University of Florida Tests May Cut Loss by Rains

Research Aide Describes Experiments Carried on by Scientists

The Florida Times-Union, Jacksonville - Friday April 22, 1949

    TAMPA,  April 21 (AP) Florida’s losses from hurricane rains may be lessened by a University of Florida experiment in radar.  

       Harry A. Owen Jr. of Palatka, a university research assistant, today described tests to try to determine the water content of clouds from cloud echoes picked up by radar.  Owen spoke at the annual convention of the Florida Engineering Society which opened here today at the Hotel Tampa Terrace.  

    If the water contents of storm clouds can be determined accurately in advance it will be an important step in fighting hurricanes. The flooding rains which accompany hurricanes usually cause more damage to unprepared areas than the winds themselves, Owen said. 

    Owen emphasized that the problem is far from solved. Results are “promising,” but the researchers are trying to get more data and investigate further.

    The hurricane angle is not the major concern of scientists in this matter, he said. The experiments were begun during the war to try to find radar channels which would not be crossed up by the raincloud echoes. The principal aim still is to learn more about the atmospheric conditions so the radar frequencies may be used which won’t be blacked out by rain.   

Explains Experiments

    Owen summed up the experiments in this manner: The rain cloud echoes were picked up by radar from many points in a relatively small area - 160 square miles. The actual rainfall was measured by  55 control stations in this area.  Complicated mathematical formulas were used to determine an accurate relation between the quantity of water which fell at any one spot and the radar wave length at that point.  

    It is this experimentation which must be carried out further before definite, dependable conclusions can be made.

    Another valuable experiment by the university along similar lines was one to determine the location of storms by measuring the noise of their lightening, Owen said.  During the war storms were located accurately as far as 2,000 miles away by measuring from three points. Experiments now are in progress to try to determine direction and distance from a single station.Work in this field was described by S.O. Hersperger, a University of Florida student from Tampa. 

    Earlier in the day the engineers had been urged by two committee heads to give better support to the state’s flood control program. Herald A. Scott of Jacksonville, head of the flood control and navigation committee, said the lack of interest was “appalling.” B.F. Gordon, also of Jacksonville, heading the sanitary and water supply committee, echoed Scott’s plea for more individual efforts on conservation and water control projects.

Engineers Told How Radar May Cut Hurricane Losses

Tampa Morning Herald - Friday, April 22, 1949

    The Florida Engineering Society opened their 33rd annual conference for three days here today, as engineers from all over the state convened at Hotel Tampa Terrace to hear technical papers and discuss new engineering methods.  

    How Florida’s losses from hurricane rains may be lessened was explained by Harry A. Owen, Jr., a research assistant at the University of Florida. He told of tests to determine the water content of clouds from cloud echoes picked up by radar, and how it may be applied to lessening hurricane damage by rain. Emphasizing the problem is still far from solved, Owen said results are “promising.”

Welcomed by Mayor   

     His was but one of the technical aspects presented by leaders in their respective engineering fields, during yesterday’s sessions, which opened with an address by Mayor Curtis Hixson, and a response by President Gillespie. The morning session consisted of society business matters, followed by luncheons and taking up again in the afternoon with technical engineering discussions. 

    Engineering discussions were grouped under four main subjects: Civil; Electrical; Mechanical and Surveying and Mapping. New processes and methods in all types of engineering were outlined by a roster of speakers, including recognized authorities in all fields listed. Flood control also popped up as a matter for discussion, and earlier in the day the engineers were urged by two committee chairmen to give better support to the state’s flood control campaign.  

 More Interest Needed    

    Harold A. Scott, of Jacksonville, head of the flood control and navigation committee, said the lack of interest among engineers was “appalling.”  B.F. Gordon, also of Jacksonville, heading the sanitary and water supply committee, echoed Scott’s plea for more individual effort on conservation and water control projects.   

     Members yesterday afternoon saw three motion pictures dealing with engineering problems, and closed yesterday’s program with a get acquainted party at the Palm Room last night.  The party was followed by entertainment presented by Florida Knights, champion quartet of Southeastern United States.

    Today’s program is to open with a general session similar to yesterday’s; a trip via chartered bus to Barstow chemical plants, and social activities tomorrow night.

    Saturday will mark the announcement of new officers and society business before adjournment.

    Member’s wives will be entertained by a sightseeing tour this afternoon.

University of Florida Engineers Plan Attendance at Tampa Meet                                                                                                                                      The Gainesville Daily Sun - Wednesday, April 20, 1949 

   The University of Florida College of Engineering will be well represented at the annual convention of the Florida Engineering Society Thursday through Saturday in Tampa. Eleven staff members and five students are slated to present technical papers, and approximately 75 students here will attend to become acquainted with Florida’s practicing engineers and their problems.  

