Indian Journal o( Radio & Space Physics Vo l. J2 . Apri I 20m. pp. 9X- 1 03

Rain structure estimation from X-band radar reflectivity measurements for radio sys tem applications in microwave and millimetre wave frequency bands

S K Sarkar. Rajcsh KUIll<lr*. Iq bal Ahillcd & M V S Pras<lcI

Rad io and At illospheric Scicnccs Di vision. National Ph ys ical Labor~lIory. New De lhi 11001 2

and

J Das & A K Dc

Elc:c troni cs and COlllllluni cati on Sc iences Unit. Ind ian Stati sti ca l Institut c. 20J. B T R()ad. Ca lculla 700 OJ.')

I?I'Cl' il '<,r/ 23 April 2()02: r l'l' isl'd 20 Decelllber 2002: {Iccepl l'd 10 .IWI/{({I'I' 2()03

The stat isti ca l morpho logy o r rad~u' relkct ivity ~IS a runcti on or horizontal and verti ca l ex tension 01 rain is ()r considcr:lhlc: intcrcs t t<>.those asscssi ng the poss ible intcrrcrence between telTestr ial l inks and earth -space satelli te links. In thi s p~lpe r . the hori 7.0 ntal ~lIld ve rti ca l ex tension or r:lin deduccd rrom I'adar plan position inci ic:ltn r (PPI ) and r.uIgc height indicator (l{H I) IllCaSU relllcnts ha vc becn preseill ed. Such rcsult s or ra in ex tcnsion ha ve al so hecn deduced as a ra n ,~ ti o n ()r radar rcflecti vity ractor (dBz). The r:ldar refl ecti vity factor is thc Illcasure or the strcng th or the scattering cros;-,ec ti on or the rain cell s, T he scallcring cross-sec tion is responsible ror causing interl'crence to the radio signals. Probab ilit y d istr ibution or radar reflecti vity (dBz) deduced from bot h hori zontal (PPI ) and verti ca l cxtension (RI-II ) or rain cell s has ~" so bcen es timated.

Intnlduction The est imation of interference of radi o signals,

caused clue to scattering of the signals from different transm itl ers, is important. Such results of interference arc essential for design ing and planning of rad io sys tems including terres trial and earth-space commun ica ti on links, and for remote sensing dev ices in microwave and millimetre wave frequency bands t

.".

In recent years, studi es on radar bri ght band have become the major interest to the radar community with the advent of wind profilerJ and MST radar~ . But the present study is on rain structure in relat ion to the app li ca tion or radi o wave propagation . A computer contro lled and sophi sticated X-band radar belonging to th e India M eteoro logica l Department operating at 9.375 G Hz has prov ided us a good opportunity to study micro-stTucture of rain in relation to its hori zontal and vertica l ex tension s. The back scattered echoes from rain ce ll s are reproduced over the plan posit ion indicator (PPI) ancl range height indica tor (RHI ) of the radarscope. The radar reflectivity (dBz) is a measure of the fraction of back scattered energy received from the prec ipitation ce ll s5

The rain events presented in this study are o f convec ti ve type. Basically, th e vertical height of the

"' Present acldcl l'ess: 2 14. Sclloo l o f Enviro nrneillal Sc icnces. Ja\Valla rl al Nehru Uni versity. Ncw Delhi 110 067

stratiform ralll is es timated from the he ight of the bright band or melting layer. The porti on that is helO\v the melting layer is ca ll ed rain hcight. Above th e melting it may not be in liquid form . But, in the cLlse of convec ti ve rain , the liquid form or hydrometeors can also be found above th e melting layer (ma inl y becau se of up and down drafts). Thi s type o f rain can more o ften be seen in trop ical or equatorial places.

2 Radar ch3l"acteristics and data processing method The charac teri sti cs o f the X-band radar are as

follows:

Freq uency Transmitted powcr Beam width Pul se width

Pul se repet iti on f requency Range for maximum time Maximum range Accuracy

= 9.375 G Hz = 200 k W = I"

= 0.8 ~l S and 2 Ils = 1250 pps ::nd 750 pps = 240 kill = 400 kill = 2 dB at 240 km for

rainfall -23 dBz ( I mm! hr)

The PPI and RHI scans during the rain events in July-August 2000 are recorded w ith th e radar. Each PPI and RHI display has been analyzed and studi ed.

