Jul 27, 1989 - Resource Technologies, Aiberta Research Council. Edmonton, Aiberta and ..... vi) The back-scatter amplitudes for scattering by raindrops of ...
Radar Measurement of Rainfali by Differential Propagation Phase: A Pilot Expe riment M. Enghsh, B. Kochtubajda and F.D. Barlow Resource Technologies, Aiberta Research Council Edmonton, Aiberta
and A.R. Hoit and R. McGuinness Mathematics Department, University of Essex Wivenhoe Park, Coîchester, Unîted Kingdom
[Original manuscript received 13 September 1990; in revised form 12 December 1990]
A pilot project concerning the measurement of rainfaîl by polarization diversuy radar, conducted jointly by the Aiberta Research Council and the Universily of Essex in the summer of I 989, is described. The objective of the experiment was to test the theory that differential propagation phase shift can give a better estimate of rainfaîl with high ram rates (about 30 mm h’ and greater) than reflectivity measurements. The projeet comprised a field experiment that was conducted in central Aiberta during the period 20 July to 2 August J989. The field experiment involved observing storms within about a 70-km radius from Red Deer with the ARC S-band polarization diversity radar and measuring rainfail at the ground through a nerwork of fixed, volunteer observers and a mobile storm-chase operation. Theory describing how differential propagation phase may be extracted from the data recorded by the radar system is given. Data collected on three days during the experiment (27 July, 29 July and 2 August) have been analysed and the resuits are presented. A total of 31 samples of total rainfaîl amount were collected on these days. AIl but three of the 31 radar rainfaîl amount predictions obtained from the d~fferential propagation phase are within a factor of 2 of the rainfali observed at the ground. in fact, the average ratio of the total rainfaîl amount predictedfrom the differential propagation phase to the total rainfaîl amount measured at the ground is 1.16 for the 31 samples. This suggests that, on average, the total rainfaîl amount predictedfrom the differential propagation phase is only 16% higher than that measured at the ground. 0f the 31 events, over a third involved some contamination of the differential propagation phase measurement through hail. Furthermore, because the KDP technique does flot rely on parameters dependent on precipitation characteristics or adjustment factors, the technique can be validated in a convenient location and then applied anywhere. On the other hand, the radar rainfaîl amount estimates derivedfrom Z—R relations represent, in general, a large overestimation of the rainfaîl amount observed at the ground, especially when hail is present. No attempt was made to adjust or calibrate the radar rainfaîl estimates with raingauge data. ABSTRACT
ATMOSPHERE-OCEAN 29(2)1991, 357—380 0705-5900/91/0000-0357$01 .25/0 © Canadian Meteorological and Oceanographie Society
358 I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoit and R. McGuinness RÉSUMÉ On décrit un projet pilote sur la mesure de la pluviosité par radar en diversité de polarisation. Réalisée par le Conseil de recherche de l’Alberta (ARC) et l’Université d’Essex à l’été 1989, l’expérience avait pour objet de vérifier la théorie selon laquelle le déphasage de la propagation différentielle donne une meilleure évaluation de la pluviosité lorsque les taux sont élevés (environ 30 mm h’ ou plus) que l’utilisation de la réflectivité. On a conduit des essais en campagne, dans le centre de 1 ‘Alberta, entre le 20juillet et le 2 août. On a observé les orages dans un rayon de près de 70 km de Red Deer en utilisant le radar de bande S de l’ARC en diversité de polarisation et pris des mesures au sol des hauteurs de pluie à l’aide d’un réseau fixe d’observateurs volontaires et d’un système mobile de « chasse aux orages ». On présente la théorie sur la façon dont on extrait le phasage de la propagation différentielle des données obtenues par le radar. Les données des 27 et 29juillet et du 2août ont été analysées et les résultats sont présentés. On a collecté un total de 31 échantillons des hauteurs totales de la pluie au cours de ces trois jours. Seules 3 des 31 prévisions de hauteur de pluie obtenues du phasage de la propagation différentielle sont près d’un facteur de 2 de la pluie mesurée au sol. De fait, le taux moyen de la hauteur totale de pluie prévue par le radar, divisé par la hauteur mesurée au sol est de 1,16 pour les 31 échantilons. Ceci laisse entendre que, en moyenne, le total de pluie prévue par le radar est supérieure de seulement 16% à celui mesuré au sol. Des 31 échantillons, plus du tiers des observations radar ont été contaminées par la grêle. De plus, comme la technique KDP ne dépend pas de paramètres sur les caractéristiques de la précipitation ou de facteurs d’ajustements, elle peut être validée à une location appropriée et ensuite appliquée n ‘importe où. Cependant, la prévision radar de la hauteur de pluie provenant de la relation Z—R représente, en général, une forte surestimation de celle observée au sol, surtout lorsqu ‘il y a de la grêle. On n ‘a pas tenté d’ajuster ou d’étalonner les estimés de hauteurs de pluie du radar selon les données obtenues des pluviomètres.
