Characterisation of Urban Rainfall in Kuala Lumpur, Malaysia

Sammanfattning: Rainfall characteristics of Kuala Lumpur, Malaysia, are studied in space and time; on a large scale, 550 km2, and long term scale to find out the monsoon influence, and on urban basin scale, 23 km2, to determine dynamic properties of rainfalls and spatial correlations, and on a point scale to determine the probabilities of very high rain intensities. The importance of spatial and temporal variability of rainfall was investigated applying cross correlation on long term historical rainfalls from Kuala Lumpur, Malaysia. Data from six rain gauges distributed over an area of 450 km2 were used. The annual and monthly rainfall correlation structures depict a pronounced effect of the monsoon. The variability of rainfall is parallel to the monsoon direction, which is perpendicular to the sea coast. The coefficient of variation for the monthly rainfalls is higher than for the annual rainfalls by a factor of two. Eight short term rainfall storms of 60 minute duration over the 23 km2 urban basin were studied to derive the average areal reduction factor curve. The results indicate that the structure of single rainfall cells is not very different from that in other climate regions. The areal reduction was well pronounced for a small urban catchment used in this study. Dynamic properties of 30 fully synchronised short term rainfall storms were investigated. The results showed significant correlation between storm velocities and observed wind velocities on the ground. The storm movements were rather slow. There was no correlation between storm movement and high altitude wind direction. The average storm velocity was 2.6 m/s compared to a mean velocity of 12 m/s obtained in high latitude regions by other investigators. A two-dimensional advection diffusion model was applied to two short term rainfall events for predicting rainfall intensities of one, three and five minutes ahead. The results enabled a reasonable physical explanation of model parameters. In addition, advection of the two rainfall events was carried out. However the results were not promising for prediction purposes. Partial duration series of short term rainfall storms recorded by seven rain gauges were analysed and empirical relationships for rainfall depths and intensity ratios as dependent variables against duration time as the independent variable were determined. The intensity-duration-frequency and the depth-duration-frequency relationship were applied to a surrounding area. Results showed that a modified intensity-duration-frequency relationship accounting for hourly rainfall intensities at the position under consideration is better than the depth-duration-frequency relationship.