Characteristics of rainfall rate useful in the estimation of attenuation due to rain are presented. Rain data collected at Ogbomoso between January–October, 2009 were used in the analysis. Result shows that power law relationship exists between the equiprobable rain rates of two different integration times. The value of conversion factor CE and CR obtained for Ogbomoso are 0.28(60) and 0.64(90) respectively. Our result then shows that different conversion factor is required for different location even within the same climatic region

Raji, T.I.

Semire, F. A.

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

PaperCharacteristics of MeasuredRainfall Rate at Ogbomoso, Nigeriafor Microwave ApplicationsF. A. Semirea and T. I. Rajia Department of Electronic and Electrical Engineering, Ladoke Akintola University of Technology, Ogbomoso, NigeriaAbstract—Characteristics of rainfall rate useful in the esti-mation of attenuation due to rain are presented. Rain datacollected at Ogbomoso between January–October, 2009 wereused in the analysis. Result shows that power law relationshipexists between the equiprobable rain rates of two different in-tegration times. The value of conversion factor CE and CRobtained for Ogbomoso are 0.28(60) and 0.64(90) respectively.Our result then shows that different conversion factor is re-quired for different location even within the same climaticregion.Keywords—characteristics, Ogbomoso, rainfall.1. IntroductionWith the rapid development currently being witnessed inthe field of information technology there is an increas-ing demand for broadband satellite services and this hascalled for satellite system that can provide rapid andreliable transmission of information from one end to an-other. Attenuation due to rain is one of the factors thatlimits the path length over which reliable radio communi-cation systems can be established. It also limits the usage ofhigher frequencies for terrestrial microwave point to pointas well as satellite communication. As the frequency in-creases, so does the impact of atmospheric conditions onthe radio wave propagation [1], which causes reduction inthe quality of signal in the case of analog transmissions,and increase in the bit error rate in the case of digital trans-mission.In this paper, some characteristic of tropical rainfall as mea-sured at Ogbomoso are discussed using rainwise rain-guagebetween January–October 2009. The effect of integrationtime on the rain rate and cumulative distribution functionsare examined. Rain rate duration characteristics are alsodiscussed.2. Measurement SystemThe precise knowledge of rain attenuation on any com-munication link improves the estimation of link availabil-ity which provides accurate knowledge of outage timeexcepted. Most attenuation prediction models required atleast 1 min rain rate statistic [2]–[4]. Rain rate statistics isspecified on a percentage of time basis, that is the percent-age of time in a year or a month that the rain rate equalsor exceeds a specific value.The experimental set-up for this measurement of rain ratecharacteristic is located at Ladoke Akintola University ofTechnology, Ogbomoso. The rain rate are measured usingrainwise tipping bucket rain guage of 0.5 mm per tip atdifferent integration time. The cumulative distributions ofrain rate for different integration times are generated fromrain data base.3. Results of Rain Rate Measurementin Ogbomoso3.1. Integration TimeThe relationship between rain rate statistics with differ-ent integration times has been studied from the results ob-tained from the rain gauge. The cumulative distribution ofthe rain rate from the different integration times generatedfrom the rain gauge data is shown in Table 1 and the cor-responding graph in Fig. 1.Table 1Time rain rate exceeded for differentintegration timesRain ratePercentage of rain rate is exceeded[mm/hr]for integration time1 min 5 min 10 min 20 min210 0.0064 0.0206 0.0386 0.110180 0.00704 0.0123 0.0193 0.0787150 0.0094 0.0185 0.0579 0.0866120 0.0200 0.0699 0.143 0.23690 0.0276 0.0864 0.135 0.30760 0.0745 0.302 0.394 0.55130 0.871 2.366 2.812 4.56785F. A. Semire and T. I. RajiAt the low availabilities, the probability of raining is animportant parameter determining the annual outage timepercentage, below which the rain attenuation has to be con-Fig. 1. Cumulative distributions of rain rate at Ogbomoso fordifferent integration times between January and October 2009.sidered in the link to be designed [5]. From the cumula-tive distribution above, a table from which the relationshipbetween