REPORT OF U.S. WEATHER BUREAU STUDIESIN RADAR HYDROLOGY

Allen F. FLANDERSHydrologie Services Division, U.S. Departement of Commerce, Weather Bureau

Washington, D.C.

SUMMARY

The progress made by the U.S. Weather Bureau on the measurement of precipi-tation by WSR-57 radar is presented. The various operational attempts, techniquesand applications made in the field of radar-hydrology show the successes and limita-tions encountered as well as progress made with the Radar Precipitation Integrator.Plans for the utilization of radar as a continuous recording raingagc arc also reviewedas a step toward automation in the radar-hydrology-computer area.

RtSUMÉ

Nous présentons le progrès réalisé par le U.S. Weather Bureau dans la mesuredes précipitations par WSR-57 radar. Les différentes expériences d'exploitation, lestechniques et les réalisations dans le champ de radar-hydrologie, montrent les réussiteset les limitations rencontrées, ainsi que les avances faites au Radar PrecipitationIntegrator. D'ailleurs, des projets d'utilisation de radar en qualité de pluviomètreenregistreur sont examinés comme une avance vers l'automation dans le domainedes calculatrices pour le radar hydrologiquc.

1. INTRODUCTION

During the past seventeen years the Weather Bureau has been utilizing radar asan observational tool for the meteorologist and the hydrologist. In the first half ofthis period, from 1946 to 1954, considerable operational experience and insight waslearned about radar — enough to warrant further professional investigation at theuniversity level for a better understanding of how to more fully utilize radar in estimat-ing rainfall. Such investigations were carried out from 1954 to 1959 at MIT, Universityof Miami and Texas A & M. (}) At this same time specifications were also beingwritten for the Weather Bureau's first radar designed for weather surveillance and in1959 the initial installation of these thirty-one WSR-57 (2) radars (Fig. I) was madeat Miami, Florida, and the last, this year, on Catalina Island, California.

Early attempts at using radar to estimate rainfall were not too successful becauseof the limitations of the equipment — converted surplus military World War II radarsets. Even with later modifications, the best that could be achieved was a coarse esti-mation of rainfall (light, moderate, heavy) and while these radar sets did markedlywell for tornado and thunderstorm detection there was no way to place a value onthe rainfall depicted on the scope. On occasions though, very heavy, persistent rainfallwas detected and the subsequent public warnings issued about possible flash floodingresulted in good verification. Not until the Weather Bureau acquired the WSR-57radar was it possible for an observer while sitting at the radar console to estimaterainfall rates with any degree of reliability. With this capability it was then possibleto enter into a more extensive radar-hydrology program (3) and a concerted effort toestimate rainfall by radar.

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Fig. 1 — Long Range Weather Radar Network

2. RADAR — RAINFALL STUDIES

1. A test of the Radar Photo Integration Analysis (RPHIA) program (3) hasbeen completed between the Wichita, Kansas, radar staff and the Tulsa, Oklahoma,River Forecast Center (RFC). During 1961 a program was inaugurated whereby theWichita radar staff made 2-hourly multiple exposure photographs of the l'PI scope,encoded and transmitted the radar data to the Tulsa RFC. Considerable loss in detailoccurred however when the radar data was encoded because of the limitations in des-cribing weather echoes by the AZRAN technique (azimuth and range). As a result theRPHIA procedure was only of limited operational value to the RFC for determiningrainfall classes. Later, however, when the projected Polaroid transparency echoimages were compared with reported rainfall amounts, the results were more meaning-ful. In many cases heavy rainfall, either its limited or wide extent, could easily besubstantiated by the multiple exposure photograph of the associated weather echoesand was of material help in defining the rainfall pattern. Based on this study and testswith other RFC's, it can be reasonably concluded that multiple exposure photographscan best be utilized on station where a hydrologist has direct access to the film andcomparative rainfall reports. The useful range that can be expected from this techniquevaries from 75 to 125 miles depending on the characteristics of the storm.

2. In the Kansas City office special use has been made of a 3 M (MinnesotaMining & Manufacturing Company) Reader Printer (4). Polaroid transparenciesof the 100 n.m. PPI scope are optically enlarged and the scope images printed on18" x 24" paper on which there have also been superimposed the drainage basinfeatures. It is also possible, using several hourly PPI scope photographs, to make amultiple exposure on the 3M map. After the area rainfall reports are plotted on themap, the multiple exposure photograph of the precipitation echoes is used to helpdefine the rainfall pattern. By taking radarscope photographs at selected db settingsthe more intense rainfall areas will be depicted by the darker shading further helpingto delineate the areas of heavy rainfall. This system has the limitation that the hydrolo-gist must have access to the Polaroid transparency within a short time in order tomake an enlargement on the 3 M Printer in time to plot the incoming rainfall datafor operational use.

