Coupling between the land surface and the atmosphere can affect how much and when it rains.

One way coupling has been uncovered in nature is through identifying when the amount of rainfall has been affected by how wet or dry the ground was before the storm. This is known as the ‘soil moisture-rainfall relationship’.

This relationship assumes that rainfall has been affected by the state of the land surface at the same location. It assumes the ground has some influence on the state of the air above it, for example in terms of its temperature and humidity.

But because wind is constantly moving air around, we also have to consider the length of time a parcel of air remains over one section of the ground. If this isn’t considered then it is unclear whether the rainfall in one location was affected by the ground directly beneath it or by ground some distance away. This makes it hard to understand the natural processes that lead to rainfall, and therefore makes it difficult to predict future rainfall.

CLEX researchers tested the importance of the assumption that rainfall has been affected by the state of the underlying land surface. They carried out the test over Australia, since how the wetness or dryness of the ground affects rainfall is currently not well understood in this region.

The researchers found that wetter ground tended to lead to more rainfall (and vice-versa) in northern Australia in the wet and transition seasons. This suggests a positive coupling is present between soil moisture and rainfall.

However, in the south and east of the country in winter, they found the opposite – that drier ground tended to lead to more rainfall (and vice-versa).

The researchers also found that if you neglect the assumption you get a very different answer. Therefore future studies of land-atmosphere coupling that use the soil moisture-rainfall relationship need to uphold the assumption that the wetness of the ground influences the atmosphere directly above it.

The researchers also found that the strength of the land-atmosphere relationship changes when you consider a smaller or larger area. When looking at scales of around 50 km, the relationship between soil moisture and rainfall was stronger than at 250 km.

Since land-atmosphere coupling has previously been studied with global climate models at scales of around 250 km, the result means that as climate models improve their spatial resolution and look at smaller and smaller areas, the strength of coupling around the world may be different to what has been previously estimated.

  • Paper: Holgate, C. M., Van Dijk, A. I. J. M., Evans, J. P., & Pitman, A. J. (2019). The importance of the one dimensional assumption in soil moisture – rainfall depth correlation at varying spatial scales. Journal of Geophysical Research: Atmospheres, 124. https://doi.org/10.1029/2018JD029762