i Introduction There are now over 100 countries operating more than 600 weather radars (Obasi, 1990). The principal uses are for flash-flood warning, severe weather warnings, weather monitoring and air-traffie surveillance. The potential of weather radar systems for making measurements of precipitation has been recognized for some 40 years. Aithougli radar techniques have practical limitations and their accuracy in rainfail rate estimation is somewhat questionable, they have the ability to survey vast areas remotely and make hundreds of thousands of measurements in minutes. The cost of a gauge network to match these capabilities in spatial continuity and rate of data sent to a central location would be prohibitive (Doviak and Zmic, 1984), to say nothing of the ability to make measurements in regions that are not readily accessible to a raingauge, such as large lakes, oceans and wildemess areas. (Rainfaîl measurements by radar are not, however, continuous in time.) Many techniques for measuring precipitation by radar have been proposed, but basically the commonly discussed methods are based on measurements of the intensity of the back-scattered radiation (radar reflectivity) and measurements of the difference between the reflectivities of vertically and horizontally polarized radiation.
Radar Measurement of Rainfaîl by Differential Propagation Phase I 359 a Measuring Rainfaîl by Reflectivity Operational radars (predominantly C- or S-band) that measure rainfali by reflectivity, relate the reflectivity Z (mm6 m3) to the rainfaîl rate R (mm h’) empirically through relations of the form Z aRb. The parameters a and b are constants, but different values of these constants are often derived for different rainfaîl situations (Battan, 1973). Values that are often used are a = 200 and b = 1.6 (Marshall and Palmer, 1948). For raindrops whose diameters are very much less than the radar wavelength, the scattering is well described by Rayleigh theory (e.g. Kerker, 1969). For Rayleigh scattering, the measured reflectivity, Z, is a function of the sum of the sixth power of the diameters of all the raindrops. Variations in drop size distribution play an important role in the lack of unîqueness of the Z—R relationship because a particular Z—R relationship is essentially based on a single parameter drop size distribution. In general, a two-parameter exponential distribution is thought to be the minimum requirement to adequately describe a raindrop size distribution. This lack of knowledge of particle sizes is, to some extent, compensated by using different Z—R relations in different situations. Such relations are normally derived by repeatedly measuring rainfaîl at the ground with a network of raingauges (or a drop disdrometer) and developing regression equations between reflectivity and rainfaîl rate (see, e.g. Richards and Crozier, 1983). Any Z—R relation that is derived for a particular radar system will also compensate, to some extent, for systematic bias in the radar-measured reflectivity due to calibration problems. An operational weather radar will, in general, measure instantaneous reflectivity, averaged over a resolution volume above the ground, at many ranges, out to 100 km or more, and at many azimuths, while the radar beam rotates about a vertical axis at a number of elevations. A constant-altitude map of surface precipitation is obtained by combining data from several elevation angles. This instantaneous reflectivity is measured as a function of time; consecutive measurements both in time and in location are normally averaged to produce more accurate areal averages of rainfall for given time periods. Some problems associated with obtaining accurate rainfaîl estimates in this manner include (Collier, 1989; Doviak and Zmic, 1984) ground clutter and anomalous propagation, height of the radar beam at a long range, beam blockage, incomplete beam filling, low-level evaporation below the radar beam, undetected orographic enhancement above hilîs, low-level wind, underestîmation of the intensity of drizzle, ram-rate gradients, vertical air