“Imagine a summer thunderstorm with sudden intensive rain showers over a short period. Compare that to rain from widespread cloud cover with lighter showers on and off over several hours. Both types can lead to extreme volumes of rain, but over completely different time periods,” says Peter Berg, a climate scientist at SMHI and one of the researchers behind a study into extreme rain showers published by Nature Geoscience.
The properties of rain showers
The volume of rain that falls in a heavy shower depends on the amount of water vapour in the atmosphere. At higher temperatures the atmosphere may contain more water vapour, thus increasing the chance of heavy showers. The properties of a rain shower are strongly dependent on the time scale being studied, from five minutes to several hours or days, for instance. The various types of precipitation are created in different processes in the atmosphere. This makes this a complex area from a research perspective.
“A summer thunderstorm is a convective type of precipitation, while widespread cloud cover is a stratiform type,” Berg explains by way of illustration.
Varying temperatures affect the type of shower
Alongside Christopher Moseley and Jan O. Haerter, Berg has studied the behaviour of different types of rain shower at varying temperatures, a process described in the latest issue of Nature Geoscience (Berg et. al., 2013).
They conclude that convective precipitation (as in summer thunderstorms) intensified far more quickly at elevated temperatures, especially temperatures around 12 to 20°C.
When they studied connected periods of steady rain, they found that the heavy convective showers become more intensive, and also that they maintain this high intensity for their entire duration. Rain from more widespread precipitation areas did not show any obvious structure at altered temperatures.
Heavy rain has far-reaching consequences for nature and society alike. Heavy rain showers also present a greater threat at higher temperatures, as they maintain their high intensity over a more prolonged period.
Processes behind heavy rain should be studied
Although the researchers have looked at extreme rain showers at different temperatures, the results cannot be directly applied in climate change scenarios. They encourage further investigation with high-resolution cloud and climate models which describe the processes involved in heavy rainfall in more detail.