danger has passed. echoes in the area south of the main storm A broader satellite imagery may give except for the thin curved line. METAR clues that the danger has not passed. As a reports from around the region show thunderstorm evolves from the mature southerly flow. However, between the stage to the dissipating stage, it becomes gust front and the main thunderstorm increasingly dominated by downdrafts northerly flow dominates. The subtle, and drops its cool rainfall. This cooler air stealthy wind shear event is well under- spreads out in all directions and forms way. what’s known as an outflow boundary. The curved line marks the region Convection is suppressed in this air and where the cooler and denser air is emerg- the sky clears. This scenario is depicted in ing. can detect these events Figure 4. Roll clouds form where the Figure 5. in regions where the radar beam is low warm inflow and cool outflow pass one The dark -free circles mark the enough. another. These are the obvious “No Fly stabilized air. The cool air spreading out Figure 7 is a depicition of how this Zones” for thunderstorms. from the decayed thunderstorm acts like a event might occur if it were approaching cold front. The cooler, denser air moves a major airport. The main thunderstorm is

New Convection Along Intersecting Boundaries

Storm movement G us t Front Surface wind

Gust front Outflow Boundaries movement

Figure 5. When outflow boundaries (blue arrows) from near- Figure 6. Thunderstorm outflow called a gust front may produce by thunderstorms intersect, new convection can begin. The strong wind shear. Sometimes the gust front is visible on NWS radar data ingested into ITWS can provide clues about wind Doppler radar. ASOS observations would provide solid ground shifts associated with these outflows. truth to confirm the presence and strength of the outflow. 3 some distance away scans just at the top of this 1500 foot deep gust front, but the radar is still able to accurately detect any internal tight circulation, or large hail within the thunderstorm. These storm attributes are sent to the ITWS for inte- gration into the TRACON display. The NWS forecasters use a display shown in Figure 8 (left side). It’s packed with an abundance of storm attributes that keep forecasters abreast of events. The NWS ITWS and MIAWS systems display gust WSR-88D fronts, hail, and possible tornadic rotation within the thunderstorm. These systems Satellite use data from NWS Doppler radar, ASR- airport 9 radar, LLWAS, ASOS, aircraft reports, G FAA output from NWS atmospheric forecast ust Front TDWR models to provide a threat assessment and Predominent short term forecast for controllers. southerly Hub flow Final Approach Emerging technology and increased number crunching by computers is doing Figure 7. Cool ouflow from thunderstorms produces a gust front. The for aviation weather what GPS, multi- may be opposite to the general winds in the area. The cold, dense air can be detected function displays, and flight management as a fine curved line by Doppler radar. (See Figure 6). The beam from WSR-88D systems has done for navigation. They’re at NWS offices may be slightly above the main portion of the gust front, but opening a new era in aviation safety. TDWRs are in prime position to detect the gust front. ASOS reports from satellite air- Never before has the technology been bet- ports in the terminal area help confirm the gust front in the ITWS system. ter or the communication better. Displays

Cell Trends

Past positions of thunderstorm

Forecast positions

Figure 8. Workstations at NWS Weather Forecast Offices identi- Figure 9. ITWS displays in the TRACON contain thunderstorm fy and track storms. Trends of storm strength, movement, hail information similar to that on NWS displays because the annota- size, and internal storm circulation help forecasters provide tion is derived from NEXRAD analysis systems. These displays excellent lead times for and severe thunderstorm warn- give present and forecast positions of gust fronts and tornadoes, ings. These storm attributes are passed to ITWS. in addition to the storm reflectivity in the six standard levels. seemingly a ways off and does not appear front has arrived, and to be a major impact yet. No alerts are the ASOS confirms a wind shift into the are available for use by the TRACON, coming from the Low Level Wind Shear north. ASOS reports this to the ITWS. Center Weather Service Unit, and users Alert System (LLWAS) yet. ASOS at the The TDWR may well begin to see with a need to know. The most effective hub airport still reports a south wind. To the outflow as it scans the low levels in coordination occurs when all parties view the west, at the satellite airport, the gust the terminal area. An NWS NEXRAD, the same image. The FAA is getting it done. 4 Automated Surface Observing System - SENSORS

Feeding a data hungry ATC system Sally Pavlow, Forecaster In order to detect freezing , the The temperature-dewpoint sensors NWS Indianapolis, IN must first communi- report between -80 F and cate with the to +130 F. To determine dew point, a chilled Major U.S. airport and scores of confirm that is actually mirror is used. The mirror is cooled to the smaller airports use ASOS (Automated occurring. Then it determines whether or point where a thin film of moisture con- Surface Observing System). ASOS not the precipitation is in the form of denses on the mirror surface. The con- reports current sky conditions, weather, freezing rain. The freezing rain sensor is a densation is detected by the reflection of , wind direction and speed, alti- nickel alloy tube which oscillates at a fre- infrared light off the surface of the mirror. meter setting, temperature and dewpoint. quency of approximately 40,000 hertz. A precision thermal sensor is embedded The sensors record data many times an When precipitation collects on the in the mirror and measures the tempera- hour and offer METAR reports hourly sensor probe, the frequency decreases and ture of the mirror which is then translated and whenever meaningful changes occur. FZRA or FZDZ is reported. into the dewpoint. The data are also used by ITWS and MIAWS to optimize safety and efficiency. WIND SENSORS Here’s brief look at the main sensors.

PRESENT WEATHER SENSOR

The visibility sensor, like the present The wind sensor measures wind speed weather sensor, uses a transmitter and and direction and computes 5 second averages of these measurements. The Univ. of Illinois receiver. A technique called forward scat- Photo tering is used. Flashes from a xenon bulb wind speed is measured by a 3 cup pho- The present weather sensor consists produce visible light for scattering As tointerruptor device and wind direction of a transmitter head and a receiver head. light passes through the atmosphere, it is is measured by a split vane along with a The transmitter head aims a beam of in- scattered. The receiver detects the light potentiometer. frared light through the atmosphere to the and measures the light level to determine receiver. When there is no precipitation the amount of light lost. A formula is Ground-To-Air falling, the amount of light received will applied which translates the light lost into Many ASOSs communicate their be nearly equal to the light transmitted. a statute visibility. data to pilots and other listeners. This is When falling precipitation is present, the done via the ground-to-air radio system. amount of light received by the receiver Frequencies and phone numbers are in head is reduced. The physical properties the Airport Facility Directory and other of each precipitation type that is detected prominent airport and navaid directories. (rain, and ) is determined by the drop size, number of drops passing Special thanks to Dave Johnson, through the sensor stream and speed at Electronic Technician at NWS which the precipitation falls. From this in- Indianapolis, for his help with this arti- formation, the present weather sensor can cle. report which precipitation type is present The Front is published bimonthly and at what rate it is falling. The reports cloud heights. and is available on the NWS Central It transmits an invisible laser beam verti- Region aviation page at: cally. As this beam is being transmitted it www.crh.noaa.gov/crh/aviation/ also senses the light return that is reflect- thefront.html , ed back toward the ceilometer by objects or on the AWC website at: in its path. The ceilometer calculates the http://aviationweather.gov/front/ time between the transmission and recep- Send comments and suggestions to: tion of the laser beam. From this, the [email protected] height of water or droplets in the [email protected] atmosphere are reported. [email protected]