Balanced and Unbalanced Flow in Primitive Equation Model Simulations of Baroclinic Wave Life Cycles

Simulations of baroclinic wave life cycles are performed in order to illustrate the wave evolution of a cyclone and diagnose possible unbalanced flow associated with the destabilization of an upper-level jet. Development of the baroclinic wave is observed using the multilevel primitive equation Weather Research and Forecasting (WRF) model. A baroclinic system is produced with an initially balanced, zonal baroclinic jet on an f-plane, whereby the geometry of the dynamic tropopause is specified. The change in geometry will result in different initial jet profiles. For each jet profile two numerical simulations comprised of different diffusion parameters are integrated to show the effect that the diffusion has on the simulation. The first simulation consists of a combination of weak horizontal and strong vertical diffusion, while the second simulation includes only strong horizontal diffusion, and is considered to be more consistent with real atmosphere characteristics. For each simulation, the nonlinear stage of the life cycle resembles a cyclonic wave-breaking system. Simulations where the vertical diffusion is strong tend to produce events of secondary cyclogenesis, which are not observed in the case of strong horizontal diffusion. Therefore, these secondary events are in all probability results of numerical instabilities at the triple point of the baroclinic system. The simulations with strong horizontal diffusion produce a crisper version of the baroclinic wave evolution cycle with sharper temperature gradients and deeper surface lows than the strong vertical diffusion case. Diagnostic calculations of the horizontal divergence and the residual of the nonlinear balance equation are shown in order to identify areas of unbalanced flow and subsequent inertia-gravity waves. Banded structures in the horizontal divergence field at the level of maximum wind speed suggest that the unbalanced flow is closely related to the upper level jet streak and possibly generated through geostrophic adjustment processes. The simulations with strong vertical diffusion contain less numerical noise and provide a clearer insight into the possible existence of inertia-gravity waves. A breakdown into the three main components of the residual of the nonlinear balance equation is shown in order to asses the contribution of each term towards the production of unbalanced flow, and indicates that the Laplacian term was the dominant factor as it was an order of magnitude stronger than the Jacobian and vorticity terms.