Relationship between surface thermodynamic contrasts and precipitation intensity in idealised monsoon simulations

While the traditional view of monsoons as continental sea breezes generated by land-sea contrasts was shown to have serious limitations, several competing alternative frameworks look promising. Within this debate, it remains unclear if the surface temperature contrast matters at all for the monsoon precipitation, and why there is a non-linear intensification of precipitation intensity with surface temperature forcing.

 

Idealised studies such as aquaplanets often help improve our understanding of basic mechanisms. But there are very few idealised simulation studies of monsoons at high resolution. Therefore, to determine if monsoon non-linearities with surface forcing come from convective processes, dynamical feedbacks, or from non-linearities in the forcing themselves, we devise a modular framework to simulate idealised monsoons at convection-permitting resolution with the WRF model, in a domain based on an aquapatch (mini-aquaplanet), but in which we can gradually add more realistic components, such as an interactive land surface. The model is forced by a season-dependent meridional contrast of surface temperature, with comprehensive physics and rotation. We compare a series of aquapatch experiments with increasingly intense smooth sea surface temperature forcings with another series including land with increasingly sharp surface temperature contrasts at the land-ocean interface.

 

By relating forcing to responses, we aim to describe the non-linearity of the relationship between surface temperature gradient (or surface Moist Static Energy (MSE) gradient, or low-level wind), and precipitation intensity (or monsoonal precipitation surface area, or monsoon onset timing). This should help clarify the actual role that surface temperature and MSE gradients play in controlling monsoon precipitation, and could potentially hint at the effect of climate change on monsoons.