Role of Moist Processes in the Tracks of Idealized Midlatitude Surface Cyclones

Abstract

he effects of moist processes on the tracks of midlatitude surface cyclones are studied by performing idealized mesoscale simulations. In each simulation, a finite-amplitude surface cyclone is initialized on the warm-air side of a zonal baroclinic jet. For some simulations, an upper-level cyclonic anomaly upstream of the surface cyclone is also added initially. Sensitivities to the upper-level perturbation and moist processes are analyzed by both performing a relative vorticity budget analysis and adopting a potential vorticity (PV) perspective.Whatever the simulation, there is a systematic crossing of the zonal jet by the surface cyclone occurring after roughly 30 h. A PV inversion tool shows that it is the nonlinear advection of the surface cyclone by the upper-level PV dipole, which explains the cross-jet motion of the surface cyclone. The simulation with an initial upper-level cyclonic anomaly creates a stronger surface cyclone and a more intense upper-level PV dipole than the simulation without it. It results in faster northward and slower eastward motions of the surface cyclone.A moist run including full microphysics has a more intense surface cyclone and induces a faster northeastward motion than the dry run. The faster eastward motion is due to the diabatically produced cyclonic circulation at low levels. The faster northward motion is explained by the stronger upper-level anticyclone due to released latent heat, together with the closer location of the surface cyclone to the upper-level anticyclone. Finally, a moist run with only condensation and evaporation exhibits less latent heat release and a slower northeastward motion of the surface cyclone than the full moist run.