Improvement in stratosphere simulation with a hybrid σ‐ϑ coordinate GCM

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Authors: Zhu, Z., Schneider, E. K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1997
• Subjects: polar night
• Online Access: http://dx.doi.org/10.1002/qj.49712354315; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fqj.49712354315; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.49712354315

Abstract This study demonstrates the practical use of a smoothly varying hybrid sigma‐isentropic (σ‐ϑ) coordinate in a general‐circulation model (GCM). Changes have been made to the vertical scheme developed in previous work in order to overcome a numerical problem associated with the crossing of levels. the hybrid coordinate, which changes gradually from a sigma coordinate at the bottom to a nearly isentropic one in the stratosphere, was implemented into the GCM. Multi‐year integrations were successfully conducted with the σ‐ϑ coordinate model. Improvements in the simulated stratospheric and upper‐tropospheric circulation are found compared with a control run using the original sigma‐pressure coordinate GCM. the greatest improvements are significant reductions of the cold biases in the winter polar stratosphere and of the wind speed of the polar‐night jet. These changes are shown to be related to a significant increase of the poleward eddy heat flux in the middle‐latitude stratosphere. Diagnostics show the wave kinetic energy, stationary‐wave phase structure, and EP flux are changed by using the nearly isentropic coordinate in the stratosphere. Possible reasons for the improvements of the stratospheric simulation are discussed, including the more accurate representation of the advection processes, and the effect of the isentropic model top.