  Precipitation extremes intensify in most regions in climate-model
projections. Changes in vertical velocities contribute to the changes in
intensity of precipitation extremes but remain poorly understood. Here, we find
that mid-tropospheric vertical velocities in extratropical precipitation
extremes strengthen overall in simulations of 21st-century climate change. For
each extreme event, we solve the quasi-geostrophic omega equation to decompose
this strengthening into different physical contributions. We first consider a
dry decomposition in which latent heating is treated as an external forcing of
upward motion. Much of the positive contribution to upward motion from
increased latent heating is offset by negative contributions from increases in
dry static stability and changes in the horizontal length scale of vertical
velocities. However, taking changes in latent heating as given is a limitation
when the aim is to understand changes in precipitation, since latent heating
and precipitation are closely linked. Therefore, we also perform a moist
decomposition of the changes in vertical velocities in which latent heating is
represented through a moist static stability. In the moist decomposition,
changes in moist static stability play a key role and contributions from other
factors such as changes in the depth of the upward motion increase in
importance. While both dry and moist decompositions are self-consistent, the
moist dynamical perspective has greater potential to give insights into the
causes of the dynamical contributions to changes in precipitation extremes in
different regions.