In the Kansas City office, however, this has not been a problem because the RFCand radar staff arc in the same building. This procedure has aided the RFC hydrolo-gists in making river forecasts by providing information on the location of heavy raincores, orientation of the more extensive rainfall areas over the basin, and for the lifehistory of storms as used in the development of river forecast procedures.

3. In the Fall of 1962 an automatic stepped gain-multiple exposure camera systemwas placed in operation at the radar station in Cincinnati, Ohio. This device, calledan APCC unit for Automatic Polaroid Camera Control, differs from the conventionalmultiple exposure technique in that an intensity parameter has been introduced. Atpreselected time intervals the APCC unit automatically takes control of the radarand steps the attenuators upward through four increments after completing one sweepat each (18, 33, 45, 51) db level. Twenty exposures of the PPI scope can be recordedon the transparency film during any one hour period.

The intended purpose of this procedure is to locate areas of potential flash floodthrough the detection of the associated bright cores as depicted on the film of theradarscope. To date no positive results are available. However, the system is quitesimple and straightforward. During periods of pronounced weather activity within100 n.m. of the radar, the APCC unit is activated. Each hour the film transparency isprocessed and placed in a slide projector. The weather echo images are projectedonto a river basin map which has the critical headwater basins delineated. Indicatedwithin each of these basins is the amount of rainfall necessary to cause flooding ascomputed by the RFC. Any hard core or persistent echoes are outlined and if warranted

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the observers in the suspicious area interrogated by phone or radio to ascertainactual rainfall amounts. This area is either kept under close surveillance by radar oran appropriate warning issued as determined by the duty meteorologist.

4. A study has also been started by the radar staff at Detroit, Michigan, to esti-mate rainfall over Lake Erie within 100. n.m. of the radar. Much interest has beenshown by many groups tor several years to ascertain the amount of rain that fallsover the Great Lakes (5). The procedure used in this study employs a 7 1/2 n.m. gridoverlay for the PPI scope with rainfall estimates being made once an hour using theRainfall Rate-Echo Intensity Chart (Fig. 2). These values will be compared withobserved rainfall amounts recorded by conventional raingages around the Lake.Any adjustments made to aline differences over the land gages as compared with theradar estimates will also be applied to the radar determined values over the Lake.Based on this technique it can then be assumed that a relatively reliable rainfall esti-mate has been made by radar over the Lake. At the writing of this report, four monthsof data have been collected but the results are pending the published hourly climaticrecords which are to be used for the comparison.

RAINFALL RATE-ECHO INTENSITY CHARTWSR-S7 RADAR 4)t SEC PULSE LENGTH

78

5a.

70 90 110 130 150

RANGE (nautical miles)

170 190 210 230 250

Fig. 2 — The chart is used at all Weather Bureau WSR-57 stations and was con-structed by Paul Hcxter based on equations in Louis J. Batten's book, RadarMeteorology.

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It is realized that many variables and unknown conditions are involved in sucha study. However, radar does seem to be one practical way to investigate rainfall overlarge bodies of water. As soon as the preliminary results arc available a more concen-trated effort can be programed, including possible raingage instrumentation of waterintake cribs and of small level islands in the Lake.

5. At the Weather Bureau mountaintop radar installation on Point Six, Missoula,Montana, a radar rainfall project is being conducted by the radar staff for the RiverForecast Center at Portland, Oregon. The first data were collected during January-April 1962, when once an hour, a yes or no determination was made in each of the6 n.m. sections of the grid overlay on the PPI scope to indicate the occurrence ofprecipitation within 100 n.m. The seasonal tallies in the sections were compared withobserved precipitation (Fig. 3a) in an attempt to place a numerical value on eachtally. It was evident that considerable adjustment for range would be necessary.Therefore a least squares fit by log transformation was attempted and a correlationvalue of .75 was derived for the 56 cases using the formula :

P - ciT" Ra

where

P is pepn in inchesT is tallyR is rangeand a, n and a are constants.All the data were then rerun to arrive at a computed precipitation value for each

case (Fig. 3b). Although the results are by no means outstanding they are importantconsidering that only one observation was made each hour.

