At our March
meeting there was some interest in QBOi coordination of “nudging” or
“constrained dynamics” (CD) experiments.
It was agreed that the initial QBOi focus would be on (i) long-term
“control” runs together with “global climate perturbation” runs and (ii) seasonal
hindcast experiments. These would use
some versions of the “full” GCMs with constraints imposed only on the initial
conditions and perhaps lower boundary conditions. In contrast, the CD experiments would (by
definition) involve some additional constraints on the model dynamics acting
continuously as the integration proceeds.
At the
meeting we arrived at a suggested time line where at least the first group of integrations
for the (i) and (ii) experiments would take up to about 18 months (to be ready
for the fall 2016 QBOi workshop). So
there may be no urgency for QBOi to plan for CD experiments now, but this blog
posting of some general ideas of mine may start an online discussion of such
experiments for interested participants.
I will
consider here only CD experiments that constrain dynamical fields (horizontal
winds and possibly also temperatures) by means of a linear relaxation. The potential experiments can then be classed
by what scales are constrained (here notably either relaxation of the full 3D fields
or relaxation of only the zonal-mean fields), what height regions are
constrained (stratosphere only, toposphere only, both), what geographical
regions are constrained (tropics, extratropics, global), and what “target” the
fields are relaxed towards (climatology, actual times series of data, idealized
profiles…).
Continuously relaxing the tropical
stratosphere zonal-mean flow
The simplest
experiments to think about are those that involve just adding an extra
zonally-symmetric momentum source so that tropical stratospheric zonal-mean
winds are forced to
(a) undergo
a prescribed (idealized) QBO cycle, or
(b) undergo
a QBO based on another model simulation, or
(c) follow
the actual observed winds for some period, or
(d) remain
nearly constant. i.e. held to some specified profile.
There are several
such experiments already reported in the literature (for example Kodera et al.,
1991; Hamilton, 1995, 1998; Balachandran and Rind, 1995; Giorgetta &
Bengtsson, 1999; Bruhwiler & Hamilton, 1999; Hamilton et al., 1984; Stenchikov et al.,
1984; Thomas et al., 2008; Mathes et al, 2010; Garfinkel & Hartman,
2011). We can imagine that the extra
forcing of the mean zonal momentum accounts for missing (or misrepresented)
eddy fluxes, and so conceptually
these experiments are somewhat similar to model simulations with QBOs generated
by highly tuned nonstationary gravity wave parameterizations. However by using relaxation to a prescribed “target”
wind field, these experiments could provide a suite of simulations performed with
different models, but with nearly identical mean flow profiles in the tropical
stratosphere through which waves will propagate. By choosing an appropriate “target” to relax
towards we can also ensure that the stratospheric mean winds in the models will
be quite realistic.
Nudging the troposphere
A set of
possibly interesting experiments could result from nudging the tropospheric
fields towards observations. This might
be most plausibly done with some kind of prescribed relaxation of the full 3D
wind and temperature field towards some global reanalysis product. This could, in principle, let one compare the
stratospheric simulation among e.g. different versions of one model with
different vertical resolutions, or
different models all with the resolved (or at least sufficiently large scale)
wave fluxes expected to be realistic. I
believe something like this approach has been tried (e.g. by a Canadian group
some years ago?), but I can’t easily locate relevant references. Of course
there are problematic aspects as well, notably how the convection
parameterization in each model will react with an effectively “imposed”
horizontal divergence.
Nudging to produce initial conditions for
free running integrations
As was
discussed in Victoria, a focus of the initial stage of QBOi will be
on seasonal hindcasts from realistic initial conditions. Some centers are no doubt set up to easily
start their models from a realistic initial state. Another option that some groups could conceivably
adopt is producing an initial state by running their model for some time with
3D relaxation to global reanalyses, and then at t=0 turning off the relaxation and
beginning the hindcast. So one application of
this approach might be for some groups to participate in the QBOi “realistic” hindcast experiments,
but one could also imagine this machinery being used for other
experiments. For example one could
compare two hindcasts made with the same model: (i) with the full 3D fields “initialized”
this way, and (ii) with just the tropical stratosphere (or even just the tropical
stratospheric zonal-mean flow) initialized.
This would allow one to see how dependent the evolution of the
zonal-mean equatorial stratospheric flow is on the details of the day to day
weather situation in the troposphere.
References
Balachandran,
N. K., and D. Rind, 1995: Modeling the effects of solar variability and the QBO
on the troposphere/stratosphere system. Part I: The middle atmosphere. J. Climate, 8, 2058–2079.
Bruhwiler,
L.P., and K. Hamilton, 1999: A numerical simulation of the stratospheric ozone
quasi-biennial oscillation using a comprehensive general circulation model. J.
Geophys. Res., 104, 30,525–30,557.
Garfinkel,
C.I., and D.L. Hartmann, 2011: The influence of the Quasi-Biennial Oscillation
on the troposphere in winter in a hierarchy of models. Part II: Perpetual
winter WACCM runs. J. Atmos. Sci., 68, 2026-2041.
Giorgetta,
M., and L. Bengtsson, 1999: The potential role of the quasi-biennial
oscillation in the stratosphere-troposphere exchange as found in water vapour
in general circulation model experiments. J. Geophys.
Res., 104, 6003–6019.
Hamilton,K.,
1995: Interannual variability in the Northern Hemisphere winter middle
atmosphere in control and perturbed experiments with the SKYHI general
circulation model. J. Atmos. Sci., 52,
44–66
Hamilton, K.,
1998: Effects of an imposed Quasi-Biennial Oscillation in a comprehensive troposphere–stratosphere–mesosphere
General Circulation Model. J. Atmos. Sci., 55, 2393–2418.
Hamilton,
K., A. Hertzog, F. Vial, and G. Stenchikov, 2004: Longitudinal variation of the
stratospheric Quasi-Biennial Oscillation.
J. Atmos. Sci., 61, 383–402
Kodera, K., Chiba,
M., & Shibata, K., 1991: A general circulation model study of the solar and
QBO modulation of the stratospheric circulation during the Northern Hemisphere
winter. Geophys. Res. Lett., 18,
1209-1212.
Matthes, K.,
D.R. Marsh, R.R. Garcia, D.E. Kinnison, F. Sassi, and S. Walters, 2010: Role of
the QBO in modulating the influence of the 11-year solar cycle on the
atmosphere using constant forcings. J.
Geophys. Res., 115, D18110, doi:10.1029/2009JD013020.
Stenchikov,
G., K. Hamilton, A. Robock, V. Ramaswamy, and M.D. Schwarzkopf, 2004: Arctic
oscillation response to the 1991 Pinatubo eruption in the SKYHI general
circulation model with a realistic quasi-biennial oscillation, J. Geophys. Res., 109, D03112,
doi:10.1029/2003JD003699.
Thomas, M.A., M.A. Giorgetta, C. Timmreck, H.F. Graf & G. Stenchikov, 2008: Simulation of the climate impact of Mt. Pinatubo eruption using ECHAM5: Part 2: Sensitivity to the phase of the QBO. Atmos. Chem. Phys. Discussions, 8, 9239-9261.
Thomas, M.A., M.A. Giorgetta, C. Timmreck, H.F. Graf & G. Stenchikov, 2008: Simulation of the climate impact of Mt. Pinatubo eruption using ECHAM5: Part 2: Sensitivity to the phase of the QBO. Atmos. Chem. Phys. Discussions, 8, 9239-9261.