equiprobable rain rates for the different integra-tion times is established. This involves developing a con-version method for time probability of one minute rainfallrate from that for integration time great than one-minute.The result is shown in Table 2 and Fig. 2.Table 2Value for equiprobable rain rates for differentintegration timesR1min [mm/hr]Rτ [mm/hr]5 min 10 min 20 min30 22 16 1060 42 29 2290 61 47 35120 79 62 49150 97 78 64180 114 93 79210 131 108 953.2. Conversion FactorsAccording to G.O. Ajayi et al. and Watson et al. [6]–[7],conversion factors CR and Ce for different integration timesare considered as:CR(t) =RTRτ,Ce(R) =eTeτ,Fig. 2. Equivalent 1-min rain rate for different integration timesat equal probabilities of occurrence.where CR is the ratio of rain rates exceeded for a givenpercentage of time t as measured by the rain gage with in-tegration times T and τ , Ce is the ratio of the exceedancesfor a given rain rate measured using the integrationtime T .The result of Ce obtained in Ogbomoso for T = 5 min andτ = 1 min was then compared with that obtained in Ile-Ife and some countries in Europe. The result is shownin Table 3.As shown in Table 3 there is a gradual decrease in thevalue of Ce with increasing rain rate for the Europeancountries while that of Ile-Ife and Ogbomoso in Nigeriadecreases rapidly with increasing rain rate. This suggeststhat Ce might be climate dependent as suggested in [8]which is also corroborated as according to the value ob-tained for Ogbomoso. The value of CR obtained in Og-bomoso is also compared with that obtained in [7] forsome stations in Europe and that of Ile-Ife, as obtainedin [6]. The results obtained in Ogbomoso agrees withthat obtained in Ile-Ife which are generally lower thanvalues obtained for other European stations. On the av-erage, the percentage difference between the values ofCR obtained for Europe and other temperate regions aregenerally lower than that obtained in tropical regionssuch as Ogbomoso and Ile-Ife. The values are depictedin Table 4.3.3. Return PeriodsReturn periods for specific rain data were studied fromthe rain data we obtained here in Ogbomoso over the pe-riod between January and October 2009. Table 5 showsthe number of occasions when rain rates of 30, 60,90, 120, 150, 180 and 210 mm/hr had particular returnperiods. It is shown also in Fig. 3.86Characteristics of Measured Rainfall Rate at Ogbomoso, Nigeria for Microwave ApplicationsTable 3Values of Ce(R) for T = 5 min and τ = 1 minLocationRainfall rate [mm/hr]Comments10 20 30 40 50 60 70 80 90Greece 0.65 0.51 2 year data for KeffalinaItaly 0.63 0.60 0.60 0.63 2 year data from RomeWest Germany 0.80 0.74 0.73 1 year data from DarmstadtUK 0.78 0.72 0.71 0.69 0.60 0.435 year data from southern UKon a network of rain gaugesNigeria0.74 0.68 0.59 0.57 0.40 0.30 0.17 0.10 0.0828-month data obtained at Ile-Ife(Ile-Ife) using a fast response rain gaugeNigeria0.37 0.28 0.1610-month data obtained at Ogbomoso(Ogbomoso) using a rain gaugeAll results except Nigeria (Ogbomoso) was obtained from [6].Table 4Values of CR(R) for τ = 1 min numbers in bracket indicate the 1 min rain rateLocation T [min] Percentage of year Comments0.01 0.0015 0.8(100)Italy 10 0.7(100) 2 year data from Rome60 0.42(100)5 0.84(20) 0.82(60)UK 10 0.77(20) 0.72(60) 5 year data from southern UK from a network of rain gauges60 0.59(20)West Germany5 0.93(20) 0.9(40)1 year data from Darmstadt10 0.81(20)Nigeria (Ile-Ife)5 0.68(80) 0.64(130)28-month data obtained at Ile-Ife10 0.52(80) 0.53(130)Nigeria (Ogbomoso)5 0.64(90)10-month data obtained at Ogbomoso10 0.53(90)Table 5Values of return periods at different rain rateRain rate Return No. of occasions rain rate[mm/hr] period was exceeded300.010 14840.423 11500.356 7280.222 573603.311 1471.147 1272.538 1021.815 709037.017 4611.574 421.815 397.407 356.993 37120 50.075 344.237 30106.383 16150 17.271 1511 111809 9142.045 127.134 1081.301 651.815 59.091 14210 156.25 1148.543 10Fig. 3. Characteristics of return period of rain rates.3.4. Rain DurationThe rain rate duration data are paramount to studythe characteristics of precipitation. The data is use-ful in the determination of communication link outagedue to rainfall. The rain duration at different rain ratesis shown in Table 6 and the corresponding graphs inFigs. 