SA RK AR el a/. : RAIN STR UCTURE ESTIM ATION FROM X-BAND RADAR REFLECTIVITY 99

The recei ved backscatte red energy (or radar re fl ecti vity) of va rio us levels o f amplitude in decibe l scale is colo ur-coded for the graphical di splay on the radarscope. Table I shows the colour code and the correspo ndi ng radar refl ecti vity. The pre­programmed, Z-R (radar refl ec tivity-rain) re lati on in the racia l' system has been used to deduce rain

intensity. The radar re fl ecti vity facto rs o f - 23 -28 dBz, 28-33 dBz, 33-38 dBz, 38-43 dBz, 43-53 dB z

and 2: 53 dBz are assoc iated with the ra in rates I mm/hr, 2 .4 mm/hr, 2.6 mm/hr, 9 mm/hr, 18 mm/hr and 75 mm/hr, respective ly.

3 Results and discussion The hori zontal ex tension of rain is very impo rtant

to deduce results on attenuati o n of radi o wave due to rain for terres tri al (horizontal ) communi cati on links6

.

Some j·esult s o n hori zonta l ex tensio n o f rain were deri ved in the pas t? by using the measured results on tota l attenuation (dB) of radi o wave due to ra in and the results o n specific attenu ation (dB/km) obtained theoretically. The typical representati ve rain events observed o n radar PPI are presented in Fig. I [(a) and (b)). It is seen fro m Fi g. I (a) ( I Aug. 2000) that the

rain of low intensity (- I mm/hr) is scattered a ll over the reg io n. Marsha ll -Pa lmer di s tributi on is utili zed to deduce rain rate. The hi gh intensity of ra in has been fo und to occur over very small regio n. Fi gure I (a) depi cts that the hi gh intensity rain is a localized phenomenon. Fi gure I (b) shows that ra in of hi gh intensity has occurred over large area and rain w ith low intensity has been found to occur at pl aces scattered a ll over the regio n. A typi cal ra in storm observed at 1352 hrs 1ST o n 15 Apr. 2000, occurring at a di stance of 30-40 km from the po int of measurements, has been fo und to be ex tended over large di stance. This rainstorm consists of rain ce ll s hav ing lo w, moderate and hi gh ra in intensity. In order to es timate the attenuation of radio wave due to rain

Table I- Co lour code vs radar refl ec ti vity fac tor

Co lour code

Green

Dark red

Yellow

Orange

Red

Dark red

Radar retlec ti vity fac tor, dB z

23-28

28-33

33 -38

38-43

43-53

<:: 53

over earth-space paths, the rain he ight is one of the most important pa rameters o the r th an the ra in intensity. Some results on prov isional rain he ight in re latio n to OuC isotherm height over di ffe rent

h· I . h b d . dS. 11l geograp Ica regIOns ave een etermlne . The typical vertica l ex tens ion o f rain observed o n 2

May 2000 at 165 I hrs 1ST and 175 I hI's 1ST are shown in Fig. 2 [(a) and (b)]. The ra in has taken place fro m the we ll-developed c lo ud and it ex tends up to the he ight of 15 km. It is a lso shown that the ra in intensified with time and moved towards the radar centre with the speed of nearl y 40 km/hr. The red and orange colours at the core of the ra instorm sign ify

strong wate r content with ra in intens ity as hi gh as -75 mm/hL Such rain occurring fro m 10 km or II km is fo und to be assoc iated with thunderstorm . Such thunderstorm is quite commo n over thi s regio n duri ng the mo nths of April and May. Another typ ical RHI pro fil e observed on 27 May 2000 is shown in Fig. 3(a) . It is seen that the ra in event is assoc iated with different rain intensity. Such ra in hav ing different intensity is assoc iated with diffe rent height. The vertical spreading of the rain is al so seen to be qui te large. The vertical ex tension o f rain observed o n I Aug. 2000 has been presented in Fig. 3(b). It is seen

that the ra in hav ing intensity of - 2.6 mm/hr is occurring fro m an a ltitude of around 6 km and the

rain intensity of - 75 mm/hr is fo und to occur fro m an a lti tude of 4 km up to the surface. The hori zonta l spread of the ra in is around 5 km . Such exac t he ight is important fo r estimatio n of attenuation of radi o wave due to ra in fo r satellite communicati on.

