1840 journal of atmospheric and oceanic technology …

1840 VOLUME 21J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y

q 2004 American Meteorological Society

Evaluation of Wind Vectors Measured by a Bistatic Doppler Radar Network

KATJA FRIEDRICH AND MARTIN HAGEN

Institut fuer Physik der Atmosphaere, Deutsches Zentrum fuer Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany

(Manuscript received 10 March 2004, in final form 2 July 2004)

ABSTRACT

By installing and linking additional receivers to a monostatic Doppler radar, several wind components canbe measured and combined into a wind vector field. Such a bistatic Doppler radar network was developed in1993 by the National Center for Atmospheric Research and has been in operation at different research departments.Since then, the accuracy of wind vectors has been investigated mainly based on theoretical examinations.Observational analysis of the accuracy has been limited to comparisons of dual-Doppler-derived wind vectorsalways including the monostatic Doppler radar. Intercomparisons to independent wind measurements have notyet been accomplished. In order to become an alternative to monostatic multiple–Doppler applications, thereliability of wind vector fields has to be also proven by observational analysis. In this paper wind vectorsmeasured by a bistatic Doppler radar network are evaluated by 1) internally comparing results of bistatic receivers;2) comparing with independent wind measurements observed by a second Doppler radar; and 3) comparing within situ flight measurements achieved with a research aircraft during stratiform precipitation events. Investigationsshow how reliable bistatically measured wind fields are and how they can contribute highly to research studies,weather surveillance, and forecasting. As a result of the intercomparison, the instrumentation error of the bistaticreceivers can be assumed to be within 1 m s21. Differences between bistatic Doppler radar and independentmeasurements range mainly between 2 and 3 m s21.

1. Introduction

Three-dimensional wind fields together with high-res-olution vertical wind profiles are one of the greatestobservational needs for regional mesoscale numericalweather prediction (NWP) and a key nowcasting pa-rameter as proposed by the World Meteorological Or-ganisation’s expert team on observational data require-ments and redesign of the global observing system intheir final report (WMO 2002). Owing to a three-di-mensional coverage in space with high temporal andspatial resolution, using weather radar data like Dopplervelocity and precipitation is favored, especially for re-gional NWP and synoptic meteorology.

Since the mid-1950s, when the first systematic ob-servations of weather echoes with Doppler radars werecarried out, Doppler techniques have already provengreat potential for short-term forecasting, severe weath-er warning, and aviation meteorology (Rogers 1990).The great progress in understanding weather phenomenaand detecting severe weather, like microbursts, prompt-ed most national weather services to deploy a Dopplerradar network (Zrnic 1996). At the same time, the usageof multiple-Doppler information has been promoted inorder to determine wind vector fields in real time. Brown

Corresponding author address: Dr. Katja Friedrich, NOAA/OAR/ETL, Mail Code R/ET7, 325 Broadway, Boulder, CO 82234.E-mail: [email protected]

and Peace (1968) were the first to present wind vectorsmeasured by two radar systems, followed by the pio-neering work of Armijo (1969) and Lhermitte (1968)who developed new methods for estimating wind andprecipitation velocities. Since then the utilization ofmultiple-Doppler analysis has increased, up to the Se-vere Environmental Storms and Mesoscale Experiment(SESAME) using seven Doppler radars. The early find-ings of the basic work were summarized at a workshopon how to operate multiple-Doppler-radar systems (Car-bone et al. 1980).

A more economic alternative to several Doppler radarsystems, the so-called bistatic Doppler receivers, weredeveloped especially for meteorological applications in1993 at the National Center for Atmospheric Research(NCAR) in the United States (Wurman 1994). Bistaticreceivers, which are spatially separated from the trans-mitter, are arranged around a monostatic Doppler radar.The theoretical framework for bistatic radar measure-ments can be found, for instance, in Wurman et al.(1993), Protat and Zawadzki (1999), de Elia and Za-wadzki (2001), Takaya and Nakazato (2002), Satoh andWurman (2003), and references therein. Network designand operation together with advantages of a bistatic ascompared to a monostatic Doppler radar system are ex-hibited, for instance, in Wurman et al. (1994) and Fried-rich and Hagen (2004a).

However, when using a bistatic compared to mon-

DECEMBER 2004 1841F R I E D R I C H A N D H A G E N

FIG. 1. Map of the bistatic multiple-Doppler-radar network at theDLR in OP consisting of POLDIRAD and three bistatic receiverslocated at Lichtenau, Lagerlechfeld, and Ried. The investigation areais restricted by horizontal antenna aperture and range resolution. Theequation system to calculate the horizontal wind field is exactly de-termined in the dual-Doppler areas (hatched) and overdetermined inthe triple- or quadruple-Doppler areas (cross-hatched). Horizontalwind fields are evaluated with measurements observed by the mon-ostatic Doppler radar located on top of Mount Hohenpeissenberg.More explanations in the text.

ostatic multiple-Doppler-radar system, not only the highcosts of purchasing and installing the equipment can bereduced, but the interpolation discrepancies as well, sim-ply owing to the fact that in the bistatic system, allDoppler velocity measurements are carried out simul-taneously and combined into a wind vector in real timesince the measurements are based on just a single sourceof illumination (Wurman et al. 1993). Until now bistaticreceivers have been used solely for experimental re-search, like the Cooperative Atmosphere–Surface Ex-change Study (CASES-97) in 1997 in Kansas (LeMoneet al. 2000), the Improvement of Microphysical Param-eterization through Observational Verification Experi-ment (IMPROVE) in 2001 in Washington State (Stoe-linga et al. 2003), and the Vertical Transport and Orog-raphy Experiment (VERTIKATOR) in 2002 in southernGermany (Lugauer et al. 2003). Although bistatic re-ceivers have been used for different field experimentsby different research groups, bistatically measured windvectors have barely been evaluated with observations.One reason is that the reference instrument has to coverthe same spatial area with a similar temporal and spatialresolution and additionally has to be able to measurewithin the same weather situation. Those demands inmind, bistatically measured wind fields can be evalu-ated, for instance, using an independent monostaticDoppler radar system. Wurman (1994) compared windfields from a bistatic dual-Doppler system with thosefrom a traditional monostatic dual-Doppler network. Hedemonstrated the ability to retrieve accurate wind fieldsand shallow surface divergence fields with a bistaticdual-Doppler network during three different weatherevents. Satoh and Wurman (1999) compared wind vec-tors derived from three pairs of dual-Doppler analysisin a stratiform weather situation. However, one disad-vantage of both investigations is that the wind velocitiesmeasured by the monostatic Doppler radar were usedto retrieve both bistatically measured and the monos-tatically measured wind vectors, respectively. There-fore, those comparisons were not achieved using twoindependent wind measurements. Point measurementssuch as in situ flight measurements and radiosoundings,on the other hand, cannot provide a spatial coverage butwill nevertheless contribute to a reliable evaluation.

