The Effect of Removing a Well-Resolved Stratosphere on the Simulation of the Tropospheric Climate, and Climate Change Michael Sigmond (University of Victoria) Michael Sigmond (University of Victoria) Paul J. Kushner (University of Toronto) Paul J. Kushner (University of Toronto) John F. Scinocca (University of Victoria, CCCma) John F. Scinocca (University of Victoria, CCCma)

The Effect of Removing a Well-Resolved Stratosphere on the Simulation of the Tropospheric Climate, and Climate Change Michael Sigmond (University of Victoria) Michael Sigmond (University of Victoria) Paul J. Kushner (University of Toronto) Paul J. Kushner (University of Toronto) John F. Scinocca (University of Victoria, CCCma) John F. Scinocca (University of Victoria, CCCma)

Motivation: Sigmond et al, 2007, (JGR, in press): Investigated robustness of the simulated response to climate change Sigmond et al, 2007, (JGR, in press): Investigated robustness of the simulated response to climate change We forced 2 AGCM with a generic SST perturbation, varied horizontal resolution, and a single tuning parameter, and compared the responses Here: investigate robustness of response to climate change to changing model top height Or: compare the global warming responses in ‘high-top’ with a ‘low-top’ model Do we need a well-resolved stratosphere to realistically model the future tropospheric climate? (Shindell et al 1998, Fyfe et al. 1999, Gillet et al. 2002)

Method:  Use different versions of the Canadian AGCM (T63 resolution)  forcing: 1) double atmospheric CO 2 concentration 1) double atmospheric CO 2 concentration 2) Forcing with 2) Forcing with ‘best-guess’ SST increase in 2xCO 2 world (repeating annual cycle) (repeating annual cycle) Ensemble average SST response of 17 AR4 models in A1B scenario ( minus )  equilibrium runs  All plots DJF

How to compare high-top with low- top models? 1) Take ‘best-tuned’ low-top model and compare it to ‘best-tuned’ high-top model HIGH: - CMAM: state-of-the-art stratosphere resolving GCM - 71 levels with top at hPa - 71 levels with top at hPa - Used in several studies with interactive chemistry for stratospheric O 3 predictions - Used in several studies with interactive chemistry for stratospheric O 3 predictions - Here: dynamical part (no coupling to chemistry) - Here: dynamical part (no coupling to chemistry) LOW: - GCM3: standard Canadian ‘tropospheric’ model - 31 levels with top at 1 hPa - 31 levels with top at 1 hPa - Used for climate prediction, e.g. in IPCC AR4 report - Used for climate prediction, e.g. in IPCC AR4 report Problem: Model versions have different settings (vertical resolution, tuning, timestep) and physics  differences can be caused by more than just the model lid height

How to compare high-top with low- top models? (2) 2) Take low-top model and add layers Problems: - we need to add physics (radiation, non-orographic gravity wave drag) (radiation, non-orographic gravity wave drag) - we need to decrease time step - we need to decrease time step

How to compare high-top with low- top models? (2) LOWERED: - lowered version of ‘HIGH’: 41 levels with top at 10 hPa, with physics and dynamics as similar to standard CMAM with physics and dynamics as similar to standard CMAM - Not trivial to construct (radiation, sponge layer) - Not trivial to construct (radiation, sponge layer) 3) Take the high-top model and remove layers above a certain height

LOWERED (only removing levels above 10 hPa) (5y, control) U T HIGH LOWERED LOWERED-HIGH

HIGH  LOWERED (- Removing all layers above 10 hPa) - Removing (non-zonal) sponge layer - Removing (non-zonal) sponge layer - Remove non-LTE LW radiation module - Remove non-LTE LW radiation module Was not ‘tuned’ for model with 10 hPa top, not needed below 10 hPa Was not ‘tuned’ for model with 10 hPa top, not needed below 10 hPa - conserve angular momentum in column - conserve angular momentum in column Instead of letting momentum of gravity waves escape to space, deposit in Instead of letting momentum of gravity waves escape to space, deposit in uppermost layer (see Shaw et al. poster) uppermost layer (see Shaw et al. poster)

