THE ROLE OF WAVE BREAKING, LINEAR INSTABILITY, AND PV TRANSPORTS IN MODEL BLOCK ONSET

To understand mechanisms responsible for the onset of atmospheric bloc ks, the authors study model blocks that form in a two-layer isentropic primitive equation model. The latter includes diabatic heating, param eterized as a Newtonian relaxation of the actual interface toward an e quilibrium interface, and a zonal wavenumber-2 orography. The study co ncentrates on four different blocking events,ne of the blocks is prese nt in the control run, while the remaining three are excited by approp riate perturbation of the model's state vector at preselected times wh en the prevailing Rows are classified as zonal. With the parameter cal ibration chosen in this investigation two phases in the formation of t he blocks are conveniently identified: The first phase consists of the formation of cutoff or nearly cutoff cyclones in the upper layer at l ow latitudes, and the second phase features a rapid intensification of the upper-layer blocking ridge, accompanied by advection of high pote ntial vorticity (PV) beneath it. While the initiation of the first pha se may be perceived as far back in lime as 6 days before the second ph ase, the latter occurs on a timescale of 1 to 2 days, giving rise to a well-defined blocking pattern. The first phase features the Simmons-H oskins basic baroclinic life cycle in the total PV field that acts as a conditioner of the large-scale Row for the second phase to occur. Th e authors hypothesize that the second phase consists of (intense) inst ability of normal mode form, very much as in the theory of barotropic and baroclinic instability of three-dimensional basic-state flows for the onset of blocks. From a different perspective, based on the concep t of interaction between different scales of motion, both phases predo minantly involve the transport of synoptic-scale potential vorticity b y the planetary waves. Planetary-planetary interactions are, however, nonnegligible.