In this paper measurements of the bistatic multiple-Doppler-radar network operated by the Deutsches Zen-trum fur Luft- und Raumfahrt (DLR) in Oberpfaffen-hofen (OP), close to Munich in southern Germany, arecompared both to those wind measurements achievedby an independent Doppler radar operated by the Ger-man Weather Service (DWD) and to wind measurementsachieved with the DLR Falcon research aircraft duringthree stratiform precipitation events (see sections 3 and4). Before evaluating wind fields, system configurationand evaluation performance, including an error sourcediscussion, are presented in section 2.

2. System configuration and internal evaluation

a. Transceiver and bistatic receiver configuration

The DLR bistatic Doppler radar network consists ofthe monostatic C-band polarimetric diversity Dopplerradar system (POLDIRAD; Schroth et al. 1988) locatedin OP at 602 m MSL and three bistatic receivers atremote sites each equipped with an antenna and a signalprocessor. It is the first bistatic radar system operatingat C band and with a magnetron transmitter. In Fig. 1the three bistatic receivers at Lichtenau, at Lagerlech-feld, and at Ried together with the respective viewingangle of the bistatic antenna are illustrated. The inves-tigation area, indicated schematically in Fig. 1, is re-stricted in range by a variable sample spacing and inazimuthal and vertical direction by the receiving powerpattern of the bistatic antennas, which have a horizontalangular aperture covering about 608 and a vertical ofabout 88.

With this configuration, the bistatic radar networkcovers an area of about 50 km 3 50 km horizontallyand a height up to 5 km within stratiform precipitation.Horizontal wind fields are determined exactly in thedual-Doppler area (hatched area in Fig. 1) and over-determined in triple-/quadruple-Doppler areas (cross-hatched area in Fig. 1). Due to a limited vertical antennaaperture of 88 oriented close to the ground, the measuredwind components are dominated by the horizontal com-ponents u and y. As a result, these measurements are


FIG. 2. Beam shielding for POLDIRAD antenna at 18 elevation with a 3-dB beamwidth of 18. (a) Spatial distribution of the shieldingfactor, Fgclu, ranging from no shielding indicated by 1.0 to total beam blockage assigned by 0.0. (b) Distance of ground clutter from POLDIRAD(gray boxes) and beam shielding factor (solid lines) as a function of azimuth angle.

used only to determine the horizontal wind vector fielddirectly.

b. Contamination sources and measurementlimitations

Radar measurements can be contaminated by a largenumber of factors like, for instance, clutter from eithernormal propagation (permanent clutter) or anomalouspropagation of the radiation, biological targets, or chaff.Other acquisition properties, like angular velocity of therotating antenna, number of averaged pulses, and alias-ing effects, also influence the quality of the measureddata. For an overview of error sources and solutions theauthors refer to other literature, for instance (Alberoniet al. 2002; Meischner 2003).

Especially in mountainous regions, ground cluttercontamination is a significant error source for both re-flectivity and Doppler velocity. Over the last severalyears great success has been achieved by detecting andremoving ground clutter based on the usage of Dopplervelocity information in signal processing. Besides con-taminating directly the backscattering signal, groundclutter can totally or partially shield the transmitted ra-dar beam. When beam shielding occurs, reduced peakintensity propagates farther, leading a priori to a reducedbackscattering signal. Behind ground clutter, receivedechos are assigned to a lower height because the mainbeam is blocked and the backscattering signal comesfrom the pulse volume edges, which are located at a

higher elevation than the main axis of the radar beam.This also holds for Doppler velocity, which is weightedby reflectivity. In the presence of vertical wind shear,the Doppler velocity measured at the pulse volume edgediffers from that measured at the beam axis. When 80%of the transmitted beam is shielded, measurements arerelated only to the upper pulse volume edge, creating aheight error, for instance, of about 0.35 km (0.48) at adistance of 50 km and a 18 beam width. Assuming avertical wind shear of 10 m s21 (km)21, the resultingvelocity error is about 3.5 m s21.

Figure 2 illustrates beam shielding at 18 elevationaround Oberpfaffenhofen for transmitting the POLDI-RAD radar beam, which has a vertical beamwidth of18. The shielding factor increases linearly between zerofor a complete shielding of the 3-dB beamwidth to onefor no shielding. The factor is valid for the respectiverange gate plus for following gates outward along eachradial. The calculation is based on a topography datasetthat was generated from measurements achieved by theEuropean Remote Sensing Satellite-2 (ERS-2) and hashorizontal resolution of 250 m and vertical of 1 m.Topography data were averaged and interpolated ontoa polar coordinate centered around POLDIRAD, whichsamples with an angular resolution of 18 in azimuth andelevation and a radial resolution of 250 m. Measure-ments using POLDIRAD are strongly contaminated byground clutter, especially southwest of OP for an ele-vation of 18, as shown in Fig. 2a. Figure 2b illustratesthe distance of the highest ground clutter value from the


FIG. 3. Dependency of resolution volume length and std dev of thehorizontal wind vector, s , on the intersecting angle b. The res-| y |h

olution volume length for bistatic reception, ab, is normalized to theconstant monostatic resolution volume length, at.

