[strat_list] Paper on cause of stratospheric SH zonal wind biases in CCMs

[Charles McLandress]

Dear Colleagues,

Attached is a paper by myself, Ted Shepherd, Saroja Polavarapu, and 
Stephen Beagley, which may be of interest to some of you. It is entitled 
"Is missing orographic gravity wave drag near 60°S the cause of the 
stratospheric zonal wind biases in chemistry-climate models?" and was 
recently accepted for publication in JAS. The abstract is:

"Nearly all chemistry-climate models (CCMs) have a systematic bias of a 
delayed springtime breakdown of the Southern Hemisphere (SH) stratospheric 
polar vortex, implying insufficient stratospheric wave drag. We use the 
Canadian Middle Atmosphere Model (CMAM) and the CMAM Data Assimilation 
System (CMAM-DAS) to investigate the cause of this bias. Zonal wind 
analysis increments from CMAM-DAS reveal systematic negative values in the 
stratosphere near 60°S in winter and early spring. These are interpreted 
as indicating a bias in the model physics, namely missing gravity wave 
drag (GWD). The negative analysis increments remain at a nearly constant 
height during winter, and descend as the vortex weakens, much like 
orographic GWD. This region is also where current orographic GWD 
parameterizations have a gap in wave drag, which we suggest is unrealistic 
due to missing effects in those parameterizations. These findings motivate 
a pair of free-running CMAM simulations to assess the impact of extra 
orographic GWD at 60°S. The control simulation exhibits the cold-pole bias 
and delayed vortex breakdown seen in the CCMs. In the simulation with 
extra GWD, the cold-pole bias is significantly reduced and the vortex 
breaks down earlier. Changes in resolved wave drag in the stratosphere 
also occur in response to the extra GWD, which reduce stratospheric SH 
polar-cap temperature biases in late spring and early summer. Reducing the 
dynamical biases, however, results in degraded Antarctic column ozone. 
This suggests that CCMs that obtain realistic column ozone in the presence 
of an overly strong and persistent vortex may be doing so through 
compensating errors."

Thanks,
Charles


Charles McLandress                email: charles at atmosp.physics.utoronto.ca
Department of Physics             phone: (416) 978-1810
University of Toronto             fax:   (416) 978-8905
60 St. George Street
Toronto, Ontario, Canada, M5S 1A7




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