When the plants wilt, the rain comes

Interactions between organized convection, soil moisture and their associated shallow circulations. Black arrow (left) for the radiatively driven circulation and grey arrows (right) for the soil moisture-induced circulation. Blue (red) shows positive (negative) soil moisture anomaly, the darker the color, the stronger the anomaly. The radiatively driven circulation maintains the convection to the previously precipitating region (left) but, as the soil dries out (right) the convection and its rain move to the previously non-precipitating region.

One basic distinction between land and ocean is the fact that the land can dry out. In a recent study, Dr Cathy Hohenegger and Prof Bjorn Stevens from the department “The Atmosphere in the Earth System” at the Max Planck Institute for Meteorology (MPI-M) demonstrated that this is of fundamental importance for the spatial distribution of precipitation over land as it brings precipitation from the previously precipitating to the non-precipitating regions. This process prevents the land-atmosphere system to sustain precipitation over the same region as deep oceans do. Against initial expectations, no evidence of a positive feedback between soil moisture and precipitation, by which the increase in soil moisture in the precipitating region would conspire to maintain precipitation in the precipitating region, is found.

For their investigations, the scientists used an idealized land planet made of a homogeneous land surface free of vegetation. For this simple situation they explored how radiation, convection and soil moisture interacted to influence and eventually determine the precipitation distribution. They found that the spatial distribution of precipitation is fully determined by the interplay between shallow circulations that spin up in the atmospheric boundary layer. On the one hand, differences in radiative cooling within the atmosphere between non-precipitating and precipitating regions lead to a shallow circulation from the relatively dry, non-precipitating, to the moist, precipitating, region. This circulation pushes the precipitation together, leads to a localization of the precipitation over a specific region of the planet and conspires to maintain the precipitation over that region as time goes by. On the other hand, as time goes by, the soil in the non-precipitating region also dries out. This leads to a strong temperature gradient between precipitating and non-precipitating regions, a gradient that generates another shallow circulation, akin to a sea breeze, directed in the opposite sense as the circulation driven by atmospheric radiative cooling; namely the near surface flow is oriented away from the cold precipitating region and toward the warm non-precipitating region. This acts to disaggregate the convection and bring precipitation from the previously precipitating to the non-precipitating region. The authors demonstrate theoretically that upon approaching its wilting point, the soil’s surface temperature increases dramatically which drives a corresponding exponential increase in the strength of the surface driven circulation, an increase that succeeds in reversing the polarity of the original circulation. Paradoxically, although dry atmospheres are known to hamper moist convection and hence the formation of precipitation, drying the soil to its permanent wilting point thus generates circulations that are strong enough to overcome this inhibition and bring moisture from the moist regions to the dry regions.

The findings help understand why tropical rain belts broaden poleward over land, the more so the drier the soils are. Moreover, they may explain why global climate models with parameterized convection have difficulties in reproducing the extent and seasonal propagation of tropical rain belts given that such models have notorious difficulties in representing the interactions between convection and all but the largest scale circulation systems. Finally, the results indicate that drying of the soil ultimately acts to resist the formation of deserts or maintenance of droughts.

Original publication:
Hohenegger, C. and B. Stevens, 2018: The role of the permanent wilting point in controlling the spatial distribution of precipitation. Proceedings of the National Academy of Sciences, 1-6. doi:10.1073/pnas.1718842115.

Contact:
Dr Cathy Hohenegger
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 302
Email: cathy.hohenegger@we dont want spammpimet.mpg.de