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.
