Evolution of CAPE in idealised simulations of tropical cyclogenesis

Several theories dealing with the intensification of tropical cyclones exist but a complete and thorough understanding of the mechanisms at work is yet to be disclosed. Several factors and conditions are being investigated whereby some authors are found to be in direct disagreement with each other. One such factor, of the greatest interest but of unclear status, is the role the convective available potential energy (CAPE) plays in the intensification of tropical cyclones.

To investigate CAPE'S role in tropical cyclogenesis, as a substitute to the 3D modeling we intend for the future, idealised axisymmetric tropical cyclone simulations have been performed using the cloud resolving non-hydrostatic model CM1, where a Dunion sounding for the base state was used along with Rotunno and Emanuel's vortex initialisation scheme and Morrison double-moment physics scheme to produce the cyclone. The experiments are conducted on a 1 km grid spacing with 1500 grid points. Two sets of experiments were performed in which the effect of perturbations on the evolution of CAPE was examined. In one of them the temperature profile of the Dunion sounding is perturbed, while in the other set the value of the surface transfer coefficient of enthalpy is modified. The temperature perturbations of the Dunion sounding temperature profile were such that the atmosphere was either warmed or cooled in the vertical with lapse rates of 0.5K/km and 1K/km. The surface exchange coefficient for enthalpy was increased or decreased by a factor of 2 and 4 in each of four experiments.

In both sets of the axisymmetric simmulations the rate of intensification varies drastically. The presence of CAPE plays a significant role in enhancing the cyclogenesis, especially in the earlier stages, when compared to the process in the later stages when the cyclone is achieving equilibrium. In all the temperature perturbation experiments of the axisymmetric tropical cyclone, CAPE was high in the initial stages of development but as the cyclone intensified, CAPE decreased in the inner core region. The decrease was less in the cooler atmosphere, even turning into an increase in the last two days of cyclogenesis, when the lapse rate was increased by 0.5K/km. The warmer atmosphere having an initial lapse rate of 1K/km supported a less intense tropical cyclone with only tropical storm wind speeds. The cooler atmosphere was more favourable to more intense storms. A higher surface exchange coefficient for enthalpy in the axisymmetric tropical cyclone did contribute to more intense and more rapidly growing storms. These consume CAPE very rapidly in their initial stages, while for lower values CAPE was depleted at a much slower rate. The intensification rate in the axisymmetric tropical cyclone is seen to be highly influenced by CAPE and thus changes in the way the cyclone intensifies. These results shed new light on the role and significance of CAPE.