In VTL method, the ensemble perturbations at three hours before and after the analysis time was shifted to the analysis time, which results in an increase of the ensemble size from 80 (ENS80) to 240 (VTL240M80) in 4DEnVar update while both experiments will still keep using the original 80 members in EnKF update. VTL240M80 only increases 10% of the cost relative to ENS80. Estimations of both the self- and cross- variable error correlation by VTL240M80 were improved over ENS80. Specifically a larger improvement was seen for the small correlation than the large correlation. There was no significant spread difference in both experiments. VTL240M80 improved both the global forecasts and the tropical cyclone track forecasts over ENS80. The analysis generated by VTL240M80 is more balanced than ENS80. While VTL240M80 increases cost by 10% and directly increasing ensemble size to 240 (ENS240) triples the cost, VTL240M80 recovers more than 50% of the improvement of ENS240.
In ITL method, combining with the original 80 members, three (twelve) lagged groups with 80 (20) members in each lagged group were selected to configure the 320-member experiment of ITL320M80 (ITL320M20). Due to the use of the longer forecasts, the cost of ITL320M80 almost doubles that of ENS80 and halves the cost by directly increasing the ensemble size to 320 (ENS320). But ITL320M20 does not incur additional cost since the 20-member longer forecasts are freely available in the Global Ensemble Forecast System (GEFS). Estimation of the error correlation in ITL320M80 didn’t show a significant difference from ENS80 while ITL320M20 show a significant degradation. Both experiments exhibited a larger spread and caused more imbalance than ENS80, especially for ITL320M20. In terms of the verification of global forecast and tropical cyclone track forecasts, ITLM320M80 generally showed neutral or negative impacts except that an improvement was seen for the temperature forecasts, while ITL320M20 showed consistent degradation.
In ITLS method, the scaling coefficients were calculated by comparing the spread in each lagged group with the original 80-member ensemble. Following the configuration in ITL method, ITLS320M80 and ITLS320M20 were carried out. Results from ITLS method didn’t show a significant difference from ITL method except that the larger spread in ITL method was reduced.