Experiment Details

Version 1.0  Drafted by John Scinocca, Tim Stockdale & Francois Lott

Five sets of simulations/experiments have been defined above:

    - EXPERIMENT 1 - AMIP,  interannually varying SSTs, sea ice, and external forcing
    - EXPERIMENT 2 - 1xCO2, repeated annual cycle  SSTs, sea ice, and external forcings
    - EXPERIMENT 3 - 2xCO2, as EXPT 2 with perturbed SSTs and sea ice and     2xCO2
    - EXPERIMENT 4 - 4xCO2, as EXPT 2 with perturbed SSTs and sea ice and 4xCO2
    - EXPERIMENT 5 - QBO hindcasts, with reanalysis initial conditions on specified     start dates.

For all experiments it is requested that all modelling groups use the same set of SST and sea ice boundary conditions, as specified below. External forcings should be followed to the extent possible, although it is recognized that models may vary in how they specify aerosols, volcanic forcing etc. For the purposes of these experiments (sensitivity studies of the QBO), what matters is that the external forcing remains constant when it is supposed to be constant, and varies as realistically as the model allows when it is supposed to vary. In all cases, the experiments are intended to be made using only reasonable efforts. Any changes in experimental details should be documented.

Ensemble sizes are given as a range, from minimum to preferred size. Each group should assess what is reasonable, given costs, resources and expected results (e.g. some models may have a highly regular or phase-locked QBO).

EXPERIMENT 1 - AMIP Cost: 30-90y

This is based on the CMIP5: Expt 3.3

Period:  30y (1979-2008)

Ensemble size: 1-3

Boundary Conditions: CMIP5 interannually varying sea ice and SSTs obtained from:

http://www-pcmdi.llnl.gov/projects/amip/AMIP2EXPDSN/BCS/amipbc_dwnld.php

External Forcings: CMIP5 external forcings for radiative trace gas concentrations, aerosols, solar, explosive volcanoes etc. obtained from: http://cmip-pcmdi.llnl.gov/cmip5/forcing.html#amip

EXPERIMENT 2 - 1xCO2 Cost: 30-90y

Repeated annual cycle simulation.

Period:  30y, after a suitable spinup (5y?).

Ensemble size: 1-3

Boundary Conditions: CMIP5 "SST Climatology 1988-2007" and "SEA ICE Climatology 1988-2007" obtained from:

http://www-pcmdi.llnl.gov/projects/amip/AMIP2EXPDSN/BCS/amipbc_dwnld.php

External Forcings: repeated annual cycle forcings. Ideally this would be some sort of climatological forcing averaged over the 30 year period used in EXPT1, but that doesn't really exist.  The suggestion is to use year 2002 of the CMIP5 external forcings:

http://cmip-pcmdi.llnl.gov/cmip5/forcing.html#amip

The year 2002 has neutral ENSO, neutral PDO, and is well-away from any historical explosive volcanoes.  Since this experiment will be the base for the 2xCO2/4xCO2 experiments, a constant value of CO2 corresponding to the average over the year 2002 should be used. Note that although these choices are not ideal (the 30 year comparison period, the 20 year SST climatology and the 2002 fixed forcing are all inconsistent with each other), the observed dependence of the QBO on changing climate through this period appears to be negligible. Thus for QBO purposes (and in particular for comparing model responses) the protocol is believed adequate, if all models use the same approach.

EXPERIMENTS 3 and 4 - 2xCO2 / 4xCO2 Cost: 60-180y

Period:  30y, after suitable spinup

Ensemble size: 1-3

Boundary Conditions: repeated annual cycle of SSTs and sea ice will be provided to modelling centres on a 1x1 degree grid as in Experiments 1 and 2. The proposal is to use an ensemble average over the CMIP5 models average over the decade centred on the time of CO2 doubling/quadrupling in the RCP8.5 scenario. [In practice we may use the decades 2050-60 and 2090-2100 from RCP8.5 runs, since 4xCO2 occurs a little later than 2100. But this dataset will be prepared centrally, to ensure we all use the same values].

