On the entrainment factor and its importance in numerical modeling of hurricanes

Abstract

ln some numerical models of hurricane development (Ogura, 1964: Ooyama, 1964), the proportionality or entrainment factor η, that relates frictionally induced vertical motion at the top of the friction layer and the release of latent heat, is taken to be constant and equal to 3. This paper presents the results of a number of experiments, using the author interpretation of Ogura's model, in·which η, is given various constant values, and also in 2 experiments is allowed to vary with time as a function of the vertical distribution of the equivalent potential temperature, which is computed at 3 levels, under sorne assumptions. The results of the experiments show that there existís a strong dependence of the growth rate of the disturbance, on the value assigned to η, This dependence was indicated in the linear analysis of Ogura (1964) and Ooyama (1964); although the critical value for increase of the maximum velocity indicated from the experiments, seems to be larger than the one indicated for development in the linear analysis. To test the reality of the assumed values η, was computed in the same way as in the experiments for 2 situations in which a hurricane was involved. and also for the hurricanes Daisy, 1958 and Hilda 1964, as well as for some mean soundigs of the tropical atmosphere. All the values obtained for η, seem to be smaller than those required to be in accordance with the results of the experiments.