FIG. 4. Spatial distribution of the (a) std dev of the horizontal wind field and (b) resolution volume length normalized by the monostaticresolution volume length at 1.6 km MSL for the bistatic dual-Doppler radar consisting of the receiver at Lagerlechfeld (Laglech) andPOLDIRAD at Oberpfaffenhofen (OP). The investigation area of intercomparison with the monostatic Doppler radar at Hohenpeissenberg(HP) is shaded gray. In this area the intersecting angle ranges between 258 and 758. Note that the maximum contour line is set to a valueof 10 in (a) and 6 in (b).

radar (gray bars) together with the shielding factor (solidline). Again, high shielding is expected between an az-imuth angle of 1808 and 2478. Hardly any shieldingeffects are assumed for elevation angles higher than 1.58(figures not shown).

When intercomparing radar data, one has to be aware

of limits in sensitivity and accuracy. For monostaticradars Doppler velocity measurements have an instru-mental error of less than 1 m s21. In addition to theinstrumental error, one has to consider errors caused bythe nature of the phenomena under investigation. Tur-bulence and wind shear, for instance, produce a spreadof Doppler velocities around the mean value resultingin higher measurement errors. As the radial distancefrom the radar increases, there is an increase propor-tionally of the size of the sample volume and the verticaldistance between ground and first radar echo the resultof beam broadening and sampling on a spherical co-ordinate system. Those effects have to be consideredwhen interpolating onto a Cartesian coordinate system.For measurements using a bistatic multiple-Doppler-ra-dar network, accuracy of horizontal wind field deter-mination, range resolution, and size of the investigationarea will be summarized shortly in the following.

Both accuracy of the estimated horizontal wind,s , and spatial resolution of bistatic measurements| y |h

depend on the intersecting angle1 between the two mea-sured wind components. The dependency of the twoparameters on the intersecting angle, b, and their spatialdistribution are illustrated in Figs. 3 and 4. For further

1 The intersecting angle is the angle between the radial velocitycomponent measured by the monostatic radar and the componentperpendicular to the ellipsoid of constant delay measured by the bis-tatic receiver. Note that it is half of the angle between transmittedand scattered paths.


FIG. 5. One-way receiving power pattern (in dB) of vertically po-larized bistatic antennas measured in azimuthal direction and for dif-ferent elevations at a DLR antenna test facility. (a) The receivingpower pattern sampled in azimuthal direction for 2.58 elevation. Thenominal horizontal angular aperture covering 6308 is indicated bythe vertical lines. (b) The receiving power pattern is measured in avertical direction for an azimuth angle of 08. The vertical angularaperture covering from 18 to 98 is symbolized by the vertical lines.Note that the 0-dB amplitude level is related to the power fed intothe bistatic antenna.

information on wind synthesis of bistatically measuredDoppler, the authors refer to Wurman et al. (1993), Pro-tat and Zawadzki (1999), de Elia and Zawadzki (2001),Takaya and Nakazato (2002), Satoh and Wurman(2003), and Friedrich and Caumont (2004). For dual-Doppler analysis, highest accuracy can be achievedwhen both wind components are perpendicular to eachother. In a bistatic network, however, the lowest theo-retical standard deviation (std dev) with values of 2.4m s21 is reached when b 5 508, as illustrated in Figs.3 and 4a (Takaya and Nakazato 2002; Satoh and Wur-man 2003). Requiring an accuracy of the horizontalwind of less than 3.5 m s21 the target area reduces toan intersecting angle limit of about 258–758 (cf. Fig. 3vertical solid lines, Fig. 4a gray marked area). Reso-lution volume length of bistatic measurements normal-ized to that of a monostatic radar is illustrated in Fig.3 (dotted line) and Fig. 4b. Close to the transceiver–receiver baseline, the resolution volume length is con-siderably larger than the monostatic one. Nevertheless,within the aforementioned intersecting angle limit, onecan expect a sufficient spacial resolution.

The area that can be observed by a broad-beam bis-tatic antenna is restricted by the receiving power pattern.By feeding a weak signal into the bistatic antenna, whilean independent microwave receiver measured the beampattern of the slotted waveguide 1808 horizontally aswell as at different elevations, the receiving power pat-terns of bistatic antennas were measured at an antennatest facility at the DLR. The distance between the mi-crowave receiver and the bistatic antenna was about 50m. As illustrated in Fig. 5a, bistatic antennas used inthe DLR bistatic Doppler radar network were designedto receive the main power with a horizontal angularaperture covering 6308 around the principal axis. In thevertical direction, however, the main power is receivedbetween 18 and 98, as illustrated in Fig. 5b. With thesharp power gradient between 08 and 18, ground-cluttercontamination should be suppressed. As a result, mea-surement can be achieved up to a maximum height ofabout 4.7–6.2 km at a range from 30 to 40 km with avertical antenna aperture of about 88.

c. Internal evaluation

In order to gain confidence in the performance ofbistatic receivers, Doppler velocities of the receivers,included in the DLR bistatic Doppler radar network, arecompared to each other. The objective of the internalevaluation is the determination of the instrumental error.For that purpose, two receivers connected to one bistaticantenna were placed at the monostatic radar site in OPso that both receivers measure almost the same velocitycomponent as the monostatic radar system. When usingthis setup, errors are neglected that can be related totime and space interpolation and to the measurementgeometry as proposed, for instance, by Takaya and Nak-azato (2002). Doppler velocity measurements were tak-

en during stratiform precipitation with low wind shearand low turbulence characteristics, reducing the ap-pearance of high Doppler velocity variations. Both re-ceivers measured normalized coherent power2 (NCP)above 0.6, indicating low turbulence for this case. Asillustrated in Fig. 6, the majority of data points varybetween about 61 m s21 with a regression line of y 51.03x 2 0.08. As a result, the velocity variation of 61m s21 is related mainly to the instrumental performance,like the accuracy of the Doppler phase measurement ornumber of independent echo samples for the time series.

