Response of Vertical Velocities in Extratropical Precipitation Extremes to Climate Change

Changes in vertical velocities affect the intensity of precipitation extremes but remain poorly understood. We find that mid-tropospheric vertical velocities in extratropical precipitation extremes strengthen in the zonal mean in simulations of 21st-century climate change. For each extreme event, we solve the quasi-geostrophic omega equation to decompose this strengthening behavior into different physical contributions. Much of the positive contribution to upward motion from increased latent heating is offset by negative contributions from changes in dry static stability and horizontal length scale. However, taking the latent heating as given is a limitation for understanding strongly-precipitating events in which the vertical velocity and latent heat release are closely related. Therefore, we also perform a moist decomposition of the changes in vertical velocities in which latent heating is represented through a moist static stability rather than being treated as an external forcing of the omega equation. In the moist decomposition, decreases in moist static stability and increases in the depth of the circulation make important contributions to the strengthening of the vertical velocities.