Do Cities Increase Rainfall? Unexpected Rain Patterns Revealed by Satellite Data

Australia’s east coast, including Sydney, has been hit by successive storms, heavy rains, and flash floods. Naturally, this raises a question: does the existence of a city alter the rain that falls over it? This question holds immense importance today, as the majority of the world’s population lives in urban areas. If urbanization even slightly alters rainfall patterns, the implications would ripple through flood countermeasures, stormwater infrastructure design, water supply planning, and urban planning as a whole.

Satellite-Observed Increase in Urban Rainfall

Previous satellite observation data have consistently shown that many cities experience more rainfall events than their surrounding rural areas. The traditional explanation attributes this to the urban heat island effect, the uneven topography, aerosols released into the atmosphere, and changes in land cover, all of which influence storm development and where rain falls. A new study published in Environmental Research Letters sheds light on this issue from a different angle. It examined a fundamental question: do the differences in rainfall patterns shown by satellite data reflect actual changes in rainfall, or are they artifacts of the observation methods themselves?

A Comprehensive Analysis Covering 15 Major

World Cities The research team used NASA’s IMERG (Integrated Multi-satellite Retrievals for GPM) rainfall data to analyze 15 major cities worldwide. The subjects included Sydney and Melbourne, covering cities in different climate zones and geographical conditions, including coastal and inland areas. The results revealed a clear pattern. Rainfall events occurred more frequently over urban areas compared to the surrounding rural regions. Notably, not all storms intensified. The most prominent signal in the satellite data was that it rained for longer durations over urban areas. When focusing on individual rainfall events, the total amount of rain falling over the city center was sometimes even less than that in surrounding regions. In other words, the main urban signal appearing in the IMERG data was “more frequent rainfall,” not “stronger rainfall.”

Two Satellite Sensors Tell Different Stories

Modern satellite rainfall data are constructed by combining both infrared and microwave observations. These two sensor technologies have distinct characteristics and limitations. Infrared sensors estimate rainfall indirectly from the temperature of cloud tops. While they offer the advantage of covering wide areas, they may miss light rain, shallow clouds, and warm rain, as such rainfall can occur even when cloud tops are not very cold. On the other hand, microwave satellites fly in low orbits and detect signals directly related to rain droplets and ice within clouds. They are particularly effective at distinguishing whether rainfall is actually occurring, offering more direct rainfall observation compared to infrared sensors. When the research team analyzed the IMERG data separated by observation type, an interesting discovery emerged. The urban signal was primarily derived from microwave observations; no urban pattern was found in the infrared estimates.

Impact of Observation Methods on Research Results

This finding highlights a critical methodological challenge in urban climate research: the type of sensor used can lead to different conclusions when interpreting satellite data. However, this does not mean the microwave signal is incorrect. Since microwave satellites detect the presence of rainfall more directly, they are more likely capturing the increased rainfall frequency over urban areas. The issue lies in the fact that previous studies have discussed urban rainfall effects without fully accounting for these differences between sensors. In fact, when comparing the scale of individual rainfall events, rainfall over urban areas tended to be less than in surrounding rural areas. This fact suggests that cities do not make all rain “stronger.”

Mechanisms Behind Increased Urban Rainfall

Multiple factors are thought to be complexly involved in the background of increased rainfall frequency over urban areas. First, there is the heat island effect created by asphalt and buildings. Urban areas are warmer than the surrounding natural environment, and this temperature difference can generate updrafts and invigorate convective activity. Second, the urban surface is far more uneven than rural areas. Skyscrapers and other artificial structures alter wind flow and cause atmospheric turbulence, potentially affecting cloud formation and precipitation processes. Third, it has been pointed out that particulate matter (aerosols) contained in car exhaust and factory emissions can act as cloud condensation nuclei. An increase in aerosols can alter the microphysical properties of clouds and potentially affect rainfall patterns. Fourth, replacing forests and farmland with asphalt and concrete can change the amount of evaporation and heat flux from the surface, potentially impacting atmospheric moisture levels and convective activity.

Key is the Integration of Satellite

Technology and Ground Observations Satellite data have become an indispensable tool for understanding rainfall. Rain gauges can accurately measure rainfall at specific points, but they are irregularly distributed, making it difficult to fully capture rainfall variations across large urban areas. Climate models can simulate urban weather in detail, but performing calculations for multiple cities over several decades at resolutions of a few kilometers requires enormous computational costs. Satellite observations play a role in bridging this gap. Satellite rainfall estimation systems like IMERG provide high-resolution, near-global rainfall estimates and are widely used in urban rainfall research. However, as this study shows, careful interpretation of satellite data is necessary. Different sensors can tell different stories, and it is crucial to analyze the data with an understanding of its characteristics.

Improving Accuracy Through Integration with

Ground Observations In the future, it is expected that more accurate understanding of urban rainfall patterns will be possible by integratively utilizing satellite data and ground observation data. Ground observations such as rain gauges and meteorological radars can provide detailed rainfall information that cannot be obtained from satellite data alone. Furthermore, by appropriately combining both microwave and infrared data, it will become possible to understand both aspects of rainfall frequency and rainfall intensity more comprehensively.

Implications for Urban Planning

Accurately understanding the impact cities have on rainfall patterns is extremely important for urban planning and disaster countermeasures. Increased rainfall frequency is directly related to the design of stormwater management systems and the assessment of flood risk. The results of this study correct the previous simplistic understanding that cities make rainfall “stronger.” However, the increase in rainfall frequency itself has been confirmed, and this point must be taken into account in urban planning. In particular, the finding that the main increase is in prolonged, light rainfall rather than short, intense downpours may require different approaches in the design of stormwater infrastructure.

Future Outlook:

Research on the relationship between cities and rainfall still holds many unsolved mysteries. While this study focused on IMERG data, verification using other satellite data and observation methods will also be necessary. Furthermore, further research is demanded on the impact of differences in city size, shape, and climate zone on rainfall patterns. Given that cities worldwide each have different characteristics, it is difficult for a single study to reach conclusions applicable to all cities. However, with advances in satellite technology, the relationship between cities and precipitation is becoming clearer than ever before. If continuous observation and research continue in the future, the full picture of rainfall mechanisms in urban areas will undoubtedly become even more apparent.

Conclusion:

The City’s “Rain-Increasing Power” is Limited What this study revealed is the fact that while rainfall events occur more frequently in urban areas, the scale of individual events is not necessarily larger compared to surrounding rural areas. The urban rainfall signal appearing in the satellite data was primarily obtained from microwave sensor observations, and no similar pattern was confirmed with infrared sensors. This reaffirms the importance of understanding the characteristics of observation methods when interpreting satellite data. The impact cities have on rainfall is not as simple as scientists once thought. However, this impact certainly exists, and as the risk of urban flooding continues to increase, these findings will undoubtedly form a vital foundation for future urban planning and infrastructure development.