Improved Skill for the Anomaly Correlation of Geopotential Height at 500 hPa
T.N. Krishnamurti, K. Rajendran, T.S.V. Vijaya Kumar, Stephen Lord, Zoltan Toth, Xiaolei Zou, I. Michael Navon, and Jon Ahlquist
This paper addresses the anomaly correlations of the 500 hPa geopotential heights from a suite of global multimodels and from a model-weighted ensemble mean called the superensemble. This procedure follows a number of current studies on weather and seasonal climate forecasting that are being pursued. This study includes a slightly different procedure from that used in our experimental forecasts for other variables. Here we construct a superensemble for the Laplacian of the geopotential based on the daily forecasts of the geopotential fields at the 500 hPa level.
The geopotential of the superensemble is recovered from solution of the Poisson equation. This procedure appears to improve the skill for those scales where the variance of the geopotential is large and contributes to a marked improvement in the skill of the anomaly correlation. We note especially large improvements over the Southern Hemisphere. Consistent day-6 forecast skill above 0.80 is achieved on a day-to-day basis. The superensemble skills are higher than those of the best model and the ensemble mean. For days 1 through 6, the percent improvement in anomaly correlations of the superensemble over the best model are 0.3, 0.8, 2.3, 4.8, 8.6 and 14.6% respectively for the Northern Hemisphere. The corresponding numbers for the Southern Hemisphere are 1.1, 1.7, 2.7, 4.5, 7.1 and 12.2% respectively. At forecast day-5 and day-6, the superensemble realizes major improvement of anomaly correlation skills. The collective regional strengths of the member models, which reflected in the proposed superensemble, provide a useful consensus product that may be useful for future operational guidance.