Analytical and adaptable initial conditions for moist baroclinic waves in a global hydrostatic model

Abstract

Abstract. This article presents a description of an analytical, stable and flexible initial background state for moist baroclinic wave simulation on an aquaplanet in order to test dynamical core of numerical weather prediction models and study the dynamics and evolution of extra-tropical cyclones. The initial background state is derived from an analytical zonal wind speed field, or jet structure, and the hydrostatic primitive equations for moist adiabatic and frictionless flow in spherical coordinates. A baroclinic wave can develop only if a unbalanced perturbation is added to the zonal wind speed field. The implementation of this baroclinic wave simulation have been done on the Open Integrated Forecasting System (OpenIFS) cy43r3, a global numerical weather prediction model developed by the European Centre for Medium-range Weather Forecasts. In total, seven parameters can be used to control the generation of the initial background state and hence the development of the baroclinic waves in the OpenIFS configuration file: the jet's width, the jet's height, the maximum zonal mean wind speed of the jet, the horizontal mean of the surface virtual temperature, the surface relative humidity, the lapse rate and the surface roughness. Nine dry and nine moist initial background states have been generated to test their stability without perturbations. The meteorological stability of the initial state is investigated by examining the spatial distributions of the equivalent potential temperature, the absolute vorticity and the Brunt-Väisälä frequency. Moreover, the Root-Mean-Squared-Error (RMSE) of the zonal wind speed has been computed to assess their numerical stability. Finally, six of these moist initial initial background state have been used with an unbalanced perturbation to ensure that the baroclinic lifecycles developing are physically realistic. The resulting baroclinic wave is shown to be sensitive to the jet's width. This configuration for baroclinic wave simulations will be used to create large baroclinic lifecycles ensemble to study how extra-tropical cyclones may evolve in the future.