Potential temperature as a prognostic variable in hydrostatic semi-implicit semi-Lagrangian IFS

Thermodynamic equation in the operational ECMWF Integrated Forecasting System (IFS) dynamical core is formulated to use temperature as a prognostic variable. An alternative formulation, using potential temperature as a prognostic variable in hydrostatic semi-implicit semi-Lagrangian IFS, is investigated. Different from temperature, potential temperature is materially conserved in the absence of sources and sinks. The new equation set is first derived. Then the performance of free-running IFS (i.e., without data assimilation) using potential temperature as a prognostic variable is compared to the control formulation using temperature as a prognostic variable. It is shown that the potential temperature formulation performs comparably to the temperature formulation under several tests, ranging from adiabatic idealized test cases to medium-range weather forecasts with full physical parametrization suite.