2 NCP, also known as signal quality index (SQI), is related inverselyto the spectral width and ranges from zero to one. It is calculated atthe bistatic receivers as NCP 5 | R1 | /R0, with R0 and R1 being thezeroth and first moment of the autocorrelation function taken fromthe Doppler power spectrum.


FIG. 6. Scatterplot illustrating Doppler velocities measured by bis-tatic receivers 1 and 2, both located at OP. The linear regression lineand its equation are presented.

3. Intercomparison to monostatic Doppler radar

a. Observation and evaluation procedure

Within the DLR’s bistatic Doppler radar network vol-ume scans were performed with a total duration time ofabout 4 min every 10 min. The vertical spacing fortransmitting was chosen to be 18 for an elevation angle18 to 48 and set to 28 for 48 up to 108 (for case 1) and188 (for case 2) elevation. The radial wind componentof the monostatic C-band Doppler-radar system operatedby the Meteorological Observatory Hohenpeissenberg(HP) of the DWD (located 1006 m MSL on top of MountHohenpeissenberg) was determined by using data fromPOLDIRAD and receiver at Lagerlechfeld (Fig. 1).Doppler velocities measured by the DLR system areboth dealiased using the four-dimensional dealiasingscheme, 4DD (James and Houze 2001; Friedrich andCaumont 2004), and quality controlled as explained inFriedrich and Hagen (2004b). Data of two successivevolume scans are interpolated both to a single referencetime (hereafter POLDIRAD reference time), applying amoving frame of reference (Protat and Zawadzki 1999)and onto a Cartesian coordinate system. The latter isachieved using a linear interpolation method based onthe usage of a sphere of influence (Protat and Zawadzki1999). The wind vector is determined based on leastsquares estimation using the wind information of allavailable receivers. From this wind vector, the radialwind component as measured by the independent radaris reconstructed. An example of the evaluation perfor-mance is given in Fig. 7 for a stratiform precipitationevent at 0558 UTC 6 June 2001. First the horizontalwind field is derived using wind information from allavailable receivers (Fig. 7a) in order to reconstruct theradial velocity component to be measured at the secondradar (Fig. 7b). Afterward the Doppler velocity mea-sured at the independent radar site (Fig. 7c) is comparedto the reconstructed wind (Fig. 7d). This case is dis-cussed in more detail in section 3c.

To evaluate the bistatically measured multiple-Dopp-ler wind fields, they are compared to radial Dopplervelocity fields measured by the HP radar (cf. Fig. 1).The system is included in the DWD radar network, per-

forming operational volume scans every 15 min (Schrei-ber 1998). The start time of each volume scan is referredto the HP scanning start in the following text. Eachvolume scan starts at the highest elevation proceedingdownward. Doppler data are dealiased using a dual-PRF(pulse repetition frequency) technique and interpolatedonto a Cartesian grid using SPRINT software (Mohr etal. 1986).

For the intercomparison both datasets were interpo-lated onto the same Cartesian coordinate system with ahorizontal resolution of 500 m and a vertical of 250 m,leading to a domain height of 4.25 km above radarheight (600 m MSL). As discussed in section 2b, thetarget area was horizontally restricted to a intersectingangle limit between 258 and 758 (cf. Fig. 3) and verti-cally between 18 and 98 (cf. Fig. 5). Therefore not con-sidered for the intercomparison were those areas of thebistatic Doppler radar network that have large spatialresolution (e.g., close to the transmitter and bistatic re-ceiver baseline, cf. Fig. 3, dashed–dotted line) and lowreceiving power of the antenna (cf. Fig. 5). An algorithmfor correcting beam shielding (Fig. 2) was not applied.The HP scanning start and POLDIRAD reference timetogether with the statistical analysis of the intercom-parison are illustrated in Table 1 for case 1 and in Table2 for case 2.

b. Stratiform precipitation event, case 1

Illustrating the reliability of bistatic Doppler radarmeasurements, a weather situation with low wind shearwas chosen for the first case study. On 10 April 2001,the trough of a low pressure system centered on theDutch coast crossed Germany within the course of themorning hours. During noon the occlusion of the vortexreached the observation area with low precipitation andwind shear. Stratiform precipitation with uniform re-flectivity was observed between 1200 and 1530 UTC.At ground level a wind-velocity gradient in an east–west direction over a length of 30 km occurred havingvalues of 14 m s21 in the west and 6 m s21 east of theobservation area. The mean wind direction was about2608, varying from 2408 to 2658.

Generally, on 10 April 2001 the velocity differencesranged mainly between 62 m s21, in some cases be-tween 66 m s21, with standard deviation values mainlyaround 62 m s21, as illustrated in Figs. 8 and 9 and inTable 1, respectively. The distribution of the Dopplervelocity difference is portrayed for 1322, 1342, 1412,and 1502 UTC at a height level of 2.1 km MSL in Fig.8. Both systems were able to observe within a variabilityof 62 m s21 1) main structures such as the wind veeringfrom southwest (Figs. 8a,b) to west/northwest (Figs.8c,d) or the increase in wind speed at 1342 comparedto 1322 UTC (Figs. 8a,b) and 2) finer structures in thewind field such as like local wind shift at 1322 UTCnorth of HP. While the intercomparison within the innerdomain of the target showed differences within


FIG. 7. Example of the evaluation performance illustrating horizontal cross sections at 1.6 kmMSL of (a) the horizontal wind (m s21) using Doppler information from POLDIRAD and thereceiver at Lagerlechfeld; (b) the radial velocity of HP reconstructed from (a); (c) the radialvelocity (m s21) measured by HP (grayscale), together with the area of horizontal wind fieldmeasurements (thick, solid line); and (d) the velocity difference (m s21) between (b) and (c) fora stratiform precipitation event at 0558 UTC 6 Jun 2001.

