M E Weber and T A Noyes Wind Shear Detection with Airport Surveillance Radars Airport surveillance radars ASR utilize a broad cosecant squared elevation beam pattern rapid azimuthal antenna scanning and coherent pulsed Doppler processing to detect and track approaching and departing aircraft These radars because of location rapid scan rate and direct air traffic control ATe data link can also provide flight controllers with timely information on weather conditions that are hazardous to aircraft With an added processing channel an upgraded ASR can automatically detect regions of low altitude wind shear This upgrade can provide wind shear warnings at airports where low traffic volume or infrequent thunderstorm activity precludes the deployment of a dedicated Terminal Doppler Weather Radar TDWR Field measurements and analysis conducted by Lincoln Laboratory indicate that the principal technical challenges for low altitude wind shear detection with an ASR groundclutter suppression estimation of near surface radial velocity and automatic wind shear hazard recognition ean be successfully met for microbursts accompanied by rain at the surface This article describes radar modifications and processing techniques that allow airport surveillance radars ASR to detect microburstgenerated low altitude wind shear Microbursts have been identified as the primary cause of 12 major air carrier accidents since 1970 resulting in the loss of 575 lives For airports with low traffic density or infrequent thunderstorm activity an upgrade to ASRs provides wind shear warnings at a lower cost than that of dedicated wind shear detection sensors Modern digital signal processing for the newest ASRs the ASR 9s generally eliminates clutter from precipitation and ground scatterers l 2 Early acceptance testing of the ASR 9 however indicated that working air traffic controllers actually made considerable use of the weather echo information on their displays To reinsert weather data in a noninterfering manner the ASR 9 s signal processor was augmented with a dedicated channel for processing and displaying six quantitative levels of precipitation reflectivity Le rain rate 2 3 This processor does not utilize the radar s coherence other than for Doppler filtering of The Lincoln Laboratory JoumQ Volume 2 Number 3 1989 stationary ground clutter echoes Techniques to extend the ASR s weather measurement capability to allow for the detection of thunderstorm generated low altitude wind shear must incorporate a signal processing for suppressing ground clutter and estimating the near surface radial wind component in each radar resolution cell and b image processsing for automatically detecting hazardous shear in the resulting velocity field Algorithms that accomplish these functions have been evaluated extensively with simulated weather signals and measurements from an experimental ASR in Huntsville Ala Our analysis indicates that a suitably modified ASR could with high confidence detectmicrobursts accompanied by rain at the surface the predominant safety hazard for aircraft in many parts of the United States The following section describes the background and potential operational role of an ASR based wind shear detection system We then discuss the primary technical issues for achieving this capability and deSCribe our evaluation of processing methods that address these issues 511 Weber et aI Wind Shear Detection with Airport Surveillance Radars are thunderstorm ou tflows whose leading edges propagate away from the generating precipitation as shown in Fig l b 5 Because the wind shear encountered by an aircraft that penetrates a gust front increases the plane s lift a gust front is considered less hazardous than the wind shear associated with a microburst The winds behind the gust front however are turbulent and the long term change of wind direction following the passage of a gust front affects runway operations Tracking and predicting gust front arrivals at major airports Background and Operational Mission Figure 1 illustrates the two principal causes of low altitude wind shear In Fig l a an intense thunderstorm downdraft encounters the earth s surface and produces a brief outburst of highly divergent horizontal winds or microburst 4 Aircraft that penetrate a microburst on takeoff or landing experience head wind totail wind velocity shear compounded by a downdraft in the microburst core Gust fronts s J 1500 CJ E Ol Qj 1000 I 500 11l 1 2000 O i 2 v 3 F Downdraft Compensating Convergence C vro i Outflow I 2000 1000 1000 0 I 2000 Distance Relative to Downdraft Center m a Turbulent Area Gust Front Envelope Multiple Surges 3000 E J 2000 CJ E 1000 20 15 10 Distance from Gust Front km Ol Qj I 5 Turbulent Area b Fig 1 a Vertical cross section of microburst wind field b Vertical cross section of gust front redrawn from Goff Ref 5 512 The Lincoln Laboratory Journal Volume 2 Number 3 1989 Weber et aI will allow more efficient use of runways In response to the hazards of wind shear the Federal Aviation Administration FAA initiated a two part enhancement to its terminal area weather information system The airport network of surface wind speed and wind direction sensors the Low Level Wind Shear Alert System LLWAS is being improved by a reworked detection algorithm and at major airports an increased number of sensors 6 In addition a dedicated microwave Terminal Doppler Weather Radar TDWR will be deployed at 45 airports to measure the radar reflectivity and radialvelocity signatures associated with low altitude wind shear 7 ASRs were initially rejected as candidate wind shear detection sensors because of their perceived deficiencies in sensitivity and groundclutter rejection as well as an inability to resolve near surface thunderstorm outflows with the broad elevation beams To the extent that these problems can be overcome however ASR 9s will complement the dedicated wind shear detection sensors in three areas 1 Airports with low traffic volume or in regions with infrequent thunderstorm activity may not warrant a dedicated TDWR or enhanced LLWAS A modified ASR could provide wind shear protection at these airports at a smaller cost than the dedicated systems 2 At airports equipped with enhanced LLWAS but lacking TDWR data from an ASR could reinforce LLWAS wind shear reports and detect wind shear in operationally significant areas not covered by the surface station network 3 At airports slated to receive a TDWR additional radar wind