A COMPARATIVE STUDY OF VERTICAL PROFILE OF RAIN DROPS FALL VELOCITY DISTRIBUTION IN AKURE, SOUTH WEST NIGERIA

Abstract

Virtually all the early investigators on vertical profile of raindrop fall velocity distribution carried out their research on convective and stratiform rain types and more researches are needed to discern and separate these rain types into drizzle, widespread, shower and thunderstorm. Again, all these early researchers carried out their measurement in temperate regions and there seems to be few of such measurements in tropics. In this study, the vertical profile of the distribution of rain drops fall velocity has been carried out using the data obtained from a vertically looking Micro Rain Radar (MRR) located at Akure (7.13oN 5.17oE ), a tropical location in South-West Nigeria. The variation of rain drops fall velocity with diameter, height and radar reflectivity for drizzle, widespread, shower and thunderstorm rain types are analysed. Three years of data were utilized for the study. Comparison of the observed raindrop size distribution shows that in all the rain types considered, velocity of fall of rain drops increases with an increase in the diameter of the rain drops. Further analysis based on different times of the day show that the rain drops fall velocity increases with height for all rain types. The distribution of shower and thunderstorm is concentrated in the diameter range of 0.4 mm to 1.6 mm while that of drizzle and widespread concentrates in the diameter range of less than 0.8 mm. One striking observation from the velocity at certain rainfall diameters is that, large rain drops tends to have sub – terminal fall velocity and small drops super – terminal fall velocity statistically, with negative deviation in large drops being much larger than the positive deviation in small drops. The result obtained can be used for effective communication links budget planning in order to build a data base of radio meteorological data for microwave propagation in the tropical region.