In May and June, 1962, data collection continued using the newly installed at-tenuators on the radar. The tally procedure was still employed but with the followingvalues derived from the RR/EI chart (Fig. 2) assigned to each section of the grid :

RANGE

36 db33 db30 db24 db18 db

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.04

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The results of this two-month test showed the radar procedure being used wasunderestimating precipitation by about 3 to 1. However, no definite conclusion shouldbe drawn from this small amount of data because of the affects of singular storms,possible overshooting, blocking, ground clutter and beam filling. A whole year'sdata will be needed for analysis before more conclusive results can be reached.

This year improved results are expected through increased frequency of obser-vation within the operational limitations of the staff. In the current program opera-tional use is being made of the radar-depicted precipitation data. It is coded in .2 ofinch intervals for transmission every six hours to the Portland, Oregon RFC where theRFC hydrologists plot the radar data on river basin maps for comparison with reportedrainfall. These radar-depicted rainfall values arc then used to help define the rainfallpattern and also relayed to the Office of the Corps of Engineers.

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6. During the 1960-61 winter season a radar-rainfall study was made over theFeather River Basin by the Sacramento, California, radar staff. Rainfall estimatesusing radar were recorded on a PPI scope 4 1/2 n.m. grid overlay according to thefollowing categories derived from the Rainfall Rate-Echo Intensity (Fig. 2) chart.

db SETTING RAINFALL RATE

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.02 in.hr.

.25 in.hr.

.75 in.hr.1.50 in.hr.2.00 in.hr.

Once each hour weather echoes over the basin were outlined on the grid overlayat 0-27-42-48-51 db settings. This produced a series of iso-db lines. The appropriaterainfall rate was then entered in each section of the grid containing a weather echo.

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These radar-determined rainfall values for the season were next compared with thecorresponding raingage charts in the basin (Fig. 4a). A range compensating formula

Pr

P =/53\3.O4

\RJwhere P is adjusted precipitation

Pr is radar estimated precipitationR is range to grid

was used to aline the radar estimated rainfall amounts with the observed rainfall(Fig. 4b).

SACRAMENTO, CALIF.1960-1961 WINTER SEASON

UNADJUSTED VALUES

O K> 20 30 «0RADAR ESTIMATEO PRECIPITATION IN INCHES

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SACRAMENTO, CALIF.1960-1961 WINTER SEASON

ADJUSTED RADAR ESTIMATED VALUES

(b)

0 10 20 30 4ADJUSTED RADAR ESTIMATED PRECIPITATION IN INCHES

Fig. 4

Another season's data were collected during the winter of 1961-62 with disap-pointing results. This could be attributed to several factors such as the relativelylower storm tops experienced that year and questionable peak radar performance.From the data collected during these two seasons, one point was evident — thatrainfall was being underestimated using the RR/EI chart.

This year's (1962-63) data has not been analyzed yet but one of the questionablefactors was removed with the implementation of a radar standardization procedure.This procedure is to insure peak operating performance of each WSR-57 radar usedby the Weather Bureau.

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Similar studies are being conducted by other of the radar staffs as part of ourcontinuing attempt to measure rainfall by radar for hydrologie application. Becauseof the operational requirements that must be met, the studies by necessity areconducted as time and staff limitations permit. It is hoped that with the establishment in1962 of the Weather Bureau's Weather Radar Laboratory at the University ofOklahoma's Research Park, Norman, Oklahoma, greater and faster strides can bemade in this endeavor.

3. MEASURING RAINFALL BY SIGNAL ATTENUATON

Since 1959 investigations have been conducted by Stanford Research Institute(SRI), Menlo Park, California, under Weather Bureau contract to estimate rainfallby measuring the attenuation effects of rainfall on short-wave radar signals over apredetermined path (6). The application of this system would be primarily over smallwatersheds or reservoirs for flash flood or control purposes. This system comprisesa modified tf-band radar emitting a CW signal which is bounced off a reflector orreflectors located in line at known distances from the fixed radar antenna. Knowingthe return signal strength on clear days a relationship can be established betweenthe signal attenuation and precipitation measured in recording raingages along thepath. Using 10 minute mean values it has been possible to estimate rainfall by thistechnique within ± .25 inch/hour. For shorter time periods of comparison, errorof rainfall estimates increases. Further study is under way to determine the designfeatures of such a low cost operational CW system as well as to acquire additionalconfirmation of the signal-rainfall relationship.

