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 change...

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Bibliographic Details
Published in:Geophysical research letters 2020-07, Vol.47 (14), p.n/a
Main Authors: Li, Yafei, Fowler, Hayley J., Argüeso, Daniel, Blenkinsop, Stephen, Evans, Jason P., Lenderink, Geert, Yan, Xiaodong, Guerreiro, Selma B., Lewis, Elizabeth, Li, Xiao‐Feng
Format: Article
Language:English
Subjects:
Air
Amplification
Atmospheric models
Dry air
Extreme weather
Heavy rainfall
Hourly rainfall
Modelling
Moisture
Rain
Rainfall
Rainfall intensity
Rural areas
Surface wind
Tropical environments
Uplift
Urban areas
Urban heat islands
Urbanization
Winds
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Summary: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. Plain Language Summary Major floods and rainfall‐related impacts are often caused by short‐duration heavy rainfall events. Although there is evidence of cities modifying rainfall in many urban areas, uncertainties still exist around their role in intense rainfall episodes. We investigate the impact of the growth of Kuala Lumpur (Malaysia) on intense rainfall using observations and modeling experiments. We find that over the last three decades hourly rainfall events have become more intense over the city than surrounding rural areas. Our modeling experiments support this finding and help us understand mechanisms behind the intensification. The relative warmth of the city with respect to its surroundings contributes to the increase. The city creates a low‐level anomaly of warm and dry air that then rises. To compensate for this, the moist surrounding air is brought into the urban area and lifted upward. This feeds the air above the city with moisture and sustains a local circulation initiated by the relative warmth of the urban area. We find that the city's influence on extreme rainfall is located over the urban area itself, as opposed to other studies that have detected a footprint downwind. This is likely due to the typical calm background wind conditions in the tropics. Key Points Observed hourly rainfall extremes have intensified more in urban Kuala Lumpur than the surrounding rural areas over the last three decades Convection‐modeling experiments provide further support that this intensification comes from urbanization, providing physical mechanisms Urbanization increases th
ISSN:0094-8276
1944-8007
DOI:10.1029/2020GL088758