motions, attenuation from precipitation, and the presence of hail and bright band. Using a radar that transmits relatively long wavelength radiation (such as at S-band) reduces errors caused by the attenuation produced by the precipitation. Enhancement of the signal by the presence of hall is the predominant factor in intense convective storms and may increase reflectivity in storm core regions by 10 dB or more above that of the ram alone (Austin, 1987). Signal enhancement by melting snow (bright band) is a significant factor in estimating stratiform rainfaîl particularly in frontal systems. Because of these factors, large differences between the radar measurements of precipitation and the precipitation reaching the ground are sometimes observed. —
360 I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoît and R. McGuinness Calibration of the radar using a few simultaneous raingauge measurements has been advanced as one of the ways to improve radar rainfaîl estimates (Hitschfeld and Bordan, 1954; Wilson and Brandes, 1979). Daily radar calibrations can be in error, and different Z—R relations might be appropriate for different rainfaîl types and even for different locations surveyed by the radar, and therefore it would be reasonable to apply different adjustment factors to different domains. Lt has been demonstrated that the domain adjustment procedure produces rainfaîl estimates that are more accurate than those derived using unadjusted radar data and also more accurate than methods using a single mean adjustment factor (Collier, 1989). The ultimate objective of the gauge-radar rainfaîl estimation technique is to combine sparse gauge and dense radar rainfaîl data to produce a rainfaîl estimate with the point accuracy of gauges and the spatial resolution and coverage of radar. The extent to which this objective can be achieved is questioned by Grosch (1989), who suggests that the relationship between Z and R is distinctly non-linear even in log-log coordinates and that conventional Z—R relations usually overestimate R for rainfaîl rates greater than 25 mm h’.
b Microwave Propagation Through Ram While electromagnetic waves propagate through a region containing raindrops, some of the power is scattered and absorbed by the medium. Consequently, the wave is progressively attenuated and changed in phase while it propagates. However, since raindrops are not spheres, depolarization, as well as attenuation, will occur. The shape of raindrops has been investigated by, for example, Pruppacher and Pitter (1971). As their size increases, raindrops become progressively more non-spherical. Their scattering characteristics can be reasonably obtained by modelling raindrop shape as an oblate spheroid, whose axial ratio varies with raindrop size (USRI Workshop Session Chairmen, 1981). That raindrops are not spherical means that scattering will depend on the polarization employed. If pure linear polarization is transmitted, the attenuation (dB km—’) and phase change (deg km’) will depend on the particular linear polarization. The difference between the phase changes when horizontal and vertical linear polarization is transmitted is termed the “differential propagation phase” (~Dp). Differential attenuation is similarly defined. At S-band wavelengths, differential attenuation is small; differential phase is the more important. At shorter wavelengths the differential attenuation increases and the differential phase decreases (Oguchi and Hosoya, 1974; Oguchi, 1983). At C-band frequencies a resonance also occurs between the wavelength of the microwaves in large raindrops and the diameter of the drops. In this situation, the polarization parameters can alter rapidly with size around the resonant drop size (e.g. Hoît and Evans, 1977). An altemative to transmitting vertical or horizontal linear polarization is to use circular polarization, which is a combination of vertically and horizontally polarized components, equal in amplitude, but differing by 900 in phase. For a review of the early history of polarimetric radar techniques see Seliga et ai. (1990).