HIGH vs LOWERED (5y, control) U T HIGH LOWERED LOWERED-HIGH

RESPONSE to Climate change (40 year equilibrium runs)

HIGH LOW AO+ LOWERED LOW-G ∆ SLP = SLP 2xCO2 - SLP control AO+ (0.001 hPa top)(1 hPa top) (10 hPa top) Model lid height? No! Just lowering model lid height does NOT change pattern of response (amplitude ~50%)

HIGH LOW LOWERED (10 hPa top) LOW-G ∆u = u 2xCO2 - u control (0.001 hPa top) (1 hPa top) -HIGH and LOWERED responses similar, but LOW response is different -Anomalous LOW response must be caused by difference in physics/model settings in LOW compared to LOWERED/HIGH

Which model setting in LOW compared to LOWERED causes the response to be so different? LOWLOW-GLOWERED # levels top: 1 hPa 10 hPa Vert res (tropopause) ~2 km ~1.2 km Sponge layer: YESYESNO non-oro gravity waves: NONOYES Gphil (~oro gravity waves; Scinocca and McFarlane 2000 ) Make setting in LOW equal to that in LOWERED/HIGH and check if responses become more similar

HIGH LOW LOWERED LOW-G ∆ SLP AO+ (Gphil=1.0) (Gphil=0.65) AO+

HIGH LOW LOWERED (Gphil=0.65) LOW-G ∆ U (Gphil=0.65) (Gphil=1.0) (Gphil=0.65) Response is more dependent on Gphil than on model lid height!!

U control HIGH LOW LOW-G Close to observations Too weak (waveguide to narrow) Closer to observations gphil=0.65 gphil=1.0

Conclusions Assessing the benefit of including a well-resolved stratosphere on the simulation of climate (change) is not straightforward Assessing the benefit of including a well-resolved stratosphere on the simulation of climate (change) is not straightforward Response in standard ‘low-top’ model (no AO response) is different from that in standard ‘high-top’ model (AO+) (for this model) Response in standard ‘low-top’ model (no AO response) is different from that in standard ‘high-top’ model (AO+) (for this model) When only lowering model lid height, the responses do not change very much When only lowering model lid height, the responses do not change very much By making the orographic gravity wave settings in the standard low-top model consistent with that in the standard high-top model, we can get a very similar response as in the high-top model By making the orographic gravity wave settings in the standard low-top model consistent with that in the standard high-top model, we can get a very similar response as in the high-top model The strength of orographic gravity waves appears crucial for response to climate change, more so than the model lid height (in this model) (pretty scary, isn’t it?) The strength of orographic gravity waves appears crucial for response to climate change, more so than the model lid height (in this model) (pretty scary, isn’t it?)

LOW vs LOW-G LOW LOW-G LOW-G minus LOW Closer to observations gphil=0.65 gphil=1.0 CONTROL 2xCO2 Climate

∆T LOWERED LOW-G HIGH LOW

LOWERED (10 hPa top) LOW-G ∆u (40 years) (0.001 hPa top) (1 hPa top)

HIGH LOW ∆u (40 years) (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top) LOW-G

HIGH LOW ∆u (year 1-20) (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top) LOW-G

HIGH LOW ∆u (year 21-40) (0.001 hPa top) (1 hPa top) LOWERED (10 hPa top) LOW-G

‘Construction’ of LOWERED step 1: removing all layers above 10 hPa U T HIGH LOWERED LOWERED-HIGH

‘Construction’ of LOWERED step 2: removing (non-zonal) sponge layer U T HIGH LOWERED LOWERED-HIGH

‘Construction’ of LOWERED step 3: Remove non-LTE LW radiation module U T HIGH LOWERED LOWERED-HIGH

‘Construction’ of LOWERED step 4: conserve angular momentum in column U T HIGH LOWERED LOWERED-HIGH