4. SPECIAL EQUIPMENT

The Weather Bureau is currently installing 30 MRT-2's (Meteorological RadarTransponders) for interrogation by WSR-57 radars. (7) These MRT's (Fig. 5) are tobe located in uninhabited areas where reports are otherwise unavailable or in highfrequency flash-flood areas and will be used to determine the rainfall intensity as wellas to report the rainfall amount. The precipitation amounts will be displayed in binarycode on the PPI scope. Measurement is by a 0.1 inch tipping bucket with the countpulse being sent to the memory unit of the MRT for later readout on demand.Although this unit will only be transmitting rainfall information (some units will havea heating element for melting snowfall) it has the capability to handle three additionalmodules, such as wind, temperature, or the water equivalent of snow as determined bymeans of a radioactive isotope system. The useful range of the MRT is 60-85 mileswith the possibility of extending the range to 100 miles with a mountainside instal-lation. The MRT operating characteristics are :

Frequency 2700-2900 MeNominal Pulse Length 1 microsecondPRF 200 ppsPower Source 115 VAC or 24 VDCAntenna 3 foot paraboloidBeam Angle 8 DegreesNominal Peak Power 1 kw.Manufacturer : HRB-Singer, Inc. State College, Pa.

In the last few years the Weather Bureau has installed some 60 two-way FMradio transceivers for the purpose of relaying river and rainfall reports from communi-cationless areas. Several of these transceivers have been installed at various radar

366

stations which have a specific hydrologie responsibility. River and rainfall informationis exchanged between the radar station and the River District Office concerned aswell as radar information on radar-depicted heavy rainfall thereby expediting flash-flood warnings. These transceivers operate from cither a 115 VAC or a 6 or 12 VDC po-wer source. The nominal range is 50-60 miles using a YAGI type antenna on a fre-

Mctcorological Radar Transponder— Model 2(MRT-2).

quency of 130-180 me with an output power of about 20 w. To further obviate thepower source problem, especially where batteries must be used, a recently marketeddevice may someday be employed. This device, called a thermo electric generator,manufactured by the Minnesota Mining and Manufacturing Company, furnishes atrickle charge to the battery thereby reducing the number of service calls for recharging.The TE generators work on the principle that a voltage is produced as a result of atemperature differential across a hot and cold junction. Heat is applied from a propaneor butane fired source to one junction while the other junction is kept cold. The TEgenerator has a number of these thermoelectric couples connected in series to providethe power output. Although none are operationally in use with the transceivers severalare undergoing evaluation to provide trickle power to the Analog-Digital Recording(ADR) River Gage.

In 1962 SRI started construction (8) of a prototype Radar Precipitation Inte-grator (RPI) for operation with a WSR-57 radar under Weather Bureau contractjointly funded by the U.S. Corps of Engineers and the Weather Bureau. This systemhas been completed and installed at the Weather Bureau Weather Radar Laboratory,University of Oklahoma Research Park, Norman, Oklahoma. An evaluation of thesystem is currently under way to determine the operational usefulness and ability ofthe RPI to perform as a digital representation of precipitation. The evaluation con-sists primarily of correlating the RPI values of rainfall with those from a dense net-

367

Rainfall 4.30 "

PPI PreaenUtion

Fig. 5a

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Fig. 5b

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work of approximately 150 conventional recording raingagcs, placed on a 3-milegrid in the Washita River basin near Chickasha, Oklahoma, and operated by theAgriculture Research Service of the Department of Agriculture. The RPI system hasthe additional feature that each observation is recorded on 5-channel paper tapepermitting transmission over existing communications lines to other offices. It is vi-sualized that with several such systems an entire RFC area could be convenientlysampled with all the radar determined rainfall information being transmitted to onelocation. Further, since the information is in digital form it is readily available forcomputer input.

The method of operation employs the LOG receiver of the WSR-57 radar. Thedata are processed in digital form and displayed (Fig. 6) in inches of rainfall on elec-tromechanical-counters which are arranged on a board to represent the geographical

Fig. 6 — Radar Precipitation Integrator

location of the area sampled by the RPI. Once every 12 minutes the RPI takes control ofthe radar, regardless of the control settings, and commences the programed observa-tion including any necessary resetting of controls. Six scans are required, one foreach of the preselected succeeding higher gain levels, to complete the three-minuteobservation of the 141 electronically gated (1° x 4 n.m.) areas. During this period

369

the operator can still view the linear receiver display on the PPI scope. At each classlevel a yes or no determination is made of an echo's presence by the RP1 and inter-preted as follows :

Class Level

123456

Threshold

Or.)

0.10 40.81.42.33.9

ClassValueOr.)

0.250.501.001.753.006.00

Increment/obs.(5 obs. an

hour)

0.050.100.200.350.601.20

This system does in three minutes what it would take a radar meteorologistsitting at the console more than an hour to accomplish.

