By combining traditional sensitivity studies with techniques that focus on the conservation and invertibility properties of Ertel's potential vorticity (PV), we illustrate the effect of latent heating on the structure and evolution of three simulated extratropical cyclones. The cases include one continental cyclone development (15 December 1987), which we examine extensively, and two cyclones over the western Atlantic Ocean (6 January 1983 and 5 January 1985) of somewhat greater intensity, which are diagnosed to assess the generality of our findings for the continental case.
Each storm featured a weaker cyclonic low-level circulation when latent heating was removed from the simulation, but the magnitude of the effect varied greatly. In all cases, the difference in intensity was attributed to velocities associated with a positive, condensation-produced PV anomaly above the warm front. The amplification of the surface thermal perturbations was not strongly altered in even the case most affected by condensation. Hence, the primary effect of condensation at low levels was simply to superpose a positive PV anomaly onto the cyclonic circulation that would exist without latent heating.
Indirect effects associated with latent heating were an increase in the translational speed of the surface thermal perturbations, intensification of the downstream ridge aloft, and an enhanced upper-level cyclonic wrapping of positive and negative PV anomalies. Much of the amplification of the upper-level ridge could be traced to upward and poleward advection of the tropopause by the irrotational and vertical motions that were augmented by latent heating. However, the scale of the changes aloft was small enough that they had a negligible effect at the surface. Therefore, the feedback of latent heating onto the interaction of tropopause PV and surface potential temperature anomalies appears small.