A research team has been studying oxygen isotopes in different types of rainfall, for example, heavy downpours and widespread low-intensity rain. They identified a strong link between the type of precipitation and the relationship between the two oxygen isotopes 16O and 18O over large parts of the world. The oxygen isotope 18O is more common in heavy downpours.
“Some years ago I worked on a study involving heavy downpours and more widespread rainfall areas, where we looked at how precipitation changes with rising temperatures. We saw that the heavy downpours intensified very rapidly with increasing temperatures. There is value in being able to separate the precipitation types to see how the climate is changing, as many studies indicate that heavy downpours are on the rise as a result of global warming,” says Peter Berg, researcher in precipitation and climate at SMHI.
Enables climate studies
The study in question has been headed by the International Atomic Energy Agency, (IAEA) in Vienna and has also been based on research from Canada, Sweden and the United States. The researchers analysed data from the monitoring system that was created by the IAEA and the World Meteorological Organization WMO to study radioactive outcomes from atmospheric nuclear bomb tests combined with earlier precipitation studies, including research from SMHI. The new findings open up new possibilities for the study of how the distribution between the two types of precipitation vary in a changed climate, and for analysing historical changes and to further develop climate models.
“This new knowledge makes it possible to use a range of historical isotope measurements to study how the precipitation has changed character,” says Peter Berg.
The various types of precipitation draw water vapour from various altitudes in the atmosphere. As the isotopic composition is different at the surface compared to higher up in the atmosphere, and the fact that 16O evaporates more easily while 18O easier is prone to condense, this all affects the ultimate composition of the raindrops.
A number of climate change studies reveal an increase in extreme precipitation which is frequently linked to severe convective storms. This new discovery makes it possible to increase the understanding of the climate variation in precipitation processes and can therefore also support the development of climate models.
Differences between the different types of rain
Precipitation is formed in two basic atmospheric processes:
- convective precipitation (short, frequent sharp bursts over a smaller area) is created through strong upward winds that lift tropospheric water vapour many kilometres up into the atmosphere with phase transitions between water and ice
- stratiform precipitation (large-scale rainfall over widespread areas) created by weak, large-scale rising air masses with little vertical extent and with the origin of the water vapour a couple of kilometres up in the atmosphere.