4 and 5.87F. A. Semire and T. I. RajiTable 6Rain duration at different rain ratesRain rate No. of occasions rain rate Rain duration[mm/hr] was exceeded [s]30 6 360Different 8 480rain events 11 660at which 20 1200rain volume 29 1740is greater 31 1860than or equal 46 2760to 100 mm 52 3120134 804060 1 603 1804 2407 4208 4809 54011 66014 84090 1 602 1203 1805 3007 4208 540120 1 602 1203 1804 2409 540150 1 602 1204 2406 360Fig. 4. Rain duration at 30 mm/hr rain rate.4. ConclusionIn this contribution, rainfall rate data between January-October, 2009 at Ogbomoso have been used in the studyof effect of integration time on the cumulative distributionof rain rate. The result shows that power law relationshipexists between the equiprobable rain rates of two differentFig. 5. Rain duration at 60 mm/hr rain rate.integration times. Our result compared to Ile-Ife followsthe same trend except that the value obtained in Ogbomosois a little higher than Ile-Ife. Although, Ile-Ife rain rateis expected to be higher because Ogbomoso experiencesNorthern climatic condition. Therefore it is recommendedthat the work of [6] need to be revisited as a result ofchange in climatic condition due effect of global warming.The conversion factors CR and CE obtained at Ogbomosoare lower as compared to those obtained at Ile-Ife. Thevalue of CE and CR obtained for Ogbomoso are 0.28(60)and 0.64(90) respectively. Our results show that differentconversion factor is required for different locations evenwithin the same climatic region for the conversion of oneintegration time to another. However, measurement periodof 10 months may not be sufficiently enough to compareother literature results. Further detailed analysis based onlonger period would be performed and presented by theauthors in the future.References[1] L. J. Ippolito Jr., Radiowave Propagation in Satellite Communication.New York: Van Nostrand Reinhold Company, 1986.[2] “Propagation data and prediction methods requirement for the designof Earth-space”, ITU-RP. 618-10.[3] S. H. Lin, “National long term rain statistics and empirical calculateof 11 GHz microwave rain attenuation”, The Bell Syst. Techn. J.,vol. 56, no. 9, pp. 1581–1604, 1997.[4] S. H. Lin, “A method for calculating rain attenuation distributions onmicrowave paths”, TThe Bell Syst. Techn. J., vol. 54, pp. 1051–1086,1975.[5] “Specific attenuation models for rain for use in prediction methods”,ITU-R Rec. P. 838-3.[6] G. O. Ajayi and E. B. Ofoche “Some tropical rainfall rate charac-teristic at Ile-Ife for microwave and millimetre wave applications”,J. Climate Appl. Meteorol., no. 23, pp. 562–567, 1983.[7] P. A. Watson, V. Sathiaseelan, and B. Potter, “Development ofa climatic map of rainfall attenuation for Europe”, Post Gradu-ate School of Electrical and Electronic Engineering, University ofBraford, U.K., Rep. no. 300, p. 134, 1981.[8] G. O. Ajayi,“Statistical properties of tropical rainfall intensity mea-sured at Ile-Ife, a southern station in Nigeria for microwave and mil-limetre wave applications”, Ann. Teelecommun., no. 37, pp. 477–483,1982.88Characteristics of Measured Rainfall Rate at Ogbomoso, Nigeria for Microwave ApplicationsF. A. Semire had her B.Sc.in electrical and electronicengineering at Ladoke Akin-tola University of Technol-ogy (LAUTECH), Ogbomoso,Nigeria in 2000. She com-pleted her M.Sc. in commu-nication option from Univer-sity of Lagos, Nigeria in 2003.She is currently working in theDepartment of Electronic andElectrical Engineering, LAUTECH and pursuing her Ph.D.in the field of radio propagation (rain attenuation) atUniversity Science Malaysia.e-mail: semireban@yahoo.comDepartment of Electronic and Electrical EngineeringLadoke Akintola University of TechnologyOgbomoso, NigeriaT. I. Raji had his B.Sc. andM.Sc. in electrical engineeringat Columbia University, NewYork, USA, between 1962 and1967. He proceeded to Univer-sity of Roschester, New York,for his Ph.D. in electrical en-gineering from 1970 to 1975.He started his teaching careeras a teaching assistant in theDepartment of Electrical Engi-neering, University of Rochester from 1970 to 1972.He joined Obefemi Awolowo University (OAU), Ile-Ife,Nigeria, in 1975 as a lecturer and rose to the positionof senior lecturer 1979. He later transferred his serviceto Ladoke Akintola University of Technology, Ogbomoso,where he became a Professor in 1990. He died on 2nd ofNovember, 2010.89

Characteristics of Measured Rainfall Rate at Ogbomoso, Nigeria for Microwave Applications, Journal of Telecommunications and Information Technology, 2011, nr 2

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Characteristics of Measured Rainfall Rate at Ogbomoso, Nigeria for Microwave Applications, Journal of Telecommunications and Information Technology, 2011, nr 2

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