October 1990 K. Nakamura, H. Inomata, T. Kozu, J. Awaka and K. Okamoto 509 Rain Observation by an X- and Ka-band Dual-Wavelength Radar By Kenji Nakamura, Hideyuki Inomata, Toshiaki Kozu, Jun Awaka and Ken'ichi Okamoto Communications Research Laboratory, Koganei, Tokyo 184, Japan (Manuscript received 2 March 1990, in revised form 7 July 1990) Abstract A rain observation by an X- and Ka-band dual-wavelength radar has been performed. The objective of this experiment is to explore the possibility of multi-wavelength radar for measuring rainfall rate. Simultaneous rain observations by C- and Ku-band radars were also performed. Correlations among measured Z- factors show that rain attenuation is significant only in the Ka-band for a short range when the observing range is a few km. After a method of obtaining an optimum averaging hit number is proposed, a dual-wavelength analysis using rain attenuation is applied to Ka- and X-band radar data and the result confirms the capability of providing an estimation of rainfall rate other than that by the conventional method. The dual-wavelength analysis is compared with a single wavelength analysis using rain attenuation and an advantage of dual-wavelength analysis is verified. The capability of the dual-wavelength analysis for retrieving a vertical structure of rain is demonstrated. A radar calibration using the dual-wavelength analysis is tested and a more accurate calibration than that comparing the rain gauge data is obtained. 1. Introduction Lion switching mechanism, but the existing trans- mitter and receive can commonly continue to be Conventional rain radars which usually operate used. Another candidate for multi-parameter radars at S-, C-, and X-band wavelengths measure the re- is a multi-wavelength radar. Radiowave scattering turn signal from hydrometeors, which means that characteristics of rain and propagation characteris- the radars measure only one physical quantity, i. e. tics in rain differ frow one frequency to another. the so-called Z-factor. However, the most impor- tant physical quantity of rain is rainfall rate which Contrary to the dual-polarization radar, the cost of a dual-wavelength radar might be expensive, be- directly determines the amount of water produced cause nearly two complete radar systems are re- by rain. The total amount of water has a close relationship not only to human activities but also quired. Also, a multi-wavelength radar has a dis- advantage in observable range which is limited by atmospheric dynamics because the total amount of rain attenuation at the higher frequency. However, rain gives the total amount of latent heat released to the atmosphere. Unfortunately, the radar-measured a multi-wavelength radar is a better candidate for a spaceborne radar than a dual-polarization radar. It Z-factor has a large variation even at the same rain- is difficult to direct the spaceborne radar beam at fall rate due to raindrop size distribution variations, large incident angles for measuring the polarization because the Z-factor is proportional to the sixth characteristics because of range smearing and the power of raindrop diameter, but the rainfall rate large range from the radar to targets. On the con- is proportional nearly to the 3.5th power of rain- trary, the disadvantage of short observable range of drop diameter (Battan, 1973). To overcome this, a multi-wavelength radar is not crucial, because the multi-parameter rain radars are investigated exten- radio path through the rain area is mainly limited sively. One of the strong candidates for a multi- by the height of the rain and is short, which results parameter radar is a dual-polarization radar. Dual- in a small rain attenuation. polarization radars have some variations but all measure the polarization characteristics of rain. One The study of dual-wavelength radar observation of the advantages of the dual-polarization radar is has a long history. The first study is Atlas and that the cost for installing the polarization capabil- Ludlam's (1961) work on hail detection as far as ity may not be so expensive, because what addi- we know. As the first trial was for hail detection, tionally are required is an antenna and a polariza- many measurements were performed for hail detec- tion. Eccles and Atlas (1973) proposed an S- and X- c1990, Meteorological Society of Japan band radar. Tuttle and Rinehart (1983) presented a 510 Journal of the Meteorological Society of Japan Vol. 68, No. 5 simple technique to correct for attenuation in dual- provide an estimation of rainfall rate over a short wavelength analyses based on data taken by S- and range. The estimation is independent of the one us- X-band radars. They also showed some effects of ing a conventional rain retrieval method where the mismatched antenna beam patterns (Rinehart and received