SEASONAL DYNAMICS AND PATTERNS OF RAINSTORMS OVER IBADAN, NIGERIA

Abstract

Rainstorm is one of the major hazards affecting urban areas in tropical region. Previous studies have largely characterised variations in rainstorm based on limited data from synoptic stations that are not evenly distributed over the city of Ibadan, hence their inability to analyse the seasonal dynamics and patterns of rainstorm at metropolitan level. This study analysed the seasonal variations in rainstorms duration, rainfall amount and speed of rainstorms on Areal Coverage (AC) of rainstorms with a view to understanding factors that account for seasonal variations in Ibadan metropolis, Nigeria.

The concept of the atmospheric circulation provided the framework. A 3x3 km grid was superimposed on the map of Ibadan metropolis and one raingauge was installed in each of the 50 resultant grids. Data on rainstorms duration (mins), rainfall amount (mm) and speed (ms-1) were recorded daily from the 50 weather stations during 2013 and 2014 rainstorm events. Data were aggregated on seasonal basis. Rain-producing weather systems were classified into thunderstorms and disturbance lines. The AC of rainstorms (km2) was measured using square method. Descriptive statistics, Multiple regression, and student t-test were used to analyse the data at p≤0.05.

Duration of rainstorms due to thunderstorm was 107±56 and 98±47 mins, rainfall amount was 15.1±9.8 and 12.5±9.3 mm, speed of rainstorms was 6.5±14.8 and 10.2±24.3 ms-1, while AC of rainstorms was 169.5±126.7 and 174.0±86.7 km2 in 2013 and 2014 respectively. Duration of rainstorms due to disturbance lines was 90±41 and 175±95 mins, rainfall amount was 12.3±9.1 and 13.6±8.2 mm, speed was 17.6±26.2 and 7.2±7.6 ms-1, and AC was 164.6±140.6 and 181.0±118.4 km2. Rainstorms duration, rainfall amount and speed of rainstorms due to thunderstorm during the early rainy seasons jointly accounted for 39.2% and 90.0% of the variation in the AC of rainstorms in 2013 and 2014 respectively, while rainstorms duration, rainfall amount and speed of rainstorms due to thunderstorm during the late rainy seasons jointly accounted for 52.1% and 14.4% of the variation in the AC of rainstorms in 2013 and 2014. Rainstorms duration, rainfall amount and speed of rainstorms due to disturbance lines during the early and late rainy seasons jointly accounted for 18.4% and 61.7% of the variation in the AC of rainstorms in 2013 and 2014. There were no significant differences in the mean speed of rainstorms due to thunderstorm between the early and late rainy seasons in 2013 and 2014. There were significant differences in the mean AC of rainstorms due to thunderstorm between the early and late rainy seasons in 2013 (t=3.36) and 2014 (t=3.82). There were no significant differences in the mean speed and AC of rainstorms due to disturbance lines between the early and late rainy seasons in 2013 and 2014. There were no significant differences in the mean rainstorms duration, rainfall amount, speed and AC of rainstorms due to thunderstorm and disturbance lines.

Rainstorms duration and rainfall amount influenced areal coverage of rainstorms in Ibadan metropolis in 2013 and 2014. There is need for sustainable city planning and flood management.

Keywords: Rain-producing weather systems, Thunderstorm and disturbance lines, Rainstorm dynamics in Ibadan, Nigeria

Word count: 496