Figure 4 shows the variatio n of hori zontal extension of rain (average results) with the radar refl ecti vity. Such results have been deri ved by tak ing all PPI measurements. It is seen that the hori zonta l

extension varies from - 14 km to 4 km , while the radar refl ecti vity varies fro m 28 dB z to 53 dBz. The large value of refl ecti vity is an indicatio n of high rain rate in the present study and is assoc iated with low hori zontal ex tension of rain . The radar re fl ecti vity of

- 43 dBz (which is a measure o f rain rate - 18 mm/hr)

is assoc iated with the hori zonta l extension of - 8 km ,

while the radar refl ectivity of - 33 dB z (whi ch is a

measure of ra in rate - 2.6 mm/hr) is assoc iated with

the ho ri zontal ex tension of - 10 km . The low radar retlecti vity t S assoc iated with large hori zonta l ex tensio n.

Figure 5 presents the results on the vari ation of ve rtical ex tension o f rain with radar refl ecti vity.

100 INDIAN J RADIO & SPACE PHYS, APR IL 2003

u

n~<: II £1 1 ~ ., f!;t,g ~o. I<

(a)

;Jon

!..J i)lJ T .i!!..

~ 27O----~~_+--~~~~~~~~~--t_--_r--_+----t_~~~--

r.,;14j r.lJ -L J 270 ____ ~~~,~~

( bJ

: ~. ~.. '.

t -

~ '"

120

\ ?\

Fig, I--Hori zontal ,'ain structures observed on rada r PPI on (a) I Aug. 2000 and (b) 15 Apr, :2000

SARKAR el al. RA IN STRUCTURE ESTIMATION FROM X-BA D RADAR REFLECTIVITY 101

25

20 E ~

t--=' 15 I <.9 10 W I

5

Reflectivity RHI Az=328.0 16.51.54 E1=1.9 May 02,2000 Rng.=240 km

(a)

o 40 80 120 160 200 240 RANGE, km

Reflectivity RHI 17.51 .55 May 02,2000

Az=343.0 Rng.=120 km

25 ~------------~----~ (b) Ref

20 E ~

t--=' 15 I <.9 10 w I

5

dBl

o 0 20 40 60 80 100 120 , ,

RANGE, km

Fi g. 2- Veni ca l ra in structures observed 0 11 rada r RHI 0 11 2 May 2000 at (a) 1(151 hrs 1ST and (b) 17.'1 1 hI's 1ST

These results sugges t the heighl from where the rain of different intensiti es starts occurring. It is seen from Fig. 5 that the verti cal ex tension of rain varies from 9 km to 5.75 km . These meas urements were taken in monsoon months. It is seen that the low vertical extension of rain - 5.75 km is assoc iated with the radar refl ectivity of - 53 dB z when the rain intensity is - 74 mm/hr. The vert ica l ex tension of rain from 8 km 10 9 km is found to be associated with 28-43 dBz

25

20 E

.:£ 15 .-: I c.9 10 w I

5

0

25

20 E

.:£- 15 l-I c.9 10 w I

5

o

Reflectivity RHI 20.01.11 May 27 , 2000

(a)

0 10 20 30 40 RANGE , km

Reflectivity RHI 15.54.53 Aug . 01 , 2000

(b)

o 10 20 30 40 RANGE. km

Az,,104 .0 E1 ,,3 .8

Rng .60 km

50

Ref dBl

60

Az,,304.9 E1"S .1

Rng.=60 km

Ref dBl

50 60

Fi g. 3--Typica l ve rti cal rain structures observed O il radar RH I on (a) 27 May 2000 and (b) I Aug. 2000

. and the rain rate varies from - 2.4 mm/hr 10 18 m m/h r.

The probabi lity di stributi ons deri ved from the observed radar refl ect ivity of the PPI and RHI measurements during July-August 2000 have been presented in Fi gs 6 and 7. It is seen fro m Fi gs 6 and 7 that the probability di stributi ons follow aimosl the same vari ati on in both PPI and R HI observat ions. The radar reflectivity of 40 dB z ~xceeds for around 2~%

102 INDI AN J RADIO & SPACE PHYS. APRIL 200]

16.-------------------------------~

E

z « 12 a: LL o Z o CfJ

~ 8 f-­X W

--.J

~ 6 4 ~ a: o I

O L-______ ~ ______ ~ ______ ~ ______ ~ 20 30 40 5 0 60

RADAR REFL ECTIVIT Y, dBz

Fig. 4- l-I or izonta l extensioll of rain from X-ha nd radar mcasuremcnt over Calculla

