An Assessment of the Quasi-Biennial Oscillation (QBO) and Sudden Stratospheric Warmings (SSWs) with Idealized General Circulation Models.

[摘要] Advancing the predictive capabilities of atmospheric General Circulation Models (GCMs) necessitates a better understanding of the weather-climate interface and, in particular, the impact of the stratosphere on tropospheric predictability. However, the representation of stratospheric phenomena, including the tropical Quasi-Biennial Oscillation (QBO) and polar Sudden Stratospheric Warmings (SSWs), is still poor in current generation GCMs. This research advances the understanding of stratospheric simulations in GCMs and stratosphere-troposphere interactions.Stratospheric QBO-like oscillations and SSW events are analyzed using different numerical methods which are provided in the four dynamical core of the National Center for Atmospheric Research’s Community Atmosphere Model (CAM) version 5. The dynamical cores are driven by an idealized forcing named Held-Suarez forcing which consists of Rayleigh friction and a Newtonian temperature relaxation on a dry and flat earth. Three of the four models in CAM are able to generate QBO-like oscillations, but with different physical characteristics. The QBO-like oscillationsare mainly driven by tropical waves that are triggered despite the absence of typical wave generation mechanisms like moist convection. The differences thereby expose the effects of the CAM numerical schemes and their dissipation mechanisms on the QBO- like circulation. In addition, SSWs are simulated using similar idealized setups even without topographically generated planetary waves which are believed to be the most important SSW driving mechanisms. Wave and instability analyses are performed to examine the wave generation processes and the dynamics of the circulations in the four dynamical cores.Furthermore, simulations with additional simple subgrid-scale processes such as gravity wave drag parameterizations and moist processes are introduced. These sim- ulations enhance the complexity of the experimental setup and lead to an increase in wave activities and shortened QBO-like cycles. The thesis demonstrates that ide- alized dynamical core simulations expose fundamental characteristics of atmospheric circulations and thereby provide insight into the numerical designs of GCMs. It is suggested that idealized GCM configurations with increasing complexity serve as a paramount tool for model developments and tests.

[发布机构] University of Michigan

[效力级别] Atmospheric

[关键词] Idealized simulation of QBO and SSW;Atmospheric;Oceanic and Space Sciences;Science;Atmospheric, Oceanic and Space Sciences and Scientific Computing