As a continuation of the previous study (Cheung 1997) on ensemble forecasting of tropical cyclone (TC) motion in which the perturbations were added to the environmental flow, this paper illustrates some possible directions of extending such work. As in Part I of the study (Cheung and Chan 1998a), Part II (Cheung and Chan 1998b) uses the same 66 cases and the Final Analyses of the Tropical Cyclone Motion (TCM-90) Experiment (Rogers et al. 1993) for running the barotropic model. The difference is that only the vortex circulation is perturbed. Two series of experiments are performed. The first consists of three experiments applied to the original analyses without any bogussing procedure, and their techniques of generating perturbations are similar to those for perturbing the environment. The second series adopts a bogussing technique to simulate the various possible uncertainties in determining the vortex structure.
The vortex structure in the analyses is found to be very sensitive to random noise added to it, and often leads to unrealistic forecast tracks. When only the initial position is perturbed, the skill differs little from the control forecast, so its utility is also low. The remaining experiments show similar average skill when verified both under the perfect model assumption and by the best tracks. In the former, improvement over the control forecast can be obtained from the ensemble mean in some cases. However, in situations in which the vorticity centre cannot be well identified during the integration of the model the forecasts failed. In experiments with a spun-up bogus vortex, the configuration with the best performance is perturbing the parameters for generating the b-gyres and persistence vector simultaneously. It can outperform the other experiments after 48 h. This set of ensemble may therefore be suitable for substituting those without bogussing when the latter is necessary.
The limitation of the barotropic dynamics in the above experiments is to be removed by generating multiscale perturbations using a full-physics baroclinic model. That is, the perturbations are bred first in a planetary scale, and then fed into regional- and storm-scale models. The purpose of this kind of experiments is two-fold: to verify the above work based on simplified dynamics, and to identify new effects when some of the baroclinic processes essential to the development of a TC are also perturbed. The results of these experiments will be presented in an integrated way at the conference.
References
Cheung K. K. W., 1997: Ensemble forecasting of tropical cyclone motion using a barotropic model. Preprints, 22nd Conf. Hurr. Trop. Meteor., Fort Collins, Amer. Meteor. Soc., 627-628.
Cheung K. K. W., and J. C. L. Chan, 1998a: Ensemble forecasting of tropical cyclone motion using a barotropic model. Part I: perturbations of the environment. Mon. Wea. Rev. (modified)
Cheung K. K. W., and J. C. L. Chan, 1998b: Ensemble forecasting of tropical cyclone motion using a barotropic model. Part II: perturbations of the vortex. Mon. Wea. Rev. (modified)
Rogers, E., S. L. Stephen, D. G. Deaven and G. J. DiMego, 1993: Data assimilation and forecasting for the Tropical Cyclone Motion Experiment at the National Meteorological Center. Preprints, 20th Conf. Hurr. and Trop. Meteor., San Antonio, Amer. Meteor. Soc., 329-330.