Strong intensification of hourly rainfall extremes by urbanization.

Although observations and modeling studies show that heavy rainfall is increasing in many regions, how changes will manifest themselves on sub‐daily timescales remains highly uncertain. Here, for the first time, we combine observational analysis and high‐resolution modeling results to examine changes to extreme rainfall intensities in urbanized Kuala Lumpur, Malaysia. We find that hourly intensities of extreme rainfall have increased by ~35% over the last three decades, nearly 3 times more than in surrounding rural areas, with daily intensities showing much weaker increases. Our modeling results confirm that the urban heat island effect creates a more unstable atmosphere, increased vertical uplift and moisture convergence. This, combined with weak surface winds in the Tropics, causes intensification of rainfall extremes over the city, with reduced rainfall in the surrounding region.

Figure 2. Urban gauges show a more rapid increase in hourly rainfall extremes than rural ones during the last three decades. Ten‐year rolling averages of the Q95 of hourly rainfall for (a) each urban station and (b) rural station, and the mean of the gauges. By Q95, we refer to the mean hourly intensity of the declustered events above the 0.95 quantile. See Figure 1 in the main text and Table S1 for locations and other station information. The station IDs are shown in the legend.