Implicit learning of convective organization explains precipitation
stochasticity
Abstract
Accurate prediction of precipitation intensity is of crucial importance
for both human and natural systems, especially in a warming climate more
prone to extreme precipitation. Yet climate models fail to accurately
predict precipitation intensity, particularly extremes. One missing
piece of information in traditional climate model parameterizations is
sub-grid scale cloud structure and organization, which affects
precipitation intensity and stochasticity at the grid scale. Here we
show, using storm-resolving climate simulations and machine learning,
that by implicitly learning sub-grid organization, we can accurately
predict precipitation variability and stochasticity with a low
dimensional set of variables. Using a neural network to parameterize
coarse-grained precipitation, we find mean precipitation is predictable
from large scale quantities only; however, the neural network cannot
predict the variability of precipitation (R 2 ∼ 0.4) and underestimates
precipitation extremes. Performance is significantly improved when the
network is informed by our novel organization metric, correctly
predicting precipitation extremes and spatial variability (R 2 ∼ 0.95).
The organization metric is implicitly learned by training the algorithm
on high-resolution precipitable water, encoding organization degree and
humidity amount at the subgrid-scale. The organization metric shows
large hysteresis, emphasizing the role of memory created by sub-grid
scale structures. We demonstrate this organization metric can be
predicted as a simple memory process from information available at the
previous time steps. These findings stress the role of organization and
memory in accurate prediction of precipitation intensity and extremes
and the necessity of parameterizing sub-grid scale convective
organization in climate models to better project future changes in the
water cycle and extremes.