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Effects of vertical differencing in a minimal hurricane model

The minimal three‐layer hurricane model developed by the authors is reformulated with a Charney–Phillips grid (CP‐grid) in the vertical instead of the Lorenz grid (L‐grid) that was used in the original model. It has been shown by others that the popular L‐grid supports a computational mode in the temperature field that can lead to inaccuracies, especially when moist processes are involved. Here we compare calculations of the hurricane model using the L‐grid and CP‐grid. We present evidence that the computational mode is excited in the L‐grid model during the period of rapid vortex intensification, which begins when grid‐scale latent‐heat release occurs in the core region. Thus it would appear that the solution in the mature stage of evolution is contaminated by this mode. We show also that the vortex asymmetries that develop during the mature stage are sensitive to the choice of vertical grid and argue that those that occur in the CP‐grid formulation are more realistic. It is possible that the computational mode is a spurious feature of many earlier studies of asymmetries in hurricane models in which moist processes are represented. Copyright © 2003 Royal Meteorological Society