THE INTEGRATED EFFECT OF CONDENSATION IN NUMERICAL SIMULATIONS OF EXTRATROPICAL CYCLOGENESIS

By combining traditional sensitivity studies with techniques that focu s on the conservation and invertibility properties of Ertel's potentia l vorticity (PV), we illustrate the effect of latent heating on the st ructure and evolution of three simulated extratropical cyclones. The c ases include one continental cyclone development (15 December 1987), w hich we examine extensively, and two cyclones over the western Atlanti c Ocean (6 January 1983 and 5 January 1985) of somewhat greater intens ity, 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 t he magnitude of the effect varied greatly. In all cases, the differenc e in intensity was attributed to velocities associated with a positive , condensation-produced PV anomaly above the warm front. The amplifica tion 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 a nomaly onto the cyclonic circulation that would exist without latent h eating. Indirect effects associated with latent heating were an increa se in the translational speed of the surface thermal perturbations, in tensification of the downstream ridge aloft, and an enhanced upper-lev el cyclonic wrapping of positive and negative PV anomalies. Much of th e 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 a t the surface. Therefore, the feedback of latent heating onto the inte raction of tropopause PV and surface potential temperature anomalies a ppears small.