Urban friction could strengthen landfalling tropical cyclone precipitation

Landfalling tropical cyclones (TCs) can pose substantial threats to densely populated and highly developed cities on the North Pacific and North Atlantic coasts. For example, Hurricane Harvey, which occurred in 2017, impacted Houston in the U.S. with record-breaking rainfall and flooding and caused over 80 fatalities and $125 billion in losses.

However, despite the vulnerability of urban areas to the extreme precipitation and flooding caused by landfalling TCs, scientists have paid little attention to how the precipitation of landfalling TCs responds to the underlying urban environment and related physical mechanisms.

In a new study conducted by researchers at Nanjing University of Information Science and Technology, China, it was found that heavy rainfall induced by landfalling TCs can be strengthened by the urban environment. The study was based on high-resolution ensemble simulations of Typhoon Rumbia, which swept through the Yangtze River Delta urban agglomeration in China in 2018. Further data analysis quantified the contributions from urban dynamic and thermodynamic drivers, revealing a dominant role played by urban friction in enhancing TC rainfall.

Results showed that the inner-core rainfall of Rumbia was strengthened by approximately 10% under the impact of the urban area close to where the TC made landfall. More specifically, the frictionally induced upward motion was found to play a decisive role in enhancing the rainfall by decelerating the tangential wind and strengthening the radial wind within the boundary layer, thereby enhancing the upward motion.

The results demonstrated that urban surface friction and related physical mechanisms make the most significant contribution to the enhancement of rainfall produced by landfalling TCs.

"Our study indicates that, although coastal urban areas only account for a relatively small proportion of the underlying surface covered by TCs, their unique frictional effect could significantly strengthen the rainfall that they produce," says Prof. Haishan Chen, the corresponding author of the study from Nanjing University of Information Science and Technology, Nanjing, China.

"Moreover, it can explain why cities on the North Pacific and North Atlantic coasts are extremely vulnerable to natural hazards induced by landfalling TCs. The results also imply that ongoing urbanization in coastal areas is highly likely to exacerbate the risks of TC-related natural disasters."

The study is published in Advances in Atmospheric Sciences.