抄録
In agreement with the experience of other centers, the high resolution GLA Fourth Order GCM has been found to exhibit a pronounced climate drift toward a state of westerly bias in the Northern Hemisphere which is not nearly so evident in the coarser resolution model. This climate drift causes the fine resolution model to lose its superiority in predictive skill over the coarser model after only about 5 days of a typical forecast.
Following Palmer et al. (1986), a simple gravity wave parameterization scheme has been implemented in the GLA model which has reduced the westerly bias in the Northern Hemisphere, has improved forecast skill by about 12 hours and has resulted in maintaining the superiority in forecast skill of the high resolution model for at least 10 days.
The error reduction due to the gravity waves is immediately apparent in the stratosphere, where the improvement is confined mainly to the zonal mean component although spurious jet streaks in the vicinity of mountainous regions are also removed. Error reduction occurs in the middle and lower troposphere only after about 5 days and is associated with a secondary meridional circulation generated in response to the gravity wave drag in the stratosphere.
Improvements are observed in the Northern Hemisphere climatology where low level westerlies are weakened and shifted poleward and, unexpectedly, in the Southern Hemisphere where the roaring forties and fifties are considerably better simulated. The positive impact on forecast skill and climate simulation of the simple gravity wave scheme has provided motivation for the development of a more advanced scheme, and the plans for such development are outlined.
