K-BAND ATTENUATION DUE TO RAINFAIX 



179 



line. The characterinties ul the flap atteuualors were 

 checked every few hours, using a K-band thermistor. 

 Each flap was calibrated and used over a 12-db range. 

 Eesettability was approximately ±0.1 db. A small 

 nozzle was used to direct the output of the signal gen- 

 erator upon the receiving paraboloid. Calibrations 

 were made before, during, and after rainfalls and were 

 witliin ±1.0 db over the 5- and 6-hour measuring 

 periods. 



10.4.4 



Analysis 



The primary attenuation curve of Figure 32, shown 

 with solid dots, was obtained by choosing periods when 

 the rainfall at all stations, including the automatic 

 gauges, was essentially uniform. Six such periods of 



25 



20 



10 



S>^- 



x 



./ 



J 



^ 



A 



- c 



40 50 



MM PER HOUR 



Figure 22. Primary attenuation curve of K-band 

 radiation in rain. K-band attenuation versus rainfall 

 intensity. (Note: Decibels per nautical mile.) 



so 



^! 



WOCAO 



Zu 



Is 20 



AVS 41.2 

 ATT 21.0 



DISTANCE ALONG PATH 



FiCiURE 23. Profile of rain intensity along the path. 

 Time 22364.5. 



uniform fall idong the path were selected covering the 

 important range of to 41 mm per hour. Figure 23 

 is a rainfall intensity profile of the highest uniform 

 fall recorded. By Humphreys' classification of rain' 

 the intensities covered by the primary curve are more 



il mm per hour, light rain; 4 mm per hour, moderate rain; 

 15 mm per hour, heavy rain; 48 mm per hour, e.xcessive rain. 

 See reference 24. 



than adequate fur normal rates of precipitation en- 

 coitutered in nature. 



Using the primary attenuation curve thus obtained, 

 it was possible to extend the curve for extremely high 

 rates of fall (cloudbursts) in the following manner. 

 Figure 24 is a rain intensity profile at an interval of 



80 



TOTAL ATTENUATION 22.3 

 ■.\KNOWN PARTIAL ATT JA_ 

 ^ 14.9 



I 



20 



AVG78.4 

 ATT 



AVG 11.8 

 ATT 2.06 



DISTANCE ALONG PATH 



Figure 24. Intensity profile during uneven precipita- 

 tion. Time 171115. 



nonuniform rainfall distribution. The area under the 

 curve is divided into sections as shown. These sections 

 cover the portions of the path where the intensity was 

 below 41 mm per lir. Hence with the primary attenua- 

 tion curve and a planimeter it is possible to assign the 

 contribution that each section makes to the total ob- 

 served attenuation (assuming that the attenuation in 

 decibels is linear with distance). 



After subtracting out the part of the attenuation 

 already known, the high intensity central portion is 

 left to account for the residual attenuation. Dividing 

 the residual attenuation by the fraction of a mile cov- 

 ered by this part of the path and plotting this value 

 against the average intensity in the interval gives a 

 point at 78 mm per hour. As a check on the method, 

 similar profiles were worked up which gave points 

 below 40 mm per hour. These are plotted as open cir- 

 cles, as shown in Figure 32. It will be seen that the 

 open circles agree quite well with the solid ones, 

 and hence considerable confidence in the high inten- 

 sity points is Justified. 



Discussion 



The total observation time of the experiment was 

 roughly 3 hours, about half of this total time being 

 represented by the figures presented with the paper. 

 It was necessary to employ a large number of precipi- 

 tation measuring stations along the path in order to 

 obtain an accurate precipitation profile. Some inci- 

 dental work on drop size was also done. The spread 

 in drop size at a given time was rather large, probably 



