volume | PIER Journals

Abstract

The quest to understand the variation of rainfall microstructures at subtropical and equatorial regions is vital to rain attenuation studies. In this study, point rainfall datasets obtained at Butare (2°36'S, 29°44'E) and Durban (29°52'S, 30°58'E), are compared at the reflectivity threshold of 38 dBz. Joss- Walvogel (JW) distrometer measurements collected from these two locations represent physical rainfall data from equatorial and subtropical climates respectively. The reflectivity threshold enables the classification of rainfall datasets into stratiform and convective (S-C) precipitation regimes. These thresholds, Rth, at Durban and Butare are analysed based on three known rainfall microphysical parameters: rain rate, rainfall Drop Size Distribution (DSD) and radar reflectivity. The results from rain rate distributions at the both regions are similar for both stratiform and convective classes. However, the sampled DSDs indicate the dominance of larger rain droplets at Butare compared to observations at Durban, irrespective of the rain classes. In addition, it is found that the reflectivity distributions at both regions, under stratiform and convective conditions, are distinct in their probability profiles. The overall S-C analysis implied that the structures of the reflectivity and DSD profiles at both regions - result in significant variation of predicted specific attenuation - at microwave and millimeter band. In comparison with other global locations, it is affirmed that the S-C transition occurs globally at rain rates between 6 mm/h and 13 mm/h.

1. Ajayi, G. O., S. Feng, S. M. Radicella, and B. M. Reddy, Handbook on Radiopropagation Related to Satellite Communications in Tropical and Subtropical Countries, 7-14, ICTP, Trieste, 1996.

2. Crane, R. K., "Electromagnetic Wave Propagation through Rain," John Wiley, New York, 1-40, 1996. Google Scholar

3. Aydin, K. and S. E. A. Daisley, "Relationships between rainfall rate and 35-GHz attenuation and differential attenuation: Modeling the effects of raindrop size distribution, canting and oscillation," IEEE Trans. Geosci. Remote Sens., Vol. 40, No. 11, 2343-2352, 2002.
doi:10.1109/TGRS.2002.805073 Google Scholar

4. Das, S., A. Maitra, and A. K. Shukla, "Rain attenuation modeling in the 10-100 GHz frequency using drop size distributions for different climatic zones in tropical India," Progress In Electromagnetics Research B, Vol. 25, 211-224, 2010.
doi:10.2528/PIERB10072707 Google Scholar

5. Chen, K. S., C. Y. Chu, and Y. C. Tseng, "A semi-empirical model of rain attenuation at Ka-band in Northern Taiwan," Progress In Electromagnetic Research M, Vol. 16, 213-223, 2011. Google Scholar

6. Tenorio, R. S., M. C. Da Silva Moraes, and B. H. Kwon, "Raindrop distribution in the eastern coast of northeastern brazil using disdrometer data," Revista Brasileira de Meterologia, Vol. 25, No. 4, 415-426, 2010.
doi:10.1590/S0102-77862010000400001 Google Scholar

7. Kumar, L. S., Y. H. Lee, J. X. Yeo, and J. T. Ong, "Tropical rain classification and estimation of rain from Z-R (Reflectivity-Rain Rate) relationships," Progress In Electromagnetic Research B, Vol. 32, 107-127, 2011.
doi:10.2528/PIERB11040402 Google Scholar

8. Bartholomew, M. J., Disdrometer and Tipping Bucket Raingauge Handbook, DOE/SC-ARM/TR-079, ARM Climate Research Facility, 2009.
doi:10.2172/1019411

9. Tokay, A., D. A. Short, C. R. Williams, W. L. Ecklund, and K. S. Gage, "Tropical rainfall associated with convective and stratiform clouds: Intercomparison of disdrometer and profiler measurements," J. Appl. Metor., Vol. 38, No. 3, 302-320, 1999.
doi:10.1175/1520-0450(1999)038<0302:TRAWCA>2.0.CO;2 Google Scholar

10. Wilson, C. L. and J. Tan, "The characteristics of rainfall and melting layer in Singapore: Experimental results from radar and ground instruments," 11th International Conference on Antennas and Propagation, Vol. 480, 852-856, 2001.
doi:10.1049/cp:20010416 Google Scholar

11. Houze, R. A., "Stratiform precipitation in regions of convection: A meteorological Paradox?," Bulletin of the American Meteorological Society, Vol. 78, No. 10, 1997.
doi:10.1175/1520-0477(1997)078<2179:SPIROC>2.0.CO;2 Google Scholar

12. Anagnostou, E. N., "A convective/stratiform precipitation classification algorithm for volume scanning weather radar observations," Meteorol. Appl., Vol. 11, 291-300, 2004.
doi:10.1017/S1350482704001409 Google Scholar

13. Begum, S., C. Nagaraja, and I. Otung, "Analysis of rain cell size distribution for application in site diversity," First European Conference on Antennas and Propag. (EuCAP 2006), 1-5, 2006.
doi:10.1109/EUCAP.2006.4584777 Google Scholar

14. Houze, R. A., "A climatological study of vertical transports by cumulus-scale convectio," J. Atmos. Sci., Vol. 30, 1112-1113, 1973.
doi:10.1175/1520-0469(1973)030<1112:ACSOVT>2.0.CO;2 Google Scholar

15. Awaka, J., T. Iguchi, H. Kumagai, and K. Okamoto, "Rain type classification algorithm for TRMM precipitation radar," Proceedings of the IEEE 1997 International Geosci. Remote Sen. Sym., 1636-1638, 1997. Google Scholar