62 m s21, large discrepancies occurred close to thePOLDIRAD and HP radars as well as between an az-imuth angle related to POLDIRAD ranging between1808 and 2128 (Fig. 8). One explanation for the lattereffects is the beam shielding of POLDIRAD. As shownin Fig. 2 and discussed in section 2b, the areas betweenan azimuth of 1808 and 2508 related to OP is favoredby total or a high degree of beam shielding. This ex-planation is supported when comparing the velocitystructure of the measured and reconstructed radial ve-locity of the HP radar (figure not shown). At the re-constructed radial velocity the zero-velocity isolineformed a bulge between an azimuth angle 2108 and 2258at a range from 15 to 25 km. On the other hand, thisbulge was less pronounced by the radial velocity mea-surements taken at the HP radar, which led to a mag-nitude in the Doppler velocity difference of more than

4 m s21, as seen in Fig. 8. Radial velocities measuredby HP radar were from 4 to 8 m s21 higher than thoseobserved by the bistatic network. This effect was sur-prisingly observed at those height levels where the mainradar beam was not shielded by ground clutter. One canhypothesize that the secondary lobe of the transmittingpower pattern hits ground clutter, creating not only ahigh backscattering echo but having also a zero Doppler-velocity component. Since the total received energy isthe sum of all backscattered energy and Doppler velocityis weighted by reflectivity, processed Doppler velocityis reduced due to the zero-Doppler-velocity part comingfrom the secondary lobe return. High velocity differ-ences close to the radars can be explained by the impactof the vertical velocity component on the measured one.Dual-Doppler analysis close to a monostatic radar sys-tem can only be achieved when one radar scans with a


TABLE 1. Statistical analysis of wind field comparison between thebistatic multiple-Doppler radar (denoted as OP) and the Doppler radarsystem at Hohenpeissenberg (denoted as HP) for 10 Apr 2001. HPscanning time (time HP), POLDIRAD reference time (time OP), num-ber of data samples (N ), mean values of the Doppler velocity dif-ferences, standard deviation (std dev), and correlation coefficient (cc)of the velocity difference are illustrated.

Time HP(UTC)

Time OP(UTC) N

Mean(m s21)

Std dev(m s21) cc

1300–13111315–13261330–13411330–13411345–1356

13121322133213421352

31 68935 08140 79742 46538 022

20.6520.4620.0420.6220.40

1.901.992.281.992.25

0.9510.9470.9540.9720.949

1400–14111415–14261430–14411430–14411445–1456

14121422143214421452

29 44923 52323 30127 83235 210

20.7520.39

0.3220.0220.22

1.781.752.101.672.19

0.9640.9700.9580.9630.963

1500–15111500–15111515–15261530–1541

1502151215221527

35 97422 42129 42120 085

0.0420.1620.05

1.25

2.122.512.443.12

0.9640.9630.9510.910

TABLE 2. As in Table 1 but for 6 Jun 2001.

Time HP(UTC)

Time OP(UTC) N

Mean(m s21)

Std dev(m s21) cc

0600–06110600–06110615–06260630–06410630–0641

06030613062306330643

42 95646 40446 54646 37945 935

0.380.440.440.660.29

1.231.441.361.641.55

0.9700.9620.9600.9450.951

0645–06560700–07110700–07110715–07270730–0741

06530703071307230733

45 81845 94346 01246 22346 206

0.580.860.410.530.70

1.341.421.441.701.90

0.9620.9610.9570.9480.939

0730–07410745–07560815–08260830–08410830–0841

07430753083208320842

46 11946 08241 17144 46537 532

20.090.17

20.040.590.32

1.601.992.342.302.14

0.9580.9330.9170.9240.935

0845–08560900–09110900–09110915–09260915–0926

08520902091109110919

35 97036 71033 16524 37510 740

1.300.650.720.860.73

2.702.282.072.131.88

0.8800.9220.9320.9180.854

higher elevation angle than the other one. As a result,the impact of the vertical wind component, thereforethe particle fall velocity, on the measured radial com-ponent is higher close to the radar than at greater dis-tances. In this case Doppler velocities that are mainlymeasured in the horizontal direction are compared withthose measured in the vertical direction.

A direct comparison of each Doppler velocity mea-surement, illustrated by scatterplots in Fig. 9, supportedthe agreement of Doppler velocities ranging within aninterval of 62 m s21. The differences in the magnitudeof the velocity are within the margin of instrumentalerror for Doppler velocity measurements using a bistaticreceiver. Furthermore, errors of about 62 m s21 are alsoassociated for traditional monostatic dual-Doppler es-timations. Nevertheless, the bistatic multiple-Dopplerradar system observed lower magnitudes of Dopplervelocities than the HP radar, which is indicated by neg-ative velocity differences in Table 1 and Fig. 9. Thiscan result from instrumentation discrepancies, interpo-lation problems, the domination of beam-shielding ef-fects, or the different observation times. The statisticalanalysis of the Doppler velocity (Table 1) showed ve-locity differences that lie mainly below 2.5 m s21 witha maximum standard deviation of 3 m s21. Mean ve-locity differences were mainly negative with valuesclose to zero. Note that the time difference between theobservations can range from 4 up to 10 min. The cor-relation coefficient between the bistatically measuredand HP-measured Doppler velocities was above 0.9 on10 April 2001 (cf. Table 1).

