Indian Journal of Radio & Space Physics Vol. 36, December 2007, pp. 502-513 Recent studies on cloud and precipitation phenomena for propagation characteristics over India S K Sarkar & Anil Kumar Radio and Atmospheric Sciences Division, National Physical Laboratory, New Delhi 110 012, India e-mail:[email protected] Received 3 August 2007 ; accepted 21 August 2007 Radio propagation above a frequency of 10 GHz is affected by precipitation in terms of attenuation due to its high dielectric constant. As far as Indian subcontinent is concerned, rain and cloud are the most important ones. Thus, there is still tremendous need to develop a strong database on these parameters. The work carried out on cloud and rain characteristics in relation to radio wave propagation over India in recent years only, from 2000 till today is reviewed in this paper. These results are useful to design as well as for estimation performance of microwave communication and for radar propagation over different locations situated in different geographical regions of India. Keywords: Precipitation, Rain, Cloud, Thunderstorm, Microwave communication, Millimeter wave communication, Attenuation, Noise temperature, Conventional radar, Doppler radar PACS No.: 92.60. Nv; 84.40.-x 1 Introduction bands are also monitored to investigate the effects of In recent years, there is a shift from C-band to rain on the performance of communication systems. higher frequency bands in India from the user point of Preferably such links should be monitored where view for radio communication and remote sensing heavy rainfall occurs. applications 1. Such higher frequency is needed due to the requirement of larger bandwidth of radio 2 Low cloud occurrences morphology over various systems 1−3. The radio propagation above 10 GHz is selected locations affected by precipitation 1-4. The measurements of Due to shift of interest to higher frequencies, many various precipitation parameters should be carried out assumptions made for lower frequency bands are no over as many locations as possible. Infact, some longer valid for higher frequency bands 10-12 . measurements have been carried out over different Attenuation due to clouds in millimeter wave and geographical regions of India in recent years by microwave frequency bands also leads to serious various scientific institutions. Parameters, which degradation in the performance of radio affect microwave communication and radar communication, especially for low noise systems. The propagation, are mainly rain rate, horizontal extension cloud morphology, particularly in relation to radio of rain, rain height, rain drop size distribution, cloud wave propagation over different geographical regions characteristics and cumulus cloud cells responsible of India is very essential and important. In view of for formation of thunderstorm 4-9. All these parameters this, systematic studies on cloud occurrence are taken as input parameters for estimation morphology over different geographical locations in performance of microwave communication systems. India have been undertaken recently. It is important to Several techniques such as rapid response rain gauge, mention here that cloud related work in relation to conventional rain gauge, radar, radiosonde, etc., are radio wave propagation over India has not been used to deduce these results 7. In addition to carried out and therefore statistical effects are precipitation measurements, communication links relatively unknown. It is well known that there are operating over both paths like horizontal and earth several types of clouds, such as altostratus, space path above 10 GHz should be monitored to cumulonimbus, cumulus, fair-weather cumulus, deduce results on attenuation of radio wave due to nimbostratus, stratus, stratocumulus, etc 10 . Among all rain and cloud 7. Some links operating in Ku and Ka these types of clouds, the cumulus type is very SARKAR & KUMAR: CLOUD/PRECIPITATION STUDIES FOR PROPAGATION CHARACTERISTICS OVER INDIA 503 important over tropical India. Each cloud type is is also the representative of the daily results of these peculiar in nature in terms of dimensions, shape, meteorological parameters 16 . The number of days moisture content, liquid water content, etc. (average of 0830, 1130, 1430 and 1730 hrs IST) and The cloud occurrences morphology over several nights (average of 2030, 2330, 0230 and 0530 hrs Indian stations, for Kolkata by Kumar and Sarkar 11 , IST) in each month during which the sky is covered over northern India by Sarkar and Kumar 12 , over totally and partially with clouds over various Hyderabad by Sarkar et al.13 , over other Indian stations 11-15 are presented in Table 1. stations by Sarkar et al. 14 and other Eastern Indian The low level cloud coverage over the