16. Gamache, J. F. and R. A. Houze, "Mesocale air motions associated with tropical squall line," Monthly Weather Review, Vol. 110, 118-135, 1982.
doi:10.1175/1520-0493(1982)110<0118:MAMAWA>2.0.CO;2 Google Scholar

17. Specifications of Distrometer RD-80, 2011, , http://www.distromet.com/98. Google Scholar

18. Alonge, A. A. and T. J. Afullo, "Rainfall microstructures for microwave and millimeter wave link budget at tropical and subtropical sites," 2013 IEEE Africon Conference, Le Meridien ile Maurice, Mauritius, Sep. 9th-12th, 2014. Google Scholar

19. Marshall, J. S. and W. M. Palmer, "The distribution of raindrops with size," J. of Atmos. Sci., Vol. 5, 165-166, 1948. Google Scholar

20. Battan, L. J., Radar Observations of the Atmosphere, 323, University of Chicago Press, 1973.

21. Feingold, G. and Z. Levin, "The lognormal fit to raindrop spectra from frontal convective clouds in Israel," J. Climate Appl. Meteor., Vol. 25, 1346-1363, 1986.
doi:10.1175/1520-0450(1986)025<1346:TLFTRS>2.0.CO;2 Google Scholar

22. Ochou, A. D., A. Nzekou, and H. Sauvageot, "Parameterization of drop size distribution with rain rate," Atmos. Res., Vol. 84, 5-66, 2007. Google Scholar

23. Sauvageot, H. and J. P. Lacaux, "The shape of averaged drop size distributions," J. Atmos. Sci., Vol. 52, 1070-1083, 1995.
doi:10.1175/1520-0469(1995)052<1070:TSOADS>2.0.CO;2 Google Scholar

24. Nzekou, A., H. Sauvageot, A. D. Ochou, and C. M. F. Kebe, "Raindrop size distribution and radar parameters at Cape Verde," J. Appl. Meteorol., Vol. 43, 90-105, 2004.
doi:10.1175/1520-0450(2004)043<0090:RSDARP>2.0.CO;2 Google Scholar

25. Houze, R., B. F. Smull, and P. Dodge, "Mesoscale organization of springtime rainstorms in Oklahoma," Monthly Weather Review, Vol. 118, 613-654, 1990.
doi:10.1175/1520-0493(1990)118<0613:MOOSRI>2.0.CO;2 Google Scholar

26. Kozu, T. and K. Nakamura, "Rainfall parameter estimation from dual-radar measurements combining re°ectivity profile and path-integrated attenuation," J. of Atmos. and Oceanic Tech., 259-270, 1991.
doi:10.1175/1520-0426(1991)008<0259:RPEFDR>2.0.CO;2 Google Scholar

27. Ulbrich, C. W., "Natural variations in the analytical form of the raindrop size distribution," J. Climate Appl. Meteor., Vol. 22, 1764-1775, 1983.
doi:10.1175/1520-0450(1983)022<1764:NVITAF>2.0.CO;2 Google Scholar

28. Liebe, H. J., G. A. Hufford, and T. Manabe, "A model for the complex permittivity of water at frequencies below 1 THz," Inter. J. of Infrared and Millimeter Waves, Vol. 12, No. 7, 659-678, 1991.
doi:10.1007/BF01008897 Google Scholar

29. Adetan, O. E. and T. J. Afullo, "Raindrop size distribution and rainfall attenuation modeling in equatorial subtropical Africa: Critical diameters," Annals des Telecommunication, 1-13, 10.1007/s12243-013-0418-z, 2014. Google Scholar

30. ITU-R Rec. P.838-3 "Specific attenuation model for rain for use in prediction methods,", ITU-R, Geneva, 2005. Google Scholar

31. Ajayi, G. O. and R. L. Olsen, "Modeling of a tropical raindrop size distribution for microwave and millimeter wave application," Radio Science, Vol. 20, No. 2, 193-202, 1985.
doi:10.1029/RS020i002p00193 Google Scholar

32. Moumouni, S., M. Gosset, and E. Houngninou, "Main features of rain drop size distributions observed in Benin, West Africa, with optical disdrometers," Geophysical Research Letters, Vol. 35, L23807, 2008.
doi:10.1029/2008GL035755 Google Scholar

33. Maitra, A., "Rain attenuation modeling from measurements of rain drop size distribution in the Indian region," IEEE Antennas Propag. Letters, Vol. 3, 180-181, 2004.
doi:10.1109/LAWP.2004.833979 Google Scholar

34. Tokay, D. and A. Short, "Evidence from tropical raindrop spectra of the origin of rain from stratiform versus convective clouds," J. Appl. Meteor., Vol. 35, No. 3, 355-371, 1996.
doi:10.1175/1520-0450(1996)035<0355:EFTRSO>2.0.CO;2 Google Scholar

35. Short, D. A., T. Kozu, and K. Nakamura, "Rain rate and raindrop size distribution observations in Darwin," Australia Proc. Open Symp. on Regional Factors in Predicting Radiowave Attenuation Due to Rain, 35-40, URSI, 1990. Google Scholar

36. Stout, G. E. and E. A. Mueller, "Survey of relationships between rainfall rate and radar reflectivity in the measurement of precipitation," J. of Appl. Meteor., Vol. 7, 165-174, 1968.
doi:10.1175/1520-0450(1968)007<0465:SORBRR>2.0.CO;2 Google Scholar

37. ITU-R Rec. P.837-6 "Characteristics of precipitation for propagation modelling,", ITU-R, Geneva, 2012. Google Scholar

Dr. Akintunde Ayodeji Alonge

Prof. Thomas Joachim Odhiambo Afullo