Prediction experiments of Hurricane Gloria (1985) using a multiply nested movable mesh model

The prediction capability of the GFDL triply nested, movable mesh model, with finest grid resolution of super(1) / sub(6) degree, was investigated using several case studies of Hurricane Gloria (1985) during the period that the storm approached and moved up the east coast of the United States. The initial conditions for these experiments were interpolated from an NMC T80 global analysis at 0000 UTC 25 September and 1200 UTC 22 September. The integrations starting from 0000 UTC 25 September were run 72 h, while those starting on 1200 UTC 22 September were run 132 h. The lateral boundary conditions were obtained from either an integration of the NMC T80 forecast model or the T80 global analysis, or were fixed to the initial value. The model's predicted track of Gloria for each integration was compared against the best track determined by the National Hurricane Center (NHC). For the case starting from 0000 UTC 25 September using a forecasted boundary condition, the model successfully forecasted significant acceleration of the storm's movement after 48 h. The 72 h forecast error was about 191 km, compared to 480 km for the official track forecast made by the NHC. To examine the model's skill in simulating the storm structure, distributions of the low level maximum wind and total storm rainfall during passage of the model storm are shown and compared with observed values. The model successfully reproduced many observed features such as the occurrence of strong winds well east of the storm center, with an abrupt decrease of the wind field along the coastline. When the storm track was accurately forecasted, the total storm rainfall amounts agreed well with the observed values. In both the model integration and observations, a significant structural change took place as the storm accelerated toward the north with little significant precipitation occurring south of the storm center and heavy precipitation spreading well north of the storm. It appears that the gross features of the structure of the storm's outer region resulted from the interaction of the vortex with its environment. Sensitivity of the model forecast to the lateral boundary condition and the horizontal resolution was also investigated. The storm's track error was greatly affected after the boundary error propagated by advection to the storm region. The impact of the horizontal resolution on the forecast was such that the model with one degree resolution produced a fairly good track forecast up to 48