| Abstract: The large-scale circulation in the Geophysical
Fluid Dynamics Laboratory "SKYHI" troposphere-stratosphere-mesosphere
finite-difference general circulation model is examined as a function of
vertical and horizontal resolution. The experiments include one with horizontal
grid spacing of ~35 km and another with ~100 km horizontal grid spacing
but very high vertical resolution (160 levels between the ground and about
85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures
in the extratropics is found to be very sensitive to horizontal resolution.
For example, in the early Southern Hemisphere winter the South Pole near
1 mb in the model is colder than observed, but the bias is reduced with
improved horizontal resolution (from ~70°C in a version with ~300 km
grid spacing to less than 10°C in the ~35 km version). The extratropical
simulation is found to be only slightly affected by enhancements of the
vertical resolution. By contrast, the tropical middle atmospheric simulation
is extremely dependent on the vertical resolution employed. With level
spacing in the lower stratosphere ~1.5 km, the lower stratospheric zonal-mean
zonal winds in the equatorial region are nearly constant in time. When
the vertical resolution is doubled, the simulated stratospheric zonal winds
exhibit a strong equatorially centered oscillation with downward propagation
of the wind reversals and with formation of strong vertical shear layers.
This appears to be a spontaneous internally generated oscillation and closely
resembles the observed QBO in many respects, although the simulated oscillation
has a period less than half that of the real QBO. |