In order to quantify the error of the Doppler velocitydifference within the three-dimensional volume the cu-mulative probability distribution function (CDF) wascalculated illustrating the percentage of data that lieswithin a respective velocity difference interval, «. Fig-

ure 12a illustrates the CDF value for each observationtime between 1312 and 1527 UTC. As a result, on 10April 2001, 65% of the velocity differences lay withinan interval of 2 m s21, while the majority reaches even70%. For e 5 3 m s21, 80% of the differences reachedthis limit, while again the majority reached even 85%.Investigations on CDF comparing wind componentsmeasured solely by the DLR’s bistatic network showedwithin low-wind-shear cases that the velocity differencewas about 1 m s21 at CDF . 90% (Friedrich and Cau-mont 2004). Monitoring high-wind-shear events, theCDF was reduced to around 80% for a velocity differ-ence of 1 m s21. Note that the CDF is also a mainparameter for the intern check procedure included in thequality control scheme for multiple-Doppler-derivedhorizontal wind fields [a detailed description of the qual-ity control scheme can be found in Friedrich and Hagen(2004b)].

c. Stratiform precipitation event, case 2

In the second case, a cold front passage was moni-tored passing the investigation area in the morning hoursbetween 0600 and 0900 UTC on 6 June 2001. In thefirst hour prefrontal winds from the southwest and westdominated. After the frontal system propagated throughthe investigation area from northwest to southeast be-tween about 0700 and 0800 UTC, wind vectors in-creased and veered to northwest (Fig. 10). Northwest-erly winds were observed until the precipitation passedthe investigation area at about 0930 UTC. The windvelocity at ground level varied between 10 and 12 ms21 and increased up to 20 m s21 after frontal passage.Generally, wind shear, in both direction and velocity,was much higher on 6 June than compared to wind fieldsobserved on 10 April 2001.


FIG. 8. Horizontal cross section of the velocity difference between the bistatically measuredsubtracted from the radial velocity observed by radar HP at 2.1 km MSL. The wind vector field(arrows) measured by POLDIRAD at OP and receiver Lagerlechfeld is overlaid. Data were sampledat (a) 1322, (b) 1342, (c) 1412, and (d) 1502 UTC 10 Apr 2001. For clarity every fourth windvector is displayed.

FIG. 9. Scatterplot illustrating the difference between the reconstructed radial velocity ( ) and Doppler velocitybiy r

measured by the radar HP ( ) at 1342, 1422, 1442, and 1502 UTC 10 Apr 2001.HPy r


FIG. 10. As in Fig. 8 but for (a) 0613, (b) 0703, (c) 0733, and (d) 0902 UTC 6 Jun 2001.Horizontal cross section was set to 2.1 km MSL.

A selection of the Doppler velocity intercomparisontogether with the wind vector measured by the bistaticmultiple-Doppler system is portrayed in Fig. 10 for fourobservation times (0613, 0703, 0733, and 0902 UTC).The complete cycle of the frontal passage is displayedwith 1) prefrontal winds coming from the west or south-west (Figs. 10a,b); 2) the frontal passage itself with anincrease in wind speed and variable wind directionsfrom west and southwest changing to northwest (Fig.10c); and 3) postfrontal northwesterly winds (Fig. 10d).

The majority of velocity differences lay within a mar-gin of 62 m s21 but increased to some extent up tovalues of even 68 m s21. Nevertheless, Doppler ve-locity differences were more variable and higher in thiscase than on 10 April 2001 (cf. Figs. 9, 11). Beamshielding effects, however, were not visible on 6 June2001, which can be related to the prevailing wind di-rection and speed. When northwest winds are observedby the POLDIRAD and HP radars, the zero-velocityisoline is oriented in a southwest to northeast direction,that is, along the baseline between the two radars. Asa result, one can disregard the impact of the zero velocity

components arising from the secondary lobe. The im-pact on the velocity magnitude is also smaller for lowDoppler velocities than for high wind speed. Both ef-fects caused lower Doppler velocity discrepancies in thearea of high beam shielding during early hours observ-ing lower velocities and after frontal passage with north-west winds.

Huge discrepancies with absolute velocity differencesgreater than 4 m s21 measured by the bistatic systemcompared to the HP radar were noticeable west-south-west of OP at a range of 20–40 km (Fig. 10d). In thisarea the velocity vector is much higher than in the re-maining observation areas. According to the wind vectorfield, both wind direction change and wind velocity in-crease occurred rapidly. Different scanning times ofeach system (both start time and scan strategy) can resultin large velocity differences.

Variance of the Doppler velocity differences in-creased with time, as portrayed in Fig. 11. One can relatethe effects to changes in wind direction and velocityduring the frontal passage. While at 0623 UTC positivevalues of the Doppler velocity difference related to


FIG. 11. As Fig. 9 but for POLDIRAD reference times 0623, 0733, 0832, and 0911 UTC 6 Jun 2001.

FIG. 12. Comparison between Doppler velocity measured by the DLR’s bistatic multiple-Doppler radar network and those observed bythe DWD’s Doppler radar at Hohenpeissenberg. The cumulative probability distribution function (CDF) of the Doppler velocity differencese is illustrated for a stratiform precipitation event (a) between 1312 and 1527 UTC 10 Apr and (b) between 0603 and 0919 UTC 6 Jun2001.

southwesterly winds (velocities toward the HP radar)dominated the area, the Doppler velocity differencesbetween 0733 and 0911 UTC were negative related tonorthwesterly winds (velocities away from the HP ra-dar). At the same time the variance increased, especiallyin the area of negative Doppler velocity differences.During the whole intercomparison period, high vari-ances correlated with high Doppler velocities.

However, the frontal passage itself was well coveredby both systems. After the wind vector finally veeredto northwest (0911 UTC), variance decreased slightly,as illustrated in Fig. 11. Unfortunately, the precipitationmoved farther to the southeast and passed the obser-vation area at about 0930 UTC (as shown in Table 2,column N, number of data points). The increase ofDoppler velocity variance during frontal passage wasalso investigated in the statistical analysis (Table 2). The

standard deviation increased between 0603 and 0852UTC and decreased after the wind vector within theobservation area veered to northwest. The same effectwas investigated using the correlation coefficient.

