After another spell of wet weather along Australia’s east coast, with storms, heavy rain and flash flooding across Sydney and parts of New South Wales, it is natural to ask whether our cities are shaping the rainfall that descends upon them.
This matters because most people now live in cities. If urbanisation changes rainfall, even slightly, the effects can reach large populations through flooding, stormwater design, water supply and infrastructure planning.
Satellite data have consistently shown that many cities experience more rain events than the countryside around them. The usual explanation is that cities themselves are involved: urban heat, rougher surfaces, aerosols and changed land cover can all affect how storms develop and where rain falls.
Our new study, published in Environmental Research Letters, asks a related question: how much of this data reflects real changes in rainfall, and how much depends on how we observe it?
Why we need satellites
Understanding rainfall over cities is hard.
Rain gauges accurately measure rainfall at a specific location, but are irregularly distributed and cannot fully capture how rain varies across a large city. Climate models can simulate urban weather in detail, but kilometre-scale simulations across many cities and decades remain computationally expensive.
Satellite observations help fill this gap.
NASA’s Integrated Multi satellite Retrievals for GPM, known as IMERG, provides near-global rainfall estimates at high resolution, and is now widely used for studying rainfall over cities.
What the satellite data shows
We examined IMERG rainfall data across 15 of the world’s largest cities, including Sydney and Melbourne. The cities span different climates and geographic settings, including both coastal and inland regions.
A clear pattern emerged. Rain events occurred more often over urban areas than over nearby rural ones. The strongest signal was not that every storm became stronger, but that satellites counted more hours in which it was raining over cities. Individual events over urban centres often dropped less water than those in surrounding areas.
In other words, the main urban signal in IMERG is more frequent rain, not heavier rain.
Different sensors, different stories
Modern satellite rainfall data combines both infrared and microwave observations.
Infrared sensors estimate rainfall indirectly from the temperature at the top of clouds. They provide broad coverage, but can miss light, shallow or warm rain because these can occur even when the tops of the clouds are not very cold.
Microwave satellites fly in low orbit and detect signals more directly linked to raindrops and ice inside clouds, making them particularly useful for identifying whether rain is actually occurring.
When we separated the IMERG data by observation type, the urban signal mainly came from microwave observations, while infrared estimates showed no urban pattern.
This does not mean the microwave signal is wrong, but it raises a potential problem for long-term studies: microwave observations have changed over time. New satellites have been launched and older ones retired,