   Faculty members scheduled to present papers include: Earl B. Phelps, research engineer on “Stream Sanitation in Florida;”  M.E. Ryberg and H.W. Burney, resident engineers on “Research in Equipment for the Production of Naval Stores;”  H.E. Schweyer, associate professor of Chemical Engineering on “Some notes on Florida Crude Petroleum;”  T.L. Bransford, assistant professor of Civil Engineering, “A Proposed Extension Course for Surveyors;” and Harry A. Owen, Jr., assistant in research, “The Use of Microwave Radar to Study Relations between Rain Intensities and Received Echo Power.”

   Also, G.E. Remp, assistant research engineer on “Alarm Systems for Condensate Return Systems in Citrus Processing Plants;” S.C.D. Lawson, associate professor of industrial engineering on “Use of Transparent Models Developing the Loci  of Conic Sections;”  S.P. Goethe, campus engineer on “ Determination of Minimum Air Requirements for Summer Air Conditioning;”  H.B. Williams, associate research engineer and W.H. Bussell, associate in research on “Machines for Watermelon Seed H arvesting;”  Frank Bromilow, associate professor of civil engineering on “Wind Forces on Buildings,” and A.L. Kimmel, assistant professor of chemical engineering, “Corrosion Prevention in Iron and Steel Water Storage Equipment.”

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THE USE OF MICROWAVE RADAR TO STUDY RELATIONS BETWEEN RAIN INTENSITIES AND RECEIVED ECHO POWER

Harry A. Owen, Jr.

Assistant in Research

University of Florida

April 21, 1949

This work was supported in part by Contract Nos. W28-099-ac-153, and AF28(099)-2 with the U.S. Air Force, through sponsorship of the Geophysical Research Directorate, Air Material Command.

       During the last few years radio has been applied to many fields of endeavor differing from the original intent of providing communication between human beings. Examples of such applications are the use of very low and very high frequency radio energy for induction and dielectric heating, electronic circuits for precise control of industrial processes and location of reflecting targets by use of radio waves. It should be unnecessary to stress the importance of the particular application of radio in the form known as radar in the winning of the last war.  Its value in locating the enemy, whether in the guise of submarine, surface vehicle or airplane is now known to all.  Certain operational problems which occur, however, have shown that this form of radio may be utilized for an entirely different purpose from that of detecting and ranging on some particular ship or airplane.  Of special importance is the phenomena that with the shorter wavelength radar sets, reflection of the radio waves from precipitation of water droplets in the atmosphere causes echoes or targets to appear on the screen of radar display indicators along with the other signals being received.  During the war when it was necessary to have a clear view  of the surrounding area within radar range, this obscuring of the screen by echoes from rain clouds was quite undesirable.  

       In the early part of 1946 the engineering and Industrial Experiment Station at the University of Florida entered into a contract with the Watson Laboratories of the Air Material Command to investigate the effect of the reduction of echo power from targets such as airplanes on the far side of clouds.  The attenuated signal was to be compared with the echo returned from the target in an atmosphere as nearly standard as possible.  Mr. Marinus H. Latour was named project leader and the project personnel began the process of obtaining the necessary equipment and theoretical background for performing the experimentation.  A total of six different radar sets were eventually obtained through war surplus releases and government loan requisitions. All but one of these radars were installed and readied for operation.  Installation of this equipment consumed considerable time since the largest set, which consists of a steel tower forty feet high in which the transmitter and antenna are located, had to be completely disassembled at Leesburg, Florida and reassembled at the University of Florida Electronics Research laboratory.

      In the first part of 1948, Watson Laboratories proposed the project objectives be modified somewhat so that investigation of radar echoes from rain clouds could be correlated to the actual rainfall intensity.  In order to perform this new program, a research team made up of three engineers, a meteorologist and a technician from the radar project at the University of Florida was sent to theClinton County Air Force base in Wilmington, Ohio in June 1948.  At this Air Force base was located a system of rain gages over an area of about 160 square miles.  This installation, which was a part of the Cloud physics Project for the U.S. Weather Bureau, was the most compete network of weather stations ever set up in such a small area. It’s locations is shown in Fig. 1 with respect to the Wilmington base.  The unique facilities for rainfall studies afforded by this arrangement accounted for the temporary transfer of the radar research activities of the University to Ohio.  The program which was followed was that suggested by M. David Atlas of Watson Laboratories and was based on experimental work of the same nature which he performed in the summer of 1947 at the same location. It consisted of containing photographs of radar cloud echoes taken in such a manner as to provide isoechoes or contours of equal received power which would be compared analytically with charts prepared from the rain gage data containing isohytes or contours of constant rainfall intensity.