Radar Measurement of Rainfail by Differential Propagation Phase I 361 c The Use of Polarization Diversity Radar Bringi and Hendry (1990) provide a review of dual-polarized radar techniques used in meteorology and present the theory underlying such radars. The use of polarization to obtain information in addition to reflectivity, appears to have begun in earnest around 1968, when two circularly polarized systems were built by McCormick, Hendry, and their associates at the National Research Council (NRC) in Ottawa. One system, operating at S-band was destined for the hail project in Alberta, while the other system, at Ku-band, was operated by NRC at Ottawa (e.g. Hendry et al., 1976; McCormick and Hendry, 1976). The systems were designed to transmit any chosen polarization, and to receive four items of data: the powers in the main and orthogonal receive channels, and the (complex) correlation of the two receive signals. If left-hand circular (LHC) polarization is transmitted, then the main retum channel will be right-hand circular (RHC), because of the reflection at the target. The systems were almost always operated using circular polarization. When transmitting circular polarization the logarithmic difference of the two receive powers is termed the Circular Depolarization Ratio (CDR), and it was hoped to use this in Alberta to differentiate hail from min. However, calculations have shown that CDR is not a good indicator of hail (Torlaschi et al., 1984), and in fact it was noted that on occasion CDR could be abnormally high in the “shadow” regions of large storms (Humphries, 1974). The theory of these radars is well described in a series of papers by McCormick and Hendry (e.g. McCormick and Hendry, 1975, 1976; McCormick, 1979). An altemative use of polarization was proposed by Seliga and Bringi (1976) in an attempt to obtain better estimates of rainfaîl than can be obtained from reflectivity alone. In their method linear polarization is used, the radar measuring only the co-polar retum power, but the transmission being altemated between vertical and horizontal polarizations. Since raindrops are non-spherical, the reflectivity factors for vertical and horizontal polarization, Zv, ZH will be different. Their logarithmic difference is termed the “Differential Reflectivity” (ZDR = 10 log ZH /Zv, Seliga and Bringi, 1976). If one assumes a two-parameter drop size distribution, then, at S-band, the parameters, N 1 and D0 may be uniquely determined from ZH and ZDR. 0
Since the drop size distribution is thus determined (within the model assumptions) the ram rate may be deduced. The method was first implemented in the United Kingdom on the Chilbolton radar of the Rutherford and Appleton Laboratory, which has a 25-m antenna (Hall et al., 1980), and has since been used on a number of different radars (e.g. Seliga et al., 1981). This method for obtaining ZDR requires fast switching since Zv and ZH are sampled consecutively, not simultaneously. The switching time must be small compared with the decorrelation time of the raindrops or else the samples of Zv and ZH will ‘The number of raindrops pet cubic mare with diameters in the range (D, D
+ dD)
is termed
the raindrop size distribution N(D)dD. For purposes of modelling, N(D) is usually assumed to be an exponential or a gamma distribution, which is of the form N(D) = NoD~exp [—(3.67 + eorresponding to ~.t = O or (nonnally) 2, where N0 and D0 are two unknown parameters (Ulbrich, 1983).
362 I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoit and R. McGuinness refer to different ram configurations within the pulse volume, and an inaccurate value of ZDR may be obtained. In the ZH, ZDR method, only powers are measured, and hence the phase information of the incoming signal is lost. However, by examining the time series of altemate H, V pulses, Sachidananda and Zmic (1986) have shown how differential phase may be estimated. Since circular polarization is a combination of linear components, it is not surprising that linear parameters such as ZDR and ~DP may be extracted from the data obtained using the Alberta or Ottawa radar (Hoît, 1988; McCormick, 1979). Though the extraction process is not exact, since it assumes that the raindrop canting angle is small, the value of ZDR obtained is not subject to error due to switching time. The use of differential phase for measuring ram rate was first proposed by Seliga and Bringi (1978). Using the large database of Alberta radar data, and the simultaneous ground truth that exists for the period up to 1985, Hoît and McGuinness (1990) showed that in heavy rainfaîl (with ram rates in excess