The interdecadal changes in southern hemisphere (SH) winter cyclogenesis have been studied using a global twolevel primitive equation instability-model with reanalysed observed July three-dimension...

The instability characteristics of three-dimensional Northern Hemisphere average winter tropospheric flow are examined in a two-layer spherical quasi-geostrophic model. All the growing modes from the fastest down to the first stationary but growing mode have been examined for three cases (1, 2a and 3) having increasingly larger static stability parameters. Comparisons with observations and with the results of the corresponding barotropic model are presented. It is found that the modes for cases 1, 2a and 3 can be divided into six approximately distinct classes. Class A modes are rapidly propagating monopole cyclogenesis disturbances with largest amplitudes in the geographical locations of the storm tracks in the Pacific and Atlantic Oceans. Class B and C modes are respectively Pacific and Pacific and Atlantic onset-of-blocking dipole modes with longer periods than class A modes and which like class A disturbances, have westward tilts with height. They also have largest amplitudes upstream of the ... Abstract The instability characteristics of three-dimensional Northern Hemisphere average winter tropospheric flow are examined in a two-layer spherical quasi-geostrophic model. All the growing modes from the fastest down to the first stationary but growing mode have been examined for three cases (1, 2a and 3) having increasingly larger static stability parameters. Comparisons with observations and with the results of the corresponding barotropic model are presented. It is found that the modes for cases 1, 2a and 3 can be divided into six approximately distinct classes. Class A modes are rapidly propagating monopole cyclogenesis disturbances with largest amplitudes in the geographical locations of the storm tracks in the Pacific and Atlantic Oceans. Class B and C modes are respectively Pacific and Pacific and Atlantic onset-of-blocking dipole modes with longer periods than class A modes and which like class A disturbances, have westward tilts with height. They also have largest amplitudes upstream of the ...

Dynamical subgrid-scale parameterizations of stochastic backscatter, eddy drain viscosity, and net eddy viscosity have been formulated and calculated for two-dimensional turbulent flows on the sphere based on the statistics of direct numerical simulations (DNSs) with the barotropic vorticity equation. A relatively simple methodology based on a stochastic model representation of the subgrid-scale eddies, but which takes into account the memory effects of turbulent eddies, has been employed. The parameterizations have a cusp behavior at the cutoff wavenumber of the retained scales and have closely similar forms to those based on eddy damped quasi-normal Markovian (EDQNM) and direct interaction approximation (DIA) closure models. Large-eddy simulations (LESs) incorporating DNS-based subgrid-scale parameterizations are found to have kinetic energy spectra that compare closely with the results of higher-resolution DNS at the scales of LES for both isotropic turbulence and Rossby wave turbulence. The methodology presented is general and should be equally applicable to parameterizations of baroclinic processes and convective processes. Applications of the parameterizations to climate models and prediction models are discussed.

The finite-time normal mode instability of four-dimensional space–time basic states has been studied for cases of block development over the Gulf of Alaska, over the North Atlantic, and over southern Greenland using a two-level tangent linear model. The authors find three generic types of finite-time normal modes, denoted as “recurring,” “traveling,” and “flip” modes. The dominant finite-time normal modes associated with block development have large-scale structures in the respective blocking regions; they tend to closely reflect the structures of the developing blocks. The time evolution in the tangent linear model of finite-time adjoint modes has been examined for each of the three cases of block development. These adjoint modes have faster than normal mode exponential growth. The initial structures of the dominant adjoint modes are characterized by small-scale baroclinic wave trains located primarily upstream of the blocking region. As the disturbances grow explosively, they increase their sca... Abstract The finite-time normal mode instability of four-dimensional space–time basic states has been studied for cases of block development over the Gulf of Alaska, over the North Atlantic, and over southern Greenland using a two-level tangent linear model. The authors find three generic types of finite-time normal modes, denoted as “recurring,” “traveling,” and “flip” modes. The dominant finite-time normal modes associated with block development have large-scale structures in the respective blocking regions; they tend to closely reflect the structures of the developing blocks. The time evolution in the tangent linear model of finite-time adjoint modes has been examined for each of the three cases of block development. These adjoint modes have faster than normal mode exponential growth. The initial structures of the dominant adjoint modes are characterized by small-scale baroclinic wave trains located primarily upstream of the blocking region. As the disturbances grow explosively, they increase their sca...