      Two microwave radars were were obtained at Wilmington for operation on the projects.  One radar was the SCR-584, operating on a wavelength of about ten centimeters.  The other radar set was a converted airborne unit, the AN/APQ-13, the wavelength of which was about three centimeters.  Both radar sets are shown in Fig. 2 along with an operation van. The two different wavelengths were employed so that the effects of return echoes from rainfall at different frequencies could be determined.  On both sets the display of targets was made on a Plan Position Indicator (PPI). this type indicator provides a plot of the targets in the surrounding area by use of a radial sweep which is rotated in synchronization with the radar antenna.  As echoes are received, they intensify the screen of the cathode ray tube used as the indicator, thus producing a bright spot for each target.  As a cloud of water droplets is swept by the narrow beam f radio energy from the antenna, the reflected signals produce a bright figure on the screen indicating the outline of the cloud.  Special cameras were attached to the indicators which exposed 35 millimeter film in such a manner that a complete scan appeared on each frame.  The film was shifted automatically at the end of each exposure to place an unexposed frame in position.  By use of an intervalometer circuit actuated by the antenna, photographs were taken on every other scan of the indicator. A block diagram illustrating the basic components of the system is shown in Fig. 3.

      The method of obtaining photographs of rain storms from which could be plotted the radar rain intensity contours was rather unusual.  Such a data run consisted of a sequence of photographs of the PPI oscilloscope taken with different gain settings of the radar receiver between the limits of maximum gain and minimum gain. These values are defined as (1) maximum, the gain setting at which the noise level of the receiver falls just below the saturation value of the incoming radar signals, and (2) minimum, the setting at which the incoming radar signals have just disappeared.  After establishing the gain setting limits, four or five convenient graduations were set up between these limits.  These values of gain settings with their associated radar pictures constitute a complete data sequences.  A sequence ordinarily was taken over a period of a minute or less so that the conditions of rainfall would not change appreciably during the interval of data collection.  A representative photographic sequence made on the AN/APQ-13 three centimeter radar set is shown in Fig. 4, with the maximum gain picture in the upper left corner.  The echoes still present at minimum gain are nearby ground targets.

      To determine the actual sensitivity of the radar receiver at the maximum, intermediate and minimum settings of the gain control, special calibration techniques were derived.  Inasmuch as the actual data collection was obtained by photographic process, it was seen that the calibration process should use the same medium to reduce errors to a minimum.  The procedure followed consists of feeding a radio frequency pulse of sufficient power to produce a spot on the PPI oscilloscope of medium brilliance with the receiver gain set for maximum into the receiver from a calibrated signal generator. A sequence of photographs is then obtained of the signal generator pulse indication on the radar indicator screen as the output power of the signal generator is reduced in increments of one decibel.  After sufficient exposures have been made to assure that the pulse has faded out on the indicator screen, the process is repeated for each of the intermediate settings of the sensitivity control and finally for minimum gain.  On the processed film the picture which follows the frame on which the last signal generator pulse is visible is located and marked as the minimum detectable signal for the particular gain setting. A plot of the actual power fed into the receiver against the setting of the gain control yields a graph of the minimum detectable signal for any gain setting.  The importance of this calibration can be easily recognized since the edges or contours of the rain clouds represent that minimum detectable signal or fade-out level.  Reference to the calibration curve just described will provide information on the echo power level represented by the cloud contour.

      In addition to the careful calibration of the receiver sensitivity, it was necessary to meter many other electrical quantities.  It was originally planned to photograph a panel containing instruments indicating these data simultaneously with the radar echoes, but since the proper equipment for accomplishing this task was never received, the information was recorded manually. The instrument readings which were noted included: time, photograph number, gain setting, transmitter current, power and frequency, receiver crystal current, indicator cathode ray tube bias and antenna elevation.

      As mentioned previously, the rain gage network, from which the data on actual rainfall was available, was the most complete network which as yet been assembled for analyzing rainfall over a small area.  The rain gages were placed in an area approximately twenty miles long and eight miles wide just south of the radar site.  The fifty-five stations were located as close as possible to intersections of squares two miles on the side.  The gage were of the recording type and indicated the total rainfall against time on the removable charts.  From these charts the rainfall intensity was computed by dividing the change in rainfall over a significant change by the time of change.  This procedure was applied to all the charts from the gage network for that particular time. From the rainfall intensity worksheets, isohytes or contours of constant rainfall intensity were plotted on a map which showed the location of each gage station.