Indian stations by Kumar and Sarkar 15 has been deduced subcontinent is also being monitored 13 by the from the low-level cloud data. The cloud data were Meteosat-5. A typical cloud coverage diagram over obtained from the India Meteorological Department the Indian subcontinent observed on 11 Feb. 2004 at and pertain to the period 1990-95. The advantage of 0000 hrs UT by Meteosat-5 is presented in Fig. 1. It is using the cloud data from the India Meteorological seen from Fig. 1 that the subcontinent is substantially Department is that these are available for a large covered with low clouds. Some scattered mid-level number of stations in India pertaining to large periods. and high-level cloud coverage were also observed. The cloud observations here in India are taken four Such satellite observations on cloud coverage are also times during the day and four times during the night useful for on-line atmospheric conditions for satellite times. The observations are taken during 0830, 1130, communication and remote sensing application work 1430 and 1730 hrs IST in the day and also at 2030, in microwave and millimeter wave frequency bands. 2330, 0230 and 0530 hrs IST in the night. The low Reddy 17 has studied the temporal variations of clouds are found to occur between 2 km and 6 km vertical structure of precipitating clouds in tropical heights. India with the 1.357 GHz lower atmospheric wind It has been seen that the times of observations of profiler (LAWP) at Gadanki. On the basis of these clouds in Indian stations were chosen in such a way observations, he has classified the precipitating cloud that four daytime and four nighttime observations are systems into (1) convective, (2) transition and (3) truly good enough to be the representative conditions stratiform. Diurnal and seasonal variations of the of day and night in Indian subcontinent 14 . The occurrence of precipitating cloud systems show that averaged monthly results observed during daytime the precipitation primarily occurs in the afternoon and and nighttime on temperature, humidity and pressure also the convective and transition clouds are most Table 1 Summaries of occurrences of low clouds during daytime and nighttime over various stations 11 -15 Station Month Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Mumbai 1 3 5 10 26 28 31 30 29 18 8 2 (1) (2) (4) (9) (26) (28) (30) (30) (28) (12) (3) (1) Nagpur 6 8 11 16 14 25 30 30 23 17 9 5 (3) (4) (6) (10) (13) (23) (27) (27) (17) (10) (3) (2) Ahmedabad 1 4 2 4 9 22 31 31 25 9 3 1 (1) (2) (1) (2) (5) (18) (29) (28) (18) (3) (2) (1) Hyderabad 10 12 16 22 26 29 30 30 28 24 17 11 (5) (7) (11) (21) (26) (28) (28) (31) (28) (25) (14) (6) Patna 4 8 5 6 13 21 28 29 25 14 4 4 (2) (3) (4) (4) (8) (18) (25) (27) (19) (6) (2) (1) Ranchi 5 11 9 14 19 22 25 26 22 18 11 5 (3) (6) (6) (8) (14) (23) (27) (25) (18) (11) (5) (2) Bhubaneshwar 8 16 18 20 23 28 30 30 28 25 15 6 (4) (9) (16) (23) (23) (25) (30) (30) (27) (21) (12) (2) New Delhi 6 9 8 8 8 14 21 20 17 3 4 (4) (6) (5) (5) (6) (12) (23) (19) (14) (2) (2) (1) Kolkata 7 8 13 18 24 28 30 30 28 23 11 8 (3) (5) (9) (15) (20) (24) (26) (26) (22) (13) (7) (4) Results in brackets pertain to nighttime 504 INDIAN J RADIO & SPACE PHYS, DECEMBER 2007 is therefore necessary and useful to estimate attenuation of the radio wave due to cloud over the region, when the cloud water particle density is maximum, as such attenuation results are important to the radio engineers for satellite communication systems. Cloud height is also required to estimate temperature of the cloud. Some results on rain/ rain bearing cloud height in relation to 0 °C isotherm height over various stations 14 are presented in Table 2. In the months of monsoon season, the variation of Hi is not substantial, particularly over inland. This is due to the fact that the rainfall in monsoon is almost evenly distributed throughout India. The study has revealed that Hi is not dependent on topographical features, but it depends more on seasons. It is useful to highlight the results of Hi over the two south-east coastal stations, Chennai and Visakhapatnam, which Fig. 1 Cloud coverage over India on 11 Feb. 2004 by are near Gadanki, where MST radar facility is located. Meteosat-5 During summer, Hi varies between 3.7 and 6 km for frequent in the south-west monsoon, while the Chennai, and between 3.7 and 5.95 km over occurrence of stratiform clouds is predominant in the Visakhapatnam. During monsoon, Hi varies between north-east monsoon. 4.05 and 6.2 km over Chennai, while over Visakhapatnam it varies from 4.2 to 5.9 km. The 3 Rain bearing cloud/rain height distribution over variations of Hi over both these stations in different the Indian subcontinent seasons are insignificant, particularly at low and intermediate probability levels.