power of radar is simply converted to rain- Tuttle, 1984). The basic concept of hail detection fall rate by a semi-empirical formula. Fortunately, is to use a large difference of the so-called effective C- and Ku-band radars were also available. We will Z-factors of hail in S-, C- and X-band radio bands, present some characteristics of data other than X- since the size of a hail is large enough to make the and Ka-band radiowaves and show that a radiowave scattering deviate from the Rayleigh condition. of more than 14 GHz is needed for dual-parameter Contrary to hail detection, dual-parameter mea- rainfall measurement for a range of a few km. The surement of rainfall rate by a dual-wavelength radar capability of an X- and Ka-band dual-wavelength uses rain attenuation along a radio path. The size of radar to provide an independent estimation of rain- a raindrop is usually small enough to allow the as- fall rate will be verified. An advantage of a dual- sumption that the scattering is Rayleigh. In this wavelength radar over a single-wavelength radar us- case, the difference of the measured Z-factors is ing rain attenuation will be presented. Though caused by the rain attenuation. Eccles and Mueller other rain retrieval algorithms are proposed (see At- (1971) studied the possibility of the estimation of las and Meneghini's review, 1988), they are methods liquid water content using S- and X-band radars. peculiar to the rain observation with a downward- Then Eccles (1979) compared the estimated total looking radar and were not applied here. A range rainfall rates from X-band rain attenuation and S- profiling capability for dual-wavelength radar will be band radar-derived total rainfall and also the rain demonstrated. Finally, an application of the dual- gauge-derived total rainfall for more than 40 days, wavelength radar for calibration will be proposed and he concluded that the estimation from rain at- and tested. tenuation is more accurate than that from the radar 2. Instruments reflectivity factors. So far, ground-based dual-wavelength radar ex- In the Kashima Space Communication Center of periments have used an X-band radiowave as the the Communications Research Laboratory, many highest frequency. The main reason seems to be meteorological instruments including a C-band rain that the severe rain attenuation limits the observ- radar and a Ku-band FM-CW rain radar are in- able range. The biggest feature of a ground-based stalled for centimeter radiowave propagation exper- rain radar is the capability of observing rain over a iments using the Japanese geostationary satellites. large area. A typical S-band radar has an excellent Experiments using CS (Medium-capacity Commu- sensitivity and the observable range is limited by the nications Satellite for Experimental Purposes), BSE curvature of the earth's surface. C-band radars have (Medium-scale Broadcasting Satellite for Experi- an observable range of more than 100 km, and X- mental Purposes), or ETS-II (Experimental Test band radars have an around 50 km observable range. Satellite Type II) have been conducted. Using this A radar with an observable range of less than a few equipment and also a dual-wavelength radar which ten km is not attractive for observations of horizon- employs X- and Ka-band radiowaves, simultaneous tal extent of rain. However, as mentioned before rain observations have been performed since June, the limited observable range for airborne or space- 1982. borne radars is not so crucial as for ground-based Figure 1 shows the instruments of the observation. radars. A spaceborne dual-wavelength radar with 10 The instruments include four rain radars, and the GHz and 24 GHz is proposed for the future manned CS propagation experimental system. The radars station (Okamoto et al., 1989). Meneghini et al. are operated at C-, X-, Ku- and Ka-band wave- (1989) performed experiments using an X- and Ka- lengths. Besides the radars, four rain gauges are band airborne dual-wavelength radar and tested var- also used. Two rain gauges are shown in Fig. 2. Two ious rain retrieval algorithms using rain attenuation. other rain gauges are located 2 km and 6 km south Fujita (1983) proposed a rain retrieval algorithm in of the radar sites. Table 1 shows the characteris- which the received powers in both frequencies are tics of the radars. Figure 2 shows the locations of combined and rainfall rate is estimated in a kind of the instruments in the Kashima Space Communica- least square sense. Fujita et al. (1985) tested the al- tion Center and the directions of radar beams. All gorithm using X- and Ka-band airborne rain radar the beams are in the direction of the CS (Az: 190*, data. El: 48*). The beams of the X- and Ka-band of Keeping the application to a future spaceborne the dual-wavelength radar coincide with each other.