      In order to make a comparison between the actual rainfall intensities, it was necessary to plot the radar photographs on a chart similar to that used for the isohyet plot.  A thin translucent paper was obtained upon which the edges or contours of the radar cloud pictures were traced by projecting the photograph on the back of the paper.  This resulted in a composite picture for each sequence showing the storm contours for five to six gain settings, dependent upon the number used in each sequence.  A bomb-setting 35 millimeter projector obtained through surplus equipment acquisitions was modified for the purpose.  The primary difficulties in tracing the enlarged radar pictures was in the problem of obtaining proper register between individual frames of a sequences and in determining the exact edge of the radar cloud echoes.

      Having obtained the isoecho storm contours for various gain settings, the next step was to convert these into equal intensity radar rain contours.  In order to do this it was necessary to employ an empirical equation which has been derived from Wexler.  The following equation which related the radar rain intensity to the radar parameters, the range of the rain cloud and the received power was then obtained of use in changing the isoechoes into rain intensity contours: Log10 Pr R2/C h Pt = 1.441 Log10 IR - 17.302, where Pr = echo power in watts; R = distance in storm in meters; C = radar constant - computed for the particular radar used; h = radar pulse length in meters; Pt = peak transmitted power in watts; IR = intensity of rainfall in millimeters per hour.  By use of a nomograph constructed for the above equation, a point by point computation was made around the perimeter of the isoecho storm contours.  Thus, rain intensity values in millimeters per hour were obtained on each of the isoecho lines for each gain setting.  With these point values available, lines of constant radar rainfall intensity were then drawn.  Fig. 5depicts a composite chart made by superimposing the rain gage isohyets shown in dotted lines on the radar rainfall constant intensity lines for the SCR-584 ten centimeter radar.

      Analysis of composite radar rainfall intensity contour-isohyet maps revealed numerous discrepancies.  If the radar beam had been exceedingly narrow and at an angle of elevation of zero degrees, then the instantaneous values of rain gage data would never would have more nearly corresponded.  However, it was calculated that for the three centimeter radar in cases of light rainfall (up to one millimeter per hour) and at distances corresponding to the farthest rain gage station, there was a time reversal of as much as thirty-one minutes between the time the light rain first entered the top of the beam and reached the ground directly underneath.  In order to correct for this difference between the observed radar pictures and the actual instant the rain struck the ground and the rain gage network, the isohyet charts were replotted by putting each rain gage on a different time basis dependent on its range from the radar site.  For the SCR-584 ten centimeter radar set, the data taken needed no correction since the beam width was much more narrow and the angle of elevation much lower.

      There is a great deal of evidence that there is a quantitive as well as qualitative relation between the radar rain intensity contours and the isohyets.  In practically all cases the storm cores or contours of highest rainfall intensity coincided with the cores on the rainfall intensity charts.  The intensity values of the radar rain cores in most cases were lower than the values of the surface isohyets.  This could be accounted for by attenuation of the signal due to the rain, errors in the empirical equation used to solve for the the radar rain intensity and errors in observation.

      While the temporary research program in Ohio was concluded at the end of August 1948, analysis of the data still continues.  At the present time corrections are being made for attenuation of the radar signal due to the rain and early results indicate an even closer correlation radar and actual rain intensity charts.  However, not only does the processing of the data  taken last summer continue but further work of the same nature is being dome in Florida.  The rain gage network in Ohio was disassembled during the latter part of 1948 and arrangements were made to secure ten of these gages for the use on the project at the university.  At this time they are being installed along a radical line from the radar site at the Electronics Research Laboratory where most of the radar equipment is located.  With this particular configuration of gages it will not be possible to plot complete contours but profiles of rainfall intensity in that area will be obtained.  In addition to the permanently installed gages, it is planned to have a mobile rain gage station along with other meteorological instruments and a special target for reflecting the radar energy back to the radar sets and to receivers located on the ground beneath the target.  The use of such a reflector, which at the present time is intended to be a sphere, will permit both one-way and two-way reception of the reflected signal from a standard target.  With this arrangement it is hoped that not only will more be learned about the relation between radar rain cloud echoes and actual rainfall but also additional information will be obtained on the attenuation of radar signals through space under atmospheric conditions.  The Engineering and Industrial Experiment Station will be well equipped for analysis of the rainfall conditions in any part of the state when the analytical process is developed more completely for correlating radar pictures and and rainfall.  By use of the large mobile radar unit, such a study would not be limited to a particular locality but could be set up for operation and investigation at almost any site.  This is one of the units which is being used in the present study at the University.

      ACKNOWLEDGEMENTS are made to the Watson Laboratories of the Air Material Command, the agency sponsoring this research, and to the All Weather Flying division and the Cloud Physics Project of the U.S. Weather Bureau at Wilmington Ohio, for their cooperation in the performance of the special summer program of 1948.