5. CONCLUSION

For operational purposes the Weather Bureau still depends largely on subjectivemanual procedures employed by the radar meteorologists to interpret the rainfallpattern and .amounts. Through the modest research studies being conducted at theradar stations, under different weather environments and physical locations, we arclearning about our capabilities and limitations within the present state of the art.

This paper is limiled to a report of the Weather Bureau's activities in the field.Advances are also being made in the meteorological radar data processor field byother groups in the United States. One which is being evaluated at this time was con-structed by the Budd Company (9) under a U.S. Air Force contract. Late in 1962 theprocessor was coupled to a CPS-9 radar at the field station of the Air Force CambridgeResearch Lab. at Sudbury, Massachusetts, under the direction of Dr. David Atlas.This system utilizes the CAPPI (10) (constant altitude PPI) type scan with severallevels of intensity where each level is a 10 db step on the logarithmic receiver of theradar. The 120 n.m. scope is divided into 5-mile-square grids requiring about 3 1/2minutes to complete each CAPPJ type observational cycle. The readout-, which is innumerical values of 1-7, represents storm intensity. The values printed out indicatethe highest intensity echo observed in any 1° x 1 mile area within the 5-mile square.

This system is more sophisticated and also more costly than the Weather Bureau'sRadar Precipitation Integrator. Nevertheless, because of its potential we are veryinterested in the processor and the results of the evaluation. Early tests appear quitefavorable in that the processor is accomplishing the job it was designed for— automaticstorm detection.

Another device which, when perfected, could make the measurement of rainfallby radar quite reliable is the "raindrop counter and sizer. "A balloon borne device, ( u )the forerunner of a ground-based electronic sensor, has just been reported on byNew York University. If rainfall measurement by radar is ever to be perfected thedrop size distribution of rainfall must be known for the echo volume sampled by theradar beam. These sensors may some day provide the answers.

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Many unanswered questions still remain in this quest to measure rainfall byradar and investigations will continue because of the tremendous potential involved.As it now stands our estimates of rainfall, using the Rainfall Rate-Echo Intensitychart, are limited to about a factor of two because of the variability of drop size in agiven volume of rainfall. It is hoped as technological advances are made, many ofthese problems will be solved. To measure precipitation by radar on a national opera-tional scale, the employment of such systems as the Radar Precipitation Integratoror other similar processors is mandatory because of the need for a high frequency ofobservations over many levels of storm intensity.

REFERENCES

0) TARBI.K, R.D., The Use of Multiple Exposure Radar Photographs in the WeatherBureau's Hydrologie Program, Proceeding 8 th Weather Radar Conference, SanFrancisco, Calif., April I960.

(2) ROCKNEY, V. D., The WSR-51 Radar, Proceedings 7 th Weather Radar Conference,Miami Beach, Florida, Nov. 1958.

(3) FLANDERS, A. F., The Weather Bureau's Radar-Hydrology Program, Proceedings9 th Weather Radar Conference, Kansas City, Mo., Oct. 1961.

C) MCCALLISTER, J.P., Radar Observations in Hydrologie Analysis, Proceedings9 th Weather Radar Conference, Kansas City, Mo., Oct. 1961.

(5) KRESGE, R., BLUST, F., and ROPES, G., A Comparison of Shore and Lake Precipi-tation Observations for Northern Lake Michigan, 13th General Assembly of the1UGG, Berkeley, Calif., Aug. 1963.

(6) COLLIS, R.T. H., Study of Techniques for Measuring Rainfall by Reference toRadar Attenuation, Stanford Research Institute, Menlo Park, California, FinalReport on Project 3948, June 1962.

(7) SOI.TOW, D.R. and TARBLR, R.D., Telemetering Precipitation Data Using a RadarBeacon, ACU Journal, Vol. 64, No. 11, 1959.

(8) COLLIS, R.T. H., Study into the Feasibility of Developing a System to MeasurePrecipitation by WSR-57 Radar, Stanford Research Institute, Menlo Park, Cali-fornia, Final Report on Project 3727, Oct. 1961.

(") PAULSEN W. H., A Radar Data Processor, Proceedings 8 th Weather Radar Conferen-ce, San Francisco, Calif., April 1960.

(10) MARSHALL, J.S., Grey Scale and CAPPI in Operation,Proceeding 8 th in WeatherRadar Conference, San Francisco, Calif., April 1960.

(") BENNETT, L. and STALDKR, J., Development of a Precipitation Particle Sensor,New York Univ. Research Report 783, June 1962.

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