of 30 mm h’) there appeared to be strong evidence that differential phase is a much better estimator of rainfaîl than reflectivity is. This is in agreement with ~DP weighting the drop424, in close accord with the ram rate factor D367~ size distribution by a factor D the reflectivity factor, however, is D6. Hence the relation between ~DP and R is nearly linear, and largely independent of dropsize distribution. (The nearly linear relation between ~l1DPand R also suggests that R measurements from ~DP will be less distorted by bail than R measurements from Z.) d The Pilot Experiment Since the ground-truth database prior to 1986 used by Hoît and McGuiness (1990) was obtained primarily to record the incidence of hail, and rainfaîl information was only incidental, it seemed worth while to perform an experiment to measure rainfaîl by differential propagation phase to test these findings. Such an experiment was conducted in the summer of 1989. Budget considerations limited the experiment to a duration of two weeks. In this paper, we describe the experiment conducted in 1989, the data collected during the experiment and present some results obtained to date. 2 The ARC Polarization Diversity Radar The Alberta Research Council Polarization Diversity Radar was the primary observing system for the experiment. In this section we describe the radar system and the underlying theory. a The Alberta S-band Radar The S-band circular polarization radar is located at the Red Deer Industrial Airport about 150 km south of Edmonton. The radar operates at 2.88 GHz and has a parabolic reflector antenna with a 6.67-m diameter dish that produces a 1.150 beamwidth in both azimuth and elevation. The radar sweeps out a helical volume scan rotating in azimuth at 480 s1 (7.5 s per revolution) and increasing in elevation 10 per revolution up to a maximum elevation of 8 or 200 depending on the proximity of the storms to the radar. The radar measures precipitation data every 0.10 in
Radar Measurement of Rainfaîl by Differential Propagation Phase I 363 TABLE 1.
Characteristics of the ARC S-band Polarization Diversity Radar System
Characteristic
Value/Description
Frequency Polarizafion Beamwidth Diameter (antenna reflector) Gain Peak power Pulse width PRF Range gate size Number ofgates Time averaging Independent samples* Measurements
2.88 GHz LHC 1.150
6.67 m 43.2 dB 450 kW 1.05 ~±s 478 Hz 1.05 km 147 21 ms per bin 7—14 j) RHC (co-polar) powcr ii) LHC (cross-polar) power iii) correlation of LHC and RHC signaIs iv) phase between LHC and RHCsignais
*The number of independent samples depends upon several parameters. Some are determined by the radar hardware, e.g. the number of pulse volumes averaged and the time each bin is sampled. Others depend on the scatsering geometry and ram decorrelasion time. That is why we quote a range of 7 to 14 independent samples.
azimuth along a 300-km scan line, but records data for 147 range gates, from 3 km to a distance of 157 km from the radar, with a range resolution of 1.05 km per range gate. The recorded data have an azimuthal resolution of 1~, which is the integrated result of the 10 scans occumng in each degree of rotation. The radar transmits a i.05-jis left-hand circular polarization pulse with 450kW peak power. The retumed elliptical signal is decomposed into a left-hand circular (LHC) component and a right-hand circular (RHC) component, which have a relative phase difference. The receiver circuitry allows four measurements from the LHC and RHC components. The powers in the two components are measured. Because the transmit pulse undergoes reflection by the precipitation target the copolar return is found in the RHC channel and gives the circular reflectivityfactor ZE. The co-polar (RHC) and cross-polar (LHC) powers are averaged over ten azimuthal scans and seven pulse volumes (1.05 km) and recorded onto magnetic tape. The third and fourth measurements relate to the correlation between the co- and crosspolar signals and their relative phase difference. Specifically, the third measurement involves passing both signals into a mixer that provides the cosine of the mean relative phase difference integrated over all possible joint signal amplitudes and phase differences. The fourth measurement involves the same procedure except that the cross-polar signal is phase shifted by 900 prior to input into the mixer, producing the sine of the mean relative phase difference. A complete description of these four measurement quantities, and their interpretation are given by McCormick and Hendry (1975, 1979). Table 1 summarizes the main features of the S-band radar system.
