Middle Atmosphere Simulated with High Vertical and Horizontal Resolution Versions of a GCM: Improvement in the Cold Pole Bias and Generation of a QBO-Like Oscillation in the Tropics

The large-scale circulation in the GFDL "SKYHI" troposphere- stratosphere-mesosphere finite-difference GCM is examined as a function of vertical and horizontal resolution. The experiments examined include one with horizontal grid spacing of ~35 km and another with ~100 km horizontal resolution 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 SH winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ~70oC in a version with ~300 km resolution to less than 10oC in the ~35 km resolution 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.