Bedymo: a Combined Quasi-Geostrophic and Primitive Equation Model in Sigma Coordinates

Abstract. This paper introduces the idealised atmospheric circulation model Bedymo, which combines the quasi-geostrophic approximation and the hydrostatic primitive equations in one modelling framework. The model is designed such that the two systems of equations are solved as similarly as possible, such that differences can be unambiguously attributed to the different approximations, rather than the model formulation or the numerics. As a consequence, but in contrast to most other quasi-geostrophic models, Bedymo is using sigma-coordinates in the vertical. In addition to the atmospheric core, Bedymo also includes a slab ocean model and passive tracer module that will provide the basis for future idealised parametrisation of moisture and latent heat release. Further, Bedymo has a graphical user interface, making it particularly useful for teaching. Bedymo is evaluated for three atmosphere-only test cases and one coupled test case including the slab ocean component. The atmosphere-only test cases comprise the growth of a cyclonic disturbance in a baroclinic environment and the excitation of Rossby waves by isolated orography, both in a mid-latitude channel, as well as the simulation of a mid-latitude storm track. The atmosphere-ocean coupled test case is based on an equatorial channel and evaluates the coupled response to an isolated equatorial temperature anomaly in the ocean mixed layer. For all test cases, results agree well with expectations from theory and results obtained with more complex models.