364
I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoit and R. McGuinness
A full description of the calibration procedures for the radar system is given in Stanley-Jones et al. (1984). Briefly, the calibration of polarization involves measuring the 4-channel values from which the separation of the orthogonal polarization channels and the alignment of the polarization ellipse can be derived. These values are then used to adjust the recorded polarization data for systematic error. This calibration is carried out by pointing the antenna at a remote, variable-polarization transmitter, situated on top of a 27.4-m tower located 400 m from the radar. Another calibration, which is performed at least once a day when the radar is operational, is carried out by injecting a known RF signal into the antenna. This calibrates the two power signals, as well as the relative phase. b Theory We make the following assumptions: i) ii) iii) iv) y)
vi)
The electromagnetic waves have a wavelength X (cm) and a wavenumber k0 (cm~’) where k0 2it/X. The refractive index of water is no(X). There is an exp(—ioet) time dependence, where o is the angular frequency. Unit vectors ~v, CH, lie parallel to the electric fields for vertical and horizontal linear polarizations, respectively. The raindrops are all aligned, and canted at an angle such that the projection of their axes in the (ev, ~H) plane makes an angle a with êv. The back-scatter amplitudes for scattering by raindrops of diameter D from polarization i to polarization j are 50(D).
Assumption (y) has been shown by Hoît (1984) to lead to negligible error assuming that the physical canting angles are small, which is generally believed (Beard and Jameson, 1983). In the analysis below we follow Hoît (1984) and Hoît and McGuinness (1990). If the radar transmits LHC, and is sensing a region with drop size distribution N(D)dD, then neglecting propagation effects, the powers W, and W2 in the LHC and RHC receive channels are given by 2dD (2.1) w1 = I/4IC~2fN(D)Is~ —SHHI w 2= 1/4~CI2JN(D)~s~+sHHI2dD
(2.2)
where 6X5 —
(2.3)
îo
Lt should be noted that one advantage of circular polarization is that the return powers W, and 1V 2 do not contain the canting angle a. This is not SO when linear polarization is used.
Radar Measurement of Rainfaîl by Differential Propagation Phase I 365 Normally, the radar beam will pass through precipitation on the path to the volume being sensed, and this precipitation will affect the signal. The vertical and horizontal components of the electromagnetic field will be affected, respectively, by factors dv,H
exp
=
[43V,H
—
(2.4)
YV,H]
which represent the one-way on-path phase change and attenuation. Thus the powers actually measured are w,
1/4ICI2JN(D)Id~,s~ —d~jSHHI2dD
(2.5)
2dD
(2.6)
w 2
=
1/4iCI2JN(D)ld~.s~ +dkSHHl
and the complex correlation is W
I/4iCI2JN(D)(~Q~
+dkSHH)(4Svv
—dkSHH)
exp(2ia)dD
(2.7)
Lt should be noted that dv and dH only depend on the ram on the path, whereas 5vv and 5HH only depend on the ram in the volume being sensed. Comparing (2.5) and (2.6) with (2.1) and (2.2), we see that the effects of propagation are to differentially alter S~ and 5HH both in magnitude and phase. Indeed, a differential change of 1800 would have the effect of interchanging the retums in the main and orthogonal channels. If we assume that a is zero, then —
W,)— ilm(W)
=
1/2iCI2JN(D)d~,dkSvvSHHdD
(2.8)
since dv and dH are independent of the ram on the path arg
(‘/ 2(W2
—
W,)
—
ilm(W))
=
Since at S-band, arg (Svv SHH) sizes then (2.9) gives 2(t3H ~DP
—
arg (d~dk)
«
13v)
=
10
+
arg (‘/2ICV JN(D)SvvSHHdD)(2.9)
(McGuinness and Hoît, 1989) for all drop
arg
=
(‘/ 2(W2
—
W,)
—
ilm(W))
(2.10)
Hence, 4DP is the total two-way differential propagation phase. The two-way differential phase shift constant, KDP, in gate n of length L is thus
K~J~
=
(‘PDP
—
~DP
)/L
Sachidananda and Zmic (1987) have shown that
KDP
(2.11) and ram rate R are related.
