A mechanical energy perspective on the lifecycle of heavy rain-producing storm systems

Extreme precipitations from convective storms produce significant harm to society and are expected to become more intense under global warming. Here we examine which storms are conducive to extreme precipitations from the perspective of the mechanical energy expended to build up such storms. Indeed, studies have indicated that about 70% of the mechanical energy of convection is used to lift water, leaving only 30% available for kinetic energy production and the maintenance of convective motions. Here we show that this partitioning holds at the scale of individual storm systems, meaning that the weight of water must have a dramatic impact on storm energy. In particular, storms must build up significant mass by performing significant work against gravity before they can produce the most extreme precipitations. The situation is complicated by the strong diversity across storm systems, characterized by varying durations and varying stages within the lifecycle of those storms. We use kilometer-scale simulations in radiative-convective equilibrium and a convective tracking algorithm to study the mechanical energy budget of storms associated to extreme percentiles of the precipitation distribution. We find that a dominant driver of the storm's ability to produce extreme precipitation is the time span during which the work done to lift water exceeds the dissipation of potential energy through precipitation, thus leading to mass build up inside the storm. Systems generating exceedances of the 99.999th precipitation percentile can accumulate mass during 4-6 hours while more typical precipitating systems only do so over 1-2 hours. Irrespective of storm duration, we find that the fraction of mechanical energy dissipated against gravity stays roughly constant at ~70% throughout the storm lifecycle, creating a drag that must be overcome to produce precipitation extremes. These results improve prospects for more accurate predictions of precipitation extremes from observations.