Isentropic Analysis on the Intensification of Hurricane Edouard (2014)

An isentropic analysis technique is adopted in this study to investigate the intensification of Edouard (2014) predicted by an experimental real-time convection-permitting hurricane analysis and forecast system. This technique separates the vertical mass transport in terms of equivalent potential temperature (Θ_e) for the rising air parcels at high entropy from the subsiding air at low entropy. It is found that, as Edouard intensifies, the vertical circulation becomes wider via the expansion of upward (downward) mass flux to higher (lower) Θ_e. In the early developing stages, the asymmetric convection dominates the vertical circulation and leads to a remarkable upward mass flux maximum center in the upper troposphere. When Edouard becomes intense, the axisymmetric convection becomes important to the upper-level vertical mass transport while the asymmetric convection still dominates the low-level vertical mass transport. Development of the warm core in the eye leads to double maxima along the Θ_e axis for both the isentropic-mean relative humidity and tangential velocity. The isentropic-mean properties such as the mid-to-upper-level relative humidity, vertical velocity, and radial outflow decrease considerably while the mid-to-upper-level vorticity enhances on the high Θ_e side before the onset of rapid intensification. The isentropic analysis also reveals that, as Edouard intensifies, the eye featured with warm and dry core firstly forms in the low-to-middle troposphere and then gradually expands upwards. The above-mentioned results indicate that the isentropic framework may have the advantages of binning common variables with Θ_e that could reflect the changes of the tropical cyclone structure in the inner-core region without a prior specification of the location of a storm center.

On the basis of the thermodynamic cycles derived from the isentropic framework, the energetic analysis further shows that the total energy input for Edouard from the ocean experienced fluctuating growth in the early development of Edouard and then increased rapidly after the start of RI until the storm reached the maximum intensity. The energy input for Edouard intensification is mainly due to the variation of entropy. The decrease of near-surface entropy in the outer-core region with lower entropy and increase of near-surface entropy in the eyewall with higher entropy both contribute to the enhancement of energy input for Edouard intensification. The former induces the initial slow enhancement of energy input to Edouard while the latter is responsible for the rapid increase of energy input to Edouard after the start of rapid intensification. The input energy from the ocean mainly converts to the mechanical energy of Edouard and only a small fraction is consumed by the Gibbs penalty, which does not exhibit evident variation during the development of Edouard.