Alleviation of the GCMs bias in the middle atmosphere of the Southern Hemisphere with GWD parameter estimation

PULIDO, MANUEL; SCHEFFLER, GUILLERMO

Sao Paulo

Taller; 2nd WCRP Summer School on Climate Model Development: Scale aware parameterization for representing sub-grid scale processes; 2018

World Climate Research Program

Current chemistry-climate models show a pronounced bias in the representation of the breakup of the stratospheric winter polar vortex in the Southern Hemisphere, characterized by a delay of around 2-3 weeks in the transitions from westerlies to easterlies in the middle stratosphere. This bias may be associated with either an inaccurate representation of the sources planetary waves or a wrong representation of unresolved gravity waves. However, it is not possible to identify which type of wave drag plays a dominant role in the appearance of these biases. In this work, we estimate optimal values for the free parameters of a non-orographic gravity wave drag parameterization. The optimal parameters should take into account the feedback mechanisms between planetary wave breaking and parameterized gravity wave drag. Two different estimation approaches are presented, based on variational data assimilation. The delay in the zonal-mean zonal-wind transition is largely alleviated by the use of optimal parameters. While the nal zonal-wind transition is mainly driven by planetary wavebreaking, we show that these are largely affected by preconditioning of the middle-atmosphere through by changes in the parameterized gravity wave drag. Additionally, planetary wave breaking modies the zonal-wind proles which in turn affects the ltering in the gravity wave drag parameterization. These type of interactions require the use of sophisticated estimation techniques.