Supplementary Information for The role of the permanent wilting point in controlling the spatial distribution of precipitation

Significance One basic distinction between land and ocean is that the land can dry out. We show that this is of fundamental importance for the precipitation distribution over land as it brings precipitation from the precipitating region to the nonprecipitating region. This process prevents the land–atmosphere system from sustaining precipitation over the same region and thus acts against drought or the formation of desert. Paradoxically, although dry atmospheres are known to hamper moist convection, drying the soil to its permanent wilting point generates circulations that are strong enough to overcome this inhibition. Our findings help understand why tropical rain bands broaden poleward over land, the more so the drier the soils are. Convection-permitting simulations on an idealized land planet are performed to understand whether soil moisture acts to support or impede the organization of convection. Initially, shallow circulations driven by differential radiative cooling induce a self-aggregation of the convection into a single band, as has become familiar from simulations over idealized sea surfaces. With time, however, the drying of the nonprecipitating region induces a reversal of the shallow circulation, drawing the flow at low levels from the precipitating to the nonprecipitating region. This causes the precipitating convection to move over the dry soils and reverses the polarity of the circulation. The precipitation replenishes these soils with moisture at the expense of the formerly wet soils which dry, until the process repeats itself. On longer timescales, this acts to homogenize the precipitation field. By analyzing the strength of the shallow circulations, the surface budget with its effects on the boundary layer properties, and the shape of the soil moisture resistance function, we demonstrate that the soil has to dry out significantly, for the here-tested resistance formulations below 15% of its water availability, to be able to alter the precipitation distribution. We expect such a process to broaden the distribution of precipitation over tropical land. This expectation is supported by observations which show that in drier years the monsoon rains move farther inland over Africa.