Major Improvements in the Simulation of Tropical Stratospheric Winds

Comparison of zonally averaged east-west wind fields, from south to north pole, in SKYHI model simulations with 40 and 160 vertical levels. The higher-resolution solution (right) shows dramatically increased wind shear near the equator in the middle atmosphere compared to the winds in the low resolution result (left).

Research Objective

To investigate the behavior of the Earth's stratosphere and mesosphere and to determine the controlling dynamical mechanisms through simulation with an atmospheric general circulation model.

Computational Approach

The GFDL SKYHI model is a comprehensive atmospheric general circulation model that has been used extensively to investigate the dynamical and chemical behavior of the Earth's troposphere, stratosphere, and mesosphere. The model has been designed to execute interchangeably on both shared-memory, parallel vector, and distributed memory cache-based processors.

A series of control integrations is continuing on GFDL's 26-processor T932/26-512 and 40-processor T3E/40-128 systems using various versions of the SKYHI model. To date, the experiments run on in-house systems include an integration of 20 years with a 3° x 3.6° 40-level model, two years with a 1° x 1.2° 40-level version, two years with a 1° x 1.2° 80-level version, and seven months with a 0.33° x 0.4° 40-level version.

All of the experiments are capable of being run on the T932 system. The experiments with 1° x 1.2° horizontal resolution up to 80 vertical levels are currently being executed on the in-house T3E. However, one additional study with 160 vertical levels was required in order to determine the sensitivity of the model solution to even higher vertical resolution. Because this model resolution required more memory capacity than was available on the GFDL T3E, a two-month experiment was performed on the NERSC T3E.

Accomplishments

The 1° x 1.2° 160-level and the 0.33° x 0.4° experiments represent integrations at unprecedented spatial resolution (at least for global climate-simulation models). Preliminary analysis of these new integrations has concentrated on the simulation of the large-scale circulation in the middle atmosphere.

Overall, the extratropical simulation appears to improve with finer grid spacing, but the effects of increasing horizontal resolution are more significant than vertical resolution.

Results from the SKYHI model experiments indicate that the tropical middle atmospheric simulation is extremely dependent on the vertical resolution employed. In all the simulations with 40 vertical levels, the east-west winds in the lower stratosphere are nearly constant in time, in contrast to the behavior of the observed winds, which exhibit a strong equatorially centered oscillation with a period between 24 and 30 months. This observed oscillation is known as the Quasi-Biennial Oscillation (QBO) and is an important feature of the real atmosphere that has been found to be absent in previous general circulation model simulations.

When the vertical resolution of the experiment is increased, the simulated stratospheric winds exhibit much more realistic behavior, including a strong, long-period oscillation and the formation of strong vertical wind shears. 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 that of the real QBO.

Significance

The finding that increased vertical resolution produces far more realistic winds in the tropical middle atmosphere is an important discovery that opens up the possibility of alleviating one of the most serious and widespread problems in atmospheric simulation.

Publications

K. Hamilton, R. J. Wilson, and R. S. Hemler, "Climatology of the middle atmosphere simulated with high vertical and horizontal resolution versions of a general circulation model: Improvements in the cold pole bias and generation of a QBO-like oscillation in the tropics," Journal of Atmospheric Sciences (submitted, 1998).