However, both statistical analysis (Table 2) and scat-terplots (Fig. 11) show no clear indication that the bis-tatic Doppler radar network measures generally lowerDoppler velocities compared to the HP radar, as inves-tigated in case 1 (section 3b). The variability within thevolume scan increased with time when comparing mea-surements at 0623 with those at 0911 UTC in Fig. 11.Both the increasing variance with time and a highervariability compared to 10 April 2001 are illustrated inFig. 12b. About 70% of the differences lay within 2 ms21 for the intercomparison period between 0600 and0800 UTC. During the entire observation period, 70%of the differences are lower than 2.8 m s21. The worst


FIG. 13. Spatial distribution of the theoretical std dev of the hor-izontal wind field normalized by 1 m s21 at 1.8 km MSL for thebistatic dual-Doppler radar consisting of receivers at Lagerlechfeldand Lichtenau and POLDIRAD at Oberpfaffenhofen (OP). The flightpath of the aircraft is marked with a dashed line.

agreement was observed at 0852 UTC where a CDF of70% was reached for 3.2 m s21.

4. Intercomparison to in situ flight measurement

In situ flight measurements taken on 11 April 2001were used as an additional independent measurement toevaluate horizontal wind fields determined by the bis-tatic Doppler radar network. The in situ measurementswere performed with the DLR Falcon research aircraft.The wind speed at the position of the aircraft, measuredby a five-hole gust probe on the tip of the nose boom,was derived from the differential and the static pressureat the five holes of the half-spherical probe tip. Thewind components, u, y, w, in an earth-fixed coordinatesystem, were derived from the differences between theairflow at the probe, velocity of the aircraft, and ori-entation of the sensor relative to the ground, which wasgiven by an inertial reference system (IRS). The ab-solute accuracy of the mean horizontal components is61 m s21 for the aircraft measurements [for more detail,see Boegel and Baumann (1991) and Quante et al.(1996)]. For bistatic dual-Doppler measurements, how-ever, the error in the horizontal wind field determinationis less than 62 m s21 (dashed line in Fig. 13 representsthe flight path).

The aircraft passed the investigation area flying be-tween 1014:15 and 1015:15 UTC from southeast tonorthwest at an altitude of 1.8 km MSL. Figure 14 ex-hibits the flight path through the investigation area. Therespective elements of UTC time and altitude are la-beled. The sensor signals were recorded with 100 Hzcorresponding to a sampling interval of typically 1.5 m.The measured data were averaged to an interval of 1 s,

which represents a length of about 150 m when theaircraft flies at a speed of about 300 kt. The radar mea-surements were interpolated from an elliptical coordi-nate system with the bistatic receiver and POLDIRADas foci onto a spherical coordinate system centered atPOLDIRAD with a resolution volume length of 150 m.

In situ flight measurements of u, y were compared tothe velocity components determined by the bistatic mul-tiple-Doppler radar network at 28 and 48 elevation (sam-pled between 1012:29–1012:32 and 1013:09–1013:11UTC, respectively). The horizontal wind vector was es-timated in spherical coordinates at 28 and 48 elevationfrom the Doppler velocities measured by POLDIRADand receivers at Lichtenau and at Lagerlechfeld. Thehorizontal wind vector field at 48 elevation underlaid byits south wind component of each sample volume isillustrated in Fig. 14.

At 28 elevation, the observation height varied between1.30 km (at a range of 19 km from OP) and 1.44 kmMSL (at a range of 24 km from OP), while at 48 ele-vation it ranged between 2.0 km (at a range of 19 kmfrom OP) and 2.3 km MSL (at a range of 24 km fromOP). The flight path was located at a height of 1.8 kmMSL. The west and south wind components, measuredby the radar at each aircraft sample point, were inter-polated vertically. Note that at a distance of 20 km fromthe radar, the sample volume of the monostatic radarhad a diameter of about 300 m and a length of 150 m(with a 18 antenna beamwidth and a 1-ms pulse length).Therefore, the spatial resolution of the aircraft mea-surements was higher than that achieved by the radar.An interpolation in time of both datasets was not per-formed due to a lack of synchronization between timemeasurements in the aircraft and at the radar. Further-more, coupled GPS and IRS measurements achieved thepositioning of the aircraft with an error ranging from100 to 200 m per 2–3 min. The error results mainlyfrom the IRS measurements.

Figure 15a illustrates time series of the west and southwind components when measured by the bistatic Dopp-ler radar network and by the in situ instruments. Thedifferences of u, y between the radar and in situ mea-surements are presented in Fig. 15b. The magnitude ofthe horizontal wind components measured by the air-craft were mostly 1–2 m s21 higher than the radar mea-surements and are therefore within the margin of the-oretical error for horizontal wind field estimation usinga bistatic Doppler radar network. As a result, this com-parison shows an agreement within 2 m s21, which wastheoretically expected. Furthermore, the Doppler veloc-ity measurements are in good agreement even with insitu measurements, which are a better ground truth thandual-Doppler estimation since only a time interpolationhas to be applied.

Differences in the horizontal wind-field componentscan be related mainly to the low values of u, y whencompared to the absolute error of the velocity mea-surement. Due to a low temporal evolution of this sys-


FIG. 14. PPI of the south wind component (in m s21) (shading) at 48 elevation taken between1013:09 and 1013:11 UTC 11 Apr 2001, overlaid by the fight path of the aircraft (solid line) andthe horizontal wind vector field (arrows). Labels on the fight path indicate time (HH:MM:SS) andaltitude MSL. Wind vectors were determined from the Doppler velocities measured by the receiversat Lichtenau and Lagerlechfeld and by POLDIRAD. PPI is centered around POLDIRAD.

tem, having weak precipitation and low wind velocity,the impact of the temporal displacement between theaircraft and radar measurements can be neglected.

For a broader evaluation, flight measurements haveto be obtained over a longer time period, a larger spatialcoverage, and in various weather situations, for exam-ple, with higher wind shear or with higher wind veloc-ities.