10

z 9 <t a: lJ.. o z 8 Q (f)

z W f--X 7 w --.J <! '=? ~ 6 w >

5 L-_____ _ .l..-____ L-___ _ L-_--'

20 30 4 0 50

RADA R REFLECTI VI T Y, dBz

Fig . .'i- Vert ica l extension of rain from X-band radar measuremcnt ove r Ca lculla

of the time in PPI case. while it exceeds for around ]4% of the time fo r RHI case. Similarl y, it has been

found th at the radar refl ec tivi ty o f - 5] dBz, which

corresponds to - 75 mm/hr exceeds for 4% or time in PPI case, wh i le it exceeds al so for around 4% of time in RH I case. It is also seen from Figs 6 and 7 that both the curves fo r PPI and R~ I follow similar variation up to 30 dBz ane! above SO dBz. T here is a difference in variat ion of the two curves shown in Figs 6 and 7 (between ] 0 dBz and SO dBz) . Such difference of

60,--- - -------------.

N 50 m

>­f--

~ t; 40 w --.J lJ.. W a: a: 8 30 <I a:

Z=23dBz,R= 1 mm / hr. Z = 28 dBz , R=2.4 rnml hr. Z=33 dBz , R= 2.6 mml hr .

Z=38 dBz , R=9 mm / hr . Z=43 dBz , R=18 mm / hr . Z=53 dBz , R=75 mm / hr.

PERCENTAGE OF T IME ORDINATE EXCEEDED

Fig. 6- Probability d is tribution 0 ;' rc fl ecti vity from radar PPI mcasurcmclll

50 N

m -0

>-f--

~ 40 f-- Z = 23 dBz ,R = I mm / hr . u w Z = 2B dBz ,R=2.4 mm l hr. --.J lJ.. Z = 33 d Bz , R=2 .6 mm l hr . w a: Z = 38 dBz , R=9 mm / hr . a: 30 Z =43 dBz , R= 18 mm l hr. <I 0 <I

Z = 53 dBz , R=75 mm l hr. a:

o 10 100 PERCEN TAGE OF TI ME ORDINATE EXCEEDED

Fig. 7- Probabilit y di stri bution of re fl cc ti vity fro m radar RI-II mcasuremellt

variation is marginal. For instance, at 50% probability level, the radar refl ect ivity is found to be 34.5 dB in case o f PPI and fo r RHI it is 36.5 dB. The different radar reflectivity is associated with difTerent rain rates. Figures 6 and 7 dep ict that rain intensity of different magnitudes ex ists for equal percentages of time both in horizontal and verti cal extension . In order to deduce the attenuation or rain , we need the probab ility d istribution o f rain rate. The probability distribution of rain rate deri ved from horizontal and vertica l extension of radar reflectiv ity measurements hy using the Z-R relat ion (Z=200 RIC,) as shown in Figs 6 and 7 can be ut ili zed for rain attenuation estimati on for both hori zontal and earth-space paths. However, still such derived rain rate di<;tributioll from radar re flectivity observations is to he validated with

SARKAR el 01.: RAIN STRUCTURE EST IM AT ION FROM X-BAND RADAR REFLECTIVITY 103

the rain rate distribution derived from the ground­based rap id respon se rain gauge measurements.

4 Conclusions Thi s study revea ls thaI the X-band conventional

radar has the capab ility to prov ide results on horizontal and vert ica l ex tension of rain, but it is to be used cautiously. It has been observed that only one Z­R relat ion has been used in thi s radar system. In fact , the inves ti gati on suggests that different Z- R relati ons for different rain types would have given more accurate resu lts, particularly , at low and very high ra in intensiti es. One Z-R relation should have been for ra in rate up to 25 mm/hr and the second one for rain ra te from 25 mm/hr to 100 mm/hr and the third one fo r above 100 mm/hr. The operati ng freq uency of the radar is 9.375 GHz. Consequently, path attenuat ion at each range gale affects the rai n esti mation through Z­N relati on. The radar refl ec ti vity factor, therefore, has to be corrected for accurate rain es timati on.

Acknowledgements The authors are grateful to the India

Meteorologica l Department (I M D) fo r providi ng the X-band radar observations. The study would not have been possible w ithout the act ive cooperation of the all the members of IMD, Delhi and Calcutta. One of the authors (S KS) is very grateful to Mr 0 Pradhan and

Mr R K Dutta (IMD) for their supports ince the initiation of thi s projec t and had several discuss ion with them in connection with the present study. Active supports from Mr Dasgupta and Mr S N Srivastava (retd.) of IMD were also received. The authors are grateful to the Department of Science and Technology , Govt. of India, New Delhi for sponsoring the proj ec t.

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