A Pathological Problem with NCEP Reanalyses in the Stratosphere

A pathological problem has been discovered in the NCEP reanalyses in the stratosphere. It is manifested most strongly as a two-delta vertical wave in the divergence of the wind field above steep topography especially where the wind increases with altitude in the stratosphere. It is present primarily above 50 mb at the topmost four levels in the NCEP model used for data assimilation and appears to be directly related to the use of the sigma (terrain following) coordinate system and the upper boundary condition in the assimilating model. Recommendations suggested for addressing the problem include switching to a hybrid coordinate system that transitions to a pressure coordinate in the stratosphere, and with a damping upper boundary condition. Certain climate diagnostics are greatly impacted by these pathologies.