5. Concluding remarks

Intercomparing measurements achieved by two dif-ferent instruments requires an extensive knowledge oferror sources that contaminate the measurements. Un-fortunately, one is mostly not even able to estimate qual-itatively and quantitatively the impact of contaminationon the measurement. Since instrumentation limits anderror sources can only be partially corrected, one hasto keep in mind that the reference instrument does notmeasure a priori correct winds. As long as all errorsources cannot be identified explicitly, one cannot con-clude that monostatically measured data are withoutmeasurement errors. The question to be answered is nowhow great is the error on Doppler velocity measurementand how variable is this margin in time and space duringthe observed weather situation.

Since the first measurements using bistatic receiverswere published, the literature has been dominated bydiscussions on measurement accuracy or contamination

sources, such as sidelobe contamination. The impressionarose that wind measurements using bistatic receiversare afflicted with large errors or they are even impos-sible. This paper does not controvert those investiga-tions but shows how bistatically measured wind fieldsare evaluated and can contribute significantly, for in-stance during a frontal passage, to research studies,weather surveillance, and forecasting.

Comparing direct measurements of two bistatic re-ceivers, Doppler velocities agree within 61 m s21,which lies within the margin of error for instrumentationaccuracy. The Doppler velocity variation is mainly as-signed the instrumental error due to the setup of re-ceivers and meteorological conditions. Note that anagreement within about 61 m s21 is also assumed formonostatic Doppler radar systems. However, the eval-uation of bistatically measured wind fields with both anindependent Doppler radar and in situ aircraft mea-surements also showed reliable results. The magnitudein Doppler velocity differences ranged mainly in allthree case studies between 2 and 3 m s21. The mainmeasurement error within a bistatic network consistsgenerally of an instrumental error, error related to themeasurement geometry, and error due to the nature ofthe phenomena under investigation. While the first twoerror sources result in a theoretical error ranging be-tween 2 and 3 m s21 [see Fig. 3 of Takaya and Nakazato(2002); Satoh and Wurman (2003)], the meteorologi-


FIG. 15. (a) The values of u, y and (b) its differences (in m s21)achieved during in situ fight measurements (denoted as Falcon) andmeasured by the bistatic Doppler radar (denoted as Radar) along theflight path for a stratiform precipitation case on 11 Apr 2001. Theaircraft passed the observation area between 1014:23 and 1015:11UTC. For the west wind component, u, the linear regression line isgiven by y 5 20.015x 1 0.68, while for the south wind component,y, it is y 5 0.026x 1 0.65.

cally induced variation is hard to quantify. Figure 11illustrates that meteorological effects influence the errorvariation to a much higher degree than instrumental ef-fects, albeit one has to link the differences also to dis-crepancies in the time and space interpolation. Never-theless, over the years error sources influencing mon-ostatic Doppler radar measurements have been ana-lyzed. Those findings can be applied to bistaticmeasurements mainly in the same way. Additionally,the quality of bistatic measurements depends stronglyon the amount of sidelobe contamination and the timeand phase synchronization stability between the trans-mitter and each receiving source. Synchronization canbe monitored by transmitting each receiver’s time andphase measurement information to the main hub com-puter. An alternative to this approach is the comparisonof Doppler velocities within areas where the equationsystem to determine a wind vector field is overdeter-mined as demonstrated, for instance, by Satoh and Wur-

man (2003) and Friedrich and Hagen (2004b). In thisintercomparison study, receivers not synchronized dur-ing the observation time were neglected. Monitoring theamount of sidelobe contamination on the bistaticallymeasured data resulting from the transmitted power pat-tern is a more sophisticated task [for more details, theauthors refer to de Elia and Zawadzki (2000)]. In orderto monitor the amount of contamination de Elia andZawadzki suggested calculating a sidelobe contamina-tion index. Nevertheless, sidelobe contamination has notinfluenced significantly the radar measurements chosenfor this case study since the reflectivity factor was lowand on the order of 35 dBZ.

The CDF values range between 65% and 80% for 2m s21 Doppler velocity differences and 82% and 94%for 3 m s21 on 10 April 2001. During the frontal passageon 6 June 2001, Doppler velocity differences are muchgreater with CDF values varying between 48% and 80%for 2 m s21 and 67% and 90% for 3 m s21. Systematicerrors in velocity measurement using bistatic receiverswere not found on 6 June 2001, although differenceshigher than 4 m s21 appeared in some areas. Since theinstrumental error is about 1 m s21, large differencesare ascribed to the aforementioned meteorological ef-fects.

The authors assume that the gross differences in windvelocity are mainly ascribed to ground clutter contam-ination and interpolation discrepancies including dif-ferent scanning strategies. Investigation showed an un-expected large impact of ground clutter on the receivedpower pattern, and therefore on Doppler velocity mea-surements, which is not necessarily visible as Dopplervelocity or reflectivity images. The amount of groundclutter contamination depends on both wind directionand speed; that is, in areas where the radial velocitiesare small the impact is less pronounced than in areaswith high radial velocity measurements.

The comparison of bistatic Doppler radar measure-ments with another independent instrument, the in situflight measurement, confirm those results achieved us-ing a monostatic Doppler radar. Again, differences inDoppler velocities are within 62 m s21.

Weather events like, for instance, a frontal passagecan be well monitored by a bistatic multiple-Dopplerradar network, as shown on 6 June 2001. One of thegreat advantages of bistatic network is the real-time dis-play of wind vectors, which allows a rapid and easydetection of signatures in the wind field, especially forusers with little or no experience in interpreting Dopplervelocities.

Acknowledgments. First the authors would like tothank Dr. Jorg Seltmann (German Weather Service,Hohenpeissenberg), together with all members of theDWD radar group, for data acquisition. We greatly ben-efited from the long and fruitful cooperation with theDWD. We also thank Robert Baumann (DLR) for pro-viding aircraft measurements. We would like to thank


Hermann Scheffold and Lothar Oswald for their tech-nical support and Edgar Clemens for measuring thebeam pattern of the bistatic antennas. We are very grate-ful to Dr. Joshua Wurman and the anonymous reviewerfor their precise and useful comments.

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