External Forcings: the forcings in these two experiments should be exactly the same as used in EXPT 2 except for the CO2 concentration, which should be doubled and quadrupled. Only CO2 forcings should be changed, not other greenhouse gases. These are sensitivity experiments, not attempts to predict specific periods in the future.

EXPERIMENT 5 - QBO hindcasts Cost: 68-150y

These are atmosphere-only experiments, initialized from re-analysis data, providing multiple short integrations from a relatively large set of start dates sampling different phases of the QBO.

Start dates: 1 May and 1 November in each of the years 1993-2007 (15 years, 30 start dates)

Hindcast length: 9-12 months

Ensemble size: 3-5 members

The boundary conditions and forcings for this experiment closely follows the prescription of the AMIP experiment (EXPT 1).

Boundary Conditions: CMIP5 interannually varying sea ice and SSTs obtained from:
http://www-pcmdi.llnl.gov/projects/amip/AMIP2EXPDSN/BCS/amipbc_dwnld.php

External Forcings: CMIP5 external forcings for radiative trace gas concentrations, aerosols, solar, explosive volcanoes etc. obtained from:
http://cmip-pcmdi.llnl.gov/cmip5/forcing.html#amip

Initial data for these dates should be taken from the ERA-interim reanalysis.  ERA-interim data is available for download from apps.ecmwf.int/datasets (registration is required; if downloading lots of start dates from this site, it may be easier to use the “batch access” method described on the site, although interactive download of each date is also possible. Data are available on either standard pressure levels or original model levels, and in either grib or netCDF. Try to download only the data you need, e.g. at 0 z on the 1st of the month).

The ensemble is expected to be generated by perturbing each ensemble member by a small anomaly, which needs do no more than change the bit pattern of the simulation.

4 comments:

  1. This comment is primarily about external forcing in the stratosphere for Experiment 5 (perhaps as offshoots, or at least issues to consider):
    1)If the goal is to do as good a job as possible on the QBO 'forecast', one might consider whether specifying the state of the solar cycle would be useful, given the possible interactions between it and the QBO. This would entail a host of somewhat problematic specifications, since models have different shortwave radiation schemes, so one might consign it to a 'Five B' experiment.
    2) Volcanic aerosols are probably less of a concern for the time period of this study, but given the start date of 1993, there was still some AOD in the tropical lower stratosphere at that time (e.g., http://www.clim-past.net/10/359/2014/cp-10-359-2014.pdf), likely providing heating that could affect QBO development. Again, the specifications would be tricky, given now both different shortwave and longwave model radiation schemes.
    3) With respect to stratospheric ozone, there has been a continual negative trend in tropical values below 22km (e.g., http://www.atmos-chem-phys.net/14/6983/2014/acp-14-6983-2014.html) that might have some influence on the lower levels of the region of interest. And since this exercise presumably will also consider the propagation of QBO effects to other latitudes, annually-specific seasonally-varying lower stratospheric ozone fields, including the ozone hole, may have an influence there. Again, maybe for subsidiary experiments...

    Also - quick comments about some of the other experiments

    Experiment 1: What CO2 level should be used (affecting stratospheric temperatures somewhat)? Presumably it has been concluded that it won't be changed during the course of those years (1979-2008), but it did vary, and some value has to be picked, for the sake of uniformity among models. This will also define the value to be used for the '2xCO2' and '4xCO2' runs.

    David

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    Replies
    1. Presumably prescribing a solar cycle in TSI as was recommended in CMIP5 could be done here too. Whether or not the CMIP5 recommended ozone would suffice for 5, is a different matter, perhaps. If it were to be used, the advantage is that there would be a solar cycle in ozone too.

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    2. Well....CMIP5 wasn't overly concerned with the stratosphere. Might be better to use our best estimates of SSI variations with the solar cycle (e.g., Journal of Climate 04/2012; 25(7):2555-2560 and associated references) given the proportionately larger solar cycle variations of UV relative to the total irradiance.

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    3. Actually, I have that wrong: I recall the solar recommendations were F10.7 not TSI. Will chase up your reference, nonetheless...

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