366
I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoit and R. McGuinness
Zmic (priv. commun., 1989) has given this as R’’5 KDP 3.172X
(2.12)
The CDR measured by the radar is CDR
=
10 10gb (Wi/W 2)
[dB]
(2.13)
and ZDR=
lOlOgio W2 + Wi 2 Re(W) W2 + W~ +2 Re( W) —
[dB]
(2.14)
Hoît (1988) and McGuinness and Hoît (1989) have shown how differential phase may be used to correct the values of ZE, CDR and ZDR for propagation effects. In this way we have derived corrected values of reflectivity, termed and also the estimate of the reduction in reflectivity due to propagation effects, Zf~ ZE, which is termed DIEF in Fig. 1. Lt may be seen from (2.10) that when CDR is O dB (i.e. = Wl), then ~DP is 90e, since the quantity (W2 W,)/2 ilm(W) becomes pure imaginary. In the absence of the propagation effects, CDR ~ 12 dB (Hoît, 1984). It should be noted that since 4DP is obtained from (2.10), h is mostly independent of the accuracy of the radar calibration. Reflectivity, however, depends crucially on accurate calibration.
4,
—
—
—
—
3 The field experiment The field experiment was conducted jointly by the Alberta Research Council (ARC) and the University of Essex in the summer of 1989. It was designed to test the theory developed at the University of Essex, that differential propagation phase shift, extracted from the data collected with the ARC S-band polarization diversity radar, can give a better estimate of rainfaîl with high ram rates than the use of reflectivity. The field project was therefore designed to obtain radar data from storms in central Alberta and also ground-truth data on precipitation. This was the first time that a field project was conducted in Alberta to obtain polarization radar data and rainfaîl measurements at the ground. In past field experiments, the objective had always been to obtain hailfaîl observations at the ground, and rainfaîl observations were only incidental. The field experiment was conducted from 20 July to 2 August. Weather forecasts were obtained from the Atmospheric Environment Service (AES) Western Region Office in Edmonton to help in planning the daily operations. Additional weather information was obtained from staff in the control tower at Red Deer Industrial Airport. Occasionally, real-time rainfaîl information was obtained from staff at Alberta Environment. The ARC C-band conventional weather radar was operated 24 hours a day to monitor rainfaîl in the area. Data from this radar were recorded continuously from 24 July to 2 August. The ARC S-band polarization diversity radar was operated only when suitable weather occurred in the project area. In total, 77 hours of S-band data were recorded.
Radar Measurement of Rainfaîl by Differential Propagation Phase I 367 JULY 27 1989 lIME 17:10:4 ELEVAlION I CENTI~ 115 I~s 358 DEG
Z
DIFF
~~~~90M
>4
Fig. i A four-way simultaneous PPI display, at 10 elevasion and at 17: 10:04 MOT, of one of the storms that crossed the projeet area on 27 JuIy 1989. Range rings are at every 10 km, from 70 to 150 km. The tick marks on the range rings are at every 50 in azimuth. Reflecsivisy (Z in dBZ) is shown in the top left picture. The top right picture shows the correction (DIFF in dB) that has been applied to the reflectivity to account for signal degradation. Differential Phase (DIFF—PR or 4op in degrees) is shown in the bottom Ieft picture, and differential reflecsivity (ZDR in dB) in the bottom right picture.
The ground measurement program comprised a network of volunteer farmers equipped with wedge raingauges, and a team of mobile chase vehicles equipped with tipping-bucket and wedge raingauges. One chase vehicle was equipped with a drop disdrometer. We refer to the data collected by this program as the “ground truth” data. a The Volunteer Farmer Network During the Alberta Hail Project (1956 to 1986), the Alberta Research Couficil (ARC) often relied on volunteers to obtain observations of precipitation events. Thus, for this experiment, ARC organized a network of 523 volunteers, distributed over an area within about 70 km of Red Deer, to measure rainfaîl amount at the ground during the field experiment and therefore to provide a database of groundtruth observations of rainfaîl with which radar predictions could be compared. The network was designed to give as dense a coverage as possible in the project area; 523 was the number of volunteers that could be enrolled in the time available.
368 I M. English, B. Kochtubajda, F.D. Barlow, A.R. Hoît and R. McGuinness Each volunteer was provided with a TRU-cHEK wedge raingauge, installed by ARC, and was asked to read and empty the gauge at 8 a.m. every moming for a twaweek period. Additionally, volunteers were encouraged to read their gauges, and record the rainfaîl amount and time, as often as they could. They were also asked to mention on their reports if bail occurred. The response from the volunteers was exceptional: 92% of them mailed rainfaîl information to ARC. Furthermore, many of the volunteer observers took rainfaîl observations before, during and after significant rainfaîl events in their area 50 that many reasonably accurate total rainfaîl amount measurements were available for particular events from these observations. Only data where the volunteer took two measurements while the radar was operating was used in the analysis; from these data we know the total amount of ram that fell while the radar was operating. (The precise duration of the rainfaîl is obtained from the radar data.) These observations form the bulk of the database that was used as ground truth in the comparisons with radar data. The TRU-CHEK raingauge is a wedge-design accumulation raingauge manufactured by the Edwards Manufacturing Company in Albert Lea, Minnesota. The gauge is a smooth one-piece moulding of heavy weather-resistant plastic, 33 cm in length, with an orifice sampling area of 37.1 cm2, capable of measuring rainfaîl from 0.25 to 150 mm. Calibrations have shown that TRU-cHEK measurements have an accuracy comparable with that of the standard U.S. govemment 8-m gauge (Huff, 1955). b Storm Chase Operation Three vehicles were equipped with maps, a radio, a tipping-bucket raingauge and a wedge-type raingauge. Each vehicle included a driver and a navigator who could record position and information about the precipitation encountered by the vehicle. The vehicles were deployed inta oncoming storms as opportunities arose. Vehicle i contained a high-resolution (0.1 mm per recorded tip) tipping-bucket raingauge, a drap size disdrometer and a wedge gauge; bath the tipping-bucket raingauge and the disdrometer were connected to a computer. The other two vehicles each contained a tipping-bucket raingauge (resolution: 0.2 mm per tip) and a wedge gauge with the tipping-bucket gauge connected to a data logger. A chase-vehicle controller was located at the radar site ta provide chase crews with real-time information about storms and rainfaîl. The controller had at his disposal a radio, an analogue PPI displaying power retumed in five shades of grey, and a PPI similarly displaying CDR. Some problems were experienced with the radio system, which had a more limited range than anticipated. The real-time radar displays in the control room were inadequate to properly position chase vehicles within regions of intense rainfaîl. This part of the experiment, therefore, was not as successful as was hoped. Sampling durations ranged from a few minutes to close to an hour long. Information recorded in log books in the vehicles far each sampling period include
visual observations of the precipitation and supplemental measurements made with the wedge raingauge (measurements from the tipping-bucket rarngauges and disdrometer being recorded continuously with data-recording systems). The sampling time and the land location of the sampling site were determined by the chase crew and recorded in the log books.
Radar Measurement of Rainfaîl by Differential Propagation Phase I 369 c The Drop Disdrometer One of the mobile chase vehicles was equipped with a unique portable system capable of simultaneous high-resolution sampling of rainfaîl amounts and raindrop size spectra. The system includes a high-resolution tipping-bucket raingauge, a disdrometer transducer, a data interface and a data-logging computer. The raingauge provided measurements of rainfaîl amount within a minimum resolution of 0.1 mm per recorded tip. The disdrometer transducer recorded the occurrence of every raindrop striking the surface area within ils measurement range of 0.3 to 5.0 mm. Timing information was provided by time marks recorded at 1-min intervals. AlI raingauge tip events and all disdrometer-sensed drop measurements were recorded. The disdrometer drop digitizer, data interface and PC computer were field-operated using battery power provided by 12-V D.C. current through an automobile cigarlighter outlet. The factory-provided disdrometer calibration curve was tested by ARC. The logged data were used to determine rainfall and total rainfaîl amount as measured by both the disdrometer and raingauge. In addition, the disdrometer data were used to calculate dropsize distribution spectra. Sainple 15 in Table 2 includes results obtained with the disdrometer. A comparison of the simultaneously sampled rainfaîl measurements made by the disdrometerlramgauge system at tIse nine sites indicated some differences between the instruments. These differences were almost evenly distributed, and the raingauge was not able to make measurements at low rainfall rates (
